The shrinking sheep of Hirta

Tartan sheep at a gathering of the Clans

. . . it’s too early to say if, in 100 years, we will have chihuahuas herding pocket-sized sheep.

The wild Soay sheep on the island of Hirta have been getting smaller for the last twenty years. A new paper suggests that the reason is climate change: milder winters mean more grass for a longer period, which removes the advantage in putting on weight fast. There is simply no need.

The Dynamics of Phenotypic Change and the Shrinking Sheep of St. Kilda

Our approach has provided several insights. First, the dynamics of body size – both the trend and variation around the trend – are primarily a consequence of environmental variation and not evolution. Second, we identified that positive viability selection on size early in life is countered by young mothers being unable to produce offspring that were as heavy as they were at birth. Finally, we report that environmental change has resulted in a reduction in lamb growth rates and that this explains why sheep are smaller than they used to be. There are two general conclusions from this analysis. First, the recent focus on evolutionary explanations for changes in heritable phenotypic traits could fruitfully be complemented with research identifying the role of environmental variation. Second, individuals and populations respond to environmental change in complex ways, and there should be no expectation for simple linear associations between environment, phenotype, genotype and population dynamics. These results reinforce the need for a theory linking genetic, phenotypic and population dynamics in age-structured populations in variable environments.

While the Hirta example is all about the phenotype – because it doesn’t address the long term – there was a fascinating recent examination of very rapid evolution in the Atlantic cod: Intense Habitat-Specific Fisheries-Induced Selection at the Molecular Pan I Locus Predicts Imminent Collapse of a Major Cod Fishery. One allele determines whether a given population spends its time near the surface or further down. The fisheries – nothing but a predator from the cod’s point of view – have wiped out the surface dwelling population, leaving us with the same species, but only half the behavioral possibilities with respect to ocean depth. Genotypic selection, real evolution, in the space of a few decades and at human hands.

As for Scotland, it is an open question if this sheep shrinkage will spread to the mainland and domestic herds. If so, the textile industry is going to have to consider the ramifications of pre-shrunk tartan.