Ecological and molecular basis of differential resource use in populations of burying beetles Nicrophorus vespilloides
A fundamental prediction of ecological theory is that competition for resources can drive the evolution of specialised resource use. One way in which costly competition can be avoided is via individual specialisation, i.e., the persistence of specialised individuals within a generalist population that utilise a smaller subset of the entire population’s resource base. This could occur through the evolution of genetic morphs that specialise on different resources. Although correlational evidence exists that is consistent with this prediction, there is surprisingly little evidence that competition causes resource specialisation. Burying beetles are an ideal species for testing this prediction. They require the carcass of a small vertebrate such as a mouse or a songbird for reproduction, but carcasses can be unpredictably distributed and competition to secure ownership is correspondingly intense. For my PhD project in Prof. Rebecca Kilner’s lab, I tested whether this fierce competition for a carcass breeding resource has driven the evolution of beetles that specialise in breeding on dead mammals or dead birds. With field experiments at three different woodlands, I tested for evidence of a bias in the type of carcass favoured by Nicrophorus vespilloides and if this bias changed across the burying beetle season (from April to October each year). I found spatial and seasonal variation within each of the three populations in the preference for dead mice over dead birds. In two populations, beetles were more likely to be trapped upon dead mice overall, but were occasionally trapped with greater frequencies on dead birds. This trend was completely reversed for the third population, where beetles were more likely to be found in traps baited with dead birds than dead mice. The patterns of resource use I observed in the field could be due to adaptive partitioning of resource type within populations. To test this hypothesis, I measured the reproductive success of wild beetles induced to breed on different types of carrion. Although I found seasonal variation in beetle reproductive success on different types of carrion, I found no evidence that this resulted from variation in carrion preferences at the individual or population level. Instead, it is more likely to be explained by variation in individual quality. In collaboration with Dr Michael Sheehan at Cornell University, we sequenced females trapped on each type of carrion within all three woodlands, to test whether carrion specialisation was associated with genetic differences. Consistent with this possibility, we found divergence at ~ 50 loci in each of the three populations. Several of these loci were associated with olfaction and sensory-system development. In the lab, I set up replicate experimentally evolving populations of N. vespilloides which were bred either on mice or chicks for ~ 20 generations. I used these populations to test whether, in principle, beetles within a natural population could become divergently adapted to specialise on different types of carrion. I found no evidence to support this possibility, perhaps because there was insufficient standing genetic variation in the founding populations to select upon. However, there was some indication that the experimental populations might have diverged in cryptic ways that I did not measure directly. To understand the chemical basis for differential resource use, I carried out several analyses in collaboration with Prof. Patrizia d'Ettorre at Université Paris, using mass spectrometric techniques. We found little evidence that the volatiles emitted from carrion differ substantially between birds and mice. We also found little evidence that a beetle’s cuticular hydrocarbons predict the carrion it will be attracted to in nature. However, we did find seasonal variation in the cuticular hydrocarbon profiles of wild-caught beetles that could be related to beetle quality or breeding status. In short, although we found some evidence for differential carrion use within wild burying beetle populations and some indication that this is associated with genetic differences among individuals, some of this variation is also due to phenotypic variation in individual quality. While it is possible that carrion specialists could evolve within natural populations, we found no strong evidence to suggest that this happens routinely.