Sunday, 14 February 2016
Having said which, it's probably not so surprising when you think about it. They do, after all, have huge gnawing teeth that look like those of rats (and which, indeed, help define the rodents as a group). Granted, they are pretty enormous as rodents go, and bulky with flat, paddle-shaped tails, but rodents they are, with animals like gophers and kangaroo rats among their closest relatives.
The beaver family, ignoring the extinct forms, contains a grand total of two species, both of which are found in Europe. One of them is the animal familiar to Americans (Castor canadensis). Descended from a population that originally lived in Wyoming and Canada, these were shipped across to Finland in 1937, and subsequently spread across the country, crossing the eastern border into Russia in 1952. A year later, some of them were moved again, this time to Austria, and in the 1970s, some were shipped all the way to eastern Siberia. All of these populations survive, although attempts to introduce the animals to France, Ukraine, and Belarus all ultimately failed.
Sunday, 7 February 2016
|Spot the male|
Sea lions and "true" (or "earless") seals belong to different, if closely related, families, but their reproductive strategy is much the same. In both cases, they spend most of their lives at seal, but they can't give birth there without their pups drowning. The majority of species breed once every year, with the females coming ashore, first to give birth, and then, almost immediately after, to get pregnant again before returning to the sea.
The precise details vary from species to species, but the South American sea lion (Otaria flavescens *) is a typical example. In December each year (that is to say, in early summer, this being the Southern Hemisphere), males haul themselves out onto beaches and fight one another for the best spots. Naturally, the largest and most aggressive males win, with the smaller and younger individuals forced out, to either wait another year, or to make daring raids when the larger ones aren't looking.
Saturday, 30 January 2016
However, opinions have differed as to quite how this should be done. How many significant sub-groups are there, which of them are deserving of the rank of "subfamily", and which animals go with which? Even with modern techniques for analysing genetics and coming up with computer-generated metrics of relatedness, the answers to these questions are not entirely clear. Partly that's because the first two questions at least are essentially arbitrary, and partly because we haven't really got enough samples from some of the more obscure species to really nail the family tree down. There could be anything up to sixteen subfamilies, or just two, or somewhere in between.
Sunday, 24 January 2016
|Tritemnodon, another early hyaenodontid|
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.
Sunday, 17 January 2016
|Insular flying fox,|
native to the Solomon Islands and Polynesia
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.
Sunday, 10 January 2016
|Myotis sodalis, the Indiana bat|
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.
Sunday, 20 December 2015
|Brown titi monkey|
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.