Exogenous testosterone inhibits several forms of male parental behavior and stimulates song in a monogamous songbird: the blue-headed vireo (Vireo solitarius)

Natural variation in circulating testosterone is thought to play a role in creating inter- and intraspecific variation in paternal care in birds. Experimental elevation of plasma testosterone in males has reduced rates of male care in many species. However, a small number of studies suggest that male care is not inhibited by testosterone when there appears to be strong selection for male care. Both genetic monogamy and male incubation are thought to be the result of selection for biparental care, yet the sensitivity of male care to inhibition by T had not been examined in a species with both characteristics. Male blue-headed vireos have been shown to be strictly monogamous and provide extensive parental care, including incubation. Males received subcutaneous implants of testosterone, flutamide (an anti-androgen), or empty controls. Consistent with previous studies, testosterone treatment decreased the relative contribution to incubation by males and increased the average duration that the nest was left unattended during incubation, relative to control and flutamide treatments. The relative and absolute rates of feeding offspring and nest sanitation by T-treated males also decreased relative to controls. Flutamide-treated males incubated eggs and brooded young more than testosterone-treated males, with control males performing at intermediate values. Testosterone-treated males sang more often and at higher rates, and flutamide-treated males sang less often, than controls. These results demonstrate that even in a strictly monogamous songbird with biparental incubation, all forms of paternal care remain sensitive to elevated plasma testosterone, suggesting a shift from parental effort to mating effort.     

Steroid hormones and immune function: experimental studies in wild and captive dark-eyed juncos (Junco hyemalis)

Monogamous and polygynous male songbirds generally differ in their breeding season profiles of circulating testosterone. Testosterone level spikes early in the breeding season of monogamists and then declines, but it remains high in polygynists. Male dark-eyed juncos (Junco hyemalis) are socially monogamous and exhibit the usual pattern, but experimental maintenance of high testosterone throughout the breeding season alters normal behavior and physiology and affects various components of annual reproductive success but not overall annual success. Because stabilizing selection predicts that alteration of naturally existing phenotypes should reduce lifetime reproductive success, we asked whether prolonged testosterone exposure might impair immune function and perhaps thereby reduce life span. We assessed immune function in captive and wild male juncos that we treated with either testosterone-filled or empty Silastic implants. Results indicate that prolonged elevation of testosterone suppresses antibody production in captive males and cell-mediated immunity in wild males. Together these results suggest that testosterone-treated males may be more susceptible to disease or parasitic infection. As earlier studies have shown, levels of corticosterone as well as testosterone are higher in testosterone-treated males, so it is unclear whether the immune suppression we observed is due to testosterone's direct effects on immunity or testosterone's influence on glucocorticoid production. We discuss results in the context of recent hypotheses regarding life-history theory and potential endocrine-immune interactions.     

Variation in testosterone levels and male reproductive effort: Insight from a polygynous human population

Recent evidence suggests that, in humans, variations in testosterone (T) levels between men reflect their differential allocation in mating versus parenting efforts. However, most studies have been conducted in urbanized, monogamous populations, making generalizations from them questionable. This study addresses the question of whether indicators of male reproductive effort are associated with variations in salivary T levels in a polygynous population of agriculturists in rural Senegal. We first show that pair-bonding and/or transition to fatherhood is associated with T profiles: married fathers (N = 53) have lower morning and afternoon T levels than unmarried non-fathers (N = 28). Second, among fathers, individual differences in parenting effort, as well as variations in mating effort, predict morning T levels. Indeed, men highly investing in parental care show lower morning T levels. Moreover, among men under 50, polygynous men show higher morning T levels than monogamous men. Taken together with previous results in monogamous settings, these findings suggest that the endocrine regulation of reproductive effort is probably a general feature of human populations.     

Life History Trade-Offs and Behavioral Sensitivity to Testosterone: An Experimental Test When Female Aggression and Maternal Care Co-Occur

Research on male animals suggests that the hormone testosterone plays a central role in mediating the trade-off between mating effort and parental effort. However, the direct links between testosterone, intrasexual aggression and parental care are remarkably mixed across species. Previous attempts to reconcile these patterns suggest that selection favors behavioral insensitivity to testosterone when paternal care is essential to reproductive success and when breeding seasons are especially short. Females also secrete testosterone, though the degree to which similar testosterone-mediated trade-offs occur in females is much less clear. Here, I ask whether testosterone mediates trade-offs between aggression and incubation in females, and whether patterns of female sensitivity to testosterone relate to female life history, as is often the case in males. I experimentally elevated testosterone in free-living, incubating female tree swallows (Tachycineta bicolor), a songbird with a short breeding season during which female incubation and intrasexual aggression are both essential to female reproductive success. Testosterone-treated females showed significantly elevated aggression, reduced incubation temperatures, and reduced hatching success, relative to controls. Thus, prolonged testosterone elevation during incubation was detrimental to reproductive success, but females nonetheless showed behavioral sensitivity to testosterone. These findings suggest that the relative importance of both mating effort and parental effort may be central to understanding patterns of behavioral sensitivity in both sexes.     

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.     

Compatibility drives female preference and reproductive success in the monogamous California mouse (Peromyscus californicus) more strongly than male testosterone measures

Female assessment of male attractiveness and how preferred qualities impact reproductive success is central to the study of mate choice. Male attractiveness may depend on traits beneficial to the reproductive success (RS) of any female, termed ‘universal quality’, and/or on behavioral and biological interactions between potential mates that reflect ‘compatibility’. The steroid hormone testosterone (T) often underlies male attractiveness in rodents and is associated with enhanced paternal care in the monogamous and biparental California mouse (Peromyscus californicus). We hypothesized that (1) T-characteristics are universally attractive to female California mice and that (2) if reproductive success is higher for females mated with preferred males, then females mated with males preferred by other females will also have higher reproductive success. Alternatively, we speculated that pair compatibility, based on emergent pair qualities, is important for a species with coordinated offspring care. We assessed individual T-characteristics in three ways: (1) T-response to GnRH challenges (2) baseline T-level and (3) T-response to a female. Testosterone-response did not predict female preference, but females spent more time investigating males with higher baseline T (accounting for only 9.6% of the variation in investigation time). None of the T-measures was associated with RS. Females paired with males they preferred produced litters more quickly and had higher RS than females paired with their non-preferred males. Naïve females who did not undergo preference tests had equivalent RS regardless of whether their mate was preferred or non-preferred by another female. These data suggest that higher male T elicits investigation, but female preference in the California mouse is more strongly linked with compatibility because individual preference was a better predictor of RS than any T measure.     

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.     

Endocrine correlates of intra-specific variation in the mating system of the St. Peter's fish (Sarotherodon galilaeus)

The Challenge Hypothesis postulates that androgen levels are a function of the social environment in which the individual is living. Thus, it is predicted that in polygynous males that engage in social interactions, androgen levels should be higher than in monogamous animals that engage in parental care. In this study, we tested this hypothesis at the intra-specific level using a teleost species, Sarotherodon galilaeus, which exhibits a wide variation in its mating system. Experimental groups of individually marked fish were formed in large ponds with different operational sex-ratios (OSR) to study the effects of partner availability on blood plasma levels of sex steroids [11-ketotestosterone (11-KT), testosterone (T), and 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P)] and gonadosomatic index (GSI). Polygyny mostly occurred in the female biased OSR groups. 17,20beta-P and gonadosomatic index did not differ among OSR groups. However, 11-KT was high in male biased OSR and positively correlated with aggressive challenges, thereby supporting the central postulate of the Challenge Hypothesis. The results of T were the inverse of those of 11-KT, probably because 11-KT is metabolized from T. 11-KT levels of polygynous males did not differ neither from those of monogamous males, nor from those of males that participated in parental care. These results do not support the expected relationships between polygyny, parental care, and androgen levels. The differences from expectations for 11-KT may be related to the fact that in S. galilaeus, the mating and the parenting phase are not clearly separated and thus, males may still fight and court while they are brooding.     

Why do pied flycatcher females mate with already-mated males?

The contribution of the pied flycatcher Ficedula hypoleuca male in parental care was studied to examine why some females mate with already-mated males. No difference in feeding rate was found between older and yearling monogamous males, when comparing nests at the same time. Monogamous and primary females were helped significantly more in parental care by the male than were secondary females of polygynous males. Females could only partly compensate for the absence of a male and of nestlings were reduced in nests with low male assistance. Differences in mate and territory quality were far too slight to make it advantageous for females to choose already-mated males instead of mating with monogamous males. We suggest that males, by being polyterritorial, deceive females into accepting polygyny; and females can be deceived since they do not have time to find out the marital status of males.     

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.     

Factors limiting polygyny in the willow ptarmigan

Willow ptarmigan (Lagopus lagopus) usually pair monogamously and males invest more in vigilance and parental care than do males of other grouse species. I tested whether polygyny is rare because male parental care is necessary for successful reproduction. By continuous removal of males I skewed the operational sex ratio, induced polygyny, and then compared breeding success and survival of females that shared a mate and those that did not. Both groups had similar clutch sizes, laid eggs at the same time, produced young of the same weight, and fledged similar numbers of juveniles. Hens that shared a mate suffered higher losses of nests during incubation, and fewer returned to the breeding range in subsequent years. Thus, although male parental care was not essential, it did improve female reproductive success and survival. I suggest that willow ptarmigan are not polygynous because territorial females are able to prevent potential secondary hens from settling, thus securing unshared access to male investment and territorial resources.     

Variation in testosterone levels and male reproductive effort: Insight from a polygynous human population

Recent evidence suggests that, in humans, variations in testosterone (T) levels between men reflect their differential allocation in mating versus parenting efforts. However, most studies have been conducted in urbanized, monogamous populations, making generalizations from them questionable. This study addresses the question of whether indicators of male reproductive effort are associated with variations in salivary T levels in a polygynous population of agriculturists in rural Senegal. We first show that pair-bonding and/or transition to fatherhood is associated with T profiles: married fathers (N = 53) have lower morning and afternoon T levels than unmarried non-fathers (N = 28). Second, among fathers, individual differences in parenting effort, as well as variations in mating effort, predict morning T levels. Indeed, men highly investing in parental care show lower morning T levels. Moreover, among men under 50, polygynous men show higher morning T levels than monogamous men. Taken together with previous results in monogamous settings, these findings suggest that the endocrine regulation of reproductive effort is probably a general feature of human populations.     

The Role of Female Behavior in Maintaining Monogamy of a Coral-Reef Filefish

The corallivorous filefish Oxymonacanthus longirostris is a predominantly monogamous species without parental care. Polygyny reduces female fitness because monogamous females benefit from male assistance with feeding–territory defense that secures both a feeding area and time to feed. I studied the strategy of females to prevent polygyny, with the help of field observations and manipulative field experiments. Monogamous females always behaved aggressively towards intruding floater females. When a caged male or female was placed within the territory of a monogamous pair, the resident female was more likely to pay attention to the caged female than to the caged male. Experimentally transplanted floater females succeeded in settling as secondary females in the monogamous territory more frequently when the resident female was caged. These results suggest that monogamous females behave aggressively towards floater females to defend their mating status; such aggressive behavior is successful in preventing polygyny. Furthermore, the removal experiment showed that polygynous (especially secondary) females moved to territories of widowed males located near them more frequently than monogamous females did. This outcome indicates that polygynous females can change their mating status and escape polygyny by moving to another territory if the opportunity occurs. Thus, female behavior can play an important role in the maintenance of monogamy and in lowering the level of polygyny within populations of this species.     

A phylogenetically controlled test of hypotheses for behavioral insensitivity to testosterone in birds

In most male birds that exhibit paternal care, extending the spring testosterone (T) peak throughout the breeding season reduces nestling provisioning. However, in some species, this trade-off between high T and expression of paternal care is absent. For example, during some or all of the nestling period, T did not affect paternal behavior in Male Lapland longspurs (Calcarius lapponicus), chestnut-collared longspurs (Calcarius ornatus), and great tits (Parus major). Two ecological constraints have been hypothesized to drive insensitivity to T after eggs hatch: (1) a short breeding season that limits breeding opportunities, and (2) a need for paternal care to ensure reproductive success. However, because two of the three species that exhibit T insensitivity are closely related, potential phylogenetic confounds limit determination of which, if either, factor constrains some males to T insensitivity. We examined the effects of supplementary T on paternal behavior in the Snow Bunting (Plectrophenax nivalis), a member of the monophyletic Calcarius/Plectrophenax clade. Male Snow Buntings are constrained to a short breeding season, but paternal care is not essential for survival of nestlings. We administered exogenous T during the parental phase to mimic the early spring T peak. T treatment increased song rates and interfered with paternal behavior such that nestlings of T-implanted males grew more slowly than controls. Our data suggest that T insensitivity in this clade is related to relatively recent constraints of the breeding environment (i.e., not simply common ancestry) and that the necessity of paternal care in some species may be a strong selective factor driving behavioral insensitivity to T during the parental phase.     

Polygyny in the tree swallow Tachycineta bicolor: a result of the cost of searching for an unmated male

The costs imposed by parental care duties on an individual's future survival and reproduction generate conflicts because parents should attempt to minimize their investment in the present brood, and exploit the parental care of the other parent. This conflict is likely to contribute to cases of both polygamy and desertion. Here, we study the costs of polygyny in the tree swallow Tachycineta bicolor , using observations on 52 nests that were attended by polygynous males over 14 y. Both females mated to polygynous males paid reproductive costs at several stages of the nesting cycle. Clutches laid by social mates of polygynous versus monogamous males did not differ in size. However, initial brood sizes for polygynously mated females were lower because a higher proportion of their eggs failed to hatch. Likewise, fledging success was lower and nest predation rates were higher, perhaps reflecting the direct or indirect effects of reduced male attention. These results demonstrate that females pay a productivity cost when breeding with reduced male parental care. In contrast, polygynous males fledge on average more young than monogamous ones and clearly benefit from the association. We suggest that a mate-search cost is leading to the few cases of polygamous males: although females are likely evaluating males for their prospective dedication to the breeding attempt, in a short-lived bird with a short breeding season, the cost to females of searching for a more dedicated male is the risk of not breeding at all.     

Preliminary analysis of reproductive success in a large mammal with alternative mating tactics,the Northern chamois,Rupicapra rupicapra

In polygynous mating systems, reproductive skew depends on the ability of males to monopolize females, which in turn may promote the development of contrasting traits in the two sexes. Although dominant individuals normally enjoy a higher reproductive success (RS) than subordinates, the use of genetic markers has shown that behavioural observations of male mating success may not provide reliable clues of RS. We report the preliminary results of the first DNA‐based paternity analysis on the Northern chamois (Rupicapra rupicapra), a scarcely dimorphic mountain ungulate described as highly polygynous, in relation to mating tactic and age. Because of sampling difficulties, the success in parentage assignment was low, and the interpretation of results requires caution. Territorial males had a greater RS than nonterritorial ones but they were unable to monopolize mating events. Age had a weak effect on paternity outcome but only males ≥ 6 years showed siring success. Although future studies are needed to assess the opportunity for sexual selection in male chamois, the concurrence of limited sexual size dimorphism, compensatory growth, unbiased sex‐specific survival, RS of alternative mating tactics and, possibly, long breeding tenure, may hint at the adoption of a conservative mating strategy in this species.     

No evidence of inbreeding avoidance despite demonstrated survival costs in a polygynous rodent

Individuals are generally predicted to avoid inbreeding because of detrimental fitness effects. However, several recent studies have shown that limited inbreeding is tolerated by some vertebrate species. Here, we examine the costs and benefits of inbreeding in a largely polygynous rodent, the yellow-bellied marmot (Marmota flaviventris). We use a pedigree constructed from 8 years of genetic data to determine the relatedness of all marmots in our study population and examine offspring survival, annual male reproductive success, relatedness between breeding pairs and the effects of group composition on likelihood of male reproduction to assess inbreeding in this species. We found decreased survival in inbred offspring, but equal net reproductive success among males that inbred and those that avoided it. Relatedness between breeding pairs was greater than that expected by chance, indicating that marmots do not appear to avoid breeding with relatives. Further, male marmots do not avoid inbreeding: males mate with equal frequency in groups composed of both related and unrelated females and in groups composed of only female relatives. Our results demonstrate that inbreeding can be tolerated in a polygynous species if the reproductive costs of inbreeding are low and individuals that mate indiscriminately do not suffer decreased reproductive success.     

Variable Social Mating System in the Sedge Warbler,Acrocephalus schoenobaenus

We studied the mating system in a local population of colour-ringed sedge warblers in south Central Sweden in 1990–92. Of 58 territorial males, 59% were socially monogamous, 14% socially polygynous and 27% unpaired. Socially polygynous males in general paired with two females: the only exception was one male that formed pair bonds with three females. Among the males that formed a pair bond, 38% resumed singing after their first female had started egg-laying or incubation and 50% of the paired males that resumed singing succeeded in attracting a second female. Hence, despite a consistently male-biased sex ratio in the population a large proportion of the males tried to become polygynous and they were often successful. The frequency of extra-pair matings did not differ between monogamous and polygynous males. Of 47 breeding females, 6.4% were sequentially socially polyandrous. In two out of three cases, the females fed the young of their first broods until independence before initiating the second brood. In the third case the female deserted her newly fledged young and these were instead cared for by a neighbouring male. DNA fingerprinting revealed that this male had not sired any of these young. Each of the sequentially polyandrous females successfully raised both their broods, and their annual reproductive success was slightly higher than the average for the polygynous males. When the sequentially socially polyandrous females initiated their second brood, their primary male (in all cases polygynous males), cared for young in their secondary female's nest. In all cases, the sequentially polyandrous females formed second pair bonds with unpaired males that were close neighbors. This suggests that females switched pair male for their second brood to obtain a mate that was more likely to provide them with direct benefits (e.g. parental care).     

Testosterone and avian life histories: the effect of experimentally elevated testosterone on corticosterone and body mass in dark-eyed juncos.

To assess whether alterations in the normal pattern of testosterone (T) secretion might be beneficial or detrimental, we studied a breeding population of dark-eyed juncos in which we elevated T experimentally and measured its effect on potential correlates of fitness. We treated both free-living and captive males with implants that were either empty (C-males, controls) or packed with T (T-males, experimentals). Timing of implant varied and was designed to mimic natural peak breeding levels except that peaks were either prolonged or premature. We bled the birds at recapture and analyzed their plasma, and that of their female mates, for T and corticosterone (B). We also measured body mass and fat score in free-living T- and C-males. In the field, T-implants elevated T and kept it elevated for at least a month. Experimental males also had higher B than controls. In captives, the effect of the implants on plasma T was detectable within 24 hr. B in captive T-males was again higher than in captive C-males. In females, neither T nor B differed between mates of T- and C-males. T-males implanted in early spring lost more mass between implant and recapture in late spring than did controls and also had lower fat scores when recaptured. When implants were inserted in summer, treatment did not influence mass. Elevated T in early spring apparently hastened the transition from the winter to the breeding mode of fat storage. We suggest that prolonged elevation of testosterone might be selected against because of the association between T and B. Premature elevation of T might be costly because of the resultant loss of mass and fat reserves, which could lead to mortality when spring snowstorms prevent access to food.