Philopatric Sex Research Articles

Sex-Ratio Selection in Species with Helpers-At-The-Nest

The data presented by Gowaty and Lennartz (1985) for red-cockaded woodpeckers, Picoides borealis, provide the first empirical evidence of a skewed sex ratio at the termination of parental care for a cooperatively breeding bird. Their findings take on added significance because they suggest that helping behavior per se may be a driving force selecting for an imbalanced sex ratio, rather than vice versa. It is well known that males outnumber females in adult populations of many, if not most, species of cooperatively breeding birds (Brown 1978, 1983; Emlen 1978, 1984). It is also true that among birds in general, and cooperative breeders in particular, males usually are the more philopatric sex, whereas females are the dispersers (Rowley 1965; Zahavi 1974; Gaston 1978; Woolfenden and Fitzpatrick 1978; Greenwood 1980; Reyer 1980; Koenig et al. 1983). The result is that males are more likely to remain in or near their natal groups and to act as helpers at the nest. These facts have led several authors to hypothesize that the skew in the adult sex ratio is brought about by heightened mortality associated with female-biased dispersal. This imbalance, in turn, imposes a demographic constraint on the ability of males to find mates, with the effect that a proportion of the male