Female–female competitive interactions lead to a number of possible fitness benefits for the winning female, suggesting that traits conferring a competitive edge among females are unlikely to exist merely as nonadaptive by-products of selection on males. Furthermore, patterns of overt aggression among females map onto the availability of resources and mates, and the direct or indirect benefits provided by those mates. These patterns reveal specific currencies of differential mating success and the evolutionary mechanisms shaping competitive interactions among females. At one end of the spectrum of relative parental investment, with extensive female-only care, sexual selection appears to play a lesser role, with fecundity and mortality selection shaping female competition to protect offspring or acquire the food necessary for prolonged maternal care. In the absence of male direct benefits, females appear to compete for access to genetic benefits from high-quality males. When males provide parental care, females also may compete for exclusive social access to those high-quality males. In either case, if all females cannot mate with the best male and if males vary in the direct or indirect benefits they provide, then sexual selection should favor females who are more adept at gaining access to these high-quality mates. In short, both natural and sexual selection favor female–female competition in a number of contexts. Whether the selective advantage of overt aggression outweighs potential costs is an empirical question for future research, one that is a key step in addressing whether the overall selection differential for female–female competition is positive.
Aspects of female–female competition appear very similar to their male counterparts, with some notable differences. The availability of mates influences female–female competitive interactions in species with standard and reversed sex roles (Darwin 1871
; Trivers 1972
; Eens and Pinxten 2000
). When males are limited, the frequency and intensity of female–female competitive interactions increases (Prediction 1). In addition to this competition over access to mates, there are additional ways in which female competition affects mating success without influencing the number of mates, per se. Overt aggressive behaviors also increase when females compete for high-quality mates that provide either direct benefits (Prediction 2) or indirect benefits (Prediction 3). While the number of males may not be limiting to females outside of role-reversed species, the number of high-quality mates may be limiting (Petrie 1983
; Altmann 1997
), and competition for these males constitutes a form of mating competition. Accordingly, sexual selection may favor females that outcompete other females for male parental care, indirect genetic benefits, and nesting sites held by preferred males.
As a consequence, it is clear that if we restrict sexual selection to competition for the quantity of mates without including competition for high-quality mates, we ignore a potentially important component of mating success, particularly in females. If mating success is strictly limited to mate number, then 2 individuals with one mate each have equal mating success. If, however, one of these mates is high quality and the other low quality, should we not also consider the former individual to have higher mating success than the latter? And, if competitive interactions between these 2 individuals determined this outcome, should we not also consider this process part of sexual selection? The answer to these questions essentially boils down to differences in the quality and quantity of mates. In light of empirical evidence that mate quality impacts fitness in many systems (Andersson 1994
), it may be overly simplistic to ignore mate quality in measuring mating success and sexual selection.
The gray area of sexual selection centers around female–female competition for territories or other mating resources that qualify an individual as a mate (Prediction 4) because many of these examples affect female survival and fecundity as well as mating success. Competition among females for nesting sites required for both mating and reproduction, for example, provides a nearly identical parallel to sexual selection via male–male competition for females and territories, although some instead consider this process social selection in both sexes (West-Eberhard 1983
). Females that are more aggressive may obtain access to resources that allow them to mate (or mate more, or mate with better mates), thus couching this process in the overarching theme of sexual selection (i.e., competition over mates). Females also may compete over resources that directly affect fertility, fecundity, or offspring survival without affecting competition for mate quantity or quality, and thus, this sort of intrasexual competition does not fit within sexual selection.
Selection should favor competition among females if variance in female competitive ability maps onto even slight variance in any component of fitness, whether sexually or naturally selected. For females, variance in mating success may be less dependent on mate number than on mate quality, which in turn may affect quality or even lifetime quantity of offspring. While quality is a more subtle measure of mating success than quantity, the critical condition for selection to favor female–female competition is a positive selection differential, even if the magnitude of the payoff in females is smaller than in males. It would be a mistake to confuse this potentially smaller fitness payoff of intrasexual competition in females with a lack of sexual selection in females.