THOUGH youth is supposedly the time of life to take foolish risks, it in fact makes more sense to save such things for old age. Young animals should be cautious, to stay alive long enough to reproduce. Old ones should be willing to gamble with their lives since there is not much time left anyway. Although this theory of age and risk makes sense (the risks young animals take are mostly connected with attracting and retaining mates, and are thus worthwhile), proof has been elusive. It requires following a population over the course of several generations. But a study of appropriate proportions has now been done, and it suggests that in one species at least, old age does indeed increase risk-taking.
The study in question was of Nile lechwe, an endangered antelope. The animals were held in an enclosure of 36 hectares (90 acres) at the San Diego Zoo’s Wild Animal Park near Escondido. The site contains ten other species of African ungulate as well as a variety of African birds. The project has resulted in the birth of 176 lechwe calves over the course of 38 years and, since this has happened under the watchful gaze of the zoo’s researchers, a lot of details about these young animals and their mothers have been recorded.
When Fred Bercovitch, one of the zoo’s researchers, and his colleagues analysed these data, they discovered an intriguing trend. As they report in the Journal of Mammalogy, they found that as the female lechwe got older, the sex and the birth weight of their young steadily shifted. Lechwe live for up to 12 years. Yearling first-time mothers had sons 57% of the time, but by the time those mothers were seven, this figure had changed to 67%.
The reason this is interesting is that a lechwe mother is three times as likely to die while giving birth to a son (or during the subsequent week) as when she bears a daughter. Why that should be is unclear. It may be connected with the fact that sons are heavier than daughters, and so are costlier to make and riskier to give birth to. The existing data, however, do not prove this.
What they do prove is that the reproductive risk to a female increases as she gets older, merely because she is more likely to give birth to a male. And that supports the theory of age and risk.
Bearing a son is risky in other ways than just the risk of it killing the mother. A male lechwe has a chance to become dominant and thus breed with many females. If he fails, he will have fought for breeding opportunities that never came. Sons are thus genetic gambles. Daughters, by contrast, are genetic insurance policies. Almost all adult females breed, so a mother will get at least some of her genes into the next generation by producing them.
How Nile lechwe mothers manage to adjust their birth sex ratio and shunt more resources into producing larger calves as they get older is not yet known. But in the evolutionary game of life, the strategy is giving them more bang for their bucks.