Effects of testosterone implants on pair behaviour during incubation in the Yellow‐legged Gull Larus cachinnans

The Yellow‐legged Gull Larus cachinnans is monogamous with bi‐parental incubation. In this study, the effects of high levels of plasma testosterone in male Yellow‐legged Gulls during the incubation period were analysed. Free‐living male gulls were implanted with testosterone (T‐males), and their sexual behaviour within the pair was observed and compared with that of control pairs. Egg temperatures, length of incubation and hatching success were also analysed. T‐males and their mates displayed more sexual behaviour than the controls. T‐males engaged in mounting behaviour with their mates, whereas control males did not. Proportionally less time was spent incubating (in relation to time present in the colony) by T‐males than control males. However, the mates of T‐males did not spend more time incubating than control females to compensate for male neglect, although they did spend more time on the territory. Egg temperature in T‐male nests was significantly lower than in control nests, but no significant difference in the length of incubation or hatching success between the two groups was found. In birds, the effects of high testosterone levels on male behaviour during incubation have only been analysed in a polyandrous species whose females usually do not contribute to incubation. The present results thus suggest that those males of a monogamous species with bi‐parental incubation that sustain high testosterone levels after laying, thus reducing their contributions to incubation, will be confronted with a lack of compensation from their mates during incubation. Finally, this lack of female compensation seems to be mediated by behavioural interactions with the male rather than by her absence from the colony.