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.

Wild Mammals of London

Arguably the single biggest threat to the continued survival of animal and plant species is loss of habitat. Even if an animal isn't actively hunted, the ever-growing human population means that there are simply fewer suitable places for them to live. Logging and the expansion of agriculture are probably the biggest factors here, at least in terms of the area affected, but it's hard to argue that the recent growth of urban sprawl isn't another.

The urban environment is obviously a difficult or impossible one for most wild mammals to exploit. House mice and rats are an obvious exception, and there are also domesticated pets, but for truly wild creatures it's a different matter. While it may no longer have (say) bison or wolves, upstate New York is still home to seven species of shrew, three moles, four hares or rabbits, 22 different kinds of rodent, ten bats, nine mustelids, two foxes, and three deer, plus coyotes, bobcats, raccoons, striped skunks, and black bears. Manhattan... not so much.

Prehistoric Mammal Discoveries 2023

Potamotherium

It's the last post of the year for 2023, and that means it's time once again to take a brief look at discoveries from the last year in the world of fossil mammals that didn't make it into this blog. Because, while dinosaurs are undoubtedly popular, the study of prehistoric mammals is also a major field, aided by that, being (mostly) relatively recent they tend to be more numerous and better preserved. Of course, everyone's heard of woolly mammoths and sabretooth cats but there's plenty more out there and, if I'm going to zip through them at speed today, I'm also going to try and cover as wide a range as possible. So, let's get going...

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. 

Prehistoric Mammal Discoveries 2021

Lesmesodon, a weasel-sized hyenadont.
A new species from Early Eocene France was
 described this year

And so another year approaches its conclusion and the pandemic seemingly isn't done with us yet. I had to self-isolate after a positive test myself for a couple of weeks (no symptoms, though) but if there's one thing that doesn't interrupt, it's blogging, so everything went smoothly here. But now it's time for what's becoming a traditional look at the paleontological discoveries of the past year that didn't get covered here but are worthy of at least a brief mention.

Large Herbivores

When we think of vertebrate fossils, the first thing that pops to mind is almost certainly going to be a skeleton of some kind, or perhaps just part of one. But there are also such things as ichnofossils - fossilised remains of how an animal affected its environment that no longer include any physical part of the animal itself. Perhaps the most obvious of these are fossil trackways - footprints of long-gone animals preserved in mud or other soft material that has since turned to stone. A study published this year examined the tracks left by two species of fossil horse. One of them, a one-toed close relative of the living species inhabiting southern Canada during the Ice Ages, turned out to have been galloping at around 34 kph (21 mph), which is quite fast for its small size (perhaps it was running from something). More significantly, however, the three-toed Miocene horse Scaphohippus was using a relatively unusual gait called the "rack" typically only seen in specialised domestic breeds today. 

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).

The Social Lives of Ground Squirrels

There are almost 300 recognised living species of squirrel. As one might expect given that large number, there is a fair degree of variety among them. To people living in Britain, for example, the word "squirrel" typically conjures up a small, tree-dwelling animal with a bushy tail. We only have two local species (one of them invasive) and they're very closely related. Take a look further out in the squirrel family tree, however, and we see some significant variations on the general theme of "squirrel-ness".

Perhaps the most obvious of these are the ground squirrels. Most ground-dwelling squirrels belong to a single evolutionary group, which includes something like a quarter of all known squirrel species. Many of these - the animals most commonly known simply as "ground squirrels" - look much like their tree-dwelling kin, but the group also includes such animals as prairie dogs and marmots as well as the semi-arboreal chipmunks.

Small British Mammals: Squirrels

Red squirrel

The majority of rodent species look, more or less, like mice or rats, depending on their size. But there are a number of exceptions among the many, many, kinds of rodent in the world. Probably the best known such exceptions in Britain are the squirrels.

By far the most common tree squirrel in Europe, and perhaps the most widespread species of squirrel anywhere in the world, is the red squirrel (Sciurus vulgaris), sometimes called the "Eurasian red squirrel" to distinguish it from the North American animal of the same name. This lives almost everywhere in Europe, apart from the Mediterranean islands, southern Greece, and Portugal, and is one of a relatively small number of native wild mammals in Ireland. Further east, they inhabit just about every temperate, forested, region of Asia, reaching as far as Korea, northern Japan, and the coast of the Bering Sea.

About twenty different subspecies are currently recognised, but there seem to be few reliable ways of distinguishing between them. For instance, while most red squirrels are, in fact, red, a significant proportion are darker, or even near-black. These are apparently more common in the dense highland forests of the Alps and the Altai Mountains, but are not a reliable indicator of subspecies, since they are found in other populations, too.

Fluorescent Pink Squirrels

In visible light...

I don't normally cover stories that have been in the news recently, but this was too good to pass up, and, anyway, it wasn't that prominent so some people may have missed it. Fluorescent pink squirrels, oh yeah!

Our story begins in May 2017 with, of all things, a professor of forestry. He was conducting a preliminary survey of the plants and lichens growing in a forest in northern Wisconsin, Many kinds of lichen, and some flowers, fluoresce under UV light, so one way of finding them is to wander around in a forest at night, carrying a UV flashlight. So far, so perfectly normal for the people who study this sort of thing. But, on this occasion, the researcher discovered something entirely unexpected: when he directed his flashlight at a nearby squirrel, it fluoresced a brilliant pink.

This is not something one generally expects squirrels to do. Clearly, more research was called for.

Prehistoric Mammal Discoveries of 2018

Gordodon,
a new non-mammalian synapsid described this year

And so another year approaches its conclusion. As usual, I will wrap up here with a post looking at things from a slightly wider perspective. This time around, as I did last year, I am going to take a brief look at a range of scientific papers on fossil mammals that were published in 2018. There's not going to be any particular theme here beyond that, merely a list of things that caught my interest, and that were not, for various reasons, included in the blog proper. So, here we go:

Beginnings and Endings

In the modern day, it's pretty easy to tell mammals and reptiles apart. But, if we go far enough back in time, that eventually ceases to be so true. A common misunderstanding is that mammals evolved 'from' reptiles, but, in reality, mammals and reptiles are separate evolutionary lines that have lived alongside one another since long before there were dinosaurs. At least, that's true if we use the modern definition of 'reptile' since, of course, the animals that mammals really did evolve from would have looked an awful lot like reptiles if we'd been able to see them in the flesh.

Prehistoric Mammal Discoveries of 2017

Albertocetus meffordorum, the post-cranial anatomy of which
was described for the first time this year.

At the end of each year, I do a slightly different post to wrap up the blog for the season. The format of these has changed over the years, and this year, again, it's time to do something slightly different from previous occasions. Not that there haven't been some interesting new species discovered this last year, with, to my mind, the Skywalker gibbon (Hoolock tianxing) being the stand-out example. This was announced early in the year, having been discovered in the Chinese/Myamar border region by a group of researchers who were fans of a certain science fiction franchise ("tianxing" literally translates to "sky-walker" in Standard Chinese), and is likely already endangered.

But this year, instead of discussing just how many new kinds of bat we discovered in the last twelve months, I'm going to note that my posts on fossil mammals tend to be more popular than those on the living sort, and take a look at a partial assortment of scientific papers published on this subject in the last year that, for various reasons, didn't end up in my regular blog posts. So here goes.

Fossilised Squirrel Brains

The fossils of mammals, and, indeed, other vertebrates, tend to consist largely of bones. That's not an absolute rule, of course, and occasionally other parts do fossilise, and there are also remains of things like footprints and poo. But, generally speaking, it's much harder to get an idea of the soft anatomy of a long-extinct mammal than it is of what its skeleton looked like.

There are many soft organs that we'd really like to understand the evolution of, but, while we can make inferences on other grounds, direct fossil evidence is always likely to fall short for most of them. The brain, however, is something of an exception, and this is perhaps fortunate, given the importance we tend to attach to it.

This is not to say, of course, that finding direct fossil evidence of brain structures in long gone mammals is particularly easy. But the brain sits inside the skull, and is fairly tightly wrapped within it. Which means that if we can get some idea of the shape of the hollow bit on the inside of a skull, we've got a pretty good idea of the shape and size of the brain it once housed. This has, in fact, been done for a number of fossil mammals, but, generally only of large ones. For smaller mammals, such as rodents, the evidence is much more patchy.

Stressed Out Squirrels

Southern flying squirrel

Stress is, it seems, an inevitable part of life. You just can't get away from it altogether, and that applies to animals as much as it does to humans. True, they tend to get stressed out about different things, not being able to ponder the future, although it's surely true that if we were about to be attacked by, say, a man-eating tiger, we'd probably find that as stressful as mice find the presence of cats.

In fact, stress is such an integral part of life that, at a biological level, it works in more or less the same way in all vertebrates. Stress results in the release of stress hormones, of which the most important is cortisol. In mammals, this is produced in the adrenal cortex, the outer part of the adrenal glands, which sit on top of the kidneys, and produce a whole range of different hormones of which cortisol is only one. So basic is this that all non-fish vertebrates have adrenal glands of some sort, although which bits of them make the cortisol (and, in some cases, how many there are) does vary between different groups. Even fish, which usually don't have adrenal glands as such, do have the relevant sorts of tissue somewhere in the general vicinity of the kidneys, so that they too can make cortisol. It really is that universal.

Learning To Be a Good Mother (If You're a Squirrel)

Like birds, but unlike many other kinds of animal, mammals spend a lot of time raising and protecting their young. It's therefore obviously important that mother mammals have some kind of instinctive understanding of what to do to look after their children. And instinct does, indeed, play a big role in maternal care - even among humans, we talk about a mother's "instinctive" desire to protect and nurture her children.

Among humans, though, it isn't all instinct; we have many ways of learning even something as basic as this. But how true is this of other mammals? In fact, there is solid evidence that other animals get better with practice. Instinct may be important, but animals are capable of learning from experience, and a second-time mother generally has a better idea of what she's doing than one who's new to the whole thing. Indeed, not all wild animals are equally good mothers. There is strong evidence, for instance, that animals that suffered the equivalent of child abuse when young grow up to be poor parents themselves.

Hunky Mother Squirrels

In general, among mammals, males are larger than females. We can see this in our own species: on average, men are taller than women, and, again on average, they also tend to have more muscles and a greater overall body mass. Compared with some other species, though, the difference isn't all that great. Seals are an extreme example, with males often many times larger than females. But, in many cases, the difference is more like that with us: it's a noticeable difference, but not a dramatic one.

The reason for this is generally thought to be male-male competition. Males often have to compete for mates, and the bigger you are, the more likely you are to win that contest. It doesn't even have to be physical; the mere fact that you've been able to 'waste' calories on bulking yourself up proves that you're physically fit, and therefore an attractive mate for females. Even the fact that you've survived long enough to grow to large size is a point in your favour.

How this manifests will depend a lot on the mating system of the species concerned. In species that are monogamous, it's not such a big deal, and it's even less so if females are highly promiscuous. It's most noticeable in polygynous species, where one male monopolises as many females as he can, perhaps defending a harem from all comers. This is what happens in seals, and it's also true, for example in deer. Stags, for instance, not only are quite a bit larger than hinds, but they have huge antlers that not only let them fight off their rivals, but also further demonstrate how many calories they've been able to waste growing something huge just for the heck of it.

Hold On to Your Nuts

American red squirrel

The squirrel family is a particularly large one, with almost 300 recognised species. About half of these are ground squirrels of various kinds, including such animals as prairie dogs and marmots. The remainder are tree squirrels, which can be found on every tree-bearing continent bar Australia. Tree squirrels aren't a single group in evolutionary terms, because chipmunks are closer to marmots than to, say, fox squirrels, but they do at least have a fair bit in common.

Life for tree squirrels isn't too bad if they happen to live in the tropics, where trees provide abundant food year round. (The Indian palm squirrel has the added advantage of being a sacred animal in Hinduism, but that's another matter). It's all a bit tougher when they live somewhere with proper winters. By and large, tree squirrels don't hibernate - although many ground squirrels do - which means that those in the north need some way to keep eating throughout the year.

The solution for many, as is well known, is to hide nuts, seeds, and other easily preserved food sources in caches, and return to them when the weather gets bad. This can be important in seed dispersal, should the squirrel forget where it put the hoard, or die from some other cause before it can eat it all.

In Britain, the native tree squirrel, and, indeed, the only native squirrel of any kind, is the red squirrel (Sciurus vulgaris). Since the 1870s, however, it has been under increasing pressure from larger, foreign squirrels introduced from the United States. These are eastern grey squirrels (Sciurus carolinensis), simply called "grey squirrels" in Britain, on the grounds that there's no other sort. Over the last 140 years or so, they have almost entirely extirpated red squirrels from England and made a good start on Wales and Ireland, too.

Squirrel Masturbation

Male masturbation appears, from a biological point of view, to be a rather bad idea: you're simply wasting sperm that you could use to fertilise mates. For that matter, you're losing some nutrients and water, as well. But, when we look at the admittedly small number of studies into this kind of thing among mammals, we find that it's not just humans who do it, or even just primates. So, why is that?

The obvious answer, from a human perspective, is that they enjoy it. Or, to put it more biological terms, that the animals have a sex drive that compels them to mate with females, and, in the absence of females, they'll take the next best option. It's worth noting that humans are unusual among mammals (although not unique) in that they don't come into heat, and are willing to have sex at pretty much any time. This means that, when it comes to non-human mammals, we would expect them to masturbate more frequently at times when the female is in heat, since that's the sort of thing they find sexually exciting. Secondly, they will do it more often when they can't get at any females, perhaps because bigger males are getting there first.

But is this what really happens? A recent study looked at this, and a number of other possible explanations, to find out just why squirrels masturbate.