Sunday, 23 April 2017

Pinnipeds: Ribbon and Ringed Seals

Ribbon seal (male)
Although there is, perhaps, more variety amongst seals than one might at first expect, when it comes to colouration, there isn't all that much to distinguish the different species. The majority are brown or grey, often with black or pale splotches of some kind. I've already described one exception, the harp seal, but arguably the most distinctive of all seal coat patterns belongs to its close relative, the ribbon seal (Histriophoca fasciata).

Male ribbon seals are black, or very dark brown, with clear, wide bands of pure white fur around their necks, shoulders, and just above the hips. Females are medium-brown with light tan stripes, so the pattern is less striking, but it's still present, and in the same shape. Neither sex is born like this; even once baby ribbon seals shed their pure white fur at around a month of age, they are initially plain in colour, only fully developing the stripes by the time they are two years old.

Sunday, 16 April 2017

Pliocene (Pt 16): The Pliocene Oceans

Over the last two and a half years, I have talked about a range of mammal species found across the Pliocene world. So far, I have only looked at the continents, and the various animals that lived on land. But the Pliocene seas were, of course, equally teeming with animal life. Much of this naturally consisted of either fish (most famously the giant "megalodon" shark) or invertebrates of various kinds. Sea turtles also existed, including some quite large ones, but, in keeping with the scope of this blog, I'm going  to focus on the mammals.

While we often tend to think that prehistoric animals tended to be larger than those alive today, this, was however, rather less true of whales, which have grown more or less steadily in size over the course of their evolution, perhaps in part to make it increasingly difficult for anything else to eat them. So, for example, the Pliocene killer whale (Orcinus citoniensis) was around 4 metres (12 feet) in length, barely more than half that of the modern species - although still quite impressive on a human scale.

Sunday, 9 April 2017

Dolphin Vaginas

If you look at any really detailed description of the physical appearance of a mammal species, you will find a lot of intricate information on the shape of the teeth (especially the molars and premolars) and the dimensions of the skull. There will also be discussions of the shape and proportion of the limbs and the exact colour of the fur, as well as any horns or antlers it might have. And there's a high probability that there will be quite a lot of information about the shape of its penis and the size of its testicles.

Obviously, reproductive anatomy is an important field if we want to really understand how an animal functions and behaves. Testicular size, for example, can tell us about its mating strategies. This is because the rule is not simply "the bigger the animal, the bigger its gonads". In order to gather a harem of receptive females around itself, a male has not only attract them to itself with suitably impressive antlers (or whatever) it also has to fight off rivals, and it's going to have to be big and muscular to do that. But if the females are sexually promiscuous, that's pointless. Instead, what you really need is to produce so much sperm that yours swamps that of your rivals. So, in those species, the males tend to be smaller, but their testicles larger (proportionately speaking). Alternatively, if your species is monogamous then neither of these things are much of a concern.

So male reproductive anatomy can tell us quite a bit. But you won't typically find, in most descriptions, is quite so much information on the female reproductive tract. There's probably some information on the shape of the uterus, which can relate to things like litter size, and maybe for a few other features besides, but it tends to be rather less than you'll find for the males.

Saturday, 1 April 2017

Deadly Demon Ducks of Doom

Since today is 1st April, although I'm not doing a spoof post, I am taking my annual break from mammals to talk about birds. Specifically demon ducks, although I nearly went with flightless boobies. It's that sort of day.

Australia is the most isolated continent to possess native mammals, so perhaps it shouldn't be surprising that the mammals that live there are particularly unusual, often only distantly related to those elsewhere. Birds, however, have an advantage that most mammals don't, in that they can fly long distances between land masses without dying. (They may not be doing so deliberately, of course, but being blown off course in a storm is the sort of risk they have to put up with). It's notable, in fact, that one of the two groups of native placental mammals in Australia are the bats, which are also also found further out in the Pacific islands.

Anyway, the upshot of this is that the birds of Australia frequently fall into taxonomic groups familiar from elsewhere in the world; Australia has owls, doves, seagulls, and parrots, among many others. At the same time, however, it is isolated enough that it does include some unique kinds of bird that are not found on other continents - less than the number of unique mammal groups, to be sure, but a number nonetheless. For example, lyrebirds are endemic to Australia, and are considered to form the oldest living branch of the songbird family tree.

Sunday, 26 March 2017

Pinnipeds: Grey and Harp Seals

Grey seal
There are four different species of seal that live off the coasts of northern continental Europe. Perhaps the best known is the harbour seal, or "common seal", which is also the most widespread of all seal species, being found across both the North Atlantic and the North Pacific. When the scientific naming of species was introduced in 1758, it was the only such species recognised from the area, but it only took a few decades for proper scientific descriptions of the others to follow, recognising that they were distinct from their "common" cousin.

The last of the four to be split off was the animal we now know as the grey seal (Halichoerus grypus), in 1791. While that's still fairly early as such things go, the fact that it was the last of the local seals to be formally identified as something different from the regular sort is likely down to the fact that it does look very similar to the harbour seal. Although, in fairness, it was first named by an entomologist, so clearly you don't have to be a mammal specialist to tell them apart.

Sunday, 19 March 2017

The Giant Chinese Badger-Otter

Otters and badgers are both members of the weasel family, the Mustelidae. As a result, while they are visibly quite different, they have a number of anatomical similarities, reflecting their shared ancestry. Indeed, some of the similarities, such as those in the precise shape of their teeth, are rather greater than one might expect simply from them belonging to the same family. On the basis of this, it was being suggested as recently as the 1990s that otters were essentially aquatic badgers - descendants of early badger species that had entered the water, developing webbed feet, a long muscular tail, and so on, in the process.

We now know, from various genetic and molecular studies, that this isn't so. The closest living relatives of otters are probably the weasels themselves and/or the zorillas and their kin, with badgers representing a rather earlier branch in the mustelid family tree. Given this, the apparent strong similarities between the two are either a case of parallel evolution, perhaps due to the fact that, by the standards of weasels, they're both fairly large animals, or, perhaps more likely, that they are ancient features of the group that happen to have been lost in their other relatives.

Sunday, 12 March 2017

A Side Order of Flies

Most species of bat eat insects, whether caught on the wing or plucked from leaves or other surfaces. But this is by no means true of all species, with the second most common diet - perhaps representing around a quarter of known species - being one based on fruit. Indeed, fruit-eating had evolved more than once among bats, with the giant fruit bats of the Old World not being especially close relatives of the much smaller ones found in the Americas.

While a great many herbivorous and omnivorous mammals include fruit as part of their diet, one estimate is that only around 10% of mammalian species rely on it as their primary source of food - most of them bats or primates. While that's not exactly a tiny proportion, given the number of mammalian herbivores in general, it isn't a huge one either. This, it has been suggested, is because, while fruit are great as a source of calories, they tend not to be high in protein, and a healthy animal needs a supply of both.

In the case of humans, eating plenty of fruit is well known to be a good thing, but trying to eat nothing but fruit for any extended period of time is likely to be a problem. Not only are you likely to suffer from lack of protein in your diet, but you will also suffer deficiencies in certain minerals and vitamins. That one of the main vitamins you would be short of is vitamin D, which promotes calcium absorption in the gut, is a particular problem, bearing in mind that there isn't much calcium in fruit to start with. For this reason, fruit-only diets can be a real problem for children, who need that calcium to grow their bones.