How Fossil Rodents Met Their Doom

Tritemnodon, another early hyaenodontid
from Wyoming

A large part of palaeontology consists, as you might expect, of looking in detail at the fossils that are dug up. This can tell us a lot about the animal and how it lived. By analysing the shape of the teeth, we can get some clue as to what sort of things the animal ate, and from the shape and proportions of the limbs and other body parts we may be able to infer other details of the lifestyle. If it's a carnivore, for example, did it chase its prey, or pounce on it from hiding and overwhelm it with sheer physical force? If the former, it should have long, relatively slender limbs suitable for running, if the latter, we'd expect powerful limbs with attachments for strong muscles.

When it comes to fossil mammals, teeth are often the best guide. For one thing, being small, and made of material even harder than bone, they are more likely to be preserved, even if the entire rest of animal is missing. For another, for most mammals, the precise shape of the cheek teeth (molars and premolars) is highly distinctive, giving us clues as to how one particular species is related to others. Even if we did find, say, an isolated rib, it might be difficult to tell what it once belonged to, and it probably wouldn't tell you much about the animal in question even if you could - beyond, perhaps, some clue as to its size.

Mammals of a Tropical Island Chain

Insular flying fox,
native to the Solomon Islands and Polynesia

There are, at last count, well over 5,000 known species of mammal on the Earth. Many of these inhabit large regions, or particular patches within continents or large islands. Visit even one of the more desolate reaches of land on a major continent (unless it's Antarctica), and there will probably be quite a few species of mammal that live there - although, certainly, you'll find more in a tropical rainforest than the depths of the Sahara.

But smaller islands are a somewhat different matter. Islands tend to have less land-dwelling species than areas of equivalent size on the mainland, due to the difficulty of crossing the water to get there. Obviously, this is less true of things like dolphins or seals, for which almost any coast will do, should they bother to approach one at all, and it can also be affected by how long humans have lived on the island, and what animals of their own they have brought with them - intentionally or otherwise.

France, for example, has something like 90 species of wild land-dwelling mammal. Yet, nip across the English Channel, and Great Britain - ninth-largest island in the world - has only 52. And that's including things like grey squirrels, which do live wild, but are nonetheless a relatively recent human introduction. To be fair, there used to be more before hunters killed them and cities and farmland expanded to cover pretty much the whole of England, since we used, for example, to have wolves. And, yes, Britain is physically smaller than France, but even so, it's a noticeable difference.

Big Bat Mothers

Myotis sodalis, the Indiana bat

A great many species of mammal exhibit some degree of sexual dimorphism, whereby one sex is larger than the other as a an adult. Usually, this is the male, something we see in our own species, where men are, on average, taller, heavier, and more muscular than women. In many animals, of course, the differences between males and females are more dramatic than they are in humans, as we can see, for instance, by comparing stags and does, or male and female seals.

The typical reason for this appears to be that larger males are better able to drive of rivals for their mates' affections, and are also able to demonstrate their fitness and ability to obtain food - and thus, hopefully, the quality of their genes - to those same mates. But it isn't always this way round, since there are a number of species where it is the female that is larger, perhaps since it makes it easier for them to care for and defend their young. (Indeed, outside of mammals and birds, this is by far the more common arrangement). Clearly, which influence wins out, and which sex ends up the larger, depends on the species concerned.

But is there some pattern to this, some rule of thumb we can use to predict which sex will be larger, and, if so, by how much? Working in the aftermath of World War II, German ornithologist Bernhard Rensch thought that there was. In 1950, he formally proposed what has since become known as Rensch's Rule. This states that, if the males are larger, sexual dimorphism will be most extreme in the largest species in any given group, whereas, if the females are larger, it will be most extreme in the smallest species.

New Mammal Species 2015

Brown titi monkey

As 2015 draws to a close, it's time to look at some of the species of mammal that we didn't even know existed this time last year. Of course, the reality is that, in most cases, we knew that the animals existed, we just didn't realise that they were a species. Some we thought were subspecies, and we now have enough information to promote to full species status, while others we just hadn't noticed were different at all.

Telling the difference between a subspecies and a species is a difficult, and rather subjective, art. It essentially relies on you being able to demonstrate that the animals don't usually interbreed in the wild when they get the chance. Not that they can't. Not that they don't sometimes. Just that they don't usually. You can really only do this by showing that their genetics are distinct, and even doing that may depend on how good your sample is. As a result, while new species of mammal are named every year, it's worth noting that this isn't a one-way process... it's not that unusual to demote a species back down to subspecies as more information comes in.

The Dog Family: Fossil Dogs

Hesperocyon, the earliest known "dog"

Despite their variety, all dog species alive today are placed in a single subfamily of the wider historical dog family. Of course, what we consider a "subfamily", rather than some other taxonomic ranking, is entirely arbitrary, but it does indicate that what we have now represents just one branch of a much older family tree.

This living subfamily are the Caninae, and thus, strictly speaking, the term "canine" can be applied to all living dogs, even the foxes,  which are "vulpine" in more common parlance. The canines have been remarkably successful. Thanks to the existence of the dingo, dogs are one of the few terrestrial mammal families with wild representatives on every continent save Antarctica. (Although not the only one, of course... mice, in particular, get everywhere). That is, at it happens, entirely due to the canines, since the other forms of dog never left their continent of origin.

Doggy is Not the Only Style

In 1999, alternative pop group The Bloodhound Gang infamously sang "You and me baby ain't nothin' but mammals / So let's do it like they do on the Discovery Channel." (Ah, talk about highbrow cultural references...) As the rest of the song lyrics made clear, they were talking about what we can technically refer to as the dorso-ventral coital position, with the clear implication that this is the standard among non-human mammals.

By and large, this is quite correct, with the alternatives being particularly rare among typical quadrupedal mammals. Doubtless that's because, for many quadrupeds, the alternatives would be particularly awkward, but that's not necessarily the only reason. It has been proposed, for instance, that in insects this position is advantageous when at least one - and ideally both - of the sexual partners still want to be able to look about them for predators or similar threats. If so, there's no reason that that couldn't also be true for at least some mammals.

The Dog Family: Raccoon Dogs and the Fate of the Warrah

Raccoon dog

The Wikipedia page on the dog family includes, on its attached discussion page, a complaint that the article includes a picture of a raccoon - an animal that is clearly not part of the Canidae. Except, of course that there isn't (and wasn't then) any such picture. The complainant was looking at a picture of a raccoon dog.

It's an understandable mistake. The raccoon dog (Nyctereutes procyonoides) does look remarkably like its partial namesake. Indeed, the second half of its scientific name, first given to it by John Edward Gray in 1834, actually means "raccoon-like". The effect is achieved primarily by that dark mask over the eyes, a feature we naturally associate with raccoons, although, aside from the lack of stripes on the tail, the rest of the animal is fairly raccoon-coloured as well.

Nonetheless, it is indisputably a dog, although quite how it was related to other dogs was long a source of puzzlement. Modern studies have shown that it appears to be related to the true foxes, albeit not very closely. Its closest relative may, in fact, be the bat-eared fox of Africa, although, again, I should stress that this isn't exactly what you'd call a close relationship... just closer than anything else happens to be.