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.

Gorilla Communities

It is, I would hope, well-known that the closest living genus to our own is that of the chimpanzees and bonobos. The second-closest is, of course, that of the gorillas. There are two species of gorilla, each with two subspecies, and both are considered "critically endangered" - literally on the verge of extinction. For one species, this is, as one would expect, largely due to their tiny surviving population and restricted range. 

There are many factors that we need to consider when attempting to reverse this, and some of them also have a bearing on the evolution of our own species. Among these is the question of how gorilla groups are socially constructed and how they interact. It turns out that here, we can't just consider "gorillas" en masse because the two species behave in very different ways. For example, while one species can have multiple silverback males in the same troop, this is rare (but not unheard of) in the other.

Delphinids: Spotted, Striped, and Spinning Dolphins

Pantropical spotted dolphin

The closest relatives of the common and bottlenose dolphins, which are perhaps the most familiar species to the western public, are those in the genus Stenella. Or at least, that might be true, because there has been a lot of confusion about the genus over the years.

It was first created in 1866 as a subgenus within Steno, the "narrow-beaked" dolphins, and contained just one species. It became a full genus in 1934,and by the end of the 20th century was agreed to contain five species - all of which had, in fact, been named before 1866. Since then, our understanding of genetics has greatly improved, and it has become clear that these various species cannot be so neatly arranged on a family tree as we might like.

Wolves, Foxes, and Food

Large scary predators are, by their nature, rare. There simply can't be too many of them, or they would run out of food to eat, something typified by the trophic pyramid often seen in biology textbooks: plants are eaten by herbivores are eaten by carnivores are eaten by bigger carnivores, with each step necessarily having a smaller total mass. The reality is, of course, somewhat more complex than this simple chain would suggest, most obviously because it ignores omnivores and decomposition.

But the basic idea holds, and apex predators - those that are large enough that nothing else normally eats them - exist in much smaller numbers than herbivores or smaller carnivores. This means that, relative to their numbers, they have a disproportionate effect on the ecosystem within which they live. Take away the apex predators and, even though there weren't very many of them to begin with, you will radically change the local ecology.

And, because they are relatively few in number, apex predators tend to be especially vulnerable to being wiped out. That's even assuming that humans don't focus on them deliberately out of fear, whether for their own lives or for the good of their livestock. Globally, apex predators are declining. (On this blog, we're mostly interested in mammals, but consider, for example, that at least eight of the 23 species of crocodile/alligator are currently thought to be endangered).

When Snow Leopards Reached Portugal

While the various species of "roaring cat" all share a great many points of similarity, most are easily distinguished from each other. Arguably, one of the most distinctive is the snow leopard (Panthera uncia). Not only does this have unusually thick fur for a wild cat, but it also has a longer tail than we would expect, and the face is shorter and wider than that of other roaring cats.

The snow leopard was first scientifically described by Johann Schreber in 1775 as a member of the genus Felis. The differences from other cats were sufficient that, in 1854, John Edward Gray proposed that it be given its own genus, Uncia. His original definition of the genus did not stand (it also included at least one species of "purring cat"), but it was resurrected again in the early 20th century, and used solely for snow leopards up until 2006. In that year genetic evidence placed it alongside the other "roaring cats" in Panthera, something that has been amply confirmed since.

Delphinids: Common and Bottlenose Dolphins

Common dolphin

When the genus Delphis was first named in 1758 at the dawn of taxonomy, it included three species of small to medium-sized toothed cetacean. By the time the dolphin family, Delphinidae, was named in 1821, one of those species (the porpoise) had been moved elsewhere, but five new ones had been added. Many more followed, but, from the mid-18th century onwards, naturalists began to notice subtle differences between animals that were probably more mysterious to them than land-dwelling mammals, and many species of dolphin began to be separated out into newly created genera.

So much so in fact, that by the time we reach the 21st century, only one species remains in the genus originally created to contain all dolphins and dolphin-like animals. That species is, of course, the one that we believe Linnaeus happened to be thinking of when he named the genus, and therefore is the defining (or 'type') species not only for its genus but for the dolphin family as a whole. This is the aptly named common dolphin (Delphinus delphis).

Call of the Mole Vole

There are over 170 recognised species of vole. The exact number is hard to determine because new species keep being described and not all will necessarily stand the test of time but we can certainly say that there are a lot. Although we might typically think of voles as being all rather similar to one another - small, mouse-like animals with short tails and rounded snouts - within such a large group there is inevitably some variation.

Most obviously different from regular voles are the giant species. Although genetic data shows us that these are voles, in the sense of being offshoots of the vole family tree rather than something distinct, we don't normally refer to them as such in English. Instead, we call them "lemmings" or, in the case of the very largest species, "muskrats". To avoid potential confusion, in more scientific language we would therefore use the taxonomic name when referring to the subfamily as a whole and say that voles, lemmings, and muskrats collectively are "arvicolines". 

Delphinid Dolphins

Of all mammal groups, those that have undergone the most dramatic change from their original form are perhaps the cetaceans. The most thoroughly adapted of all mammals to life in the water, there is little else they could possibly be mistaken for. The cetaceans have been considered a distinct order of mammals since the dawn of taxonomy in 1758, and today are divided into fourteen living families. The largest of these families, in terms of the number of species, is the dolphin family.

Technically known as the Delphinidae, this was named by John Edward Gray in 1821 as part of one of the earliest formal lists of mammal families. Gray's original definition basically included all toothed cetaceans other than narwhals and sperm whales, encompassing four genera, only one of which is still placed in the family today - Delphinus, from which it takes its name. He didn't list how many species he thought that genus contained, but it would certainly have included the two named by Linnaeus in 1758, and probably at least three others that had been described in the interim. Over the following decades, the number expanded considerably, with Gray himself identifying several of them, notably when he catalogued the observations and specimens collected by the Ross Expedition of 1839-43.

Oligocene (Pt 13): The First Porcupines

Prosotherium

The dawn of the Oligocene is marked, in part, by the formation of a deep water channel between South America and Antarctica. To what extent there had been a true land bridge between the two continents in the preceding epoch is debatable, but the separation was certainly more complete than it had been previously, beginning a long period of isolation that allowed unique animals to evolve and flourish. Furthermore, most of the large mammal groups we are familiar with today originated on the northern continents, so much of the fauna of South America was already strange to modern eyes.

 

Much of it, but not all. The exact date of the oldest rodent fossil from South America has been disputed, but recent analysis suggests that it may have lived shortly after the beginning of the Oligocene. How it reached the continent is also unclear. Logically, it must have made a long sea journey, presumably clinging to some storm-tossed piece of debris, either from North America or Africa (the Atlantic being narrower then than it is now). The latter seems the more likely, since South American rodents are more closely related to the African sort than to their equivalent northern counterparts.

The Struggles of a Pollinating Bat

Note the dusting of pollen...

When we think of flowers being pollinated, for most people, the first things that are likely to come to mind are insects, especially bees. But other creatures, including mammals, can also be a part of the pollination process, and this is particularly true of bats. Nor are we talking about some occasional event that merely gives the flower some sort of backup - for some the presence of pollinating bats is crucial to their survival.

Even aside from the obvious importance that this gives bats to the wider ecosystem, this can also have direct economic importance to we humans. For example, sour pitayas are an important cash crop in parts of Mexico. Similar to the much sweeter dragonfruit (although not closely related), they grow on a particular type of cactus that is native to the country but is also commonly cultivated. As it turns out, this cactus relies on bats for pollination. While they are not essential, crop yields drop by over a third when the bats are prevented from reaching them, which would clearly be devastating for a Mexican farmer who may be living on the edge of profitability to start with.

Miniature Mediterranean Mammoths

Insular dwarfism is a phenomenon that has occurred many times throughout evolution. What happens is that a population of some large animal becomes trapped on an island, out of contact with its mainland kin., Because the island has a limited size, it also has a limited amount of food on it, and this is a problem for a large animal that needs plenty to eat,

If the island is particularly small, of course, the animals in question are likely to die out, if not immediately, then dwindling over a few generations until they lack the genetic diversity to sustain themselves. On the other hand, if the island is a large one (such as, say, Britain) then there may not be a problem at all, and nothing happens beyond the usual genetic drift between isolated populations.