The Trials of Life by David Attenborough

Author:David Attenborough [Attenborough, David]
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2022-10-28T12:00:00+00:00

If there is a great disparity in size between the two participants in such a relationship, each is bound to have a very different attitude to its partner. For the smaller, the vast body of its host is simply another environment with its own rewards and hazards like any other. To the bigger, its tiny guests may be so small that they are scarcely noticeable. Sometimes they may be helpful and even worth encouraging. But once the relationship has been established, consciously or unconsciously, willingly or unwillingly, it may become increasingly intimate and have profound effects upon both.

The three-toed sloth, hanging from a branch in the South American rainforest, for most of the time apparently fast asleep, seems so careless of its own personal toilet and so disinclined to any kind of violent action that there is little to prevent any animal that wants to do so from living within its coat of coarse shaggy fur. And many organisms do. Algae grow on the filaments of its long outer hairs. These have a form that is quite different from other hair with surface scales beneath which the microscopic algal cells lodge. Whether these structures developed specifically as accommodation for the algae, and if they did, what benefit the sloth might get from them, no one knows. There are also large numbers of small moths scuttling about in the fur. A single sloth may have as many as a hundred of them. It was once believed that the moths and their caterpillars fed here, grazing on the algae. Now it has been shown that they do no such thing. The female moths lay their eggs on the sloth’s dung which it deposits in special middens on the ground. There the caterpillars feed and pupate. The adult moths, when they emerge, use the sloth’s coat as a moving carpet to transport them from one breeding ground to the next. It probably also provides them with the opportunity during the journey to mate.

A little mouse that lives in the forests of Costa Rica regularly carries as many as a dozen beetles in its fur in a similar way. They cling to its ears and neck and crawl all over its face. They are seldom found anywhere else except with the mice. It used to be thought that these also fed on their host, perhaps sucking its blood, for they have very large jaws. Mysteriously, however, those mice with the largest number of beetle passengers, far from being debilitated or anaemic as might be expected, seemed particularly healthy. In fact the beetles’ jaws do no more than give their owners a firm grip on their host as it runs through the forest at night. They feed during the day. At that time the mouse is in its burrow and the beetles leave its fur to hunt for the fleas that abound in the mouse’s nest. Because the beetles keep down the number of fleas, the more beetles a mouse has, the healthier it is likely to be.

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