Which Animal Looks Like a Hedgehog, an Otter and a Shrew? All of Them, on One Island

The group is a family of small mammals found on Madagascar and, in a few species, in parts of central Africa, and its members look so different from each other that they were long classified apart. Genetic work has established that the Madagascan species descend from a single colonisation event, with ancestors arriving from Africa across several hundred kilometres of sea at some point in the last sixty million years, probably on floating vegetation. Their closest relatives are not hedgehogs, shrews or moles but the group containing elephants, hyraxes, aardvarks and manatees, which is not an obvious family resemblance.

The resulting variety within one family is remarkable:

• •Spiny species resembling hedgehogs, complete with a defensive roll
• •Long-nosed species resembling shrews and occupying the same role
• •Burrowing species resembling moles, with reduced eyes
• •An aquatic species with webbed feet resembling a small otter
• •Tree-climbing species with long tails
• •Sizes from a few grams to well over a kilogram

The pattern is a textbook case of a single lineage diversifying to fill roles that other animals occupy elsewhere, which happens on islands because those other animals are absent. Madagascar separated from other landmasses long before most modern mammal groups existed, so it was never colonised by hedgehogs, shrews, moles or otters, and those ways of making a living were unoccupied. A group arriving with no competition therefore radiated into all of them, and natural selection produced bodies resembling the animals that fill the same roles elsewhere, because similar problems have similar solutions. The resemblances are convergence rather than relationship.

The group faces the same pressure as everything else on the island and the situation is serious. Madagascar has lost a very large proportion of its original forest cover, and the species dependent on forest are declining accordingly, with several listed as threatened and a number known from very few localities. Hunting is a factor for the larger species, which are eaten. Against that, some species tolerate cultivated land and gardens well and are among the more resilient small mammals there. The aquatic species is among the most threatened, since it needs clean fast streams and those are affected by erosion from cleared slopes upstream.

Several features are genuinely odd rather than merely convergent. One species produces the largest litters known in any mammal, with up to thirty two young recorded. Several can lower their body temperature and metabolic rate substantially, entering torpor for long periods, and one hibernates for months at a stretch in a tropical climate, which is unexpected. One species has specialised quills on its back that it rubs together to produce sound used to keep a family group together, which is stridulation and is otherwise essentially unknown in mammals. And several have a single opening serving both waste and reproduction, which is a feature normally associated with much older lineages.

One colonisation of Madagascar produced a family whose members resemble hedgehogs, shrews, moles and otters, none of which they are related to, since the island had none and those ways of living were unoccupied. Their nearest relatives are elephants and manatees. One species has up to thirty two young, one hibernates in a tropical climate, and one rubs quills together to make sound.