Effects of experimentally elevated testosterone on plasma glucocorticoids, body mass, and recapture rates in yellow-pine chipmunks, Tamias amoenus

In male yellow-pine chipmunks plasma levels of glucocorticoids (GCs) are low while plasma testosterone (T) levels peak during the mating season, suggesting that T suppresses GC levels. To test this hypothesis, free-living, post-reproductive males were implanted during summer with either a T-filled (T-males) or an empty silastic implant (controls or C-males). Body mass and plasma levels of corticosterone, cortisol, and T were measured immediately before and 1 month after implantation. Exogenous testosterone increased T to high physiological levels typical of reproductively active males. By 1 month after implantation, T-males decreased their mean body mass and plasma GC levels, while C-males maintained their mean body mass and GC levels. Even though T-males lost mass, recapture success 1 month after implantation for T-males (71%) was equal to that of C-males (71%). However, the overwinter recapture rate of C-males (83%) was significantly greater than that of T-males (20%). The results support the hypothesis that high plasma T of males during mating has a suppressive effect on plasma GC levels. Additionally, experimentally elevated T significantly reduced the rate of recapture during the following spring, and this may reflect a reduction in local overwinter survival. The suppression of adrenocortical activity by T may contribute to the reductions in prehibernation body mass and post-emergence recapture success. J. Exp. Zool. 287:378-383, 2000.     

Testosterone increases activity but not daily energy expenditure in captive male dark-eyed juncos, Junco hyemalis

Plasma testosterone (T) levels in male dark-eyed juncos peak early in the breeding season, then decline. If T enhances opportunities for reproductive success, as suggested by previous experiments, why does elevated T not occur naturally? To address this question, we prolonged the early peak level throughout the breeding season and explored potential energetic costs of maintaining elevated T. We measured daily energy expenditure (DEE) of treated males (T-males) and controls (C-males) using doubly labelled water (DLW). We also conducted behaviour scans of T- and C-males housed in outdoor aviaries. DEE was not higher in T-males than in C-males. However, T-males did increase locomotion and foraging and decrease rest and self-maintenance. These results suggest that elevated T may increase the contribution of some components of DEE and lower the contribution of others. Furthermore, the T-induced decrease in allocation of time to rest and maintenance may represent a long-term cost that has led to selection against the maintenance of elevated T beyond the natural early spring peak. Copyright 2000 The Association for the Study of Animal Behaviour.     

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

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.     

Phenotypic engineering: using hormones to explore the mechanistic and functional bases of phenotypic variation in nature

Perhaps the best way to determine whether and how traits of organisms are currently adaptive is to alter them experimentally and compare the relative fitness of altered and unaltered individuals. We call this method phenotypic engineering. To the extent that natural selection moulds organisms on a trait-by-trait basis, we would expect fitness of unmanipulated (control) individuals to be higher than that of experimentally altered individuals. However, other outcomes are possible and of interest. If, for example, a single trait were altered and the fitness of manipulated and unmanipulated organisms were found to be similar, we might conclude that selection is not currently operating on the altered trait. Phenotypic engineering with hormones describes an experimental approach to the study of adaptive variation in suites of traits that are hormonally mediated and correlated in their expression. A likely outcome of such manipulations is that some traits would be altered so as to elevate fitness but that changes in other, correlated traits would lower fitness. If the net effect were to depress fitness, a process by which natural selection shapes and maintains organisms as integrated units would be demonstrated. We have employed this approach in studies of the Dark-eyed Junco Junco hyemalis, a small passerine whose reproductive success varies with the abundance of nest predators. We treated males with testosterone, documented the phenotypic consequences and related these to various measures of fitness. Summarizing results to date: Behavioural comparison of males treated with testosterone (T-males) and control males (C-males) shows that T-males sing more frequently, are less attentive to offspring, have larger home ranges and are more attractive to females. Physiologically, testosterone accelerates entry into breeding condition in spring (loss of winter lipid stores) and results in higher levels of corticosterone. If exposure to testosterone is prolonged beyond the breeding season, pre-basic moult is delayed or prevented. We are currently comparing T- and C-males with respect to corticosteroid binding proteins, sperm reserves, response to nestling vocalizations and neuroanatomy. The relationship between testosterone-induced phenotypic variation and fitness is still under study. When treatment extends well beyond the breeding season, testosterone significantly reduces survivorship; otherwise it does not. With respect to apparent reproductive success (i.e. estimates of paternity that are not based on genetic analysis), more young leave the nests of C-males than of T-males, but treatment groups do not differ in the number of young that reach independence. Preliminary data on realized reproductive success (i.e. number of genetic offspring sired) suggest that production as the result of extra-pair fertilizations is greater in T- than in C-males but that T-males lose paternity of more of the offspring of their social mates to other males. Continued investigation will, we hope, reveal the factors governing the trade-offs between male mating effort and parental effort and between survival and current reproduction, as well as the frequency with which the typical phenotype outperforms one that has been experimentally altered.     

The role of testosterone in male downy woodpeckers in winter home range use, mate interactions and female foraging behaviour

Studies of the role of testosterone (T) in birds have typically focused on sexual or aggressive behaviours of males during the breeding period, but males of nonmigratory species may invest in mate and territory long before breeding, and the influence of T in facilitating nonbreeding-season behaviours is poorly understood. We gave free-living male downy woodpeckers, Picoides pubescens, T-implants during the winter to determine whether elevated levels of T increased a male's ability to exclusively occupy territory-based resources, and whether elevated T strengthened a male's investment in an existing pair bond relationship. We also explored how a female's foraging efficiency might be affected by her mate's behaviour if he had elevated T. We found little difference between control and T-implanted males with regard to home range exclusivity. Surprisingly, male-male display rates were significantly lower in T-implanted males than in controls. Regarding male-female interactions, T-implanted males that experienced high incursion rates from other males maintained more frequent spatial association with their mate, suggesting that T facilitates male behaviours that could restrict the mate's access to other male birds. Female mates of T-males showed reduced foraging rates, but because male-female aggression was similar between treatment groups, the cause for this reduction is unknown. The results indicate that exogenous T during winter affects a variety of behaviours in male woodpeckers, and proximate influences on pair bond maintenance in winter may be a fruitful avenue for future research.     

Consequences of elevating plasma testosterone in females of a socially monogamous songbird: evidence of constraints on male evolution?

To explore whether selection for testosterone-mediated traits in males might be constrained by costs of higher testosterone to females, we examined the effects of experimental elevation of plasma testosterone on physiological, reproductive, and behavioral parameters in a female songbird, the dark-eyed junco (Junco hyemalis). We used subcutaneous implants to elevate testosterone (T) in captive and free-living female juncos. In captive birds, we measured the effects of high T on body mass, feather molt, and brood patch formation. In the field, we monitored its effects on the timing of egg laying, clutch size, egg size, egg steroid levels, incubation, and nest-defense behavior. Females implanted with testosterone (T-females) had significantly higher circulating levels of testosterone than did control females (C-females). Captive T-females had lower body mass, were less likely to develop brood patches, and delayed feather molt relative to C-females. Among free-living females, the interval between nest completion and appearance of the first egg was longer for T-females than for C-females and egg yolk concentrations of testosterone were higher, but there were no significant differences in estradiol levels, clutch size, or egg size. Incubation and nest defense behavior were also similar between T- and C-females. Our results suggest that selection on males for higher testosterone might initially lead to a correlated response in females producing changes in body mass and feather molt, both of which could be detrimental. Other possible female responses would be delayed onset of reproduction, which might reduce reproductive success, and higher yolk testosterone, which might have either positive or negative effects on offspring development. We found no reason to expect reduced parental behavior by females as a negative fitness consequence of selection for higher testosterone in males.     

Testosterone affects reproductive success by influencing extra-pair fertilizations in male dark-eyed juncos (Aves: Junco hyemalis)

Monogamous male birds typically allocate less effort to courtship and more to parental behaviour than males of polygynous species. The seasonal pattern of testosterone (T) secretion varies accordingly. Monogamous males exhibit a spring peak in plasma T followed by lower levels during the parental phase, while males of polygynous species continue to court females and maintain T at higher levels. To determine whether testosterone underlies the trade-off between mating and parental effort, we treated male dark-eyed juncos (Junco hyemalis) with exogenous T and compared the reproductive success (RS) of T-treated males (T-males) to that of controls. T-males had lower apparent annual RS than controls, probably because elevated T reduced parental care. Nevertheless, annual genetic RS of the treatment groups was similar because (i) T-males suffered fewer losses in genetic RS due to extra-pair fertilizations (EPFs), and (ii) T-males gained more genetic RS through their own EPFs. This is the first hormonal manipulation of an avian phenotype shown to have influenced male RS through EPFs. Together with other studies, it suggests that testosterone may have mediated the evolution of inter- and intraspecific differences in allocation of reproductive effort to mate attraction and parental care.     

Prenatal Exposure to Androgen Excess Increases LH Pulse Amplitude During Postnatal Life in Male Sheep

Prenatal exposure to excess testosterone has a profound impact on reproductive and metabolic functions in young and adult female sheep. Nevertheless, few studies have addressed the impact of prenatal exposure to an excess of androgens on reproductive and metabolic functions in males. The aim of the present study was to assess the impact of prenatal exposure to an excess of testosterone or dihydrotestosterone on the luteinizing hormone (LH) pulse characteristics during sexual development in male sheep. Control male sheep (C-males) and males born to mothers exposed to twice weekly injections of 30 mg testosterone or dihydrotestosterone from day 30-90 and 40 mg from day 90-120 of gestation (T-males, DHT-males) were studied at 5, 10, and 20 weeks of age, ages that represent infancy, early prepubertal, and late prepubertal stages of sexual development in this species, respectively. Patterns of LH pulsatility showed that T- and DHT-males exhibited a higher secretion of LH during the 6-h study and a higher amplitude of the LH pulses compared with C-males. Moreover, nadir of the pulses was higher in T- and DHT-males compared with C-males. Frequency of LH pulses, however, was not different within ages or between groups. These results show that males can be responsive to prenatal androgenization and suggest that treatment transiently alters the amplitude of LH pulses probably as the result of defects in the pituitary responsiveness pattern or in the gonadotropin-releasing hormone (GnRH) release pattern.     

Effects of testosterone implants on pair behaviour during incubation in the Yellow-legged Gull Larus cachinnans

The Yellow-legged Gull Larus cachinnans is monogamous with bi-parental incubation. In this study, the effects of high levels of plasma testosterone in male Yellow-legged Gulls during the incubation period were analysed. Free-living male gulls were implanted with testosterone (T-males), and their sexual behaviour within the pair was observed and compared with that of control pairs. Egg temperatures, length of incubation and hatching success were also analysed. T-males and their mates displayed more sexual behaviour than the controls. T-males engaged in mounting behaviour with their mates, whereas control males did not. Proportionally less time was spent incubating (in relation to time present in the colony) by T-males than control males. However, the mates of T-males did not spend more time incubating than control females to compensate for male neglect, although they did spend more time on the territory. Egg temperature in T-male nests was significantly lower than in control nests, but no significant difference in the length of incubation or hatching success between the two groups was found. In birds, the effects of high testosterone levels on male behaviour during incubation have only been analysed in a polyandrous species whose females usually do not contribute to incubation. The present results thus suggest that those males of a monogamous species with bi-parental incubation that sustain high testosterone levels after laying, thus reducing their contributions to incubation, will be confronted with a lack of compensation from their mates during incubation. Finally, this lack of female compensation seems to be mediated by behavioural interactions with the male rather than by her absence from the colony.     

Territorial aggression and song of male European robins (Erithacus rubecula) in autumn and spring: effects of antiandrogen treatment

Male robins aggressively defending a feeding territory in winter have low levels of testosterone, while males defending a breeding territory in spring have elevated levels of testosterone. Song is an integrated part of territorial defense during both phases. We investigated whether testosterone is involved in the expression of these behaviors by treating free-living and captive male robins during both phases with the antiandrogen flutamide. Results suggest that, similar to species in which territoriality is restricted to the reproductive phase, aggressive defense of a breeding territory by male robins is facilitated by androgens. Territorial defense during the nonbreeding season, however, does not require androgenic activity. Singing frequency, on the other hand, was not significantly reduced during either phase by flutamide application. Since the quality of male song changes with season we suggest that the incorporation of sexual signals into male song during breeding depends on testosterone, but that the year-round production of territorial "keep out" signals is independent of testosterone.     

The cost of producing a sexual signal: testosterone increases the susceptibility of male lizards to ectoparasitic infestation

According to current evolutionary theory, advertising traitsthat honestly indicate an organism's genetic quality might becostly to produce or maintain, though the kind of costs involvedin this process are controversial. Recently the immunocompetencehypothesis has proposed that testosterone (T) stimulates theexpression of male sexually selected traits while decreasingimmunocompetence. Even though some recent studies have shownan effect of T on ectoparasite load, the dual effect of thehormone has not been addressed in free-living populations. Herewe report results of an experiment in a free-living populationof the lizard Psammodromus algirus during the mating season.Males implanted with T had larger patches of breeding colorand behaved more aggressively than control males. In T-implantedmales, the increase in number of ticks during the mating seasonwas significantly higher than in control males and this negativelyaffected several hematological parameters. T-males sufferedsignificantly higher mortality than control males during theexperiment The results from the manipulation of T are consistentwith the dual effect of this hormone.     

A condition dependent link between testosterone and disease resistance in the house finch.

Testosterone has recently been proposed as a link between male quality and health and the expression of sexual traits. We investigated the relationship between testosterone and measures of the individual condition and health of males in a natural population of house finches (Carpodacus mexicanus). We also conducted a captive experiment in order to test for the effects of testosterone on resistance to coccidia, which is a common parasite of house finches. Free-living males in better condition had higher testosterone levels and lower corticosterone levels than free-living males in poor condition. In our captive experiment, increased testosterone accelerated the rate of coccidial infection as compared with sham-implanted or gonadectomized males. Although the differences were not significant, free-living males infected with coccidia had lower levels of testosterone and higher levels of corticosterone than males that were not infected. Thus, experimentally elevating testosterone levels in captive males resulted in a higher percentage of infected males, while free-living males with coccidial infection had low testosterone levels. This apparent discrepancy between captive and free-living males in the association of testosterone and disease may be explained by the condition dependence of testosterone. These results suggest that the testosterone-dependent sexual traits reliably indicate male overall condition and health and, thus, females could benefit from assessing potential mates based on these traits.     

Elevated spring testosterone increases parasite intensity in male red grouse

The expression of testosterone-dependent sexual traits mightsignal the ability of their bearers to cope with parasite infections.According to the immunocompetence handicap hypothesis (IHH),such signals would be honest because physiological costs oftestosterone, such as a reduced ability to control parasiteinfections, would prevent cheating. We tested whether testosteronewould affect the outcome of a standardized parasite challengein red grouse, using a main parasite of the species, the nematodeTrichostrongylus tenuis. We caught males in spring, removedtheir nematode parasites, and implanted them with testosteroneor empty implants, as controls. After 1 month, they were reinfectedwith a standard dose of infective T. tenuis parasites. Whenchallenged, testosterone males had relatively less globulinrelative to albumin plasma proteins than control males, an indicationthat they had experienced increased physiological stress. Testosterone-treatedmales had significantly more T. tenuis parasites than controlsin the next autumn and also had more coccidia and lost moreweight than controls. Testosterone-treated males neverthelessbenefited from their elevated spring testosterone: they hadbigger sexual ornaments than controls both in spring and autumn,and they tended to have a higher pairing and breeding successthan controls. Our results supported the IHH in showing thatelevated testosterone impaired the ability of males to copewith a standardized challenge by a dominant parasite. Testosteronethus plays a key role in mediating trade-offs between reproductiveactivities and parasite defense, and testosterone-dependentcomb size might honestly signal the ability of red grouse tocontrol T. tenuis infection.     

Seasonal changes in the blood composition of captive and free-living White-crowned Sparrows

Summary Seasonal changes in several blood components (cholesterol, phospholipid phosphorus, glyceride glycerol, free fatty acids, and calcium), hematocrit, and body mass were studied in captive and free-living groups of White-crowned Sparrows (Zonotrichia leucophrys gambelii) over a 2-year period. Cholesterol, phospholipid phosphorus, and glyceride glycerol levels were correlated with body mass and all of them changed in parallel during the year (Table 2 and Figs. 1–3). These lipids were elevated during premigratory and migratory periods, minimal during the summer breeding period, and reduced during periods of body molt. Concentrations of them were consistently higher in captive birds than in free-living ones. Free fatty acid levels were highly variable and not correlated with levels of other blood lipids or with body mass (Fig. 4). However, they too increased during premigratory periods. On the other hand, they were consistently higher in free-living sparrows than in captives. Plasma calcium was relatively constant at 3–5 mEq/l between July and the following March (Fig. 5). It increased during the spring, but earlier than preparations for migration by 2–3 weeks. It was also elevated in egg-laying females. The hematocrit rose during the vernal migratory period, but not during the autumnal one; was minimal in breeding birds; and declined during periods of body molt (Fig. 6). Calcium and hematocrit levels were similar in captive and free-living birds. It appears that captive populations of White-crowned Sparrows generally provide relibale information concerning changes in blood composition that are taking place concurrently in the field.This investigation was done to satisfy partially the requirements for the Ph.D. degree at Washington State University by deGraw and Kern. The project was supported by an NIH Training Grant (GM01276) from the National Institute of General Medical Sciences     

The influence of testosterone on territorial defence and parental behavior in male free-living rufous whistlers, Pachycephala rufiventris

We studied a population of rufous whistlers, Pachycephala rufiventris, throughout a single breeding season in central New South Wales, Australia. We evaluated the relation between plasma testosterone (T) and reproductive behaviors using both simulated territorial intrusions (STIs) and subcutaneous T implants. We compared circulating T values to aggression levels of males (using STI) during pair bond and territory establishment and again during incubation. Although plasma T levels were significantly lower in the latter period, male responsiveness to STI, in terms of proximity to decoy, call rate, and number of attacks on the decoy, was indistinguishable between the two breeding stages. T levels of males exposed to STI were not different from the levels of unexposed free-living males at the same breeding stage. The effect of exogenous T on parental behavior was examined by comparing duration of incubation bouts of males and their mates prior to and after T treatment. T males significantly reduced the amount of time they incubated following implantation, whereas Control males maintained their incubation effort. After cessation of breeding activities, T males displayed significantly higher call rates due to increased use of the primary intersexual advertisement call in this species. The reduction of incubation behavior following T implantation emphasises the functional significance of the rapid decline in T in free-living males during incubation. The results from both experiments suggest that intersexual advertisement, rather than territorial aggression, may be dependent on high T levels in this species.     

Effects of captivity and testosterone on the volumes of four brain regions in the dark-eyed junco (Junco hyemalis)

This study investigates the effects of captivity and testosterone treatment on the volumes of brain regions involved in processing visual and spatial information in adult dark-eyed juncos (Junco hyemalis). We treated captive and free-living male juncos with either testosterone-filled or empty implants. Captive juncos had a smaller hippocampal formation (HF) (both in absolute volume and relative to telencephalon) than free-living birds, regardless of hormone treatment. Testosterone-treated males (both captive and free-living) had a smaller telencephalon and nucleus rotundus, but not a smaller HF or ectostriatum, than controls. We found that free-living testosterone-treated males had larger home ranges than free-living controls in agreement with earlier experiments, but we found no corresponding difference in HF volume. We discuss the implications of the effect of captivity on HF volume for past and future laboratory experiments.     

Testosterone does not affect basal metabolic rate or blood parasite load in captive male White-plumed Honeyeaters Lichenostomus penicillatus

We examined physiological and morphological responses of reproductively active male White-plumed Honeyeaters Lichenostomus penicillatus to exogenous testosterone (T). Birds used in the experiment were held in captivity for two weeks and then were given subcutaneous implants which were either empty (controls) or T-filled silastic capsules (T-treated). Basal metabolic rates (BMR) measured just prior to implantation and again 4-weeks later were not affected by treatment and were indistinguishable from BMR measured in free-living White-plumed Honeyeaters, averaging 3.2 ml O2/g/h. Plasma T levels measured at the time of the final BMR evaluation were 1.2, 1.8, and 5.5 ng/ml in free-living, control, and T-treated birds, respectively. Body mass was unaffected by T treatment. All captive birds used in the experiment displayed Haemoproteus sp. infection, but blood parasite loads did not differ temporally between T-treated and control birds during their 6-week captivity. Corticosterone secretion during a 60-min capture-handling protocol did not differ between control and T-treated birds, but corticosterone levels at each sampling period in captive birds were significantly lower than those of free-living males. Three morphological variables of White-plumed Honeyeaters were affected by T treatment: T-treated birds had significantly higher haematocrits and larger testes than control birds and also showed no change in bill colour. Control birds, by contrast, showed substantial lightening of the proximal half of their bills. We conclude that BMR, resistance to Haemoproteus infection, and stress responsiveness are not directly affected by T levels in male White-plumed Honeyeaters.     

Elevated testosterone reduces choosiness in female dark-eyed juncos (Junco hyemalis): evidence for a hormonal constraint on sexual selection?

Because testosterone (T) often mediates the expression of attractive displays and ornaments, in the absence of constraints sexual selection should lead to an evolutionary increase in male T levels. One candidate constraint would be a genetic correlation between the sexes that leads to a correlated response in females. If increased T in females were to have deleterious effects on mate choice, the effect of sexual selection on male T would be weakened. Using female dark-eyed juncos (Junco hyemalis), we tested whether experimentally enhancing female T would lead to a decrease in discrimination between two classes of males, one treated with T (T-males) and one control (C-males). The two female treatments (T-implanted and C-females) spent equal amounts of time with both classes of males, but T-treated females failed to show a preference for either male treatment, whereas C-females showed a significant preference, albeit in an unexpected direction (for C-males). T-females were less discriminating than C-females, irrespective of the direction of their preference. To our knowledge, this is the first study to show that circulating hormones can alter female choosiness without reducing sexual motivation. Our results suggest that hormonal correlations between the sexes have the potential to constrain sexual selection on males.     

Intra-sexual competition alters the relationship between testosterone and ornament expression in a wild territorial bird

In a reliable signalling system, individual quality is expected to mediate the costs associated with ornamental displays, with relatively lower costs being paid by individuals of higher quality. These relative costs should depend not only on individual quality, but also on levels of intra-sexual competition. We explored the current and delayed effects that testosterone implants have on bird ornamentation in populations with contrasted population densities, as a proxy for intra-sexual competition. In a replicated experiment, we manipulated testosterone in 196 yearling male red grouse Lagopus lagopus scoticus in autumn in populations of high and low levels of intra-sexual competition. Males were assigned to one of three exogenous testosterone (T) treatments: empty implants (T0), small T implants (T1) or larger T implants (T2). We monitored subsequent changes in testosterone levels, ornament size and carotenoid-based colouration, carotenoid levels and body condition from autumn to spring. Testosterone implants increased testosterone levels, comb redness and comb size, and decreased body condition but these effects depended on levels of intra-sexual competition. Specifically, T2-implanted birds increased testosterone levels and comb size more, and reduced body condition more, in populations where intra-sexual competition was low. In the following spring, testosterone levels of T2-treated birds kept increasing in populations where intra-sexual competition was high but not in populations where intra-sexual competition was low. Our results highlight that levels of intra-sexual competition alter the relationship between testosterone levels and ornament expression, influencing their condition-dependence; they also indicate that the outcome of standard hormone manipulation conducted in free-living animals vary depending on the population context.     

Testosterone effects on avian basal metabolic rate and aerobic performance: facts and artefacts

We examined the effects of cage size and testosterone (T) levels on basal and peak metabolic rates (BMR and PMR, respectively) and on pectoral and leg muscle masses of male house sparrows (Passer domesticus). Birds were housed either in small birdcages or in flight aviaries for at least 2 weeks prior to the initial metabolic evaluations. They were then implanted with either empty or T-filled silastic capsules and remeasured 5-6 weeks later. Birds treated with single T implants achieved breeding levels (4-6 ng/mL) and one group given double implants reached 10 ng/mL. There was no effect of T on BMR or PMR in any group studied, but there was an effect of caging. Caged birds showed significant reductions in PMR over the course of captivity, whereas PMR in aviary-housed birds were indistinguishable from their free-living counterparts. Testosterone treatment significantly increased leg muscle mass in caged birds, but had no effect on muscle mass in aviary-housed sparrows. We conclude that testosterone has no direct effect on sparrow metabolic rate or muscle mass, but may interact with cage conditions to produce indirect changes to these variables.