Prehistoric Mammal Discoveries 2025

Computer reconstruction of the skeleton of the Miocene
 right whale Megabalaena, published this year

As another year draws to a close, it's time to take another dash through the mammalian palaeontological findings of the last twelve months. As always, this will be a rapid survey of some discoveries that I find particularly interesting without any detailed discussion, but hopefully covering as wide a range of prehistoric mammals as I can.

Large Herbivores

Land mammals don't come much larger than mammoths, and such animals, and their mastodon relatives, have shaped the world around them through their mere presence and the food they chose to eat. A study on southern mammoths (Mammuthus meridionalis) uncovered at a million-year-old site in northern Spain showed that the climate at the time was already Mediterranean, a warm gap within the Ice Ages, causing them to feed more on grasslands than trees. While hyenas had fed on some of the carcasses, they were not alone, as the evidence also shows clear signs of butchering with stone tools.

Elsewhere, a new study found that the South American gomphothere, Notiomastodon, which had entered the continent when the Panama Land Bridge formed, had a diet rich in fruit. When they went extinct as the climate warmed 10,000 years ago, their disappearance spelt trouble for the plants with large, fleshy fruit that had relied on them for seed dispersal.

But if Spain had a reasonably modern Mediterranean climate one million years ago, that obviously didn't last. Although we know they must have existed earlier, until this year, the oldest known cold-adapted mammals from Spain dated to no more 190,000 years ago, during the second-to-last Ice Age. This year, however, a reindeer tooth was described from the country that could date back as far as 300,000 years, during the Ice Age prior to that - it's the most southerly known reindeer fossil ever discovered. 

On the subject of deer, a new analysis of the DNA of the Toronto Subway deer (Torontoceros hypogaeus) discovered during the construction of the eponymous transport system suggested that, while undeniably a distinct species, it might be more closely related to the living white-tailed deer than previously thought. It lived in open habitats, and was driven to extinction at least partly by the spread of the forests that now cover the area.

Studies of isotope ratios in the short-legged hippo-shaped rhinceros Teleoceras from Nebraska showed that they lived in much in the same areas throughout their lives, not migrating, or even moving as far as we would expect when they reached adulthood and left their parents. This may have been because they relied on habitats with plenty of water to wallow in, and they just couldn't find them elsewhere. Over in eastern Siberia, the longest ever fossilised horn from a woolly rhino (Coelodonta antiquitatis) was described; it was over 164 cm (65 inches) long, far larger than those of any living rhino.

Fossils are not necessarily of things like bones and horns, however. This year, the oldest known fossilised cowpat was described from 20 million-year-old deposits, also in Nebraska. At least it looks like a cowpat - it's about the right size and shape, and it's obviously dung - but cows didn't exist that far back, and, honestly, neither did anything closely related to them. So, probably a ruminant of some sort, but what kind will likely always remain a mystery.

Carnivores

Europe is devoid of big cats today, unless you count the occasional lynx. This was not always so, however, and a publication this year expanded the range of the European jaguar (Panthera gombazagoensis) to Poland and to Suffolk in England between 700,000 and 350,000 years ago. Another large cat, the famous sabretooth Smilodon fatalis, was discovered further south than ever before - in Uruguay. 

We already knew about leopards (the modern species) in the Pyrenees during the Ice Ages, but their fossils are rare. A new study of their overall distribution and history showed that they became steadily larger as the Pleistocene wore on, and also that males and females were more similar in size than in living members of their species. Moreover, they showed a preference for mountainous habitats, and their body shape shifted towards that of modern snow leopards prior to their local extinction.

The largest mammalian carnivore during the Ice Ages in North America was the giant short-faced bear (Arctodus simus), with some individuals estimated to have weighed up to 950 kg (2,000 lbs). However, some fossils were significantly smaller than this and had been suggested to represent a distinct subspecies. Genetic analysis this year was unable to find any significant differences between the larger and smaller specimens except for one: all the large ones were male, and the small ones female...

And, well, yes, I should probably mention this, since it was all over the news. This year, a company claimed to have returned the dire wolf (Aenocyon dirus) from extinction, in the form of three, rather cute, white puppies. Almost nobody outside the company with any expertise agrees that this is really what they have done, simply adding a few dire wolf-like genes to what are rather obviously grey wolves.

Cetaceans are also carnivores, although descended from creatures that were not. An analysis of the skull of the very primitive dolphin-sized Protocetus showed that it had already developed the large brain that cetaceans are known for today but, more importantly, that it still had a keen sense of smell. Modern whales and dolphins have no use for smell but Protocetus likely spent at least some of its time on land, giving it a very different lifestyle.

Among more recognisable whales, Idiophorus is either the most primitive known sperm whale or a close relative of their last common ancestor. Recovered from Miocene age deposits in Patagonia, it had a long snout shaped like a wine bottle (so hardly like a modern sperm whale) and was around 6.6 metres (22 feet) in length. The key fossil was thoroughly re-examined this year for the first time since it was uncovered in the 19th century, revealing that it was a deadly predator feeding on relatively large vertebrate prey, unlike other known sperm whale species living in the area at the time. 

At the other end of the Americas, and a few million years earlier, Fucaia was a Canadian whale belonging to a now-extinct group. Analysis of a well-preserved fossil this year showed that, like Idiophorus, it was an agile and active predator, although, partly because it was rather smaller, its hunting style was probably most like that of a modern sea lion. Despite this, its closest living relatives include the toothless krill-feeding baleen whales.

Other Placentals

Glyptodonts were gigantic relatives of armadillos, their heavy armour plating making them the mammalian equivalent of tanks. A study this year provided the first analysis of damage to that armour in three South American species, adding to those already performed on more common ones. It also confirmed that, like certain dinosaurs, they likely did wallop each other with the clubs on their tails, likely while competing for mates or territory.

Ground sloths have a long evolutionary history, much larger relatives of the small(ish) tree-dwelling animals that are their only remaining close relatives. A new analysis shows how they gradually increased in size as the climate shifted, but that, probably to the surprise of few people, it was not climate change that eventually drove them to extinction, but the arrival of humans. 

Look at any reconstruction of a ground sloth, and it will inevitably show a large, somewhat shaggy animal. But it had been argued that, like elephants and rhinos, the largest ground sloths may not have had much fur at all, being so big that they did not lose much body heat anyway. It was a minority view, but geochemical analyses and simulations published this year seem to put the nail in its coffin. Given the relatively cool environments in which they lived, the simulations suggest that even the largest species would have needed a pelt of fur 10mm (0.4 inches) thick, and some would have needed fur three to five times that long. This, despite the fact that palaeothermometric analysis in the same study showed that they had unusually low body temperatures for their size.

Several studies this year took a look at the development of increasing brain size during primate evolution - primarily monkeys and apes. They did not all come to the same conclusions, which, while they don't necessarily contradict one another, either, suggests that there was probably a lot going on rather than one single answer. One study pinpoints visual processing as the main factor, with visual parts of the brain becoming disproportionately large before the rest caught up, something that would make sense for an animal that had to navigate through tree branches.

Another points to socialisation, with parts of the brain responsible for some of our most complex behaviour starting to enlarge early on. A third points out that brain enlargement correlates, to some extent, with the elongation of our thumbs (which are obviously very different from those of most non-primates), implying that fine manipulation of objects may have been another key element in the drive for greater intelligence.

Going further back in time, another study this year challenged the traditional view of primates originating in tropical jungles. Instead, it suggests that the very first primates appeared in cold and temperate habitats in the north, the changeable climates of such areas promoting the evolution and dispersal that allowed them to reach the tropics later on.

Among the rodents, analysis of a new fossil and comparison to others already known shed light on the origins of the group that includes the gophers. The fossil belongs to an unusually large relative of the ancestor of gophers and kangaroo rats, and seems to have already been a skilled digger. This supports existing theories that the ancestral gopher looked like, and had a similar lifestyle to, today's pocket mice.

An unusually complete fossil free-tailed bat found north of Marseilles in France dates back over 30 million years to the Early Oligocene. The shape of the wings and muscular anchors in the body shows that it was fast and capable of sustained flight, probably catching insects in the air as it flew over what was then a large lake.

Conoryctes was a medium-sized burrowing mammal living in North America just 3 million years after the extinction of the non-avian dinosaurs. It's probably a placental mammal, but it has been argued that it may actually be an early relative that's simply closer to true placentals than to marsupials. A study of the microscopic structure of its bones shows that it must have grown rapidly, reaching adult body size in a single year and that it would have been weaned at about the age we would expect for a true placental of its size. So even if, by some chance, it wasn't literally a placental mammal, it lived very much like one.

Marsupials and More

The best-known and distinctive marsupial species are probably the kangaroos and wallabies. All but one living species belong to a subfamily that spread through Australia starting around 11 million years ago. The oldest known fossil belonging to this subfamily belongs to Dorcopsoides, which may hold some clues as to why the group became so succesful on their home continent. A new analysis published this year found that it was a more efficient hopper than had previously been thought, suggesting that it was already adapted to open environments, something that was just starting to become more common at the time, as the Australian outback began to dry out. 

It may have helped that, as another new analysis of the wear patterns on their teeth showed, Ice Age kangaroos had a wider diet than was apparent from their anatomy alone. On the downside, if they were adaptable, the fact that so many kangaroo species went extinct around 40,000 years ago is probably not primarily due to the change towards a colder climate...

While fairly detailed DNA analysis is possible for mammoths and other Ice Age animals preserved in permafrost, it is much harder for animals of similar age in warmer climates. A new study this year therefore used the molecular structure of collagen from sinews and other tissue to confirm the relationships between some of the marsupials that went extinct in the last 100,000 years. Perhaps the most significant finding was that the closest living relative of the extinct carnivorous marsupial lion Thylacoleo may be the koala, rather than it being an earlier relative of the koala-wombat common ancestor.

On the subject of carnivorous marsupials, these were at least as prominent in South America in prehistoric times, dominated in particular by the large sparassodonts. A new analysis of the skulls of South American marsupials this year showed that, while most had similarly shaped (elongated) brains to modern marsupials, that of the sabretooth marsupial Thylacosmilus was more rounded, due to its shorter, cat-like face. More significantly, revised estimates of its body weight showed that its brain was smaller than previously thought, in the range of some of its earliest ancestors, its size having evolved less than expected.

Similar analysis of an armbone belonging to the mysterious mammal Kryorectes confirmed that it was, as previously thought, a monotreme. Living alongside Australian dinosaurs a whopping 106 million years ago - 40% further back in time than Tyrannosaurus rex - the study also showed that it was probably a semiaquatic burrowing animal. The platypus lifestyle may be truly ancient. 

Finally, the multituberculates were neither placental, marsupial, nor monotreme but a fourth grouping that survived well into the Age of Mammals. An analysis this year of their habitats and distribution across North America suggested that, at least on that continent, they were restricted to damp temperate forests dominated by redwoods and swamp cypress and that they finally disappeared when those forests dramatically declined as the world cooled at the end of the Eocene. This contradicts the popular theory that it was competition with newly arrived rodents that killed them off, since they were apparently living elsewhere at the time.

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Synapsida is taking a break for the holiday period and will return on the 4th January

[Image from Yanaka et al. 2025, available under CC-BY-4.0]

Age of Mammals: The Eocene (Pt 1)

When Scottish geologist Charles Lyell first created the system of epochs we now use for dividing the Age of Mammals, he designated four of them. This was in 1833, so he did not know the true age of the Earth, let alone the timespans of the epochs he was naming - he was basing them purely on geological strata and the types of fossil seashells found within them. We now know, however, thanks to the wonders of radiometric dating techniques, that the oldest of the four epochs he defined spanned over half of the Age of Mammals, longer than the other three put together.

Two epochs have been carved out at either end since, so the Eocene is not quite as long now as it was when Lyell named it. However, it remains the longest of the seven epochs since the extinction of the non-avian dinosaurs, still occupying a third of that entire stretch. As currently defined, it runs from 56 to 34 million years ago. Compared with the entire age of the Earth, that's not very much, but from the point of view of most mammalian palaeontology, that's unusually long. 

Viverrids: Civets Great and Small

Small Indian civet

Most species of true civet weigh somewhere between 7 and 9 kg (16 to 20 lbs), somewhat larger than a domestic cat, and similar to a King Charles spaniel or a Highland terrier - albeit of a more slender shape. The large Indian civet (Viverra zibetha) is at the upper end of this range, the second-largest species of true civet. The name of the large Indian civet, however, tends to imply that there must also be a smaller version in the same general area, and so there is.

The small Indian civet (Viverricula indica) is the smallest of the true civets, between 48 and 68 cm (19 to 27 inches) long, not including the tail, and weighing just 2 to 4 kg (4½ to 9 lbs) - more of a toy poodle or a Pomeranian in terms of dog weight. Otherwise, it looks much like its larger cousin, although it has block spots like a Malay civet, rather than the fainter blotches of its larger namesake, and does not have the crest of erectile hair running down its back. The smaller size caused it to be placed in a separate genus from the other civets as early as 1838, and modern genetic analysis has shown that this is fair; the two parted company about 12 million years ago, while the other Asian civets are much more recent than that.

Climate, Cloud Forests, and Cotton Rats

The Cricetidae is the single largest family of mammals in terms of the number of species, at least according to the current count from the American Society of Mammalogists. While the name translates as "hamster family", the great majority of species are not themselves hamsters. In fact, there are five subfamilies of cricetid: the hamsters, the voles, and no fewer than three with members that basically look like mice or rats... even though the true mice and rats belong to the second-largest family, the Muridae.

The largest of these subfamilies is the Sigmodontinae, consisting of mouse and rat-like animals primarily native to South America, although one species lives as far north as Virginia, and several others reach Arizona and New Mexico. The group is named for the S-shaped pattern on the molar teeth and was originally coined as the genus name for the cotton rats in 1825.

Splitting the Troop

Primates are, for the most part, social, group-living animals. This underlies many aspects of human behaviour and likely played a role in our development of intelligence. Often, these groups have a fission-fusion structure, where new members come and go, but, in other species, they can be long-lasting and stable. Either way, just as with nation-states or tribal societies among humans, nothing lasts forever. Groups die and new groups form.

This can be due to disaster or misfortune, but it can equally well be due to success. If a group becomes too large, there may no longer be enough food in the local area to keep it healthy, or parasites or disease may spread too rapidly within it. Or it may simply become too large for dominant individuals to control. In fission-fusion societies, this may lead to a temporary break-up into local subgroups that otherwise remain in contact. Sometimes, however, the pressure is too great and the only solution is for a new group to form.

Viverrids: Civets of Southeast Asia

Large Indian civet

Our definition of the Viverridae family has, as I noted previously, undergone some ups and downs over the centuries. It was first named, in 1821, for the genus Viverra, which, when it was first described, had contained five species. Even by 1821, four of those had been moved elsewhere - and three are no longer even in the family. But, by the rules of scientific naming, unless we scrap the family entirely, Viverra must remain within it. Which leaves one species that, in a sense, defines the family, and against which everything else in it is compared.

That species is the large Indian civet (Viverra zibetha). Thus, even though "viverra" literally means "ferret" in Latin, this means that we can reasonably call the Viverridae "the civet family", as I will be doing from here on in.

The Air Conditioning in Your Nose

The nasal cavities are not mere holes running through the head. Anatomically, at least, that's broadly true at either end - in the vestibule immediately behind the nostrils, and in the nasopharynx above the throat. But in between, in the area above the mouth and separated from it by the palate, the air instead must pass through defined channels. 

These channels are formed by the "conchae", projections from the outer side of each nasal cavity stretching almost to the inner surface, so that most of the air is forced through the narrow slots between them. These conchae are, in turn, formed by the turbinate bones, delicate, paper-thin, sheet-like structures rolled up like a scroll, and covered in the same sort of fleshy lining that we find in, say, the trachea (windpipe). 

Oligocene (Pt 18): Sawfish-Dolphins and Baleen Whales with Teeth

Olympicetus, a simocetid whale

Although the oldest fossils of seal-like animals may date back to the end of the Oligocene, there are very few of these, and the dating may not be wholly reliable. Whales, however, are a different matter and were already well established even at the beginning of the epoch, 33 million years ago. In fact, the Oligocene marks an important phase in their evolution, since it was at this time that the oldest living groups first appeared and that, potentially, the last common ancestor of all living whales roamed the seas.

Even so, especially towards the end of the epoch, it is possible to place some Oligocene cetaceans into groups we are familiar with today. For example, there was Kentriodon, which is better known from the Miocene, but first appeared in the southern oceans at the tail end of the Oligocene. Although it is not placed in any living family, it is the oldest member of a branch that diverged from the common ancestor of dolphins and porpoises around this time. It likely looked rather similar and had a similar fish-and-squid-based diet.

Viverrids: Rise and Fall of a Wastebasket

The carnivorous mammals have been recognised as a taxonomic order since the official dawn of biological classification in 1758. In that first publication, Systema Naturae, Carl Linnaeus listed 36 species. The concept of "families" was a later innovation, but Linnaeus used the rank of "genus" much as we would use families today, and, in the case of what we now call the carnivorans, there were six.

These were the cats, dogs, bears, weasels, seals, and a sixth group that he called (in Latin) "ferrets".

The Family Life of a Spectral Bat

It may not be Halloween just yet, but it is October, so that's as good a time as any to talk about an animal that goes by the name of the spectral bat (Vampyrum spectrum).

You might think from the scientific name that this is a close relative of the vampire bat but, while it does belong in the same family, vampire bats are a side-branch of that family thought to have diverged from the main branch around 50 million years ago. Its closest relative may be the far less fearsomely named "big-eared woolly bat" (Chrotopterus auritus), with which it shares some of its unusual feeding habits. 

Delphinids: Killer Whales

Orca / killer whale

The dolphin family is scientifically defined as including all the species more closely related to the common dolphin than they are to porpoises. The great majority are what we'd normally think of as "dolphins" but four species are so much larger that, instead, we tend to call them "whales". Three, including the two species of pilot whale, are of roughly similar size to each other, but the fourth is noticeably larger still.

It's the biggest "dolphin" of all: the orca or killer whale (Orcinus orca).

That it is a dolphin has never been seriously doubted from a scientific perspective. It is one of just three species of dolphin to be listed as such in the first catalogue of scientific names in 1758 - and one of the other two is a porpoise, and so has since been moved elsewhere. On the other hand, it has been recognised as belonging to a distinct subfamily with the dolphins since 1846, and modern genetic studies confirm that its ancestors diverged from those of most other dolphins unusually early. 

South Africa, 14,000 BC

Paleoecology is the study of how animals and their environments interacted in the distant past. While the basic idea has been around almost since we started the scientific investigation of fossils, it really only became a field in its own right around the 1950s. That's largely because it isn't easy, becoming harder the further back we go.

The basis of the field is to look, not at individual fossils, but at the whole array of fossils at some particular site, correlating them with what we can determine of the climate and environment at the time. Which, among other things, requires a good understanding of exactly what that time was and at least a reasonable confidence that the fossils in question are all around the same age. Often, it relies not just on good and plentiful fossils at a particular site, but on us being able to say what the animals' lifestyles were. Which is a lot harder for those that don't resemble the ones we have today - dinosaurs being an obvious case in point.

Hungry Hippos

The hippopotamus (Hippopotamus amphibius) is an unusually large animal. Among land-dwelling mammals, only the elephants and some species of rhinoceros are larger. It is also, like elephants and rhinos, herbivorous and, since it prefers to eat plants that aren't especially nutritious, this means it needs to eat a lot. 

As in, it eats 35 to 50 kg (77 to 110 lbs) of food each day. 

Which is fine if the hippo happens to be out in the wild, far from human interference. But the reality is that there are fewer and fewer such places around these days. It's not so much the urban sprawl that humans bring, or even the roads and other infrastructure of an expanding African economy, but more the cropland that's required to feed us all. Although hippos are hunted for the ivory in their teeth, the number one threat to their survival is probably the expansion of farmland. Compared with many other animals, this is exacerbated by their reliance on large amounts of fresh water, so even if the farmland isn't near them, they suffer if water is diverted to where it is needed for crops. 

Bats in the Belfry

Mammals, like other animals, need a safe place to sleep. For large animals living in herds on the open plains, safety in numbers may be the best they can do, with some keeping guard while the others perhaps try to hide in long grass. Hiding in trees or sleeping on rocks out in the ocean are valid options for those in the right habitat. For many others, however, especially the smaller ones, some kind of den, nest, or burrow provides just the ticket. In the case of bats, we have roosts.

When it comes to bat roosts, it's likely that most people initially think of caves. Caves can hold communities of thousands of bats, often with many different species sharing the same one. Caves are ideal roosts for bats, because they provide a stable environment safe from the weather, few predators will enter them, and, when you're nocturnal anyway, you don't care that it's dark. 

Delphinids: Pilot Whales

Long-finned pilot whale

The term "dolphin" is not, strictly speaking, a scientific one. It refers, in common parlance, to any small cetacean, often even including porpoises. Even ignoring the porpoises, however, not all dolphins are members of the dolphin family, technically referred to as "delphinids". This is because some freshwater animals are not closely related to the dolphins proper (or, indeed, to the porpoises). We call them "dolphins" because they're about the right size, a similar shape, and... well, we don't have a better word, at least for them all collectively.

But it works the other way, too. Not all members of the dolphin family are commonly called "dolphins". With the exception of the melon-headed whale, which it's hard to think of as anything other than a dolphin, this is because they're too big. We call them "whales" - another term that doesn't map to anything scientifically - since that's what we call any large cetacean.

Oligocene (Pt 17): Dawn of the Kangaroos

Ekaltadeta

Kangaroos are perhaps the single most iconic Australian mammals. As marsupials, we tend to think of them as lost relics of an earlier evolutionary period, and, indeed, they have been around for a long time. Of course, they have been evolving during that time, rather than standing still, but if we had a time machine, we could go back millions of years into Australia's past, and still find animals that were, more or less, kangaroos. But, obviously, there is a limit.

Exactly how far back that limit is partly depends on how kangaroo-like you want your kangaroos to be. But even then, there are some gaps in our knowledge that don't have direct counterparts on other continents. The obvious place to start is with the fossil record, and, here, at least, we can provide a clear answer. The oldest known fossil kangaroos date to around 28 million years ago, towards the end of the Oligocene.

Delphinids: Right Whale Dolphins

Northern right whale dolphin

Dolphins are familiar animals. We see them at aquaria and boat trips to see them in the wild are relatively common. In recent decades, there has been a rise in 'swimming with dolphins' tourist experiences, which studies have shown to be good in the short term for humans, but less so in the long term for the dolphins. Either way, we know a fair amount about them, both culturally and scientifically and, depending on the part of the world you live in, there may be many different species that you can see.

Some dolphin species, however, are less well-known than others. I've covered some already, but perhaps the most obscure are the right whale dolphins. 

Fishing for Salmon (When You're a Bear)

Bears like eating fish. Among the most iconic images of brown bears (Ursus arctos) are those that show them wading out into a wide river or by a waterfall, and catching salmon for their food. Yet this isn't necessarily an image of everyday ursine behaviour.

This is because wide rivers, whitewater rapids, and so on, aren't all that common. Or at least, they don't form the majority of bear habitat. We watch and photograph bears feeding in such places because it looks dramatic and, more importantly, it's relatively easy to do. It's the same with other predators. We know a fair amount about the hunting habits of wolves and lions because we can watch them in Yellowstone Park or the Serengeti, where the terrain is wide open. That allows us to safely observe their behaviour from a distance, so, understandably, we'd prefer it where possible.

Delphinids: Newest and Largest Dolphins

Fraser's dolphin

One of the largest features at the British Natural History Museum is a full-scale model of a blue whale, occupying a large chunk of one of the mammal halls. This was installed in 1938 by Francis Fraser, a Scottish zoologist with a lifelong interest in cetaceans. Eighteen years later, still working at the museum, he was put in charge of reorganising their collection of cetacean skeletons and came across one that hadn't been closely examined since it had arrived in 1895.

It had been donated by Charles Hose, a colonial administrator and amateur naturalist who had found the skeleton on a beach near a river mouth in Sarawak (then a British Protectorate). Hose hadn't been quite sure what it was, and simply labelled it "white porpoise ? Lagenorhynchus sp." before sending it on. When Fraser examined it, however, he soon realised that it couldn't possibly be what Hose had guessed and that it was, instead, an animal previously unknown to science.

American Moles in a Spanish Crater

Eastern mole

Moles are unusual animals. Most species are highly adapted for digging, spending almost all their lives underground, making them vulnerable to predators when they have to venture onto the surface. One might think, therefore, that they would not have dispersed widely across the globe and that it should be easy to trace their evolutionary history.

However, this is not the case. For one thing, moles are found across the Northern Hemisphere, in Europe, Asia, and North America. A million years is, after all, a very long time and moles have been around far longer than that - including some times when the Bering Straits were dry land. 

How the Lemming Got its Coat

The Ice Ages were, without doubt, the most dramatic natural climatic changes on Earth in the last few million years. The last one was particularly severe, with vast ice sheets covering much of northern Europe and northern North America. This, naturally enough, forced many species of animals in the Northern Hemisphere to move south. Even those well-suited to the cold, such as reindeer, musk oxen, and woolly mammoths, would have had to avoid the barren ice sheets, even if they were happy in the broad tundra belt to the south.

In Europe, in particular, there is only so far south you can go before hitting the coastline. This meant that many animals were forced into small areas, some of which may still have been marginal habitat for them, to avoid extinction. These areas are called "refugia", and their small size and isolation were a driver for evolutionary change. Sometimes populations were split apart for so long that they became separate species and, for example, we can date many species of northern birds to this time, even though, for them, the Mediterranean would not have been an issue. 

Delphinids: Dolphins in the Irrawaddy

Irrawaddy dolphin

While we normally think of dolphins as being sea-dwelling animals, there are no fewer than eight species referred to as such that are commonly found in rivers. Six of these, however, are not true members of the "dolphin family", or Delphinidae, their ancestors having split off from that group even before those of some of our modern whales did. Of the two exceptions, one is entirely freshwater, and lives in South America. The other is more varied in its habitat and lives in Asia.

The Irrawaddy dolphin (Orcaella brevirostris) was first described in 1866, from a specimen caught, not in a river, but off the northeast coast of India. We now know that this is at the far western edge of its range, and that it is also found all along the coast from northeast India, around the Malaysian Peninsula, to as far east as southern Vietnam. It is also found further south, around Borneo and along the north coasts of Sumatra and Java. In 1999, a very small population was discovered in the Philippines, living in a couple of isolated bays very far from the remainder of the animal's range, presumably the result of some having been swept away in a storm decades or even centuries before.

Oligocene (Pt 16): The First Monkeys in South America

A modern South American monkey

Much like the rodents, the presence of monkeys in South America has long been a puzzle. We know that monkeys evolved in Africa and that the monkeys still living in the Old World share a common ancestor distinct from, but related to, the common ancestor of the American sort. Genetic evidence shows that the split between the two lineages, which must have happened in Africa, happened a very long time ago. At some point then, early monkeys from what we now call the 'New World' group must have crossed the Atlantic, likely rafting on a floating patch of vegetation.

The Atlantic was narrower then than it was now, and ocean currents were different, but it's still a remarkable feat. It may also have been a lucky escape, since the African relatives of this first American migrant died out not long after, perhaps outcompeted by the ancestors of today's langurs, baboons, macaques, and apes. In South America, however, its descendants got almost free rein, diversifying into the five families we have today.

How to Drink Nectar

Orange nectar bat

When most people think of what bats eat, the first thing that likely comes to mind is insects. It probably doesn't make much further thought to remember that fruit bats also exist. And these are the two most common sources of food for bats, although 'fruit' in particular can cover quite a wide range of specific food types. But bats are the second largest order of mammals, after the rodents, and there is considerable variety amongst them.

This is particularly true of the leaf-nosed bats, or phyllostomids. While most formally recognised families of mammals have names almost everyone is familiar with - cats, bears, dolphins, horses, gibbons, etc. - and most of those that don't at least sound like they're actual names - binturongs, tuco-tucos, tenrecs, colugos - bat families tend to lack anything we could reasonably describe as a common name. Instead, we have bulldog-bats, and sucker-footed bats, and disc-winged bats, and so on. 

So it is with the leaf-nosed bats, which are the second-largest family of bats in terms of number of species, beaten only by the vesper bats. The family is usually divided into no fewer than eleven subfamilies, all of which have equally obscure-sounding names, and, in some cases, not even that much. It may not be obvious that, say, the spear-nosed bats are a subgroup of the leaf-nosed bats, but they are. And it's even less obvious that stenodermatines are phyllostomid, but kerivoulines are not.

Antlers and Ageing

Ageing is an inevitable fact of life. Without it, populations would rapidly expand to the point that insufficient resources existed to maintain them, unless we also do away with reproduction. And, if we do that, then the creature in question will never be very numerous, and will be wiped out by the first accident, natural disaster, or change in climate conditions to come along. This is something that has been the case since well before mammals existed, even if the nature and pace of ageing might be different for, say, an oak tree or a coral colony, or conceptually vague, as in a mycelial network.

When it comes to reproductive senescence, however, there is a difference in the way this affects male and female mammals. Females are born with a finite supply of eggs, although, in practice, this is far more than they will need, so they don't cease to be fertile simply because they run out. What actually triggers the menopause in humans is complex, even assuming no confounding health conditions, but the number of remaining egg follicles falling below a required level and thereby lowering the production of certain hormones is thought to be key.

Delphinids: The Freshwater Dolphins of Brazil

Tucuxi

In this series so far, I have generally been referring to the Delphinidae as the "dolphin family". That's a literal translation of the name and serves to distinguish it from, say, the porpoise family. However, as I mentioned in the first post, not all animals commonly referred to as "dolphins" belong in the family. Thus, when zoologists want to distinguish the family from those other animals, but want to avoid saying "delphinids", the more common term is "Oceanic dolphins". Oceans are, after all, where they are found.

With, it turns out, one exception.

The tucuxi (Sotalia fluviatilis) is unique among Oceanic dolphins is being an exclusively freshwater animal. It lives in the Amazon River and its major tributaries, mostly in Brazil, but also further upstream into Colombia, Peru, and Ecuador. Indeed, it was first formally described, by Paul Gervais in 1853, from an animal sighted in Peru, about 2,500 km (1,500 miles) from the mouth of the Amazon... and they are known to get further upriver than that, until they are stopped by features such as waterfalls.

Bast, Sekhmet, and the Egyptian Hyena-cats

Megistotherium, a Miocene hyena-cat

The majority of the land-based mammalian predators we are familiar with today belong to the order Carnivora. This is a diverse order, including such mammal families as the cats, bears, dogs, weasels, and seals. In fact, if we ignore the cetaceans and a few kinds of marsupial, they are the only large carnivorous mammals alive. But, as so often, this was not always the case, and they once shared the world with at least three other orders of predatory placental mammal (plus some marsupials far more fearsome than any Tasmanian devil). 

Two of these orders died out relatively early on, but one of them survived for much longer, producing multiple diverse species that lived across Eurasia, North America, and Africa. These were the hyaenodonts, named for Hyaenodon itself, first identified from a fossil all the way back in 1838. With so many species, they must have been successful in their day, but their numbers declined until the last two species died out in Africa and India around 9 million years ago, perhaps due to competition from the carnivorans, perhaps due to long-term climate change. Or, more likely, both.

I Ain't Afraid of No Owls

Northern bat

There is no particular reason why bats should be nocturnal. True, nocturnality has many advantages, but so does daytime activity; the real question is why so few bats fly during the day. I looked at this last year, where I mentioned that one of the main theories is that since birds evolved flight before bats did, the bats originally flew at night so that daytime predators, such as hawks and eagles, didn't try to eat them.

If so, it may be an effective strategy, since there isn't very much that eats bats on a regular basis. That isn't to say that there isn't anything, however. The bat hawk is, as its name implies, probably the single most specialised bat predator, but studies in Africa have shown that hobbies (which overwinter there), Wahlberg's eagles, and African goshawks also attack bats with some frequency, and they're probably not alone. In this part of the world, bats, as one might expect, take measures to reduce their risk of attack. Flying in large flocks may help, but it's also known, for example, that they avoid flying on moonlit nights, and, when they have to, they don't fly above the trees as they normally would, keeping themselves out of view.

Delphinids: Dolphins of the Deep Seas

Rough-toothed dolphin

To our human eyes, it's easy to distinguish the major habitat types on land. There are pine forests, tropical jungles, open prairies, deserts, mountains, and so on. When it comes to the sea, however, it's less obvious. Most maps show the sea as a solid mass of blue which is, of course, what most of it looks like from the surface. But there are different environments and habitats within it, even if they aren't necessarily arranged in quite the same way.

While for some dolphin species the only real limitation is pack ice preventing them from surfacing to breathe, most have more specific requirements. Temperature is the most obvious, with some species preferring tropical or subarctic seas, but the depth of the underlying water is also significant. Species such as common and bottlenose dolphins are most comfortable over the continental shelves, where nutrients well up from the sea bed to feed the fish and squid on which they prey. Rather more species prefer shallow waters, close to the coast. Here, the water is shallow enough for light to reach the bottom, allowing seaweed or coral to grow, which benefits a different kind of fish than those further out.

Sea Lions v. The Blob

The waters off the coast of Alaska are supposed to be cold, especially in winter. Yet, in the autumn of 2013, they cooled far less than they normally would. A great mass of warm water, 2,000 km (1,250 miles) across and around 100 metres (330 feet) deep, remained trapped in the North Pacific. Nicknamed 'The Blob', this was caused by the weather patterns over the region remaining stuck in a high-pressure mode, preventing the warm water from dissipating with the winds as it should do. With temperatures stuck at up to 4°C (7°F) warmer than normal, the high pressure did not dissipate for eight months.

When it did, tropical winds pushed the warm water up against the American coast, from southern Alaska to southern Mexico, where it basically sat until El Niño kicked off in 2015... and that kept things unusually warm for another year. Inland, this disrupted weather systems leading, among other things, to frequent thunderstorms that sparked what was (at the time) the worst wildfire season in California's history.

Oligocene (pt 15): Land of the Fire-Beasts

Pyrotherium

Even at the dawn of the Oligocene, South America had already long been isolated from the northern continents, giving it the opportunity to evolve its own distinct mammalian fauna, with many animals quite unlike those seen elsewhere. Most of these strange animals would die out millions of years later when the Isthmus of Panama finally formed. The armadillos are among the exceptions.

Armadillos first evolved on the continent during the previous epoch, if not earlier, but most of the older fossils are incomplete, making it difficult to trace their detailed relationships. The oldest reasonably complete armadillo skulls belong to Kuntinaru, first described in 2011. This lived in Bolivia towards the end of the Oligocene, around 27 million years ago and would already have looked much like modern armadillos, albeit somewhat smaller than the species most familiar to North Americans. 

Delphinids: Small Dolphins of Shallow Southern Seas

Commerson's dolphin

While related, dolphins and porpoises are regarded as distinct types of animals. Each is placed in its own family, with the two separating at least 15 million years ago. However, telling the two apart is not always easy, at least on a superficial look at their external anatomy. Porpoises are, generally speaking, smaller than dolphins and they have a blunt nose rather than a 'beak'. The problem is that we can say exactly the same about some species that really are dolphins.

In 1766, naturalist Philibert Commerson accompanied explorer Louis-Antoine de Bougainville on what would become the first successful French circumnavigation of the globe. While passing through the Straits of Magellan the following year, he spotted an unusual-looking dolphin close to the ship and sent a description of it back to France. (As a side note, later on in the voyage, it was discovered, much to the crew's shock, that Commerson's assistant was secretly a woman; she is now remembered as the first woman to circumnavigate the globe).

Cheating Little Vixens

The majority of mammal species are either polygynous, where one dominant male mates with multiple females, or promiscuous, where both sexes have multiple partners. However, while monogamy may be less common, it isn't exactly rare, either, with it having evolved several times in widely separated mammalian groups. In some cases, this is what we would term "facultative" monogamy, where animals (often large predators) are sufficiently widely spaced that it's simply difficult for a male to find multiple partners, or, if he can, they live sufficiently far apart that he can't plausibly defend more than one of them from his rivals. In others, monogamy is an essential part of the breeding process, typically because the young are too much effort for one parent to raise alone.

Delphinids: White-sided Dolphins

Atlantic white-sided dolphin

The taxonomy of dolphins is far from settled, with exactly how we should classify some species having been an open question for years. There is a good chance that the scientific names I am using for some species in this series will not still be in use in a decade, as old genera are split and the family tree re-arranged. Such is the case, for example, with the dolphins of the genus Lagenorhynchus.

The genus was named by John Edward Gray in 1846 for a specimen of a previously unknown species sent to him for analysis at the British Museum, after having been caught somewhere off the coast of Norfolk. It translates as "bottle-nose", for the shape of the beak... which is, perhaps, unfortunate, given that the animal we refer to in English as the "bottlenose dolphin" is something else entirely. Over the centuries since, five new species have been added to the genus, giving us the six we recognise today.

The Diets of Parallel Pigs

Peccaries from South America

The word "ungulate" refers to a broad category of mammals that are generally large and herbivorous. They form a natural evolutionary group, although only if you include the cetaceans, which are descended from vaguely hippo-like ancestors. The word comes from the Latin word for "hoof", and the great majority of living ungulates have hooves of one kind or another. Even ignoring the cetaceans there are exceptions, but it doesn't take a great stretch of the imagination to see that camels, for instance, are at least the same general kind of animal as deer or cows.

All living ungulates can be placed into one of two mammalian orders: the artiodactyls, which includes the cloven-footed kinds, and the perissodactyls, which includes, among others, the horses. If you start looking at fossils, however, it becomes clear that other ungulate orders once existed. Most were very early, dying out in the first half of the Age of Mammals, representing early branches in the great ungulate family tree that ultimately proved unsuccessful.

Of Pregnancy and Progesterone

Female mammals, like those of many other creatures, are not permanently fertile. Instead, they go through regular cycles, ovulating at intervals under the control of hormonal signals produced by the pituitary gland and the ovaries. In humans, this manifests as the menstrual cycle, but this is a relatively unusual feature of our species.

Whether or not other mammals menstruate may depend on your exact definition of the term. Chimpanzees certainly do (and, indeed, rarely experience menopause), and it's present to a variable extent in other apes and Old World monkeys. In New World monkeys it's microscopic and it's completely absent in lemurs. At least some bats menstruate, as do sengis (elephant shrews) and, so far as we know, just one species of rodent.

The Earliest Landfowl

Gallinuloides

This post will be the latest as of 1st April, so if you've been following this blog for a while, you'll know what that means... 

The humble chicken (Gallus domesticus) is a member of the pheasant family. This is a moderately-sized family, with around 180 species. Alongside the chicken and its wild ancestor, this also includes, not just pheasants, but many similar ground-dwelling birds, such as grouse, partridges, true quails, turkeys, and peacocks. 

It is, in turn, a part of a larger taxonomic group technically referred to as the Galliformes, or more commonly the "landfowl". The other four living families in this order have fewer species and are generally less well-known, but they share the same features of being generally plump, often quite large by avian standards, and having short, rounded wings unsuited for long-distance flight. 

Delphinids: Humpback Dolphins

Indo-Pacific humpback dolphin

The genus Stenella, to which many of the closest relatives of the well-known bottlenose and common dolphins belong is, genetically speaking, a mess. A combination of interbreeding and rapid speciation have made it very difficult to determine how its family tree should be constructed or if, indeed, there is even a clear pattern to find. 

Our best evidence suggests that it probably isn't a "real" group, in the sense of one consisting of a common ancestor and all of its descendants. That's because three other genera of dolphin appear to be descended from that common ancestor, forming part of the same cluster of what we might describe as "typical-looking" dolphins. The common and bottlenose dolphins form two of these interspersed groups, while the third is represented by the humpback dolphins.

As it turns out, their classification has also had to undergo significant revision in recent years, albeit for different reasons.

Oligocene (Pt 14): The Southern Horses That Weren't

Scarritia

30 million years ago, South America obviously lacked horses, since those (still three-toed at the time) originated in the north, and the southern continents had long been isolated from their homeland. What it did have, however, were a group of mammals called the notohippids, a name that literally translates as "southern horses". While they might not be closely related, the name might lead one to suspect that they had at least a resemblance to the modern animals.

They didn't.

Well, not much. When the name was originally coined, for the Miocene genus Notohippus, back in 1891, it was assumed that they really were horses, or at least closely related. This is because of the shape of their teeth which, did indeed resemble those of equines. It only took until 1914 to realise that, teeth aside, they weren't very horse-like. That their teeth were similar suggests a similar diet with plenty of tough vegetation, and their head was elongated in an almost horse-like fashion to accommodate them... but that's pretty much where the resemblance ends. For one thing, they had claws, not hooves; their bodies were also stockier, albeit with long limbs that may have given them a certain agility.