Experiment of the month

Any runner knows that if you want to cover a long distance, you shouldn't start too fast. And that may be a sound rule of thumb for a runner's entire lifetime. Experiments with people as well as with laboratory animals continually demonstrate that rapid growth leads to early death. Now it appears that the same trend holds for some wild animals, too.

Mats Olsson, now at Goteborg University in Sweden, and Richard Shine of the University of Sydney in Australia captured pregnant southern snow skinks (Niveoscincus microlepidotus) at the summit of Mount Wellington on Tasmania and then placed their newborns in pens on the same mountain. The pens encompassed the skinks' natural habitat, and the baby animals were individually marked and given plenty of extra worms to eat. Four times during the first three months of their lives, the little reptiles were caught and weighed to establish their growth rate. Then they were released into the wilderness at the mountain's summit.

Twice in the next four years the investigators recaptured the skinks across an area that far exceeded the animals' capacity for travel. Individuals that weren't recaptured were thus presumed to have perished. As the biologists expected, skinks that had grown fast as youngsters--raised, one might say, with a silver spoon--figured prominently in the group of missing individuals. That silver spoon, say the authors, "may sometimes be tarnished."

The physiological reason for the link between fast growth and lower life expectancy is still unclear, but the implication for evolutionary studies is important. Although fast growers generally outcompete their rivals during any given reproductive season, in the course of a lifetime they may not leave more offspring in the next generation, contrary to what has commonly been assumed. Olsson and Shine say the slow starters may compensate for their languid pace by living longer and getting more chances to breed. ("Growth to death in lizards," Evolution 56:1867-70, September 2002)

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