Tuesday, 22 August 2006

Autumn 'epidemic' hits common shrews

In my travels of recent weeks I have come across a number of dead common shrews, most of which have shown no external signs of injury. This late summer/early autumn phenomenon is well known and has been termed the “autumnal epidemic” by researchers. Despite the name attributed to it, this mass mortality is not the result of some disease but is, instead, the consequence of an increasing population and senescence of breeding individuals. Shrews are not long-lived animals and, following the breeding season, most of the breeding adults die, leaving a population almost entirely composed of animals born that year. Adult shrews show several, quite distinct, signs of ageing. Increasing numbers of grey hairs, often coupled with cessation of the autumn moult leave ageing individuals looking somewhat unkempt. Perhaps more important in terms of mortality, is the deteriorating condition of the teeth. Unlike the incisors of rodents, shrew teeth do not grow throughout life and so gradually wear down with use. With a diet composed of invertebrates, with their chitinous exoskeletons, and earthworms, packed full of soil particles, the teeth of shrews take quite a battering. As such, it is not unusual for the teeth of the oldest shrews to be worn down to the gum line. This makes feeding very difficult. Another important factor may simply be the sheer number of shrews around at this time of the year. High densities of these highly strung and territorial animals may lead to greater numbers of confrontations and increased stresses.

So why should this matter? Why should increased aggressive encounters, which incidentally only rarely end up in actual physical confrontations, lead to higher levels of mortality among older animals? Quite simply, shrews live on an energetic knife-edge. Small in size, they need a high metabolic rate in order to maintain their body temperature. This, in turn, demands a high food intake and requires that shrews spend most of the day feeding. If feeding time is lost to territorial disputes then an individual may simply not have sufficient time to find the food needed to keep itself alive. Since young males are dominant over older (post-breeding) males, the oldest males end up wandering about without a territory, getting into disputes and are thus prone to starvation.

This may all seem rather bleak, especially when you consider that only 20% of the shrews born will survive to breed themselves. However, the system clearly works, since shrews are one of the most successful groups of mammals, in terms of the habitats they occupy, their global distribution and the densities they can reach. I guess we should view the sight of a dead shrew, apparently untouched, as an individual that has probably enjoyed a full and productive life.

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