Thinhorn sheep Ovis dalli in Yukon Territory, Canada have been exposed to selective harvesting (based on horn size) and a warming climate over the past four decades. Both of these factors could potentially have an effect on horn growth (hunting would be expected to decrease growth, while a milder climate would increase growth), and in this paper we had the mighty task of looking at over 50,000 growth measurements in over 8000 hunted individuals to try and figure out what has been happening with the horns.
Not surprisingly, horn growth was mostly affected by weather, and annual fluctuations in spring weather were mirrored by the horns. Over the period we studied, we found that growth in horn length increased slightly over time, following in step with the climate trend. With horn growth increasing with spring warming it seemed unlikely that selective hunting had a detrimental effect on horn growth, but there was still the possibility that the increase in horn growth as a response to the warming trend could also be accompanied by an evolutionary response…
To attempt to figure out how likely an evolutionary response to the warming climate might be, we looked at the size of the individual effect in horn length. It turned out that only 2.6% of variance in horn length growth could be explained by an individual effect. If horn length growth is genetically determined to a large extent, we would expect a large individual effect, but in this case the role of individual genotype appears to be miniscule. So there is little potential for selection to produce a response in horn length over the time period studied.
For me one of the most interesting aspects of this study is that I think it serves as a cautionary tale that I would title “Things are not always as they seem”. These populations are fairly heavily hunted with approximately a quarter of all legal rams being harvested each year. Sheep that grow their horns faster can be hunted at a younger age, which might make one believe that there has to be an evolutionary response to the selection. But all is not as one might expect. Horn growth is increasing over time and closely following the climate warming trend, and then on top of that there is a small individual effect. Sometimes the data pry us away from preconcieved notions and force us to think differently.
Loehr, J. Carey, J. O’Hara, R.B., & Hik, D. The role of phenotypic plasticity in responses of hunted thinhorn sheep ram horn growth to changing climate conditions. Journal of Evolutionary Biology, in press.