The Smallest Mammal Ever?

When it comes to fossil mammals, or indeed, any other kind of fossil animal, our attention is inevitably drawn to the giants. We are often fascinated by the mammoths or glyptodonts or the largest of Irish elk or the most muscular of sabretooths. Outside of mammals, it seems there's a never-ending battle to find the "largest dinosaur ever".

Indeed, one might almost get the impression that everything prehistoric was larger than today. In a number of cases, that's because larger animals really did exist in the past, perhaps being wiped out by a combination of the harsh realities of the Ice Ages and the arrival of human hunters. It's also an artefact of larger bones being easier to find in rock layers and being less fragile and likely to fragment when they fossilise. And that's before we add in the fact that most popular books on the subject tend to have a focus on the biggest and most dramatic animals of their kind.

Prehistoric Mammal Discoveries of 2019

Nehalaennia, an 8 million-year-old rorqual
from the Netherlands, first described this year

As the year - and decade - approach their inevitable conclusion, it's time again to look back at a few palaeontological findings of 2019 that didn't, for whatever reason, make it into the regular Synapsida posts. As always, there is no theme to this list, just a sample of what seemed interesting linked only by when it happened to be published.

Small British Mammals: Water Shrews

Two of the three species of shrew found in Britain are very similar to one another in appearance and habits. This is hardly surprising, given how closely related they are. The odd one out, however, is not quite such a close relative, and is rather more distinctive.

This is the Eurasian water shrew (Neomys fodiens), which unsurprisingly, is simply called the "water shrew" in Britain. Despite its differences, even at first glance, it's pretty obvious that it is a shrew: it's a small, long-tailed animal with short fur, tiny ears, small eyes, and a narrow, pointed, snout filled with sharp teeth. However, by the standards of shrews, it's unusually large. Fully grown adults can reach as much as 10cm (4 inches) in length, and weigh up to 25g (0.9 oz.), closer in size to a typical mouse than to other native shrews.

The Smallest British Mammals: Shrews

Common shrew

For some reason that I no longer recall, a picture of a shrew cropped up while I was talking with a friend on the internet. "That's a mouse," she insisted. "No," I said, "it's a shrew." "Mouse!"

It was only later that I remembered that the German word for "shrew" is spitzmaus (literally "sharp/pointed mouse") and, since my friend was Austrian, she wasn't necessarily entirely wrong. From a certain perspective, perhaps to German-speakers, all these small, furry, long-tailed things are mäusen.

And it's certainly true that shrews do look, superficially, quite like mice. They tend to be smaller, with narrower snouts, and very small ears and eyes, but there is something quite mouse-like about them. Nonetheless, shrews (unlike, say, voles) are not rodents. In fact, as placental mammals go, they aren't even particularly related to rodents - humans are more closely related to mice than shrews are.

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.

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.

New Mammal Species 2015

Brown titi monkey

As 2015 draws to a close, it's time to look at some of the species of mammal that we didn't even know existed this time last year. Of course, the reality is that, in most cases, we knew that the animals existed, we just didn't realise that they were a species. Some we thought were subspecies, and we now have enough information to promote to full species status, while others we just hadn't noticed were different at all.

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.

Spot the Difference

Common European shrew

The shrew family contains what are, by the usual method of measuring such things, the smallest of all living mammals. Yet they are actually quite a large group, with well over 300 species so far identified, and, given their small size, probably quite a lot that we don't yet know about - especially in the tropics. Indeed, in the grand list of mammal families, they come fourth in terms of sheer number of species, beaten only by the rats and mice, the cricetids (most, of which, frankly, are also mice, although the group also includes the voles and hamsters), and the vesper bats. They may not have the greatest range of physical variation, at least to human eyes, but that's still quite a lot of diversity, and they certainly aren't "all the same".

It's perhaps worth pointing out, though, what a shrew is, and what it isn't. Firstly, while they may look kind of like mice, shrews are not rodents. Rodents are defined by the presence of great gnawing incisor teeth, and by not having the second set of incisors that rabbits have. Shrews don't look even remotely like this, having lots of small sharp teeth ideally suited for biting into insects. In fact, shrews' closest relatives are actually the moles and hedgehogs - relatively small animals that also eat a lot of invertebrates.

Growl Whistle Squeak

While scent marking, for example, is great for leaving long-lasting messages, for other purposes, vocal communication has a number of advantages. However, since non-human animals can't talk, this necessarily imparts less information than it does in our own species. But how much less? Or, to put it another way, how complex can animal vocalisations get?

The number of different sounds an animal can make depends on a number of factors. Many of these are physical, due to the way that their larynx and vocal cords are set up, and to their ability to modulate the sounds it produces with their mouth, lips, and so on. For instance, while there has been at least some success in getting chimps and gorillas to use human sign language, they can't actually speak because of purely physical limitations in their upper respiratory tract. Added to this is the matter of just how much complexity they need to get across anyway.

Broadly speaking, the more sociable an animal is, the more need it has for complex communication. If you rarely come across other members of your own species, you probably don't need to say much when you do. A simple "go away" is probably about as much as you need, and you can co-opt the same threat against hostile members of other species, too. Beyond that, you may need some kind of mating call, and a means for mothers to find their offspring, and you're pretty well sorted.

Freedom to Dive

Mediterranean water shrew

There are well over 5,000 different species of mammal in the world. From a human perspective, a great many of them are very similar - at least a thousand of them can reasonably be described as either 'mice' or 'rats'. Where they live in different continents or unconnected parts of the same continent, that's not really an issue: European beavers don't care what the American sort are up to. It's a different matter when they live together.

But, of course, they often do. For example, there are fifteen different species of insect-eating bat in Britain alone. But if you have two different kinds of animal living in the same place, in the same way, eating the same thing, they have no choice but to compete. Inevitably, one of them will be slightly better at it than the other, and, given enough time, one of them has to either die out or change what it's up to. So how come there are so many similar animals?

Generally speaking, the answer is that they're not all doing exactly the same thing: there's some subtle difference in what they're up to. In some way or another, they're partitioning the available resources. One simple possibility, for instance, is that they aren't eating the same thing after all. If (to pick a rather unlikely hypothetical example) one of you eats only raspberries and the other eats only blackberries, there's plenty for both, and you don't have to fight over who gets to eat what.