What is a Marsupial?

A possum

In America, the word "possum" is usually used to describe a moderately-sized, somewhat rat-like, animal that has grey fur, sometimes pretends to be dead, and has far too many teeth for any self-respecting land-based mammal. Officially, this creature is an "opossum", and more specifically, the Virginia opossum (Didelphis virginiana). The word comes from the language of the Powhatan people of Virginia, and has been in use in English since at least the 17th century.

Over on the other side of the world, in Australia, the word "possum" is, however, used to refer to an entirely different animal. These are nocturnal, tree-dwelling creatures, typically with large eyes and long tails, and the majority of the seventy or so species are herbivorous. Early settlers, who had probably only vaguely heard of the American animal, nonetheless decided to give it the same name. Like the Americans, over time they confused "opossum" with "a possum", and shortened the word. Unlike the Americans, their shortened word became not merely colloquial, but the one formally used in zoological texts.

The Pig Family: Bush Pig, Bushpig, Red River Hog

Red river hog (boar)

As zoological knowledge has advanced, the number of species that we know about has changed regularly, and not just because we keep discovering new ones. Animals that we knew about, but previously thought were subspecies get promoted to full species, and animals that we thought were separate species turn out not to be.

In fact, over time, we can sense something of a trend here. Beginning with Linnaeus in 1758, several new species are named, often by naturalists unaware, in the days before fully up-to-date reference libraries, let alone the internet, that somebody else had already given a name to the same thing. That process continues through the 19th century, with minor differences being seized on as evidence of speciation, even where it was possible to make decent comparisons. Through the 20th century, the number of genuinely new species being discovered dramatically tails off (at least, for mammals), but there's also a tendency to tidy up the great mass of inherited names from the past, merging similar animals together. Finally, from the late 20th century onwards, an increasing understanding of genetics results in numerous subspecies being promoted, often with our Victorian predecessors turning out to have been right all along.

The Fast Lives of Early Sperm Whales

Livyatan melvillei, a Miocene species
(It was supposed to be 'Leviathan', but the name was
already taken)

Sperm whales are magnificent and highly distinctive animals. While they may not be quite as large as the great toothless whales, they are still pretty huge: a fully grown male is typically about 16 metres (52 feet) in length and weighs over 40 tons. So distinctive is it, in fact, that today it is usually considered to be the only living species in its family, the Physeteridae.

The caveats in that last sentence - "today", "usually", and "living" - are all significant. On the first two points, the sperm whale family used to be considered to contain no less than three living species, and some researchers still define it that way. The other two species are the dwarf and pygmy sperm whales (Kogia spp.), and they're typically (but not always) placed in their own family these days. As their names suggest they are much, much smaller than the "true" sperm whales, being more like the size of a large dolphin.

Broken Bones and Missing Toes

Woodland jumping mouse

Life can be hard, especially out in the wild. Injuries can be common, and among the most painful are broken bones. As humans, we can mitigate bone fractures using splints, braces and so on, but wild animals have no such luxury. Nonetheless, bones do heal by themselves, a process that starts with the formation of a tough fibrous scar at the injury site that at least helps to keep things fixed partially in place. Over time, the scar is mineralised to form weak, but functional, bony material, and then eventually rebuilt with as much of the original structural integrity as possible.

Without splints and braces, this is likely to be an imperfect fix, even assuming that the injury doesn't prevent the animal from feeding or otherwise kill it before the process completes. If the animal does survive, the signs of the injury are always going to be there in its skeleton, and are quite likely to cause at least some ongoing problems. But, survive they do, and bone healing wouldn't have evolved in the first place if it never worked in the wild.

The Laziness of Venomous Shrews

While there are some bats that could challenge them for the title, by most measures, shrews are the smallest of all mammalian species. They typically weigh no more than about 20 grams (two-thirds of an ounce), and are often much smaller than that. Being so small poses a number of challenges, but there's one in particular that affects mammals of this size that would not affect, say, beetles.

This is because mammals are warm-blooded, and need to generate internal heat in order to keep functioning (unless they're hibernating). The smaller you are, however, the more rapidly you lose heat through your body surface, which means that shrews need a fierce metabolism to keep burning enough calories to keep themselves warm. This means that they have to eat almost constantly, but it also means that they want to exert themselves as little as possible while doing so, so as to waste the minimum amount of energy in the process.

The Pig Family: The World's Largest and Smallest Pigs

Pygmy hog

By some definitions, the term "pig" can really only be applied to those species that are most closely related to the domestic animal; those included within the genus Sus. This includes the wild boar, and the various kinds of warty and bearded pig that inhabit the Indonesian and Philippine islands.

But these are not the only pig-like animals to inhabit Asia. In 1847, Brian Hodgson, a naturalist and former colonial administrator who was living in Darjeeling at the time, described and named the pygmy hog (Porcula salvania), an animal he considered so different from regular pigs in the shape of its teeth and feet that he placed it in its own, newly defined, genus, Porcula. (The scientific name, incidentally, translates as "piglet from the Sal Van", the latter being a forest that only coincidentally sounds like the Latin "silvae" meaning "woodland").

Miocene (Pt 7): Hornless Rhinos, Long-Tusked Elephants, and Three-toed Horses

Anancus arvernensis

As the climate cooled around 11 million years ago, the forests of Europe began to thin out once more, something that favoured fast-running animals such as horses. Until this time, the only kind of horse in Europe, however, was the small, three-toed, Anchitherium, which was likely adapted to dense woodland and not so suited to the new climate. Its own ancestors had reached the continent from the east, having crossed over during one of the periodic rises of the Bering Land Bridge, but now, not coincidentally, given the colder climate, the Land Bridge rose again, and a second kind of horse followed it out of the Americas.