Effect of rapid modulation of circulating plasma testosterone concentration on begging, aggressive behavior and competition for food in black-headed gull (Larus ridibundus) chicks

Territory aggression and energy budget in food-restricted striped hamsters

Food resource availability is one of the most important factors affecting interindividual competition in a variety of animal species. However, the energy budget and territory aggression strategy of small mammals during periods of food restriction remain uncertain. In this study, metabolic rate, body temperature, territory aggression behavior, and fat deposit were measured in male striped hamster (Cricetulus barabensis) restricted by 20% of ad libitum food intake with or without supplementary methimazole. Serum thyroid hormone (tri-iodothyronine, T₃ and thyroxine, T₄), and cytochrome c oxidase (COX) activity in liver, brown adipose tissue, and skeletal muscle, were also measured. Attack latency, total attack times and duration, and the interval duration between attacks of resident hamsters were not significantly changed during food restriction, which was not significantly affected by supplementary methimazole. Metabolic rate and body temperature was significantly increased in food-restricted hamsters following introduction of an intruder, which was not completely blocked by supplementary methimazole. Serum T₃ and T₄ levels and BAT COX activity were not significantly changed following aggression, and were significantly decreased by supplementary methimazole. These findings suggest that striped hamsters increase energy expenditure for territory aggression during food restriction, and consequently lead to excessive energy depletion. Territory aggression behavior may decrease the capacity to cope with food shortage, which may be independent of thyroid hormone.

The effect of aggression II: Acclimation to a high ambient temperature reduces territorial aggression in male striped hamsters (Cricetulus barabensis)

Thyroid hormones have a profound influence on development, cellular differentiation and metabolism, and are also suspected of playing a role in aggression. We measured territorial aggression, body temperature (T_b) and serum thyroid hormones levels of male striped hamsters (Cricetulus barabensis) acclimated to either cold (5 °C), cool (21 °C) or hot (34 °C) ambient temperatures. The effects of methimazole on territorial aggression, food intake, metabolic rate and serum thyroid hormone levels, were also examined. Territorial aggression was significantly lower in male hamsters acclimated to the hot temperature compared to those acclimated to the cool or cold temperatures. T_b significantly increased during aggressive territorial interactions with intruders but did not significantly differ among the three temperature treatments. Serum T₃, T₄ and cortisol levels of hamsters acclimated to 34 °C were significantly lower than those acclimated to 21 °C. In addition to significantly reducing territorial aggression, treatment with methimazole also significantly reduced serum T₃ and T₄ levels, T_b and metabolic rate. These results suggest that exposure to high temperatures reduces the capacity of hamsters to dissipate heat causing them to lower their metabolic rate, which, in turn, causes them to reduce territorial aggression to prevent hyperthermia. The lower metabolic rate mediated by down-regulated thyroid hormones inhibits territorial aggression and could thereby determine the outcome of territorial conflicts.

Response of testosterone and corticosterone plasma levels to the challenge of sibling competition: a study in common terns

The hormonal response to social challenges has been widely studied, however, most work focused on adult behavior in a reproductive context although developing animals also encounter important social challenges early in life. We studied the relationship between acute sibling competition and plasma corticosterone (CORT) and testosterone (T) in common tern (Sterna hirundo) chicks, a species whose young compete for access to food by scramble interactions. Blood samples were taken in nests with two and only one single chick both immediately after a feeding bout and in non-challenged controls. We found that T levels were lower in siblings challenged by a feeding bout as compared to controls, which may be explained by the fact that T suppresses begging behavior and is only elevated in response to territorial intrusion but not sibling competition in a related species. Singletons had, corrected for body condition, generally lower CORT levels than siblings suggesting that growing up with siblings creates a competitive environment in which high CORT levels are sustained irrespective of a social challenge. CORT levels were also negatively correlated with body condition and were higher in males than in females. The latter may be related to sex-specific food requirements and susceptibility to stress. Our results suggest a possible suppressive effect of acute sibling competition on T secretion, and a positive effect on CORT levels by longer term sibling competition. The degree to which these dynamics are related to begging or aggression, or both, needs further experimental work.