Sunday 9 December 2012

Mazes and Mole-Rats

The African mole-rats are a family of bizarre rodents, and one that I've discussed before. Their most famous member is the exceptionally weird naked mole rat, but the other, furrier, species are still pretty odd. They spend almost their entire lives underground, feeding on plant roots, and, as a consequence, are virtually blind. Famously, naked mole rats are 'eusocial' mammals, living in the style of ants or bees, with a single breeding queen and a number of sterile workers. However, some other African mole-rats live solitary lives, and others, while not as extreme as the naked species, are still strongly social.

We know very little about some of the species, but others have, on account of their peculiarity among mammals, been well studied. One of these is the Zambian mole-rat (Fukomys anselli). Much of the work on this species has concerned their hearing abilities. They turn our to be particularly good at hearing low-pitched sounds; the sort of noise that rumbles through the soil, and echoes through their narrow tunnels. Not only that, but they are also able to detect magnetic field lines, presumably to orient themselves in the absence of any cues from the sun.

However, the great majority of these studies are of the animal under laboratory conditions. Which is fine, if what you want to do is determine how good they are at hearing, how well they can see, or even whether breeding affects their longevity. However, it is obviously also useful to get some idea of how they live in the wild, if only to put the lab studies into context. While not the first to do so, a recent paper by Jan Šklíba and co-workers gives us a relatively detailed look at the natural history of a strange animal, little known outside the research community.

Zambian mole-rats were only identified as a species in 1999, having previously been thought to be local examples of the more widespread common African mole-rat (Cryptomys hottentotus). They aren't exactly widespread, being known only from one relatively small region of central Zambia, near the capital, Lusaka. Which at least meant that the researchers knew exactly where to go in order to find them.

One of the first things they looked at was the structure of the animals' burrows. Most of the tunnels turn out to be relatively shallow, no more than 12 cm (4.7 inches) below the surface. These are feeding tunnels, dug by the mole-rats in their search for tasty roots, and they form a branching net-like pattern below the ground, with numerous side-tunnels. In all, these tunnels typically stretch for over 1 km (about three-quarters of a mile).

On the face of it, that's not so surprising, since the tunnels of naked mole-rats are about the same length, albeit less complex. However, Zambian mole-rats are much smaller animals than their naked cousins, so, all things being equal, we would expect their tunnels to be much shorter. Indeed, the researchers calculate that, relative to their body size, Zambian mole-rats dig longer tunnel complexes than any other mole-rat (African or otherwise).

Heck, it isn't just "relative to their body size", either. While the average length of the tunnel complexes, was, as I say, not so unusual, one complex the researchers uncovered now holds the world record for the longest set of tunnels ever dug by a mammal. This particular complex reached a whopping 2.8 km (1.7 miles) - pretty remarkable for an animal that weighs less than 100 grams (3.5 oz). Nor are the individual complexes necessarily separate, as had previously been believed. Some of them connect with neighbouring complexes, allowing members of one colony to visit their neighbours. Quite why they'd do this isn't clear; the obvious assumption would be that they're doing it to breed, but there's no clear evidence either way that this is what they're actually up to.

At the heart of this sprawling network lies a three-dimensional maze of closely-spaced tunnels, plunging deeper under the ground. This surrounds a single sleeping chamber, lined with grass, where the animals can shelter and raise their young. Elsewhere within this central maze are latrines (they have to poo somewhere, and doing it in the middle of a regularly travelled passage is hardly a good plan), and a number of storage chambers. The latter are used as larders to hoard items of food to large to eat at a single sitting, including the tubers of hyacinth beans and African potatoes. Interestingly, the mole-rats were not alone in their tunnels, sharing many of them - for no clear reason - with Senegalese running frogs.

Frogs aside, each tunnel complex complex was shared by a colony of about ten mole-rats, with the largest colony having thirteen members. This is quite a bit less than among naked mole-rats, but fairly typical for close relatives of the Zambian species. Like the naked mole-rats, however, the Zambian colonies included just one breeding pair, even though there were, in most cases, at least some other fully grown adults.

Two colonies, however, didn't have a breeding pair at all. In one case, the solitary adult shared the complex with a number of immature individuals - perhaps he'd recently lost his partner. The other was an unusually small tunnel complex shared by just two mole-rats, both adult females. Most likely they were sisters who had only recently arrived from elsewhere and begun digging, and had yet to find a male to join them.

But why were these tunnel complexes so large, and why does only pair of adults bother to breed? As I mentioned when I previously discussed African mole-rats, the usual explanation for the colony structure is that the breeding pair needs non-breeding assistants to dig out all the tunnels. Essentially, many hands make light work, without everyone having to divert some of their energy into looking after young. If so, we would expect that the environment that Zambian mole-rats live in is a particularly harsh one with, for example, plenty of hard-packed soil. This would also explain the length of the tunnels - if food is scarce, the mole-rats have to dig further to find enough of it.

This study, however, did not support that idea. Seemingly, the environment in which Zambian species lives is much the same as that of the silvery mole-rats (Heliophobus argentocinereus) of eastern Africa. Yet that species is not social, with animals living alone in relatively simple tunnels, apparently without any help at all. Has the environment changed since they evolved? That's a possible explanation, but perhaps there's something else going on that we haven't thought of.

In a way, that can be the value of studies like this: challenging what we thought we already knew.

[Picture by "Headster", from Wikimedia Commons]

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