I promised a long time ago to write about the talks at SICB 2014 that changed the way I think about sexual selection. I managed one post about signalling in paper wasps before getting sidetracked into writing about other things. It’s now been far too long since the meeting for me to remember enough about Nathan Morehouse‘s talk in the “Stress, Condition, and Ornamentation” symposium to deliver an accurate summary of the empirical work he presented (unfortunately my notes are as dreadful as my memory). So instead I’m going to explain the concepts as best I remember them and refer you to a paper–Morehouse et al. (2010)–that address some of the same issues as the talk.
What Morehouse demonstrated clearly and memorably in his talk is that our usual framework of sexual selection is terrible at making predictions. Imagine that you are studying a species where males have a large, colourful ornament (a Sitana perhaps?), and suppose that you’ve measured some index of condition (say parasite load), as well as some index of investment in the ornament (size or colour, for example) for these males. Now imagine that you find that parasite load decreases with increased ornament investment. Can you explain this? Sure! Individuals in this species must vary in some underlying “condition” variable, and males in better condition can invest in both their immune system (leading to fewer parasites) and their expensive ornaments.
But imagine you found that parasite load was positively correlated with investment in the ornament. You could explain this too, by positing that males who invest heavily in their ornaments don’t have enough resources left over to also invest in immunocompetence. That we can explain two diametrically opposite results using the same framework suggests that we need to reconsider and clarify the processes underlying the patterns we seek to explain.
Morehouse traced this contradiction to the imprecision with which we think about resource acquisition and allocation–how creatures obtain particular resources from their environment, and how they then invest those resources in particular traits. The scenario posed above could easily be clarified if we knew what nutrients were responsible for ornament quality and parasite resistance, whether these nutrients were limited in the environment, and whether there was variation among individuals in their ability to acquire these nutrients. Imagine two individuals that differ in ornament quality: if we know that, in this species, individuals vary in their abilities to acquire resources but not in allocation strategies, we’d expect that the male with the better ornament to also have a lower parasite load. In contrast, if individuals in this species don’t vary in resource acquisition strategies but instead differ in how they divide up their resources among different body parts, we’d expect the flashier male to have more parasites than the drab male. In real life, individuals will probably vary in both acquisition and allocation, but this variation is potentially quantifiable, and therefore can be used to make predictions about the relationships we expect to see between ornaments and parasites.
In his talk, Morehouse presented some results on nitrogen acquisition and allocation to UV reflective patches in male cabbage butterflies that demonstrated the power of this approach. Consider this post a teaser for future coverage of the butterfly story. He made the case for holometabolous insects–insects that metamorphose from larvae to adults–as particularly good systems in which to ask questions about resource acquisition vs. allocation, because the two processes are completely separated between the larval and adult stages. Larvae are little eating (=resource acquisition) machines, whereas all the allocation to reproductive structures and ornaments only occurs at metamorphosis.
I’m not yet sure how widely this framework of being explicit about resources has been adopted in studies of sexual selection. Morehouse et al. (2010) has been cited roughly 20 times, and a quick scan of the titles of these papers suggests that the field is well on it’s way to being more nutritionally explicit and, one hopes, more predictive.