Bateman in Nature: Predation on Offspring Reduces the Potential for Sexual Selection

Sexual selection is driven by competition for mates, and the advantage of a competitor is determined by the number of offspring it produces. Early experiments by Angus Bateman characterized this interaction, and the quantitative relationship between a male's number of mates and number of offspring is known as the Bateman slope. Sexual dimorphism, one of the most obvious results of sexual selection, largely requires a positive Bateman relationship, and the slope provides an estimate of the potential for sexual selection. However, natural selection from the environment can also influence male success, as can random effects, and some have argued for inclusion of the latter in calculations of mate success. Data from pronghorn (Antilocapra americana) reveal the presence of a positive Bateman slope in each year of a 10-year study. We found no evidence that random effects skewed male mating success; however, substantial yearly variation in the Bateman slope due to predation on fawns was evident. These results support the validity of the Bateman relationship, yet they also demonstrate that environmental or extrinsic influences can limit the potential for sexual selection.