Bateman Gradients in Field and Laboratory Studies: A Cautionary Tale1

SYNOPSIS. Since tools of molecular genetics became readily available, our understanding of bird mating systems has undergone a revolution. The majority of passerine species investigated are socially monogamous, but have been shown to be genetically polygamous. Data sets from natural populations of juncos suggest that multiple mating by females results in a sexual selection gradient as steep for females as for males (a result that does not support Bateman’s predictions). However, in males, fitness is enhanced directly through fertilization success with multiple matings; in females fitness benefits may be enhanced immediately through direct access to food, protection against predators, or other resources received from males, or they may be delayed through improvement in offspring quality (e.g., through good genes, or greater genetic compatibility between the female and the extra-pair male). But a steep sexual selection gradient for females can be difficult to interpret. If all females copulate with multiple partners that are equally likely to fertilize eggs, then females that produce larger clutch sizes, for any reason, will appear to have copulated with more males. That is, multiple sires have a higher probability of detection in larger clutches than in smaller ones, giving the impression that females that mate with multiple males increase their reproductive success. Yet, in most studies in which there is a correlation between number of offspring produced by females and number of extra-pair males, causation has not been clearly established and other factors may explain the results