Effect of testosterone on the behaviour of yellow-legged gulls (Larus cachinnans) in a high-density colony during the courtship period

Alonso-Alvarez, C. & Velando, A., 2001. Effect of testosterone on the behaviour of yellow-legged gulls (Larus cachinnans) in a high-density colony during the courtship period. Ethology Ecology & Evolution, 13(4), pp.341-349.

Yellow-legged gulls breed in high density areas, and the condition dependent hypothesis suggests that birds with high physical condition can obtain breeding benefits in high-density areas because they are able to pay off the energetic costs of aggressive behaviour and territory defence. This study and others showed a relationship between aggressiveness or copulation behaviour and nest-density during the pre-laying period in gulls. The link between density and behaviour can be explained by the strong competition for space and mates. Testosterone regulates male behaviour and can play an important role in the condition-dependent hypothesis. We tested the effects of testosterone implants on male breeding behaviour. In a high-density colony, testosterone-implanted male yellow-legged gulls showed higher aggression and copulation frequencies than controls during the courtship period. In addition, these testosterone-treated birds acquired larger territories than the controls. Thus, a high testosterone level can increase individual fitness in densely populated areas given the benefits derived, on the one hand, from a large territory and, on the other, from an increased copulation rate that would guarantee the male’s paternity of the chicks born in his own nest. Since testosterone increases energetic requirements, only high-quality males would be able to pay off the costs of high levels of testosterone and so obtain the benefits of breeding in high-density areas. Testosterone could therefore provide a proximate mechanism regulating nest density in gull colonies.

Female control in yellow-legged gulls: trading paternity assurance for food

Velando, A., 2004. Female control in yellow-legged gulls: trading paternity assurance for food. Animal Behaviour, 67(5), pp.899-907.

Females in many socially monogamous birds copulate hundreds of times more than necessary for fertilization, although little is known about the benefits of this excess. Females may not directly benefit from high copulation rates, but instead may exploit male interest in copulating to obtain benefits. In species with courtship feeding, females may trade copulations for food (immediate benefits hypothesis). I tested this hypothesis by analysing female behaviour during courtship in yellow-legged gulls, Larus cachinnans. Female gulls to some extent controlled sperm transfer, because they moved during copulation bouts, and this behaviour influenced the number of cloacal contacts per mounting that the male achieved. Female control was related to previous feeding by the male, and hence the male courtship feeding rate correlated with the cloacal contact rate. Males that give more food probably enhance their chances of fathering offspring. By analysing within-individual female behaviour, I also found that the number of cloacal contacts was higher when the male fed the female than when he did not, which indicates that female gulls followed a decision rule to resist copulation when food is not given. Overall, these results support the hypothesis that female gulls manipulate their mates to obtain food.