The evolution of fighting structures in hartebeest

Question: Is sexual selection for the evolution of larger horns and related fighting structures opposed by food constraints in bovids? Data studied: Horn circumference and length, pedicel height, skull weight, and body size (skull length) were measured on 382 skulls belonging to all eight hartebeest subspecies (Alcelaphus ssp.). We used mean group size and length of the breeding season for each subspecies as surrogates for the intensity of sexual selection through the potential for polygyny; mean annual rainfall and index of seasonality of each subspecies range quantified habitat primary production. The phylogeny of the hartebeest tree and the branch lengths were based on Flagstad et al. (2001). Search methods: Within- and across-taxa variation infighting structures: One-way analysis of variance and independent sample t-tests. Phylogenetically uncorrected: Dimorphism, as Log (M/F), regressed against independent variables (surrogates of sexual selection and habitat productivity, plus Log female size to check for allometry) with stepwise regressions weighted by the sample size of specimens measured for each ssp. Phylogenetically corrected: Independent contrasts using the program CAIC (Purvis and Rambaut, 1995). Conclusions: The length of the breeding season was the only predictor for dimorphism in pedicel height and skull weight and the best, albeit non-significant, predictor for horn circumference. Only mean annual rainfall predicted horn length dimorphism. Thus, taller pedicels and heavier skulls appear to be under sexual selection, while natural selection seems to affect the evolution of fighting structures by limiting investment in longer horns. We argue that the pedicel, by increasing the defence to the head during clashes, might reduce the selective pressures on horns as protective structures.