Soay sheep have greatest sexual fitness when they have two versions of a gene that determines horn size. Credit: Laurie Campbell/NHPA/Photoshot

Alpha Red 78 — a ram with horns like elephant tusks — sired 95 lambs before he died at the ripe (for a ram) old age of nine. A gene with a role in horn growth explains his fertility and his longevity, finds a study of sheep on a remote Scottish isle. The work also explains how variation can persist in traits that offer big reproductive boosts.

Ample horns are a ram’s ticket to reproductive success. During the breeding season, males fight for access to females, and those with the largest horns win. But if big horns are a sexual asset, the genes underlying the trait should have become ubiquitous, says Susan Johnston, an evolutionary biologist at the University of Edinburgh, UK, who led the research. Yet some male sheep have short horns or none at all. “From an evolutionary perspective, it doesn’t really make sense,” Johnston says.

Johnston’s team turned to the sheep living on Hirta, an island 160 kilometres west of the Scottish mainland. The animals, a primitive breed called Soay (Ovis aries), are known for their diminutive size and their agility on cliffs.

Two years ago, Johnston’s group reported that a single gene, RXFP2, explains horn variability in the sheep (S. E. Johnston et al. Mol. Ecol. 20, 2555–2566; 2011). One version of the gene, Ho+, is linked to large horns; another allele, HoP, is associated with small ones.

In the latest study, published in Nature, Johnston’s team related the RXFP2 genes of 1,750 sheep to three factors: horn size, reproductive success and lifespan (S. E.Johnstonet al.Nature;2013). Males with one or two copies of the Ho+ allele had the biggest horns. They fathered twice as many lambs as those with two copies of the short-horned allele, averaging 3 (versus 1.6) each year, says Johnston. But where lifespan was concerned, rams with two copies of HoP had an edge, she says, with a 75% chance per year of surviving the harsh Hirta winter, compared with a 61% chance for those with two long-horned alleles.

The scientists found that rams with one version of each allele (heterozygotes) had the best of everything: they were big-horned, fecund and long-lived. And this explains why short-horned rams persist. “I’m just impressed by the simple elegance of this story,” says Hopi Hoekstra, an evolutionary geneticist at Harvard University in Cambridge, Massachusetts.

Johnston says that to learn more, scientists will need to study the gene: in humans and mice, it is involved in sexual development and bone density. She adds that heterozygotes such as Alpha Red 78 end up with more offspring largely because they outlive homozygous big-horned males, which tend to die young.

The ram probably wasn’t winning on his looks. “He was quite an ugly sheep,” she says.