Sunday, 21 August 2016

The Sound of Squirrels

It is, of course, well known that bats and cetaceans use ultrasound to help them navigate the world using sonar. However, there is no particular reason why animals should always make sounds that happen to be in the range of human hearing (and, arguably, some good reasons why they might not want to). So, taking the usual definition of ultrasound as anything higher than 20kHz - the approximate upper limit for the hearing of a young, physically fit human - it shouldn't be too surprising that other mammals use ultrasound for purposes like communication. Or, in the case of shrews, for actual echolocation.

There has been significant laboratory research on the use of ultrasound by mice and rats since at least the 1970s, but it has also become clear that they are far from the only rodents to make sounds beyond our hearing range. Last year, I talked about the potential use of ultrasound for sexual seduction among hamsters (and, to be honest, didn't reach much of a conclusion) and we also know that, for example, baby voles call to their mothers ultrasonically if they become separated.

In the case of squirrels, though, we know rather less, and much of what we do know applies to various kinds of ground squirrel. As recently as 2013, however, it was discovered that flying squirrels also use ultrasound, presumably for social communication. Indeed, the sounds were quite complex and varied, not mere repetitive squeaks, implying that they might impart rather more information that one might think at first glance.

Sunday, 7 August 2016

Bovines: The Largest Living Antelopes

Common eland (male)
Over the last few months I have looked at the various species of cow-like bovine, including such creatures as yak, bison, and water buffalo. But we've known, since at least the 1950s, that there are also a number of other animals that are more closely related to cattle than they are to other kinds of bovid. Although the choice of terminology is ultimately arbitrary, these are commonly considered to be members of the subfamily Bovinae, and thus can also be described as "bovines".

They are not, however, physically very cow-like, and so are instead described by the more generic term "antelope" - which really just means "any bovid that's not obviously some kind of cow, sheep, or goat". The great majority of antelopes are therefore not "bovines" in any sense, belonging to their own distinct subfamilies. The majority of those that are are commonly referred to as spiral-horned antelopes. While all bovines are said to have some degree of spiral growth pattern to their horns, it's only in these antelopes that it's really obvious, with multiple turns of the spiral clearly visible to the observer.

Saturday, 30 July 2016

Pliocene (Pt 12): From Rabbits to Rhinos

Around three million years ago, as the Pliocene began to draw to a close, North and South America collided, the island chain that had previously stretched between them rising to become the Isthmus of Panama, and much of the rest of southern Central America besides. With a solid land bridge in place, land animals from both continents were finally free to mingle, merging two faunas that had, until then, been quite distinct.

The North "won" the resulting battle of competition, in that far more creatures successfully moved south than travelled in the opposite direction. These were, on the whole, creatures we're familiar with across the Northern Hemisphere, because the proximity of North America to Asia had already led to some blurring between the animal groups there. So, it's as a result of this, the Great American Interchange, that South America has foxes, deer, cats (most obviously jaguars and ocelots), mice, rats, weasels, otters, and rabbits.

Not only that, but some of the animals that headed south later died out in their northern homeland, so that today, we think of them as being uniquely or primarily "South American". These animals include the llamas and the tapirs, and, to a lesser extent, the peccaries or javelinas, which still survive in Mexico and the American south.

Sunday, 24 July 2016

The Echidna's Incredible Expanding Testicles

The echidna is a pretty strange mammal. Most notably, it is the only mammal other than the platypus that lays eggs. I've discussed in an earlier post why that doesn't disqualify it from being a mammal, but it is undeniably unusual. It's not even the only odd thing about echidnas, since they can also sense electricity through their snout, and the males have a four-headed penis.

I'm not making this up.

There are, in fact, four different species of echidna. The one most people are likely familiar with, though, is the short-beaked echidna (Tachyglossus aculeatus), on the grounds that it's the only one found in Australia. (The others, if you're wondering, live in New Guinea). In fact, they're found pretty much across the whole of Australia, albeit generally in open woodland or rainforest, rather than true desert. Even so, that's a fairly broad range of habitat types and variations in climate, and their behaviour does change somewhat across the continent.

Sunday, 17 July 2016

Bovines: Mini-Buffaloes

Anoa (probably lowland species)
One of the themes that we see played out from time to time in evolution is that of insular dwarfism. Here, a population of some large animal is trapped on an island or (less commonly) some other isolated locale from which they cannot easily escape. Faced with a limited food and other resources, over sufficiently large periods of time, they become smaller, making the best of whatever there is. (There is also a related phenomenon of insular gigantism, whereby really small creatures become larger, likely because they no longer need to hide from predators that happen to absent on their island).

Today, there are no tiny elephants or rhinos, but bovines - where the males of some species can weigh upwards of a ton - are a different matter. At least two, and probably three, species of living bovine are examples of exactly this principle in action.

The reason that I'm vague about the numbers here is that there is still some dispute about the nature of the anoa. There are usually regarded as being two species: the lowland anoa (Bubalus depressicornis), and the mountain anoa (Bubalus quarlesi). But not everyone agrees that the two are distinct. The first formal suggestion that they might be separate species was made by Dutch scientist Pieter Ouwens, better known for being the first scientist to describe the komodo dragon. That was back in 1910, and at least as late as the 1960s, there was still general agreement that the two forms were physically distinct, and that they likely favoured different terrain.

Sunday, 10 July 2016

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.

Sunday, 3 July 2016

The Importance of Monkey Grandmothers

Humans are almost unique among mammals in experiencing menopause a long time before their actual death. That is, they continue living for an extended period after they cease to be able to have children and pass their genes on to the next generation. Evolutionarily speaking, one might think, there is no longer any point to them; their task is done. But, obviously, that's not the case, as we can see from the importance of grandmothers and other female elders in, for example, tribal societies. For example, their greater knowledge of the world, and the ability to share in the childcare of their descendants, can mean that the latter have a better chance of survival.

Examples of this effect among other mammals are much less common. At the extreme, some mothers simply drop dead as soon as they have raised even a single batch of children to adulthood, but even among the majority of species, it's being a mother, not a grandmother, that's really important. Perhaps the best known exception is among elephants. Here, the leader of a family group is typically the oldest living female, who helps the herd through her knowledge of the outside world (where watering holes may be found in the dry season, for example), experience of predators, and a lifetime's worth of social knowledge.