TESTOSTERONE DOES NOT INCREASE IN RESPONSE TO CONSPECIFIC CHALLENGES IN THE WHITE-BELLIED ANTBIRD (MYRMECIZA LONGIPES), A RESIDENT TROPICAL PASSERINE

Auk, The, Jan 2006 by Fedy, Bradley C, Stutchbury, Bridget J M

ABSTRACT.-

Resident tropical passerines that exhibit year-round territorial aggression do not fit well into the temperate-zone model, because testosterone does not increase substantially during the breeding season. We studied patterns of testosterone secretion in the White-bellied Antbird (Myrmeciza longipes), a resident tropical species in Panama that maintains territories year-round and is capable of aggression throughout the year, regardless of its stage of reproduction. Levels of plasma testosterone were low (mean = 0.30 ng mL^sup -1^) throughout the breeding and nonbreeding seasons and did not differ between them. Testosterone also did not increase in response to simulated conspecific intrusions. When we used temporary removal experiments to induce natural, extended conflict between males, testosterone levels did not increase in response to the extended social instability that resulted. White-bellied Antbirds demonstrate an apparent uncoupling of testosterone and territorial aggression throughout the year. Received 6 September 2004, accepted 11 May 2005.

Key words: Myrmeciza longipes, territoriality, testosterone, tropical birds, White-bellied Antbird.

La Testosterona no Aumenta como Respuesta a Desafíos de Individuos Coespecíficos en Myrmeciza longipes, un Paserino Residente de la Zona Tropical

RESUMEN.-Las aves paserinas residentes de la zona tropical que exhiben agresión territorial a lo largo del año no encajan correctamente en el modelo de la zona templada, pues su testosterona no se incrementa de modo sustancial durante la época reproductiva. Estudiamos los patrones de secreción de testosterona en Myrmeciza longipes, una especie tropical residente en Panamá que maintiene territories a través del año independientemente de su estado reproductivo. Los niveles de testosterona en el plasma fueron bajos (media = 0.30 ng mL^sup -1^) y no difirieron entre la época reproductiva y la no reproductiva. La testosterona no aumento como respuesta a intrusiones territoriales simuladas. Cuando empleamos experimentos de remoción temporal para inducir la ocurrencia natural de conflictos extendidos entre machos, los niveles de testosterona no aumentaron como respuesta a la inestabilidad social extendida que se generó. Myrmeciza longipes parece exhibir un desacoplamiento entre la testosterona y la agresión territorial durante todo el año.

THE STEROID HORMONE testosterone has been shown to have an important influence on the behavior of birds. In most temperate-zone birds, an increase in testosterone, testes size, territoriality, aggression, and reproduction all coincide during a brief period in the spring (for overviews, see Balthazart 1983, Wingfield et al. 1990, Ketterson et al. 1992, Wingfield 1994). Wingfield et al. (1990) proposed the "challenge hypothesis," which predicts that in socially monogamous species, territorial challenges should increase testosterone, which would function to boost an individual's persistence during periods of social instability (challenges). However, that hypothesis is based generally on temperate species. Tropical bird species that maintain year-round territories do not seem to fit well into this temperate-zone model. Data for tropical birds suggest an uncoupling of territoriality, testosterone, aggression, and reproduction (Wikelski et al. 1999a, b, 2003). Wikelski et al. (1999a) have demonstrated that (1) tropical birds with year-round territories can be aggressive at any time of the year, (2) mating and social systems may play a role in the pattern of testosterone secretion, and (3) testosterone can be high despite entirely regressed gonads. These data hint at a system that is much different from the fairly well-understood temperate-zone model.

Although high levels of testosterone are not typical of tropical birds, testosterone can still have an effect on singing and aggressive behavior. Captive Spotted Antbirds (Hylophalyx naevioides) with experimentally increased testosterone levels showed higher levels of aggression (Hau et al. 2000). Wikelski et al. (1999b) have also shown that testosterone levels will increase in Spotted Antbirds after extensive (2 h) simulated intrusions using playbacks of conspecific songs. Bay Wrens (Thryothoms nigricapillus) are another tropical species that are highly aggressive year-round but do not have measurable levels of testosterone (Levin and Wingfield 1992). Bay Wrens had low levels of testosterone, even when individuals were exposed to simulated territorial intrusions (Levin 1996). The uncoupling of territorial aggression and testosterone is emerging as a common theme in tropical passerines (Wingfield et al. 1991, 1992; Wingfield and Lewis 1993; Wikelski et al. 2003; Moore et al. 2004).

We studied patterns of testosterone secretion in a tropical passerine species in Panama, the White-bellied Antbird (Myrmeciza longipes), a resident species that exhibits aggressive territorial behavior year-round. In another component of this research, we found that males are not more aggressive during the breeding season than during the nonbreeding season (Fedy and Stutchbury 2005). Given these results and the findings of previous studies on year-round territorial species, we expected that testosterone levels would not vary between nonbreeding and breeding life-stages. We also predicted that testosterone levels would not correlate with the duration of simulated territorial intrusions using playbacks of conspecific songs. We used removal experiments in an attempt to create extended social disruption in a natural context. We temporarily removed territorial males until a replacement male bird moved into their territory and began duetting with the widowed female. We then released the removed individual back onto his territory, which resulted in the male competing with the replacement individual to regain his original territory. This created a situation of extended challenge for the focal birds. We then captured the males and measured testosterone levels