Responses to Mate Loss by Herring Gulls Larus argentatus and Great Black-Backed Gulls Larus marinus

Herring Gulls and Great Black-backed Gulls were used to examine the reactions of individuals to mate loss. Late in incubation, one member of each of 41 incubating pairs was experimentally removed and its mate's response observed. We predicted that deserted individuals unable to obtain a replacement mate, would also desert the reproductive attempt, and that there would be no sexual difference in responses. All individuals either abandoned the initial clutch or brood. Only one individual replaced a lost mate that season. Males, however, abandoned their eggs significantly earlier than did females. There was no significant difference in the responses of the two species, nor any significant species-sex interaction. We review and suggest a number of possible explanations for why males and females should react differently. These include cumulative reproductive effort, the tertiary sex ratio, the physical reserves of the deserted mate, the possibility and consequences of mixed reproductive strategies, and potential differences in replacement costs between the sexes due to differences in what each sex regards as its primary asset.