Sunday 31 October 2021

Flesh-eating Bats

Greater noctule bat
(Well, it is Halloween...)

At least in the parts of the world with a temperate climate, the great majority of bat species are insectivorous. Of course, there are a number of vegetarian bats, especially in the tropics, including the large fruit bats of the Old World as well as smaller, often fig-eating, species in the Americas. But insect-eating does seem the default, even though there is general agreement these days that bats are more closely related to the larger carnivores and to the hoofed mammals than they are to the other small insectivores, such as shrews and hedgehogs.

From an ecological, trophic web, perspective, insectivores are a type of carnivore - after all, they eat other animals, even if they're small ones. But, when talking about mammals (and similarly sized vertebrates) most researchers tend to draw a distinction between those that hunt invertebrate prey and those that eat comparatively large vertebrates. So there's a distinction between leopards and killer whales on the one hand, and shrews and anteaters on the other.

Sunday 24 October 2021

Miocene (Pt 29): Fake Elephants and Otter-Shaped Herbivores

At the dawn of the Miocene, South America was home to no fewer than three orders of moderate to large hoofed herbivore, compared with the two it has today (one containing the deer and llamas, the other including the tapirs). There is, of course, a degree of arbitrariness in what gets designated an 'order' as opposed to some other ranking. But nonetheless, these animals were distinctive, related to the two orders of ungulate we have today, but different from both of them. 

Two of these South American groups would outlast the Miocene, only dying out relatively recently. These were the litopterns and the notoungulates, both of which bore most of their weight on the third toe of each foot, and which may, indeed, be related to the group containing the tapirs, horses, and rhinos, which does the same today.

Sunday 17 October 2021

All the World's Deer: Small Deer of South America

Grey brocket
Deer first entered South America when that continent first connected with its northern counterpart a little over two million years ago. Finding nothing similar already there, they rapidly diversified into several distinct species, taking advantage of a wide range of different habitats. Many of the resulting species were medium to large animals, resembling their northern relatives, such as white-tailed and mule deer. But others instead took on a role closer to the muntjacs of eastern Asia. 

How many species of these small South American deer are truly distinct is a question that is not fully settled yet. In common with the scientific fashions of the time, a great many species were named in the 19th century, only to be merged in the 20th and then split apart again in the 21st (not always along the same lines, of course). As a result, many of the species we recognise today weren't considered such until recently and have not been individually studied to any great extent beyond demonstrating that they exist. On the other hand, the reason that we confused them for so long - and still aren't entirely sure how many they are - is that they are all quite similar. This is especially true of the various kinds of brocket, all of which are currently placed in the genus Mazama.

Sunday 10 October 2021

Baby's First Dive

Perhaps the most distinctive feature of mammals is that they provide their young with milk. This allows the young mammal to continue growing after it has been born, but before it is ready to fend for itself. The downside, of course, is that it leaves the infant entirely reliant on its mother for survival, something that we don't see, for example, in most reptile species. 

At some point, however, the infant has to survive on its own. Before it can truly leave home it needs to be able to find its own food, so there's going to be a period between it feeding entirely on milk and it becoming fully independent during which it's learning the necessary techniques of foraging or hunting. We see something similar in birds; while they obviously don't produce mammalian milk, young birds do necessarily rely on their parents for food until, as a minimum, they get the hang of flying. (It seems plausible that the same was true of pterosaurs, although there is evidence that at least some species learned to fly very quickly and that the period of dependence was correspondingly short).

While the reliance on suckling means that all mammals do this to some extent, the closest obvious analogy to birds would be bats - and, indeed, it does take them time to learn how to fly. But another group of mammals that has to learn how to do something nearly as dramatic before becoming self-sufficient are the pinnipeds (seals and their relatives).

Sunday 3 October 2021

Why Polar Bears Still Make Dens

Hibernation is something that's generally associated with smaller animals, such as bats and rodents. It's likely to be a particularly useful trait for them, since they otherwise need to eat regularly to survive and this would be difficult in winter. Bears are a clear exception to this, snoozing away the winter despite being particularly large animals, and it seems that hibernation has been a key feature of their evolution for some time, distinguishing them from other carnivorans, such as wolves, that live in similar climes.

One of the first things we have to point out in this respect is that it isn't universally agreed by scientists that bears do, in fact, hibernate. This is obviously a debate about terminology, rather than the bear's actual behaviour, and it hinges on whether a bear's body temperature drops enough while it's denning for the activity to be truly considered as "hibernation". It seems fair to say that the usual tendency these days is to consider that what they're doing is close enough to count, but not everyone agrees. For today's purposes, however, I'm going to use what some might argue is a layman's understanding, rather than a strict biological term.