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.
5 Comments
Very interesting! It’s good to see how we challenge or reject the role of evolution in the response of different beasts to fisheries, climate change, and now, hunting. Seemingly the only crateures showing some signs of evolution are sticklebacks (and fruit flies, I suppose).
But jokes aside, the last sentence in this post is a really good one. It’s so tempting to assume a priori that OK, these guys have horns, horns must increase reproductive output (via success in female preference and/or male combat) but decrease survival (costs of developing and ‘wearing’ horns), and there is a balance depending on individual quality. Add hunting to the equation and the costs will be increased and horns get smaller. One would think that it is not even necessary to study it. And then…
But what does the data really tell us? That 2.6 % individual contribution to the total variance (I don’t even want to think about the genetic contribution within this 2.6 %) does all the business in male success? Or horns does not count? Body size is the key? Or spirit (= personality?)
If I have got it right, the individual effect is relative to all other sources of variation: if the other effects are big, then the individual effect becomes minuscule relative to them. This does not mean that heritability would neccesserily low. In standard genetic analyses the yearly variation would be standardisized out before estimating the individual (genetic) effect. It is a bit tricky issue how the individual effect should be estimated: depends a bit on the context and question.
At any rate, great paper and nice to see it published!
Of course, there’s the question of whether the yearly effect should be taken out like this: IIRC there was a paper discussing this a year or two back.
Aha, I see now (or Bob will tell me why I don’t).
This everything-is-relative thing confused me a bit. So horn size can be highly heritable, and all sorts of selective forces can actually shape it, but if the environmenal effects are huge, it would seem tiny. Does this mean something like if one is considering horn size variation in the last handful of years, the genetic component / evolution is of minor importance, but can be (and is) highly important in the long run?
It is similar (in a way) to the long dispute about should we seek for (statistically) significant effects or only effects responsible for a big part of the variation should be considered…
Yes, Gabor, you’ve got it. What makes it worse is that people will add or not add different terms into the heritability calculations, and get very different results.