CONDITION DEPENDENT CONTROL OF PATERNITY BY FEMALE PURPLE MARTINS - IMPLICATIONS FOR COLONIALITY

Proposed causal links between extra-pair copulation (EPC) and colony f ormation in socially monogamous birds hinge on the question of which s ex controls fertilizations. We examined in colonial purple martins Pro gne subis (1) whether EPCs were forced or accepted by females, and (2) the degree to which apparently receptive females were able to obtain EPCs against their mates' paternity defenses, Paternity analyses of mu ltilocus DNA fingerprinting confirmed previous findings of a marked re lationship between age class and extra-pair fertilizations (EPFs), wit h young males losing paternity of 43% (n = 53) of their putative offsp ring compared to 4% (n = 85) by old males, All assignable extra-pair o ffspring were sired by old males, with one male obtaining most EPFs ea ch year, Contrary to the hypothesis that EPCs are forced, EPF frequenc y within age class did not increase with seasonal increases in the num ber of males per fertile female. Whereas the male control hypothesis p redicted that the male age class that mate-guarded more would be cucko lded less, the reverse was true: young males guarded significantly mor e intensely. The male age class difference in cuckoldry could not be e xplained by the possibility that young and inexperienced females (whic h are usually paired to young males) were more vulnerable to forced co pulation because EPFs were unrelated to female age, These findings sug gest that females (1) pair with old males and avoid EPCs, or (2) pursu e a mixed mating strategy of pairing with young males and accepting EP Cs from old males. The receptivity to EPCs by females paired to young males put them in conflict with their mates. Two factors determined th e paternity achieved by young males: (1) the relative size of the male to the female, with young males achieving much higher paternity when they were larger than their mates, and (2) the intensity of mate-guard ing, Both variables together explained 77% of the variance in paternit y and are each aspects of male-female conflict. Given female receptivi ty to EPCs, mate-guarding can be viewed as male interference with fema le mating strategies. We conclude that EPCs are rarely or never forced , but the opportunity for females paired to young males to obtain EPCs is relative to the ability of their mates to prevent them from encoun tering other males, Evidence of mixed mating strategies by females, co mbined with other features of the martin mating system, is consistent with the female-driven ''hidden lek hypothesis'' of colony formation w hich predicts that males are drawn to colonies when females seek extra pair copulations.