Testosterone and autumn territorial behavior in male red grouse Lagopus lagopus scoticus

Abstract

In many bird species, males exhibit territorial aggression outside the breeding season, when testosterone concentrations are low and may not regulate territorial behaviors. The hormonal regulation of aggression at this time of year has only been studied in passerine birds. Here, we investigated the role of testosterone in the regulation of aggression in a non-passerine bird, the red grouse Lagopus lagopus scoticus. Male red grouse are aggressive in early spring when breeding starts, in autumn when they establish territories, and sporadically through much of the winter. We first describe seasonal variations in plasma testosterone concentrations and in the size of males' sexual ornaments, their red combs, which relates to aggressiveness. Testosterone concentrations and comb size were correlated. Both increased in autumn to a peak in October, and then increased again in spring, to a greater peak in early April. Secondly, we experimentally investigated the effects of testosterone, and of an anti-androgen (flutamide) used in combination with an aromatase inhibitor (ATD), on autumn territorial behavior. Males were treated with either empty implants, as controls (C-males), testosterone implants (T-males), or with flutamide and ATD implants (FA-males). One month after implanting, both T- and FA-males had higher concentrations of testosterone than C-males. Comb size, aggressive call rate, and response to playbacks of territorial call all significantly increased in T-males. However, the increase in testosterone in FA-males did not increase comb size or aggressive behavior. In the following spring, after the content of implants was used, FA-males had significantly lower testosterone than C-males, and had a reduced seasonal increase in comb size. The results suggest that testosterone plays a significant role in regulating red grouse aggressive behavior in autumn. However, the observation that flutamide and ATD treatment did not reduce territorial behavior, suggests that estradiol may also be involved in the regulation of non-breeding aggression.

Introduction

Sex steroids, such as testosterone, play a major role in regulating the territorial behavior of many vertebrate species during the breeding season. In most seasonally breeding bird species, both aggressive and sexual behaviors are activated by increased testosterone concentrations and are tightly coupled with the reproductive phase (Wingfield et al., 1987, Wingfield et al., 2001a). The mediation of territorial behavior by testosterone can occur by direct action through androgen receptors or through conversion to estradiol by the enzyme aromatase (e.g., Canoine and Gwinner, 2002, Schwabl and Kriner, 1991, Soma et al., 2000b). Some species also exhibit territorial aggression outside the breeding season, and the hormonal regulation of such behavior has so far been studied in passerine birds only. Amongst these species, aggression outside breeding might not be regulated by gonadal testosterone. Males then have regressed gonads and plasma concentrations of testosterone that are often basal, much lower than during the breeding season (e.g., Wingfield et al., 2001a). Moreover, elevated testosterone can have major costs during the autumn and winter months, such as reduced condition and fat stores or impaired immune function and decreased survival probability (e.g., Folstad and Karter, 1992, Grosmann, 1985, Hillgarth and Wingfield, 1997, Ketterson and Nolan, 1992, Wingfield et al., 2001b). The mechanisms regulating aggression can vary seasonally, and territoriality outside the breeding season can be regulated by hormones other than testosterone (e.g., Logan and Wingfield, 1990, Soma et al., 2000a, Soma et al., 2000b, Wingfield and Soma, 2002a).

We studied the hormonal regulation of autumn territorial behavior in a non-passerine bird, the red grouse (Lagopus lagopus scoticus). This tetraonid species is temperate, inhabiting the heather moorland of the UK, and breeds in summer, between April and August (Cramp and Simmons, 1980). Male red grouse exhibit aggressive territorial behavior in autumn (non-breeding season), sporadically throughout winter, and in spring, at the onset of the breeding season (Cramp and Simmons, 1980, Watson, 1985). Aggressiveness during the autumn territorial contests plays an important role in regulating grouse abundance, and determines subsequent breeding density (Moss and Watson, 2001, Mougeot et al., 2003a). Autumn territorial behavior is crucial to males, because those that fail to establish or maintain a territory usually die over winter or do not pair and reproduce (Mougeot et al., 2003b, Watson, 1985). During aggressive interactions, males use territorial calls, ground and flight displays, and erect their supra-orbital red combs or wattles (Cramp and Simmons, 1980, Watson and Jenkins, 1964). Combs are the main sexual ornament of red grouse and function in intra-sexual competition. Their size is testosterone dependent, and a good proxy of aggressiveness: males with bigger combs are dominant and more aggressive (Moss et al., 1979).

We investigated the role of testosterone in the regulation of territorial behavior in free-living male red grouse, both descriptively and experimentally. We first described seasonal variations in plasma testosterone concentrations and comb size. In some birds, testosterone peaks in autumn (e.g., Wingfield et al., 1997) and we looked for such a seasonal pattern in red grouse. Second, we tested the effects of treatment with testosterone, or treatment with an aromatase inhibitor in combination with an anti-androgen, on territorial behavior during autumn. We assigned males to one of three treatments: (1) control, i.e., sham implanted (C-males); (2) implanted with testosterone (T-males); or (3) implanted with the anti-androgen flutamide (F), trifluoro-2-methyl-4′-nitro-m-proprionotoluidide, in combination with the aromatase inhibitor ATD, 1-4-6-androstatrien-3,17-dione (FA-males). Flutamide and ATD have been used in a number of studies on passerine birds to block the actions of androgens (e.g., Canoine and Gwinner, 2002, Hau et al., 2000, Moore et al., 2004, Soma et al., 1999), but their combined effects have not yet, to our knowledge, been investigated in a non-passerine bird. Flutamide is an androgen receptor antagonist, while ATD blocks the aromatase enzyme and therefore conversion of androgens to estrogens where they can act through estrogen receptors to activate aggressive behaviors. We investigated the effects of these treatments on plasma testosterone concentrations, comb size, territorial call rate, and response to playbacks of territorial calls. We expected males treated with testosterone (T-males) to have bigger combs, to call more frequently, and to respond more aggressively to playbacks. In contrast, we predicted that blocking the actions of testosterone by the combined actions of flutamide and ATD would reduce territorial behavior, and thus expected FA-males to have smaller combs, reduced territorial call rate, and less aggressive responses to playbacks as compared to controls.