The Great Impersonators
The animal in the picture above looks so much like a miniature hedgehog that you’d be forgiven for thinking that it actually is one. But October here at The Nature Nook is Madagascar Month, and since true hedgehogs don’t live on Madagascar, it must be a different type of creature. And indeed it is – but what? It has a pointed snout, it’s protected by a carpet of spines, and it can even roll up into a ball if it feels threatened – perhaps not as perfectly as a real hedgehog, but nonetheless sufficiently well to serve as an adequate defence and to present predators with little more than a very prickly meal.
Despite these many similarities, both physically and behaviourally, this animal is not closely related to hedgehogs whatsoever. It’s a tenrec – the aptly-named lesser hedgehog tenrec, to be precise. The great resemblance between this creature and the hedgehog family is a striking example of convergent evolution (which you can read more about here).
And it isn’t just hedgehogs that tenrecs so closely resemble. Some members of the group scurry around like large shrews. A few resemble moles or mice or opossums. All of these animals, though seemingly different, are actually closely related to one another; a widely diverse family that were among Madagascar’s earliest inhabitants after it broke away from the ancient supercontinent known as Gondwana. Since then, with little competition from other small mammals, the tenrecs have evolved into a great variety of forms to take advantage of the previously unoccupied ecological niches.
The ancestors of today’s tenrecs are believed to have rafted over to Madagascar from what is today mainland Africa around 30 or so million years ago. Their closest relatives still live in Africa today. They are the otter shrews: a poorly understood group of elusive, semi-aquatic mammals, of which there are just three species. Two of them – the Nimba otter shrew and the Ruwenzori otter shrew – live in tiny areas of West Africa and Uganda respectively, and look like large aquatic shrews. The third and largest species – the giant otter shrew, which more closely resembles a member of the mustelid family – can be found across Central Africa. Like true otters and mink, the otter shrew has fine, dense fur, which has long been coveted by humans. Hunting and habitat destruction has greatly reduced its numbers, though no one is really sure how many are still left in the wild.
Due to their very ancient lineage, tenrecs seem to be quite primitive mammals. They possess a cloaca, a single orifice for urinary and excretory purposes that is commonly seen in reptiles, birds and amphibians, but hardly ever in placental mammals (the only others to possess a cloaca are the golden moles and a few species of shrew). Tenrecs also have an unusually low body temperature, while males don’t even possess external testes. These two characteristics are, in fact, related. Testes don’t work well inside the body because the temperature is normally too warm for the optimal production of sperm, so most mammals have a scrotum that dangles down between the legs to keep them a few degrees cooler. However, because the tenrecs have such a low internal temperature anyway (30-35°C), they can afford to keep their testes hidden away within their bodies.
The largest species of tenrec, which is about the size of a rabbit, is the common or tailless tenrec. This was the first tropical mammal found to hibernate for long stretches without arousal period, up to nine months at a time. The female common tenrec has up to 29 teats (more than any other mammal), and that’s just as well because she can give birth to that many underdeveloped young – or even more – in one go.
Having such a large family means that some youngsters occasionally get lost in the undergrowth, but one species of tenrec, the lowland streaked tenrec, knows how to deal with this problem. This species has barbed quills on its back that it uses as defensive weapons; it rushes at would-be assailants and attempts to drive these quills into their delicate noses or paws. But the streaked tenrec also has specialised ‘stridulating’ quills, which can be rubbed together to make high-pitched sounds. These sounds, which easily cut through the noise of the forest, allow family members to keep in touch with one another as they forage. This is the first – and so far only – instance of a mammal using stridulation as a form of communication; it is normally only seen in insects such as grasshoppers.
The tenrecs are a fabulous array of oddballs, there’s no doubt about that – but there’s a lot more Madagascan wildlife to get through before the month is out. In the next post, we’ll be taking a brief detour from the animal kingdom and looking at a Madagascan plant that gave Charles Darwin himself something to think about.