Michaela Hau Lab
Department of Ecology & Evolutionary Biology
Princeton University, Princeton, NJ 08544

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Year-round territorial aggression and testosterone in tropical birds

Many tropical birds are sedentary and maintain permanent territories year-round. In temperate zone birds, territorial aggression is regulated by elevated plasma concentrations of the hormone testosterone. Testosterone also has important functions in the control of sexual behavior during the breeding season. However, elevated levels of testosterone may be detrimental for immune function and many other physiological processes, when kept up over long periods of time. Do tropical birds have constantly elevated testosterone titers, or do they bypass potential costs of testosterone by using other hormones or by keeping testosterone low?

Spotted Antbirds: Year-round territoriality

Although reproduction in these birds is seasonal, Spotted Antbirds maintain and defend all-purpose territories throughout the year. Yet these birds can have baseline testosterone (T) levels, even during the breeding season. This differs dramatically from most temperate birds, which have increased T titers throughout the reproductive and territorial period.

However, free living male Spotted Antbirds can increase plasma concentrations of T in response to a territorial challenge.

Furthermore, experimental increase of T titers and pharmacological blockade of T actions indicate this hormone is nevertheless involved in regulating aggression in Spotted Antbirds. We hypothesize that Spotted Antbirds have developed novel ways to regulate territorial aggression through T, perhaps to avoid potentially detrimental side effects of this hormone. To this end, we are currently investigating the neuroendocrine basis of year-round T production and steroid receptor dynamics in Spotted Antbirds. This project is a collaboration with Virginie Canoine and Barney Schlinger.

 

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Seasonal profiles of testis volumes (uppermost graph), and plasma concentrations of testosterone (T, middle graph) and luteinizing hormone (LH, lowest graph) of free-living male Spotted Antbirds. Birds show significant seasonal cycles in testis size and LH concentrations, but not in T. Furthermore, overall plasma concentrations of T are very low. From Wikelski et al., 2000.


Male territorial Spotted Antbirds have low plasma concentrations of T when we caught them alone (no territorial challenge occurring). Plasma concentration of T in a territory owner increased when additional birds were caught on his territory on that day, indicating a territorial challenge. From Wikelski et al. 1999.



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