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.

Hair, Fur, Bristles, and Spines

Trinomys

Hair is one of the key defining features of mammals. Apart from cetaceans, such as dolphins, no mammals are entirely hairless, although, for example, hippos and some babirusa species come very close. The original purpose of hair was almost certainly to maintain body heat, something that matters to warm-blooded animals in a way that it doesn't to, say, reptiles. Since then, however, hair has adapted to fulfil many other functions as well.

To begin with, there is colouration, a function that hair needs to take over once it obscures the skin. The most obvious way that hair colour can help an animal is camouflage. While this is instinctively true, there have been statistical studies that demonstrate certain hair colours are, indeed, more common in animals living in certain environments. For instance, multi-species analyses of lagomorphs and cloven-footed mammals have shown that animals with grey fur are more likely to live in rocky environments. The same studies, as well as similar ones on carnivores, show that pale fur is associated with open environments, especially deserts, while dark fur is most common in those animals living in jungles, dense forests, or heavily vegetated swamps. And, surely to the surprise of almost nobody, white fur is associated with the Arctic, or with the winter coats of those living where it snows.

Mice at the Oak Tree Cafe

Forests, it should come as no surprise, rely on a complex set of interdependencies among the native species. The animals that live in the forests rely on the existence of the trees for shelter or food, or feed on other animals that rely on the trees for food. But the trees also need the animals, or at least some of them.

Obvious examples include the reliance of many plants on insects and other animals for pollination. Another is the fact that plants have edible fruit specifically so that animals will eat them and spread the digestion-resistant seeds in their dung. That doesn't work where the animal obtains nutrition from the seed itself, as is typically the case, for example, of plants that produce nuts. But, even here, the plants may rely on scatter-hoarding.

That is to say, many seed-eating animals in temperate regions store food in caches hidden across the landscape - squirrels being among the better-known examples. Some of those caches won't be found again, or the animal that made them will die from other causes before it has the chance to use them. And then, the seeds can germinate - the great majority won't, but it happens often enough that this simple process is of key importance to the survival and growth of some nut-bearing trees.

Drought and the Mother Rhino

You may be surprised to discover that the white rhinoceros (Ceratotherium simum) is not internationally listed as an endangered species. This is because it is reasonably widespread across southern Africa, poaching of the species has been in decline since 2014, largely due to effective enforcement methods. While it did almost go extinct in the late 19th century, well over 10,000 of the animals are thought to be alive today, with populations in some areas still rising in recent decades. In fact, it meets all the usual criteria for a species of "least concern", one that we wouldn't normally consider even close to being threatened.

This, of course, hides a fair bit of complexity.

Squirrels, Advance!

The rapid growth of human population over the last century or so has led to a decline in many species. As I talked about last month, however, some animals can live alongside us even in urban environments, and there are many more than can tolerate us in rural - yet not truly wild - habitats, such as cropland or pasture. Any species that can do this clearly has an advantage, in many cases being able to move into new parts of the world previously inhabited by some similar, but less human-tolerant species. Thus, we can see some native species replaced by foreign invaders, as has happened, for example, with mink in continental Europe and jackrabbits in the American southwest.

In Britain, the most familiar example of this is probably the replacement of our native red squirrels (Sciurus vulgaris) by invasive eastern grey squirrels (Sciurus carolinensis). Red squirrels were once common across the British Isles, but have now vanished from most of England and Wales, surviving in the far north of England and a few pockets elsewhere, but otherwise replaced by the greys. In large part this is due to the greys carrying a virus to which they are immune but the reds are not, but simple competition is another factor.

When the Desert is Too Dry

The round-tailed ground squirrel lives
further east, and is not threatened

Many mammal species are territorial, carving out a patch of land for themselves which they then defend from same-sex members of their own species. Typically, they are less bothered about members of the opposite sex, for obvious reasons, and such territories will often overlap. Male territories tend to be larger than those defended by females, making it easier for them to meet as many females as possible. 

The size and relative location of such territories naturally vary between species, but also depend on the local conditions of terrain, climate and so on. The harder it is to find food, for instance, the larger your territory will need to be. As young animals grow up and leave home, they will need to find unoccupied territories to inhabit, or else somehow drive an existing resident out and take over. Males commonly travel further than females so that they don't end up with only their sisters or close cousins as potential mating partners, although there are a few species where it works the opposite way around.

Oligocene (Pt 2): Europe's Big Break

Eomys

The dawn of the Oligocene is marked by a sudden cooling of the Earth's climate, of which the most obvious consequence was the creation of the Antarctic ice sheets. These locked up so much water that sea levels dropped worldwide, reshaping coastlines. Nowhere were the consequences of this more apparent than Europe, despite its great distance from Antarctica.

Prior to the Oligocene, it would have been possible for a hypothetical traveller to sail from what is now the eastern Mediterranean, through the Paratethys Sea (now the Black and Caspian Seas) due north and into the Arctic Ocean. The body of water that made this possible, the Turgai Strait, was already becoming shallower and narrower as the Oligocene approached, and the sudden dip in sea level finished it off altogether, closing off the sea route that had once run along the eastern flank of the Ural Mountains. 

Fruit Bats of Madagascar

While most attention tends to focus on large charismatic animals such as rhinos and tigers, it will probably come as no surprise to discover that many species of bat are endangered. Here in Britain, all bats are legally protected by the Wildlife and Countryside Act 1981, making it an offence to disturb their roots. That's perhaps an unusually robust example, but other protections and conservation efforts exist across the world.

On the other hand, it is true that the majority of bat species are not especially threatened, at least on a worldwide scale - although things may be different locally. Bearing in mind that around one in six bat species are so recently identified and so little studied that we simply don't know how common they are, only around another one in six are rare enough to be listed as "threatened" by the International Union for the Conservation of Nature. Still, that's not exactly a small proportion, and since there are somewhere around 1,400 named species of bat, it's not small in absolute terms, either.

Tha bats are a highly diverse group, representing no fewer than 21 taxonomic families, none of which are likely as familiar to the layman as terms such as "cat family" or "deer family". Some of these contain very few species, representing oddities that don't quite fit into any of the main subgroups, but there are still five families with over a hundred species each. Of these, the one that contains the highest proportion of threatened species is the fruit bat or "flying fox" family, the Pteropodidae.

Tree-Dwelling Almost-Lemurs of the Canadian Arctic

Northern Canada, it has sometimes been noted, can get a bit cold. The northern coasts, and the islands beyond them, are covered in treeless tundra, the permanently frozen subsoil preventing anything that needs deep roots from growing there. The interiors of the more northerly islands don't even have that much, just permanent fields of ice and snow. 

The most northerly island in Canada is Ellesmere Island, whose most northerly point is not far from being the most northerly piece of solid land on the planet, only beaten by parts of Greenland. Midsummer temperatures reach a daily high of about 9°C (49°F) in midsummer, and it often snows in July. Winter temperatures regularly drop below -35°C (-31°F) on January nights. So, yeah, that's uncomfortable.

Ain't No Mountain High Enough

Almost anywhere you can go on Earth, you will find life. A field of molten lava might be going a bit far, but otherwise, life seems to have adapted to almost every environment, from ridiculously hot springs at the bottom of the sea bed to barren ice fields to microscopic cracks in apparently solid rock far underground. Much of this life is, of course, far too small to see with the naked eye - bacteria or their cousins the archaea. Mammals are somewhat more limited; we don't find them inland in Antarctica, for instance. But what are the limits beyond which even the best-adapted mammals cannot live comfortably?

One of the easier limits to measure in this way is altitude. This is, after all, a fixed value, whereas factors such as temperature can vary throughout the year. Certainly, it would be useful to know whether a given species can survive as the world warms, but that can be a complex question to answer. When it comes to altitude, we simply have to go and look. For this reason, when describing the habitat of a creature in a conservation catalogue or the like, the altitudinal range of the animal is often described in numeric terms, while the preferred climate is described more vaguely.

Out of the Amazon: A History of the Opossums

Tate's woolly mouse-opossum,
a marmosin species

When most people think of marsupials, it's likely that Australia is the first place to spring to mind; the land of kangaroos, koalas, and wombats among many others. In fact, even the "Australian" marsupials aren't restricted to that country, since they are also found further north, notably in New Guinea - which, for example, has its own species of wallabies. But, more significantly, marsupials are also found outside of Australasia, in the Americas.

The best known of these American marsupials is surely the Virginia opossum (Didelphis virginiana). This lives across most of the US outside of the western mountains and interior deserts, and is also found just across the border in parts of southern Canada, through the whole of Mexico, and well into Central America. It's the only species of opossum found north of Mexico, but it's very far from being the only species of opossum anywhere.

A Good Winter's Sleep

Mammals have an advantage over reptiles in that they don't need the weather to be warm in order to stay active. This makes it easier for them to live in parts of the world that have cold winters, but even then, the scarcity of food at such times of the year means that they often need some additional survival strategy. Some, of course, simply migrate somewhere warmer during the winter - which typically means moving downhill from summer grounds on mountainsides, the long-distance migration of birds being less of an option. Others, such as polar bears, are just good at surviving cold weather anyway, and may not need to do anything significantly different in winter.

But, leaving those possibilities aside, three basic options for surviving the winter present themselves. They could do something behavioural, such as storing food during the summer and coming back to their hidden caches later in the year when food is short. Or they could change physically, such as by building up fat over the summer or having an extra-thick winter coat that falls out in the spring. (And these are not, of course, mutually exclusive).

A History of the Bamboo Rats

In terms of number of species, the mouse-like rodents are the most successful group of mammals alive today. The group is typically considered to consist of six families, with the vast majority of species crammed into just two - the mouse family itself, and the cricetids, which includes the voles and hamsters. These, however, are relatively late arrivals, and there are two families that split from the main line of mouse-like rodents long before the ancestors of the mice and voles diverged from one another.

One of these consists of a grand total of three species (maybe), living in the forests of southern Asia. The other goes by the technical name of the Spalacidae, and it consists of animals that have the rather unusual trait of spending almost their entire lives underground. There are at least 35 species of these animals, found in various places across Europe, Asia, and Africa. This is a fairly wide distribution, which raises the interesting question of how they managed to spread that far when they seem to have fairly narrow habitat preferences. Until recently, the evolutionary history of these oddities has been something of a mystery but now, it seems, we are starting to get enough information to piece together an overview.

Digging in a Winter Wonderland

Humans have a somewhat ambivalent attitude to snow, greeting it either with joy and wonder or with concern and frustration, in large part depending on how old you happen to be. It decorates Christmas cards, covers the countryside in pristine white, and provides plentiful opportunities for play. On the other hand, it makes travel difficult, and may even interrupt deliveries of food, making our lives awkward. (Plus, if you're in a city, it may not stay white for very long).

If heavy snow makes it difficult for us to get around and obtain sustenance, the same obviously goes for animals that live in the relevant parts of the world. Different species have different strategies for how to cope with its arrival. Some hibernate, while others avoid the problem by migrating somewhere else for the winter, but there are, of course, a number of species that simply have to put up with it. Those that live particularly far north may even have to do so for over half the year, and the difficulty of doing this is probably at least part of the reason why relatively few species do.

Lots of Little Deer Mice

If a sexually reproducing animal is to preserve its species without population loss, each pair of parents has to produce, on average, two offspring that will themselves live long enough to reproduce. This is just basic arithmetic, but there are at least three different ways that an animal can achieve this result and which one is used varies from species to species.

One approach is to maximise the chances of each of your offspring surviving. This is called the K-selection strategy, and results in the population being as close to the maximum capacity of the local habitat as possible. (The 'K' stands for 'capacity'... in German). Such animals don't need to reproduce very often, or produce very many offspring when they do, but they have to invest a lot of resources in their survival. They tend to be relatively large animals, with few predators... elephants, primates, and whales are all good examples among mammals. Humans are a particularly extreme example, given how long it takes us to raise our children, and, as with other strong K-selectors, twins are rare in our species.

The Jungles of British Columbia

It's changed a bit since...

Earth's climate has changed dramatically over the billions of years of its existence. The recent warming events may be unusually fast, but they are by no means unusually large. Not much more than 12,000 years ago the world was in the grip of the Ice Ages, when the polar ice caps stretched much further than today, and places like southern Italy and northern California were cold, sparsely forested steppeland.

On the other hand, there were times when the world was much hotter than today, warm enough that there appear to have been no ice caps at all, even at the poles. During the Age of Mammals - the eon of time since the extinction of the non-avian dinosaurs - the hottest of these was the Eocene Climactic Optimum. This took place a little over 50 million years ago, and it lasted rather a long time, punctuated by short periods of even greater heat. (That's 'short' as in they lasted tens of thousands of years, but not millions).

Bats in a Changing World

Daubenton's bat is doing fine, thank you

After rodents, bats are the second largest order of mammals, in terms of number of species. Being small animals, they are also quite numerous in terms of their absolute population figures. Bats are found on every continent, except Antarctica, being among the few native placental mammals in Australia. Yet, despite this diversity, many bats are struggling to survive, with 14% of species worldwide being considered threatened, according to the latest (2008) edition of the IUCN Red List.

Now, in fairness, we do have to put that into a broader context. 14% may sound like quite a lot, and it is, in terms of actual number of species... but as a proportion, it's not unusually bad. It's about the same as for rodents, and considerably less than the 19% or so - nearly one in five - for all mammal species, worldwide. But still, bats are a fairly good indicator of how mammals in general adapt to the changing world around them.

There are a number of reasons for this that don't apply, for example, to rodents. Much of this has to do with their reproduction. Rodents and rabbits breed like... well, rabbits. They have multiple offspring, several times a year, that are themselves able to breed within months, or even weeks. This means that they can adapt rapidly to change; if it's a bad year, a lot of them will die, but the population zooms straight back up again if the next year is good. For bats, it's not quite so easy.

Does Colder Mean Bigger?

Amne Machin, Qinghai

In 1847, German biologist Christian Bergmann formulated what has since become known as Bergmann's Rule. Over the following 165 years, the Rule has been somewhat modified and re-interpreted from Bergmann's original statement, but a common modern version is that warm-blooded animals tend to be larger the colder the environment they live in. Bergmann was originally thinking of closely related species; for example, polar bears tend to be larger than most brown bears. However, the rule is now often applied to individuals and populations within species; for instance, Siberian tigers are larger than those that live in the tropics.

The rule has, as I've noted before, been the focus of some controversy. Is it, in other words, actually true? Certainly, there are many counter-examples - snow leopards are the smallest cats in their genus, for example. But are these rare exceptions to a general rule?