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.

Caprines: Browsing Goats of the Western Mountains

Alpine chamois

The caprines as a whole can be divided into two groups, depending on which of two different lifestyles they happen to follow. The best known of these, since it is the lifestyle of sheep and "true" goats, is that of the grazing caprines.

Grazers feed fairly indiscriminately, munching down lots of grass and similar plants. In the case of the caprines, they survive in marginal habitats by eating pretty much anything that's available. They range across large areas in search of food, and adopt safety in numbers by packing themselves together in herds. Today, the great majority of caprine species adopt this lifestyle, and it's often what we think of when we think of goats and sheep.

But it appears that, in the evolutionary history of goat-like animals, it's a relatively recent innovation, one that was given a significant boost by the arrival of the Ice Ages. Before that, goats had adopted a rather different lifestyle, and there are still a minority of species - perhaps no more than five - that still live this way.

These apparently "primitive" goats were once grouped together in their own tribe, and given impressive sounding technical names like "rupicaprines" or "naemorhadins", but it's now less clear that they're really related. Instead, they represent at least two evolutionary lines within the goat-like animals, each a relic of the distant past when all goats were like this. Around them, some of their relatives switched to grazing, and, in the long run, proved the more successful.

Pleistocene (Pt 9): Before the Bison

Megatherium

The Pleistocene in North America can be divided into three 'land mammal stages', defined by the kinds of animal that inhabited the continent. The first, which was only recently added to the Pleistocene, is the one that comes before the Ice Ages proper, so that the Ice Ages themselves are, as I mentioned in part 8, divided into just two. As I said back then, the first of these starts with the arrival, not only of the ice, but also of the first American mammoths. The second, called the Rancholabrean after the famous tar pits, is much shorter, and essentially refers only to exceptionally bitter cold of the Last Ice Age. In terms of animal life, though, it is defined by the arrival of bison.

Today, when we think of America before the white man arrived, vast herds of bison are often part of our mental picture. And that, on the whole, is pretty much accurate, and is why their first appearance is deemed so significant in the ongoing evolution of North American wildlife. But what was the American wilderness like before there were any bison? What dominated the continent during the earlier, "Irvingtonian", stage?

As always, of course, the most common animals were the smallest. But, important though the teeth of (for example) voles are for the precise dating of geological deposits from this time, the eyes of any time travelling visitor would inevitably have been drawn to much larger animals. Obviously, mammoths and mastodons are a large part of the answer, and the presence of large elephantine animals crossing the plains would have been enough to tell our time traveller that, while he might still be in Kansas, it isn't really the same one he left. But there are many other beasts that would provide just as quick a clue.

Deer, peccaries, and horses are all examples of animals that aren't so unfamiliar today, although in many cases, the exact Pleistocene species were different from those alive now. Horses, for example, had, by this point, evolved to the modern single-toed form, but the last North American native horses died out around 12,000 BC, when the Pleistocene ended, leaving the ancestors of the domesticated forms behind in Asia. Exactly how many species of wild horse there were in America at this time, though, is unclear, with as many as fifty having been named - in my view, it's somewhat unlikely that they're really all distinct. There were, however, at least two broad types, one similar to the modern domestic horse, and the other, the so-called stilt-legged onager (Equus francisci, among others), which looked much more like a wild ass.

Lost Dolphins of the Mediterranean

The common dolphin (Delphinus delphis) still lives
in the Mediterranean

The largest family of cetaceans is the dolphin family, which includes well over 30 species. Most of them are what we'd normally think of as dolphins, but both pilot whales and killer whales are also included. However, as I've mentioned before, exactly how they're all related to one another if far from clear. We also have rather less fossils than we might expect, given the number of species and individuals alive today.

This may partly be due to something of a gap in the fossil record - a time for which we don't have many deposits of the right age from the right place - but there are other possible reasons as well. Many mammal fossils are identified from quite small parts, and often just the teeth. Mammalian teeth, especially those at the back of the mouth that grind up food rather than biting into it, have complex shapes that vary from species to species, enabling an expert to tell quite a lot from looking at only one. At least enough, in most cases, to tell what family it belongs to, even if you can't precisely finger the species in question.

But the majority of dolphin teeth look more or less the same. They have no cheek teeth at all, only stabbing teeth for grabbing onto fish, and those are usually quite simple in shape. So, while we may have plenty of dolphin teeth, and other scattered bits of their anatomy, they tell us rather less than they might for land-based mammals, making them useful than we might like for building up a picture.

One Gazelle or Two?

Mountain gazelle (probably)

According to the International Union for the Conservation of Nature, there are sixteen living species of gazelle. However, as I've mentioned before, counting species isn't always as simple as we might like to think, because the lines between them often blur. Are those two, similar-looking, animals separate species, or just two subspecies of the same thing? And how would we know?

You might suppose that the answer is something along the lines of 'do they cross-breed to produce fertile offspring or not?' Horses and donkeys are different species, you might think, because, while they can interbreed, the result is a sterile mule. Similarly, tigers and lions can also cross-breed, but the results are usually infertile. (Actually, fertile mules and "ligers" do exist, but they seem to be rare). If they don't cross-breed at all, of course, the answer is even simpler. So, take the two gazelles you're wondering about, mate them, and see what happens. Right?

Well, not really. For one thing, that's much easier said than done, especially if one of the possible species is endangered. It's hard enough getting pandas to mate with other pandas to be sure that mere lack of mating proves anything at all. Not only that, but there are, believe it or not, a number of cases where animals that are clearly different species can, indeed, interbreed to produce fertile offspring. Polar bears and grizzly bears, for instance, to give one example becoming more common of late.

Caprines: Half-Goats and False Sheep

Himalayan tahrs

Sheep and goats are fairly closely related animals. In a sense, sheep are just goats that don't like climbing. However, even once we acknowledge that ibex, say, are really just another kind of goat, it turns out that there are a number of species that are also closely related to sheep and goats, without really being either.

Quite how they're related, and which ones are on the goat side of the tree, and which on the sheep side, has been a matter of some debate, and depends largely on whether or not you think that physical appearance is more important than genetic similarity. Even if you look only at the genetics, it seems that they're all close enough to one another that it makes a fair difference which genes you happen to consider most important.

Still, we can say that some look more like goats, and some look more like sheep. On the goat-like side are the tahrs - not to be confused with turs, which really are goats. There are three species of tahr, all of which look fairly similar to one another, and have traditionally been placed together in the genus Hemitragus - a word that literally means "half-goat". In the traditional, appearance-based, scheme it's thought that these are the closest animals to true goats, without actually being them. Genetic analysis, however, shows that this is just an illusion, and the three animals aren't especially close relatives. Two of them, therefore, have been removed from the genus, and given new scientific names to reflect their distinct nature.

Fossil Giraffes of Ethiopia

Giraffes on the Serengeti

There is some debate about exactly how many species of giraffe there are alive today. The official answer is 'just the one' (Giraffa camelopardis), but there may be two or even three, depending on how you define a 'species'. Certainly, it isn't very many, and if there really is more than one, then they all look pretty similar.

Looking slightly further afield, the entire giraffe family contains just one other living species - the okapi. Superficially, okapis may not look much like giraffes, but that's largely because, when we think 'giraffe' we tend to focus on the incredibly long legs and neck, and, perhaps, on the crazy-paving coat pattern. An okapi is quite different in both of these respects, but, when you look at the head alone, its resemblance to giraffes is much clearer.

Still, an okapi is not an actual giraffe, any more than a fox is a wolf. There may, therefore, only be one species of true giraffe alive today. But, as is so often the case with these small groups of animals, the family becomes rather larger once we include in all the extinct species. True, giraffes have never been as numerous as some other kinds of large herbivore, and the total number of species that we know of is far less than we have of, say, bovines. But it's certainly more than one.

Perhaps one of the most important questions we'd like such fossils to help us answer is why giraffes are so tall when okapis, for example, are not. This isn't as easy as it might appear. If you visit somewhere like the British Natural History Museum, or the Smithsonian, you'll see huge articulated skeletons of dinosaurs on display. It's perhaps natural to assume that what you see is more or less what somebody dug out of the ground. Sure, the original may have been squashed or distorted a bit, but generally speaking what they found was a single skeleton. Right?