Pleistocene (Pt 6): Lions, Hyenas, and Bears, oh my

Cave lions
(original artist unknown, c. 33,000 BC)

The iconic Ice Age carnivore is, of course, the sabretooth cat. However, as I mentioned when discussing the Mid Pleistocene, sabretooth cats when extinct in Europe well before the Last Ice Age. (Well, maybe not... but at any rate, they seem to have been very rare). So what were the dominant predators in Europe during that final, and most extreme, period of intense cold?

It probably depends on how small a predator you're willing to count. Animals like weasels, stoats, and shrews were presumably very numerous, even if their skeletons are often too delicate to preserve well. Moving up the scale, the most numerous carnivoran fossils from the time belong to foxes. Both the modern red fox and the Arctic fox were widespread in central Europe at the time, although the latter was presumably more comfortable.

Another modern animal that we wouldn't be surprised to find among the snowy forests and open tundra was the grey wolf. Indeed, grey wolves may well have originated in Europe from an American ancestor, before heading back over the Bering land bridge. If so, they were probably quite quick about it, since they are found across the Arctic on both sides of the Pacific from an early date.

All of these are creatures that are still found in Europe today, although not necessarily in large numbers - wolves are more common in the wilds of Asia and North America than they are in densely settled Europe. But I've been harping on through this series about how "African" European wildlife looked at the time, and that's as true of the big carnivores as it is of herbivores such as elephants/mammoths and rhinos. One of the clearest instances of this is the presence in Pleistocene Europe of lions.

Being a Little Bit Pregnant

Sun bear

In my ongoing series on the members of the weasel family, I have, on a number of occasions, referred to the concept of delayed implantation. A number of members of the weasel family exhibit this phenomenon, and they are far from alone among mammals.

To begin with, the egg cell starts to divide immediately after fertilisation. In most mammals, it then rapidly forms into a hollow ball of cells that move down through the reproductive tract into the womb. During this time, it really doesn't change much in size; the ball of cells, known as a blastocyst, is not much larger than the unfertilised egg, and the individual cells within it are much smaller.

Once it reaches the womb, the blastocyst attaches itself to the uterine lining and begins to send out tiny tendrils of cells that penetrate the wall in search of blood vessels. Once it finds them, the embryo can obtain nutrients from the mother, and starts to grow and develop. The side of the structure in contact with the uterine lining develops into the placenta, while the remainder forms the embryo proper, and the amniotic membranes surrounding it. This even occurs in marsupials, although, in their case, the placenta is primitive and short-lived, and, for that matter, there's something similar in some reptiles, such as sea snakes.

Making your Mark

For most humans, the dominant sense is that of vision. Although the other senses are important, vision is particularly important to us, and it is generally well-developed in other primates, too, perhaps because most of them spend a lot of their time swinging through trees, where a good idea of the location of nearby branches is definitely helpful. But, for most mammals, the sense of smell is much better developed than it is in us, and far more important to the way they interpret the world.

One of the many ways that they make use of this is through scent marking. Apart from those that spend most of their lives in the water, the great majority of mammals scent mark in some way, and humans are a bit of an oddity in this respect. To assist them in scent marking, many mammals possess special glands producing smelly secretions that they can place on prominent landmarks, or in other locations that fellow members of their species might come across. And, of course, any cat or dog owner is familiar with the concept of using urine as a mark - the urine contains unique chemicals that can provide far more information to the animals concerned than it does to us.

The actual purpose of scent marking varies depending on the nature of the animal, and, particularly, its lifestyle and habits. For many animals, it can be used to stake out patches of territory, or to advertise sexual receptivity, for example, and that will depend both on how the animal claims territory, and on the details of their mating habits, such as how promiscuous they might be.

Along with polar bears, brown bears (Ursus arctos) are amongst the largest of all carnivorans. Like many carnivores - although unlike, say, wolves, lions, or otters - they are solitary animals as adults. Scent marking is therefore particularly useful for them, allowing them to record their presence and leave information for any other bears that may happen to wander through the same area. However, they are not particularly territorial. They do, like most other mammals, have a "home range" through which they habitually travel, but this regularly overlaps with those of other bears, of either sex, especially where food is abundant, and several bears live relatively close together.

How Black Bears Trekked to Ontario

Can you show me the way to Ontario...?

The Pleistocene is the geological epoch immediately before our own. It ended just ten thousand years ago, a time so recent on a geological timescale that the modern epoch isn't even visible on the chart of the Age of Mammals shown below - its effectively just the line at the very top of the column. It's hardly surprising therefore that, for the most part, the animals alive in the Pleistocene are the same ones that are alive now, barring those that have gone extinct over the course of human history.

The Pleistocene, however, represents the Ice Ages, when vast glaciers covered much of the landmass of the northern hemisphere (rather less so in the south, though, because Australia and Africa, in particular, don't get as close to the south pole as Europe, Asia, and North America do to the northern one). It ends with two events that occurred roughly at the same time: the warming of the world at the end of the last Ice Age, and the dawn of human agriculture. In the case of North America, humans only arrived at the very end of the Pleistocene, meaning that it represents the last time that the continent was untouched by human hands.

The Polar Bear's Bite

Bears are generally omnivorous animals, and will eat pretty much whatever is available. Although they look fairly fearsome, and do, of course, eat meat, they also consume a relatively large amount of plant material, such as berries, leaves, grass, nuts, and so on. This is useful, especially for a large animal, since they are unlikely to go very hungry for long, and plants are always more readily available than animals.

The polar bear (Ursus maritimus) is, of course, an exception. Unlike all other living bears, they feed exclusively on meat, and really don't eat plants at all. They are, to use the technical term, hypercarnivores, more similar in this respect to cats than they are to their own relatives. Nor do the differences end there. Most obviously, they are adapted to bitterly cold environments, and, indeed, they soon get heat stroke if taken to places that most other bears would be perfectly happy - anything much above 10°C (50°F) makes them uncomfortable. Not only that, but they are also much better swimmers than other bears, and have claws that grip into the ice when they walk.

Given these differences, you might think that polar bears represent an early branch from the lineage that led to all the other bears, one that adapted itself to both an extreme environment and a (for bears) unusual diet. But that turns out not to be the case, with polar bears having diverged from their closest living relatives, the brown bears, long after that line split from the other species.

 Polar      Brown     Sun Bear     Black
 Bear       Bear                   Bears
   |          |          |           ^
   |          |          |           |         Sloth Bear
   ------------          -------------             |
        |                      |                   |
        |                      |                   |
        ------------------------                   |
                   |                               |
                   |                               |
                   ---------------------------------
                                  |
                                  | 


Rather more surprising, perhaps, is just how recently this happened. Genetic analyses show that the first polar bears evolved no more than 0.7 million years ago, which really isn't very long in evolutionary terms - in contrast, most other species of bear diverged around 5 million years ago. In fact, its quite probably a lot less than 0.7 million years. The oldest known skeleton of a polar bear is only around 130,000 years old, which is sufficiently young to retain a good deal of DNA.When that DNA was analysed earlier this year, it seemed to show that the animal in question was so incredibly close to brown bears that it probably lived at around the time that the two diverged, and would therefore have been one of the first members of its species - a rare find, indeed.

What this means is that, after they first evolved, polar bears must have undergone a very rapid sequence of changes, quickly adapting themselves to their environment and new diet. Judging from the shape of their skulls, they must have changed at least twice as quickly as any other bear species did, and that's probably an underestimate.

How has the polar bear managed to adapt to a new diet in such a short time? Generally speaking, hypercarnivores such as lions tend to have strong skulls with powerful jaw muscles and large shearing teeth, suitable for slicing through tough meat and bone. This is much less true of bears, adapted as they are (in most cases) to a more omnivorous diet. But, compared with brown bears, polar bears have a long, rather sleek skull, which is even weaker than the heavy rounded skulls of their relatives. This is probably, at least in part, to make it easier for them to nuzzle through holes in the ice to catch seals.

Some recent stress analyses using the sort of computer models normally used in engineering design have shown that while the jaw muscles of polar bears are just as strong as those of normal bears, the skull itself is less able to take up the strain of a powerful bite. Granted, that's assuming the computer models are right, but even so, it does seem a bit odd. Similarly, while they do have less premolar teeth than many other bears (a feature they share with cats, for example), the teeth overall seem somewhat weaker than one would expect for a purely carnivorous animal.

One reason may be what the polar bears are eating. Their diet consists almost entirely of seals, which, unlike the antelopes eaten by lions, have a lot of soft blubber. If their food is less tough than the meat favoured by other hypercarnivores, they don't need such strong jaws, and the evolutionary change required to adapt to that diet may be less than it first appears. It may also help that, while animals like leopards and wolves regularly take down prey  larger than themselves, polar bears obviously don't. Seals just don't get that big!

There is, however, a clear downside to this rapid evolution and specialisation. As ice sheets melt, the habitat of polar bears is shrinking. But what's bad for them can be quite good for their relatives, the brown bears. Brown bears (Ursus arctos) are a diverse species, including both the grizzlies and Kodiak bears of North America, and the "common" brown bears of Europe and Russia, among others. They're pretty adaptable, and can eat a wide range of food, making them at home in dense forests, open woodland, or even tundra. As the polar bears' preferred environment shrinks, brown bears can move steadily further north.

The sort of things that brown bears eat are often tougher than seal blubber, and their teeth and bites are accordingly stronger. This means that, even if polar bears were to start eating such things (which isn't, perhaps, very likely in the first place), the brown bears would still do better. Brown and polar bears are still so closely related that they can interbreed to produce hybrids, at least some of which are apparently fertile. If that continues to happen, and the brown bears muscle in on the polar bear's territory, out-competing them for food, then it's the browns that will eventually win out, diluting the purebred polars into non-existence.

As if the great white bears of the north didn't have enough problems to be going on with...

[Picture from Wikimedia Commons]