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It Pays to Be a Weakling Sometimes

To be the best is not always the best thing – at least it’s true for rams. Study has found that rams with bigger horns are more likely to get the girls – but they also die younger. The tradeoff helps explain a long-standing puzzle about why the best genes for mating don’t prevail.

Charles Darwin, the father of evolutionary biology, had pondered the peacock’s elaborate plumage and proposed that the drive for sex-and for producing offspring-was a powerful driving force in evolution. Defeating potential rivals or vying for a female’s attention lead to more extreme males—bigger, stronger and more colorful. Such process, known as sexual selection, can also influence the genes as natural selection does. Hence, only versions of genes that result in these enhanced qualities should exist. However, this is not what happens in real world. There are large quantities of small, weak males among the supermen.

Rams with stumpy horns (left) are difficult to get mates, but they are better survivors than their bigger horned peers. *Image source: Nature, Peter Korsten

Let’s take sheep as a good example. Males with the biggest horns are good at getting the ewe more often—the big horns provide an advantage when they tussle with each other over mates. However, some guys still have stubby horns. “It remains a mystery as to why there would be so much variation in horn type,” says evolutionary biologist Susan Johnston of the University of Edinburgh in the United Kingdom.

In order to resolve this paradox, Johnston and her colleagues analyzed long-term data about wild sheep living on an island off Scotland’s coast. They have been collecting information on the Soay sheep since 1985 and kept records on how long these sheep live, what offspring they give birth to, and other life history details. For 20 years, they analyzed the long-term data of 1750 sheep in total. The researchers also collected genetics data and eventually located major genes responsible for deciding horn size.

The researchers discovered two versions, or alleles, of the horn gene—a large-horn allele and a small-horn allele. Rams with two large-horn alleles sported the biggest horns and sired more lambs—around three each year, while on the contrary, male rams carrying two small-horn alleles only sired about 1.6 offspring every year. However, males with two big-horn alleles lived shorter, having a 61% chance of surviving each year, compared with a 75% chance in males with two small-horn alleles. “Horn length has opposite effects on reproduction and survival,” says Thomas Flatt, an evolutionary biologist at the University of Lausanne in Switzerland.

Johnston believes that this influence on survival can be indirect: Small-horned males might spend more time eating and taking care of themselves since they don’t have females to watch over and court.

An animal’s lifetime fitness—or say “how well it does in life”—is a combination of how long it lives and how many offspring it produces. Study says that the individuals with one allele of each type share the best of both sides—strong fertility and longevity. Given the advantage of having one of each allele, “Natural selection can’t get rid of one of the alleles, “says Flatt.

This study offers a very clear, compelling confirmation that genetic variation in important traits is sustained by tradeoffs between components like reproduction and survival. Johnston points out that this way to maintain genetic variation is fundamental to many different real-life problems. For instance, in Africa the allele that results in sickle cell anemia persists in humans due to its ability to protect against malaria. Genetic variation in crops is more appreciated as well, because that diversity can help plants better withstand pathogens. And for species forced to adapt to climate change, variation might be the only thing to save them.