Strong, convergent male mate choice along two preference axes in field populations of black widow spiders

Abstract

Strong male mate preferences have been documented in species with otherwise conventional sex roles in the laboratory, and across taxa, male preferences generally focus on indicators of female fecundity and sperm competition risk. However, the few field studies of male choice rarely show equally strong effects. This suggests that costs of choice in nature may make the expression of preferences unlikely, or that interacting assessment cues may lead to unpredictable and variable outcomes in the wild. Field studies are therefore critical for testing whether or not male mate choice is sufficiently strong to exert sexual selection on females. Here we examine male choice in nature in response to two experimentally manipulated cues of female reproductive value. We recorded the attraction of male black widow spiders, Latrodectus hesperus, to females in a field enclosure and in a unconstrained natural setting. Stimulus females varied in two ways: sperm competition risk (low–unmated; high–mated) and fecundity (high–fed; low–unfed). Females and their silk were caged to prevent behavioural interactions while allowing the spread of airborne pheromones that attract males. Males in both experiments showed strong, nearly unanimous choice for unmated, well-fed females. In field enclosures, 80% of males made this choice, and wild males made the same choice in 94% of trials. We conclude that male choice in L. hesperus is mediated by detection of airborne pheromones, is strong and consistent despite natural costs of mate searching, and is not affected by natural interactions between (sometimes conflicting) cues of female reproductive value. Thus, universal mate choice may exist among males in nature. We propose general features of systems in which this may be likely and discuss implications for the study of sexual selection on females under conventional sex roles.