Swimming With Dinosaurs

At the highest level, all mammals alive today can be placed into one of three broad taxonomic groups. Around 94% of the known species are placental mammals, a vast group that includes everything from bats to dolphins and from anteaters to humans. Virtually all the others are marsupials, with the strange egg-laying monotremes representing just a handful of species.

But there were once others, many of them existing before placentals and marsupials (and presumably monotremes, although we don't have much in the way of fossils for them) came into existence. Technically speaking, this includes a few side-branches from the two man lines that arose before the last common ancestor of the living forms - for example, creatures close to the line that eventually gave rise to the placentals, but that arose early enough that we can't be sure that they literally had a placenta. But even once we trim out those, there are still quite a few left.

Pinnipeds: Ross and Weddell Seals

Weddell seal

The majority of 'true' seal species live in the Northern Hemisphere, with just five found south of the equator. While they were once more widespread, today, all five of these live in the extreme south, with the most widespread - the southern elephant seal - reaching Tasmania, southern New Zealand, and Patagonia. The remaining four, which are all relatively closely related to one another, do not range so far north and are exclusively Antarctic and sub-Antarctic.

The Weddell seal (Leptonychotes weddellii) is a typical example. With fully-gorwn males up to 3 metres (10 feet) in length, and weighing about half a ton, they are noticeably smaller than elephant seals, but still pretty large by the standards of seals in the north. They were first discovered during the expeditions of navigator James Weddell, who, in the 1820s, sailed further south than anyone had previously travelled, into the sea that now also bears his name. Since the seals are named for him, rather than for the body of water where they were first found, it's perhaps unsurprising that they are not unique to that sea, and are equally common right round the frozen continent. During the winter, they can travel as far north as South Georgia and other islands of the extreme South Atlantic, but they don't normally reach (for example) the Cape Horn at the southern tip of South America.

When the Hippos Changed

The hippopotamus family contains just two living species, including, of course, the well-known common hippo (Hippopotamus amphibius). As is so often the case, though, the family has a long fossil history including a number of other species and genera, although, while the fossil species were more widespread than the living forms are today, it's fair to say that it was never a huge or diverse group in the way that, say, antelopes are.

Among the fossil species, several are very closely related to the living common hippo, including both the stalk-eyed hippo (H. gorgops), which probably weighed over 3 tonnes, and the pig-sized Maltese hippo (H. melitensis). Quite where the living pygmy hippo (Choeropsis liberiensis) fits in the fossil family tree is much less clear, but there are a number of species that aren't particularly close to either of the surviving forms.

Taking a broad view of the fossil history of the family, then, palaeontologists have tended to group the hippos into two subfamilies. One are the "hippopotamines", a group of broadly "modern" hippos that includes both of the living species. At least two other genera are also considered to belong to this group, one of which, Hexaprotodon, lived everywhere from Madagascar to Spain and Indonesia, taking in much of northern Africa and southern Asia on the way. Most of these lived during the Pleistocene and Pliocene epochs, which is to say, the Ice Ages and the epoch immediately preceding them. The earliest forms, including the other genus, Archaeopotamus of Kenya and Arabia, lived during the late Miocene, first appearing somewhere around 8 million years ago.

When Dolphins Emigrate

While most mammals tend to have a "home range" in which they spend most of their adult lives, there are many reasons why they might wish to move elsewhere. Perhaps most obviously, there is the movement of juvenile individuals leaving home for the first time, and so travelling out of their parent(s) home range to establish their own territory. There's also the issue, mostly with males, of travelling about in the hope of encountering fertile members of the opposite sex. On a larger scale, there can be regular migration events, including short-range movements such as goats moving down mountain slopes in winter to avoid the worst of the weather.

But large-scale movements on a one-off basis are relatively rare. In social animals, individuals may move from one herd (or other group) to another for all sorts of reasons, although, in many species, even that can carry risks. Incidents in which a large portion of a herd ups sticks and moves to a place already occupied by another herd, before trying to integrate with the locals, are relatively rare. Which also makes them difficult to study, since it largely relies on the luck of happening to already be looking at a given group when it happens to occur. It's perhaps particularly hard to do so for cetaceans, which live underwater, and can be difficult to track.

Mice, Mice, and More Mice

A common feature of my blog posts, pretty much since the beginning, has been the inclusion of cladograms, tree-like diagrams that show how different species, or groups, of mammal are related to one another. Our knowledge of these relationships is constantly evolving, as we get more and more information, or find different ways of doing the necessary measurements.

In general, though, what happens is that scientists measure a number of different features from the animals that they want to study, and compare which ones have the most in common. These days, these are most likely to be physical measurements, such as fine details of the shape of the skull or teeth, if at least some of the creatures we're looking at happen to be fossils. Otherwise, it's much more likely that the things being compared are stretches of genetic code. The further apart two animals are, evolutionarily speaking, the more differences there are likely to be, and if we pick a gene that both animals possess, and that can accumulate a reasonable number of changes without stopping working altogether, we have a good basis for comparison.

Pinnipeds: The Mighty Elephant Seals

Northern elephant seal (male)

The largest of all seals are, of course, the elephant seals. These were one of the original species of seal to be scientifically named, back in 1758. They were originally given the name Phoca leonina, which literally translates as "seal-lion", suggesting some possible confusion on Linnaeus's part about which exact animal he was describing. For a long time, it was thought there was only one species, but we now know that the populations in the Northern and Southern Hemispheres represent two different, but closely related species.

Northern elephant seals (Mirounga angustirostris) live off the western coast of North America, with breeding colonies on offshore islands and occasional isolated patches of continental coast in California and Baja California. It's probably here that they are more commonly seen, since they not only spend the winter breeding season ashore, but they also visit the same beaches during the summer for their annual moult, when they are unable to enter the water for weeks at a time. Surprisingly, then, they spend the rest of the year somewhere else entirely, travelling hundreds of miles from their colonies to feed in the north-east Pacific, and spending spring and autumn as far north as the Aleutian Islands and, on occasion, as far west as Hawaii. Quite why they'd do something as seemingly daft as to make the same long-distance migration twice a year isn't entirely clear, although it presumably has something to do with the ideal weather for each activity.

Bed Time for Wild Hamsters

Golden hamster

After the mice, the second largest family of mammals is the hamster family, with somewhere in the region of 600 known species. The vast majority of these species are, however, not hamsters, and I refer to it as the "hamster family" only because that is the literal meaning of its scientific name, the Cricetidae. Over half of the cricetids, for instance, are "New World mice" visibly more or less indistinguishable from the "true" mice of the Old World. Most of the remainder are voles, and a mere couple of dozen or so are actually hamsters.

The species most people think of when they think of hamsters is the golden hamster (Mesocricetus auratus), which is the animal commonly seen in pet shops. While a few other species are sometimes also kept as pets, most of the varieties named on the basis of things such as hair colour and length are just domesticated breeds of the golden hamster. They are found in the wild only in one relatively small area on the Turkish-Syrian border, just north of Aleppo. Which, right at the moment, does make it somewhat difficult to study their behaviour in their natural environment.

But then, given that there are so many of them in captivity, and that they aren't exactly obscure animals, you might think that we know pretty much all there is to know about them. Certainly, compared with many other species, they are well-studied creatures. Indeed, they are common laboratory animals, due to the ease of breeding them in large numbers and their apparent comfort in indoor environments. On the other hand, while there are obvious advantages, such as not being eaten by predators, hamsters do not naturally live in cages... so maybe they behave differently when given the free run of the countryside?