Social Stimuli, Testosterone, and Aggression in Gull Chicks: Support for the Challenge Hypothesis

Abstract

We tested the challenge hypothesis for the hormonal regulation of aggression in chicks of the black-headed gull, Larus ridibundus. Chicks of this species are highly aggressive toward conspecifics, but never to peers that hatched from the same clutch (modal clutch size is three). Therefore, in the first experiment small families were housed together in large groups (challenged condition) and compared to families kept isolated (nonchallenged condition). As expected, in the challenged condition during the initial stage of territory establishment basal levels of testosterone (T) were clearly higher than those in the nonchallenged condition. In the second experiment we tested the effect of a short social challenge on short-term T-fluctuations. The design was based on an earlier experiment, showing that after temporary T-treatment chicks become very sensitive to social challenges while having low basal T-levels. We now show that these social challenges induce brief elevations in plasma T-levels. These peaks are similar to those in previously untreated chicks but untreated chicks do not respond with aggression to a challenge. Therefore, we conclude that the initial exposure to elevated T-levels increases the sensitivity to brief changes in T induced by social challenges. In this way exposure to T, that may be detrimental for development, is minimized while birds remain able to defend territories. This is the first report showing that the challenge hypothesis as established for adult birds, is also applicable for aggressive behavior in young birds outside the sexual context. Furthermore we suggest that a phase of priming with T is necessary to obtain the high behavioral responsiveness to a challenge. © 2002 Elsevier Science (USA)

The hormonal control of begging and early aggressive behavior: Experiments in black-headed gull chicks

“…Control birds received sham surgery. Similar implants but with testosterone propionate were shown in an earlier study to induce moderate levels of T (1.45 ng/ml; Groothuis and Meeuwissen, 1992), within the physiological range of birds in social groups (1-2 ng/ml, Ros et al, 2002).…”

“…As expected, the T-treatment elevated the androgen level in the blood plasma to the upper part of the physiological range (Malickiene, 1999;Ros et al, 2002). Based on the cross reactivity of the antibody with DHT (about 50%), the actual circulating DHT level in the DHT group will have been around that of T in the T-treated birds.…”

“…These chicks frequently perform both conspicuous begging displays and aggressive behavior, including threat displays. In this colonial breeding species, chicks defend small territories around the nest and young chicks produce testosterone contingent on the level of aggressive challenges (Ros et al, 2002). Furthermore, within individuals, these testosterone levels correlate with the frequency of aggressive behaviors, while testosterone treatment strongly facilitates aggressive behavior (Groothuis and Meeuwissen, 1992;Ros et al, 2002).…”

“…In this colonial breeding species, chicks defend small territories around the nest and young chicks produce testosterone contingent on the level of aggressive challenges (Ros et al, 2002). Furthermore, within individuals, these testosterone levels correlate with the frequency of aggressive behaviors, while testosterone treatment strongly facilitates aggressive behavior (Groothuis and Meeuwissen, 1992;Ros et al, 2002). Together, the available information suggests that the gull chicks face the problem that androgens, required for territorial defense, at the same time inhibit begging behavior, which is needed for obtaining food from the parents.…”

“…In that case, the suppression of begging is a possible cost of producing testosterone that is needed for territorial defense, in addition to other costs of the hormone such as reduced growth (Ros, 1999), the development of a less cryptic plumage (Ros, 1999), and malformation of the syrinx (Groothuis and Meeuwissen, 1992). To reduce these costs, territorial behavior becomes increasingly sensitive to relatively low levels of androgens in the course of the first 4 weeks after hatching (Groothuis and Meeuwissen, 1992;Ros et al, 2002). One mechanism underlying this increase in sensitivity and avoiding the costs of testosterone exposure might be to increase aromatase activity that converts testosterone into estrogen.…”