Staying Away from the Boys

Many large, hooved mammals live in herds. This has obvious advantages of safety in numbers, spreading the work of looking out for predators, as well as benefiting from the knowledge of more experienced herd members as to the best places to find food or shelter. The structure and composition of these herds vary from species to species, but one common trait is that males and females often form single sex herds that travel apart for much of the year.

This is referred to as "sexual segregation", and was first formally described by Charles Darwin in The Descent of Man. It isn't unique to mammalian herd animals, being seen in everything from fish shoals to bird flocks, as well as in non-hooved mammals (dolphins, bats, primates, etc.) Most zoological research, however, has tended to focus on large cloven-hooved mammals, such as deer and antelopes.

The Sounds of Mother and Calf

While for humans, speech is our primary means of communication, for many mammals scent-marking is more important. This is not to say, however, that audible signals are irrelevant. This can be especially true for social mammals (such as ourselves), which often have a need for something more immediate and complex than the sort of long-lasting "keep away" or "I'm ready to mate" signals that scent marks can provide. Communication through sound can help keep a herd together, signal aggression, provide instant warnings, and so on. It's key to many primates, for instance, because most of them live up in the trees where it may be difficult to keep sight of all troop members when they are out foraging.

It's also important for many hoofed herd animals and, at least in the wild, few North American mammals are more sociable than the bison (Bison bison). While herds are no longer as vast as they were 200 years ago, recovery plans for the species are underway, and, in many cases, may rely on some degree of fencing or other containment at least for the time being. Understanding bison behaviour, including communication, could help with that, making it easier to assess how comfortable the animals are feeling - and, perhaps, the likelihood of a 750 kg (1,600 lb) bull deciding it's had quite enough of that fence and heading off somewhere it doesn't realise is less safe.

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.

Bovines: Prehistoric Bison and Buffalo

Steppe bison, from a cave in Altamira, Spain

In the course of this series I have described 22 species of bovine. Whether that number accurately reflects the number of species alive today depends, in part, on what you consider to be a species - not least whether or not long-term domestication is a sufficiently dramatic thing to constitute the creation a new species. But that's the number I came up with, and it's worth noting that no less than seven of these species - almost a third - are currently considered "endangered", and at high risk of extinction. Indeed, the kouprey may well already be extinct, while the species to which domestic cattle belong is extinct in the wild.

(On that last note, incidentally, I received a note after posting on the subject reminding me of the Chillingham Cattle of Northumberland. These are sometimes claimed to be aurochs, or genuinely wild cattle, but this has no foundation, and, while they have been living wild for centuries, they are actually feral animals, likely descended from escaped medieval stock).

At any rate, there have been many other species of bovine that have gone extinct over the millions of years since their first appearance, in most cases for reasons that have nothing to do with humans. So, albeit with a focus on the genuinely cow-like ones, rather than the spiral-horned antelopes and their kin, today I'm going to look at some of them.

Bovines: Bison bison bison, the Bison

American bison

The only bovine native to the Americas is the bison (Bison bison). Indeed, it is one of only five species of bovid that fit that description - the other four all being members of the goat subfamily. To give at least some variety, however, there are two generally recognised subspecies of American bison, one of which, following the standard rules for naming subspecies, necessarily goes by the wonderfully tautonymous trinomial Bison bison bison.

This, in fact, is the plains bison, which was once found across pretty much the whole of what is now the contiguous United States, leaving aside only the Pacific and Atlantic coasts and the arid deserts of the Southwest; they also lived in far northern Mexico and up as far as central Alberta. The other subspecies, commonly known as the wood bison (Bison bison athabascae) is native to north-western Canada and to Alaska. Physically, it's slightly larger than its southern relative, which means that, in the absence of American rhinos, elephants, and so on, it is, in fact, the largest living, land-dwelling animal of any kind native to the Americas.

Because bison are, indeed, pretty big. A fully grown male can be anything up to 195 cm (6' 6") tall at the shoulder, and can weigh as much as a tonne (2,200 lbs). Females are, admittedly, quite a bit smaller, although at a maximum of about 180 cm (6') and 550 kg (1,200 lbs) they're still pretty big - and closer in size to the males than, say, those of yak. The horns are also about the same size in both sexes, although (relative to the rest of the animal) fairly small in comparison to those of most other bovines.

Pleistocene (Pt 10): Of Armadillos and Bison

Glyptotherium

When the continents of North and South America collided, shortly before the dawn of the Pleistocene, a range of creatures wandered across the newly formed isthmus of Panama. Most were heading south, giving us such "normal"-looking South American animals as jaguars, ocelots, and grey foxes. The ones heading north were a lot stranger, and many of them didn't do so well in the long term. These include such wonderfully weird creatures as the giant ground sloths, which managed to get as far north as modern Canada before dying out.

But they weren't alone, and not necessarily any stranger than some of their fellow immigrants. The same event that saw the arrival of the ground sloths also saw the coming of the armadillos. Today, there is only one species of armadillo in the United States, and only two outside of South America (the other gets no further north than southern Mexico). But, back during the Pleistocene, there were others, and some of them were not quite what we'd expect today.

The Fate of the Bison

Before Europeans arrived, vast herds of bison inhabited the Great Plains of America, ranging from Manitoba to Mexico and Pennsylvania. The story of their decline due to massive overhunting is a dramatic one that seems to be well-known in America, an object lesson in mankind's ability to damage nature. Today, around 30,000 American bison (Bison bison) survive in the wild, and those raised in captivity are often hybrids with domestic cattle, rather than genetically pure survivors of the original species. American bison are officially considered "Near Threatened", and it is only the ongoing conservation efforts that prevent them from being a truly threatened species.

If you think that's bad, the story of what Europeans did to the bison of their own continent is even more depressing.

The European bison (Bison bonasus) looks very similar to its American cousin, but is, nonetheless, a distinct species. It's somewhat slimmer than the American species (which admittedly isn't saying much - they're both very muscular), and the horns are a different shape, but that's pretty much it. To avoid confusion between the two, the European bison is sometimes called by its German name of "wisent", but I'll stick with the more common English name.

While we're on the subject of names, though, it's worthwhile mentioning the difference between bison and buffalo. In America, the two words mean the same thing; they're both referring to the American bison. In Europe, however, the term "buffalo" is reserved for a number of African and Asian species, such as the water buffalo, which are clearly cattle, but look quite different from bison.

Once, herds of European bison ranged across most of central Europe, much as their American counterparts did across the Atlantic. They did not sweep across wide grassy plains, because Europe doesn't have many of those, and instead preferred woodland environments, much as the American bison in Canada do today. There were three subspecies, one on the south-eastern edges of Europe in the Caucasus mountains, one further west in the Carpathian mountains, and the other more widespread across the lowlands.

While the American bison did not begin to die out until the nineteenth century, the European species began to suffer much earlier, partly due to hunting, and partly due to the loss of their woodland habitat to encroaching agriculture. As early as the Middle Ages, only scattered populations remained, and the situation only got worse as human population soared. By the nineteenth century, the Carpathian subspecies was extinct, and only two populations of European bison survived: one in the Caucasus, and the other in a carefully managed royal hunting reserve in the Białowieża Forest in Poland.

In 1914, World War One began to sweep across Europe, and by 1919, the last lowland bison in Poland had been eaten by the German army. The Caucasian subspecies struggled on only for a few more years before, it, too, went extinct in 1927. Except... the lowland bison, at least, were not quite extinct. There were still animals in zoos - only a tiny number, to be sure, but that might just be enough to save them.

Of these captive animals, five bulls and seven cows successfully bred, and all European bison alive today are descendants of these twelve animals. In 1952, a group of forty purebred lowland bison were released into the wild, back in the Białowieża Forest. The herd prospered, and today, there are nearly 2,000 lowland bison in existence. Over half of them still live in the same forest, with the remainder scattered across Poland, Belarus, and Lithuania.

These animals are, however, remarkably inbred. They are descended from just seven of the founding twelve animals, and the male lines of all but one of the bulls have since died out - meaning that all living bulls have an essentially identical Y-chromosome. In domestic cattle, its considered unwise for animals to be more than 12.5% inbred, or they will begin to lose fertility, die young, and generally perform poorly. In the lowland bison, inbreeding has been estimated to approach 50%, although there don't seem to be any major problems. There are a number of possible reasons for this, but it could just be that we haven't noticed yet, what with most of the animals being out in the wild. On the other hand, they have suffered from a recent outbreak of balanitis, although this doesn't appear to be connected with inbreeding.

But what of the Caucasian bison? It turns out that one bull of the founding five, appropriately named "Kaukasus" by his keepers, belonged to this subspecies, not the lowland one. He died in 1925, just two years before the last living member of his subspecies out in the wild, but he had managed to sire a number of calves. As the only animal of his kind in captivity, the only cows he could mate with were lowland bison, and so all of his calves were hybrids, and the Caucasian subspecies is therefore officially extinct. But his calves did survive, and they went on to breed with other lowland bison.

Because of this, the hybrid descendants of this solitary Caucasian bison are also descended (through the female line) from all eleven of the other breeding bison alive in the 1920s, and are much more genetically diverse than their purebred kin. Indeed, the hybrids are about 94% lowland, with only 6% of their genes coming from Kaukasus himself. There are estimated to be slightly more of these hybrids alive today than there are purebred lowland bison, although about half of them live in captive breeding centres. Nonetheless, some have been released into the wild in Poland, Slovakia, Russia, and Ukraine. They have even recolonised the Carpathians, former home of the long lost third subspecies.

Inbreeding among these hybrids is estimated at around 26%, just marginally worse than the 25% you would get if a brother and sister had children, but not as bad as in the purebred lowland bison. Even so, they seem to be worse off, although that may just be because so many of them are in breeding centres where such things are more obvious to researchers. For instance, the hybrids seem to suffer from abnormalities of the skull, and an increased chance of dying within a month of birth.

Because the total population of European bison is increasing - albeit from an incredibly low starting point - as of 2008, it is no longer considered an Endangered species. It is however, classified as Vulnerable, which is only one grade higher, and makes its position worse than that of the American bison. As you might expect from a species down to just twelve animals less than a hundred years ago, it is extremely inbred, and its continued survival is by no means assured. The effort that has been put into saving the European bison over the last century has reaped great dividends, but the task is far from over.

[Picture from Wikimedia Commons.]