The Spikiness of Tenrecs

Being a small mammal (or, indeed, any other kind of animal) can be tough; there's always something out there that wants to try and eat you. There are a whole host of adaptations that animals use to try and avoid this fate. Running fast and being good at hiding or camouflage are, perhaps, the most obvious and widespread defensive methods. Another approach is to live somewhere that it's really difficult for most predators to find you, such as spending your entire life in an underground burrow.

Fighting back, as larger horned animals might do, generally isn't so effective, although skunks (for example) have certainly found a way of doing that. But there is also the option of body armour. It's something we see quite a lot in reptiles and, in living mammals, it's perhaps best developed in armadillos. But body armour doesn't have to be a bony or keratinous shell, for something that wants to avoid being eaten, spines can also be an effective deterrent.

The thing is, body armour, whether spiny or bony, requires a trade-off. Aside from the weight, every calorie you are diverting into making it is unavailable for anything else. Clearly, it's a price that many mammal species have found well worth making, which is why we have hedgehogs and porcupines, among others. But many animals don't, and some of them are quite closely related to the spiny sort, so what sort of differences favour, or discourage, the development of such defences?

Given that this blog has been going for over eight years now, it's perhaps notable that there are entire groups of mammals that I haven't really looked at before. And, while I have mentioned them in passing a few times, tenrecs are among them.

Tenrecs are small(ish) mostly insect-eating animals that are found only on the island of Madagascar. Estimates for how long they have been there range from 29 to over 37 million years, but nothing like as far back as the island's separation from the mainland, meaning that they must have crossed the sea to reach it. Once on the island, however, they diversified into a number of different forms, perhaps taking advantage of limited competition, as well as a very long period of isolation. Some climb trees, some swim in rivers, some burrow in the ground like moles, and yet others move about in the jungle undergrowth.

The closest living relatives of tenrecs are the equally obscure otter-shrews of mainland Africa. Indeed, until recently, they were considered to actually be tenrecs; we now think the difference is too great for this to be a useful classification. Excluding those, then, there are at least 30 living species of tenrec, and they can be grouped into three subfamilies. One of those contains just one living species, the slightly odd large-eared tenrec, while the other two are the furred and the spiny tenrecs.

Those names are appropriate; the furred tenrecs look somewhat like shrews, while the spiny tenrecs can be remarkably like hedgehogs, and have a rather similar lifestyle. Even within the spiny tenrecs, however, there is some variation, with some species having a relatively small number of long, but flexible, spines, and others having a dense covering of sharp and solid spikes. Which means that we can ask the question as to what other features of the animals correlate with how spiny they happen to be.

When researchers decided to look into this exact question, the first thing they had to do was to try and figure out a scientifically sound definition of "spininess". The one they came up with involved measuring the length and thickness of the largest hairs on the neck and back of the animal, and performing statistical analysis on the results to produce a number that could be measured against other, more easily defined, features - such as the animal's weight. They then performed this analysis on multiple preserved skins from each of five spiny, and eight furred, tenrec species, with that of an otter-shrew for comparison.

What this showed was that the spinier a tenrec species was, the larger it was likely to be. Add this to the observation that spiny tenrecs are also more likely to live in open woodland than dense jungle (compared with the furred sort) and one plausible reason for the evolution of the spines becomes apparent: it's more difficult for them to hide.

A similar pattern has been seen more generally with the evolution of body armour in animals. When species of animal that are initially quite small begin to evolve (relatively) larger sizes, it becomes more likely that they will also evolve some form of body armour to protect themselves. Animals that start out large are much less likely to do so, perhaps finding fast running to be a more effective mean of escaping the kind of predators they are more likely to fear.

Interestingly, though, with the exception of the common tenrec (Tenrec ecaudatus), which averages around 890g (2 lbs), the spiny ones aren't as large as one would normally expect. Granted, they aren't as spiny as, say, a porcupine, but even so, they seem to have evolved defences at a rather smaller size than one might expect, based on similar animals elsewhere in the world. It's hard to be certain why this is, but one possibility is that most predators on Madagascar aren't terribly large, either.

In fact, there is only one species of mammalian predator on the island that's much larger than about 2.5 kg (5.5 lbs), which is certainly not something one could say about mainland Africa. Now, it's certainly true that even the spines of the common tenrec are no use as a defence against that one predator species when it comes calling (the fossa, if you're wondering) because both they, and the local boas, have been seen eating them. But more typically-sized local predators, such as the ring-tailed vontsira, seem to have no luck at all against the medium-sized spiny tenrecs, and find even the small ones a considerable challenge. Which may well be enough.

But, while the more general lifestyle of the various kinds of tenrec - that is, whether they spent more time digging burrows or climbing trees - turned out to have no relevance to how spiny they were, there was another factor that did seem to correlate. This is the encephalisation quotient, a measure of how large an animal's brain is compared with how large we would expect it to be for a mammal of its size. By definition, for an average mammal, this should be 1.0.

Tenrecs have slightly smaller brains than we'd expect, and this is true regardless of what species of tenrec we're talking about. This may be because their diet isn't very nutrient rich, compared with some others, and brain tissue is energy-intensive to maintain. Furthermore, since they aren't very sociable animals, they don't need to develop a high level of intelligence to interact with others in their group, as primates, elephants, dogs, and whales do. In fact, furred tenrecs turn out to have an EQ of somewhere between 0.6 and 0.9, which is low, but not remarkably so.

Streaked tenrecs (Hemicentetes spp.), which fall into the 'spiny' subfamily, but have relatively weak, flexible, spines, and not very many of them, have an EQ of about 0.45. And, while the greater hedgehog tenrec (Setifer setosus), the spiniest species of all, has an EQ of around 0.55, all of the other spiny tenrecs are even lower than the streaked sort - around 0.3 for the common tenrec. This is unusually low, especially for a placental mammal.

The correlation isn't perfect, but it does suggest that spiny tenrecs don't need brains as larger as the more physically defenceless sort. That may partly be due to the "trade-off" effect of growing the spines, but it's perhaps more likely to be because furred tenrecs have to rely on vigilance, stealth, and effective hiding skills to avoid being eaten. Spiny tenrecs, on the other hand, can afford to be less intelligent, relying on their physical defences rather than their smarts.

This is something we see in other armoured mammal groups, too. For example, hedgehogs have a lower EQ than shrews, and the EQ of armadillos is also quite low. It's far from a universal rule, since porcupines are reasonably large-brained for rodents, even after accounting for their larger size, so clearly other factors can be at play.

Nonetheless, this does give us some insight as to what might drive an animal to develop physical defences, what it might be able to give up in order to do so, and under what circumstances it really might not be worth the cost.

[Photo by Frank Vassen, from Wikimedia Commons. Cladogram adapted from Stankowich & Stensrud, 2019.]