The effect of age on encounters between male crab spiders

In males that compete aggressively for females, size and age may determine which males obtain access to these females. In the present study, we use the crab spider, Misumena vatia, a species with males that do not grow after becoming sexually mature adults, to test the hypothesis that age affects the success of males competing for access to females. M. vatia is an excellent species to test this hypothesis because it is possible to disentangle age from size, characters that typically vary together in the species usually tested. We staged encounters between similar-sized older and younger adult male M. vatia in the presence of a female to determine the role of age in male access to females. Encounters between the males occurred during 63.3% of these pairings. Younger males won significantly more (70.2%) of the encounters than did older ones, but did not initiate significantly more encounters than did older ones (62.5%). Although older males won only 29.8% of these encounters, they initiated significantly more (76.5%) of them than predicted by chance. This design also allowed us to test Parker's hypothesis that older individuals should exhibit a higher level of aggression than younger ones. However, attacks by younger males were most likely to include extensive bodily contact, whereas attacks by older males involved significantly less contact. These results counter the frequent assertion that older individuals usually prevail over younger ones in contests for access to females, and that older males are more likely to engage in highly overt aggression than are younger ones. Aging may decrease reproductive opportunities and success rates of male M. vatia, affecting as many as nearly one-fourth of their encounters. Copyright 2004.