Testosterone decreases the potential for song plasticity in adult male zebra finches

Zebra finches are age-limited learners; males crystallize their songs at 90 days and do not subsequently alter those songs. However, a variety of interventions, including deafening and syringeal denervation, result in long-term changes to the crystallized song. These changes can be prevented by lesioning nucleus LMAN. As different social contexts for song production result in differential activation of LMAN, we asked whether the social context experienced by adult males would affect their ability to alter their songs in response to syringeal denervation. Males able to see and direct their songs to females made fewer changes to their songs than did males that could hear but not see females, but this trend was not significant. The volume of a male's HVc, a forebrain song control nucleus, also failed to predict the degree to which a male would change his song. However, testis mass was significantly correlated with the number of changes made to the song, indicating that variations in testosterone modulate adult song plasticity. We directly tested the effect of circulating testosterone on adult song plasticity by implanting adult males with either testosterone or flutamide, a testosterone receptor blocker, and tracking song changes triggered by ts nerve injury. As predicted, males implanted with testosterone changed their songs less than did males that received flutamide implants. These results suggest that the high testosterone concentrations associated with sexual maturity and song crystallization in zebra finches continue to act in adult males to reduce the potential for vocal plasticity.     

Effects of testosterone and corticosterone on immunocompetence in the zebra finch

The original immunocompetence handicap hypothesis (ICHH) suggested that testosterone has a handicapping effect in males by both promoting the development of sexual signals and suppressing immune function. A modified version, the stress-linked ICHH, has recently proposed that testosterone is immunosuppressive indirectly by increasing production of corticosterone. To test both the original and stress-mediated versions of the ICHH, we implanted male zebra finches taken from lines selected for divergent maximum stress-induced levels of corticosterone (high, low and control) with either empty or testosterone-filled implants. Their humoral and cell-mediated immune responses were then assessed by challenge with diphtheria:tetanus vaccine and phytohemagglutinin respectively. We found no effect of the hormone manipulations on either PHA or tetanus antibody responses, but found a significant interaction between titers of both testosterone and corticosterone on diphtheria secondary antibody response; antibody response was greatest in individuals with high levels of both hormones. There was also a significant interactive effect between testosterone treatment group and corticosterone titer on body mass; the body mass of males in the elevated testosterone treatment group decreased with increasing corticosterone titer. These results suggest that, contrary to the assumption of the stress-mediated version of the ICHH, high plasma levels of corticosterone are not immunosuppressive, but are in fact immuno-enhancing in the presence of high levels of plasma testosterone. Equally, the central assumption of the ICHH that testosterone is obligately immunosuppressive is also not supported. The same individuals with the highest levels of both hormones and consequently the most robust antibody response also possessed the lowest body mass.     

Effect of isolation and conspecific presence in a novel environment on corticosterone concentrations in a social avian species, the zebra finch (Taeniopygia guttata)

Zebra finches are a highly social and monogamous avian species. In the present study, we sought to determine the effect of social isolation (separation from the flock) in a novel environment with and without a conspecific present on the adrenocortical activity of paired and unpaired individuals of this species. With regard to paired birds, we hypothesized that the presence of the mate during isolation from the group would act as a social buffer against the stressful effects of isolation. We observed that 10 but not 30 minutes of social isolation resulted in elevated concentrations of corticosterone in unpaired and paired male zebra finches in comparison to baseline concentrations of corticosterone. Furthermore, the presence of a mate during isolation in a novel environment did not have a buffering effect against increases in corticosterone concentrations. Additionally, to compare concentrations of corticosterone in response to isolation (in a novel environment) to a previously well-established stressor, we subjected groups of birds to restraint. We observed that 10 or 30 minutes of restraint led to significantly higher concentrations of corticosterone as compared to baseline. Finally, to rule out the possibility that merely handling a bird would result in significantly elevated concentrations of corticosterone as compared to baseline samples, we measured corticosterone concentrations 10 or 30 minutes after handling involving capture and release only. Our results suggest that handling alone might have contributed to the elevation of corticosterone in birds exposed to 10 minutes but not 30 minutes of restraint. Handling by itself did not account, however, for the elevated corticosterone in birds socially isolated for 10 minutes.     

Speed of exploration and risk-taking behavior are linked to corticosterone titres in zebra finches

The existence of consistent individual differences in behavioral strategies ("personalities" or coping styles) has been reported in several animal species. Recent work in great tits has shown that such traits are heritable and exhibit significant genetic variation. Free-living birds respond to environmental stresses by up-regulating corticosterone production. Behavior during mild stress can occur in accordance to two types of coping styles, i.e. active and passive. Using artificially selected lines of zebra finches that vary in the amount of corticosterone produced in response to a manual restraint stressor we ran three "personality" experiments. We show that birds in the different corticosterone lines differ in their exploratory and risk-taking behaviors. There was an increase in exploratory behavior as corticosterone titre increased but only in the low corticosterone line. Birds in high corticosterone line showed greater risk-taking behavior than birds in the other lines. Thus, in general, higher levels of circulating corticosterone following a mild stress result in greater exploratory behavior and greater risk taking. This study shows that lines of animals selected for endocrine hormonal responses differ in their "coping" styles or "personalities".     

Social facilitation of male song by male and female conspecifics in the zebra finch, Taeniopygia guttata

Zebra finches are a ubiquitous model system for the study of vocal learning in animal communication. Their song has been well described, but its possible function(s) in social communication are only partly understood. The so-called ‘directed song’ is a high-intensity, high-performance song given during courtship in close proximity to the female, which is known to mediate mate choice and mating. However, this singing mode constitutes only a fraction of zebra finch males’ prolific song output. Potential communicative functions of their second, ‘undirected’ singing mode remain unresolved in the face of contradicting reports of both facilitating and inhibiting effects of social company on singing. We addressed this issue by experimentally manipulating social contexts in a within-subject design, comparing a solo versus male or female only company condition, each lasting for 24 h. Males’ total song output was significantly higher when a conspecific was in audible and visible distance than when they were alone. Male and female company had an equally facilitating effect on song output. Our findings thus indicate that singing motivation is facilitated rather than inhibited by social company, suggesting that singing in zebra finches might function both in inter- and intrasexual communication.     

Effects of 17-β-estradiol treatment of female zebra finches on offspring sex ratio and survival

Treatment of female zebra finches (Taeniopygia guttata) with 17-beta-estradiol leads to a female-biased sex ratio in their offspring at the age of independence [Horm. Behav. 35 (1999) 135]. It is unclear whether this is due to a bias of the primary sex ratio or to sex-specific survival. We replicated this experiment and found again a significantly higher total number of daughters than sons at independence in the estradiol-treated group. This was due to higher embryonic survival of daughters compared with sons in the estradiol-treated group and the reverse in the control group. There was no effect of the hormone treatment on the primary sex ratio. Treatment with 17-beta-estradiol led to a significantly shorter hatching time and to heavier offspring at day 7 after hatching. This weight was correlated with maternal plasma estradiol levels on the day of the first egg, which were significantly higher in the estradiol-treated group than in the control group. The results do not support the idea that maternal estradiol levels influence the primary sex ratio. They indicate that maternal estradiol differentially affects survival of sons and daughters via an influence on the embryonic environment, possibly enhancing offspring growth.     

Song recognition memory in juvenile zebra finches: effects of varying the number of presentations of heterospecific and conspecific songs

Seven male and three female zebra finches were exposed to 14 zebra finch (CON) and 14 starling (HET) songs during their sensitive period for song learning and then tested for their recognition memory of both the CON and HET songs in two separate memory tests. Amount of song exposure was varied by presenting individual songs either 3, 9, 27, or 81 times per day for nine consecutive days. After song exposure the birds were trained to discriminate two of the exposed, familiar songs (FAM) from two novel songs (NOV) in a go/no-go operant discrimination procedure, with FAM songs as "go" stimuli. Following discrimination training, untrained FAM and NOV songs were presented as probe songs without reinforcement. Birds responded more to FAM than NOV songs at all levels of song exposure, indicating that the songs were recognized. There were no differences in recognition memory for CON and HET song at any level of song exposure. The results suggest that selective song learning does not result from selective memorization of conspecific song.     

Tactile contact is required for early estrogen treatment to alter the sexual partner preference of female zebra finches

Sex steroid actions during early development appear to play a role in the development of sex differences in sexual partner preference (SPP, preference for males vs. females) in several species of mammals and in the socially monogamous pair bonding zebra finch (Taeniopygia guttata). Female finches treated with estrogen as nestlings exhibit varying degrees of masculinized SPP as adults, but only if they have been housed in all-female groups during the juvenile and young adult period, suggesting that the estrogen effect may involve social experience and possibly sexual imprinting. Because tactile contact is important for consolidation of imprinted preferences in this species, it was predicted that early estrogen treatment would alter preferences of females only if they were allowed to have tactile contact with other females. Subjects were injected with estradiol benzoate or with oil (normal controls) daily for the first 2 weeks post-hatching. At age 45 days, they were housed in a mixed sex aviary (normal controls), in an all-female aviary allowing tactile contact (group EB-TC), or in an all-female aviary with no tactile contact (group EB-NTC). At 100+ days, birds were given two-choice SPP tests followed by aviary tests of SPP. EB-TC females did not show the sex-typical preference for male stimuli, and differed significantly from the controls on several measures. EB-NTC females preferred males and never differed significantly from controls. These results show that tactile contact after age 45 days is essential for an EB effect on SPP, supporting the hypothesis that hormones and sexual imprinting together contribute to SPP.     

Vasotocin treatment inhibits courtship in male zebra finches; concomitant androgen treatment inhibits this effect

Zebra finches evolved in arid areas of Australia. Their reproduction is stimulated by water availability, which is unpredictable. Cheng (Poult. Sci. Rev. 5 (1993) 37) hypothesized that the primary mechanism controlling reproduction in species relying on unpredictable cues should be inhibitory. The onset of stimulatory environmental conditions terminates the inhibition, allowing rapid initiation of reproduction. As the primary hormone regulating water balance in birds, arginine vasotocin (AVT) appears a likely candidate to modulate reproduction in finches. Drought conditions cause sustained AVT release, which in other species inhibits androgen production. To determine whether increased AVT inhibits reproductive behavior, intact males were tested with females and divided into three groups matched for courtship behavior. Osmotic minipumps containing (a) saline, (b) 264 ng AVT, or (c) 1320 ng AVT in saline were implanted subcutaneously and males tested 48 h later. AVT-treated males socialized with females, but the high dose significantly reduced singing and courtship displays. To determine whether AVT acted by depressing androgen secretion, additional males were given subcutaneous androgen implants and divided into two groups matched for courtship behavior. Males were then implanted with minipumps containing (a) saline or (b) the high AVT dose. Males treated with AVT plus androgen showed no deficits in courtship behavior. These data suggest that AVT secretion during periods of drought may inhibit reproduction by inhibiting androgen production. Inhibition of reproductive behavior by AVT may be a more general phenomenon. Large quantities of AVT or, in mammals the closely-related peptide vasopressin (VP), are released when animals are stressed, and high levels of AVT/VP may inhibit reproductive behavior. The extremely short half-life of these peptides means that once proximal factors become more favorable, the gonads should rapidly be released from the peptides' inhibitory actions.     

Carotenoids modulate the trade-off between egg production and resistance to oxidative stress in zebra finches

The allocation of resources to reproduction and survival is a central question of studies of life history evolution. Usually, increased allocation to current reproduction is paid in terms of reduced future reproduction and/or decreased survival. However, the proximal mechanisms underlying the cost of reproduction are poorly understood. Recently, it has been shown that increased susceptibility to oxidative stress might be one of such proximate links between reproduction and self-maintenance. Organisms possess a range of antioxidant defenses, including endogenously produced molecules (e.g., enzymes) and compounds ingested with food (e.g., carotenoids). If reproductive effort increases the production of reactive oxygen species, the availability of antioxidant defenses may partly or fully counteract the free-radical damages. One could, therefore, expect that the trade-off between reproduction and oxidative stress is modulated by the availability of antioxidant defenses. We tested this hypothesis in zebra finches. We manipulated reproductive effort by either allowing or preventing pairs to breed. Within each breeding or non-breeding group, the availability of antioxidant compounds was manipulated by supplementing or not supplementing the drinking water with carotenoids. We found that although birds in the breeding and non-breeding groups did not differ in their resistance to oxidative stress (the breakdown of red blood cells submitted to a controlled free-radical attack), one aspect of breeding effort (i.e., the number of eggs laid by birds in both breeding and non-breeding groups) was negatively correlated with resistance to oxidative stress only in birds that did not benefit from a carotenoid-supplemented diet. This result therefore suggests that carotenoid availability can modulate the trade-off between reproduction and resistance to oxidative stress.     

Age effect of deafening on stereotyped song maintenance in adult male bengalese finches Lonchura striata domestica

Birdsong is a complex learned vocal behavior that relies on auditory experience for development.However,it appears that among different species of close-ended songbirds,there are some variations in the necessity of auditory feedback for maintaining stereotyped adult song.In zebra finches,the deterioration of adult songs following deafness depends on the birds' age.It is unknown whether this age effect is a general rule in other avian species as well.Therefore,we chose Bengalese finches,whose songs show more...     

Mate recognition by female zebra finch: analysis of individuality in male call and first investigations on female decoding process

Zebra finches are monogamous birds living in large assemblies, which represent a source of confusion for recognition between mates. Because the members of a pair use distance calls to remain in contact, call-based mate recognition is highly probable in this species. Whereas it had been previously demonstrated in males [Vignal, C., Mathevon, N., Mottin, S., 2004. Audience drives male songbird response to mate's voice. Nature 430, 448-451], call-based mate recognition remained to be shown in females. By analysing the acoustic structure of male calls, we investigated the existence of an individual signature and identified the involved acoustic cues. We tested to see if females can identify their mates on the basis of their calls alone, and performed preliminary experiments using modified signals to investigate the acoustic basis of this recognition. Playback tests carried on six individuals showed that a female zebra finch is able to perform the call-based recognition of its mate. Our experiments suggested that the female uses both the energy spectrum and the frequency modulation of the male signal. More experiments are now needed to decipher precisely which acoustic cues are used by females for recognition.     

Effects of central vasotocin and mesotocin manipulations on social behavior in male and female zebra finches

Male and female zebra finches (Taeniopygia guttata; total n = 40) were fitted with chronic guide cannulae directed at the lateral ventricle and were tested for aggression, affiliation, and partner preference following infusions of mesotocin (MT), vasotocin (VT), their antagonists, and vehicle control. Aggressive behavior was tested in a mate competition paradigm and tests of intersexual affiliation and partner preference were conducted following 1 day of cohabitation with an opposite-sex individual. These tests also provided data on male courtship singing. The results demonstrate a modest dose-dependent facilitation of aggression by VT, but not MT, in both male and female finches. However, only males were sensitive to infusions of a vasopressin antagonist, suggesting that endogenous VT is more important for behavioral modulation in males. Peptide effects were specific to aggression, as no treatments influenced intersexual affiliation, partner preference, or male courtship singing. Thus, in contrast to rodents, partner preference is not readily induced by VT or MT in this species. However, the potential necessity of endogenous VT and MT for natural pair-bond formation remains to be tested.     

Use of labelled methionine to investigate the contribution of muscle proteins to egg production in zebra finches

Many bird species show a loss of female muscle mass at the time of egg formation. In this study we investigated whether there was a causal link between the loss in muscle condition and the formation of egg proteins by feeding ³⁵S-methionine to female zebra finches to label muscle proteins. When these birds subsequently bred the isotope was transferred to the egg proteins: isotope loss from female muscle tissue was significantly greater in birds which had bred than in control groups which had not.     

Food, stress, and reproduction: Short-term fasting alters endocrine physiology and reproductive behavior in the zebra finch

Stress is thought to be a potent suppressor of reproduction. However, the vast majority of studies focus on the relationship between chronic stress and reproductive suppression, despite the fact that chronic stress is rare in the wild. We investigated the role of fasting in altering acute stress physiology, reproductive physiology, and reproductive behavior of male zebra finches (Taeniopygia guttata) with several goals in mind. First, we wanted to determine if acute fasting could stimulate an increase in plasma corticosterone and a decrease in corticosteroid binding globulin (CBG) and testosterone. We then investigated whether fasting could alter expression of undirected song and courtship behavior. After subjecting males to fasting periods ranging from 1 to 10 h, we collected plasma to measure corticosterone, CBG, and testosterone. We found that plasma corticosterone was elevated, and testosterone was decreased after 4, 6, and 10 h of fasting periods compared with samples collected from the same males during nonfasted (control) periods. CBG was lower than control levels only after 10 h of fasting. We also found that, coincident with these endocrine changes, males sang less and courted females less vigorously following short-term fasting relative to control conditions. Our data demonstrate that acute fasting resulted in rapid changes in endocrine physiology consistent with hypothalamo-pituitary-adrenal axis activation and hypothalamo-pituitary-gonadal axis deactivation. Fasting also inhibited reproductive behavior. We suggest that zebra finches exhibit physiological and behavioral flexibility that makes them an excellent model system for studying interactions of acute stress and reproduction.     

Sex steroids modulate changes in social and sexual preference during juvenile development in zebra finches

Zebra finches, like many other animals, have close social relationships mainly with the family at young ages but begin to express interest in opposite-sex extra-family animals as they enter the late juvenile period and sexual maturity. This experiment tested a set of hypotheses that sex steroids are involved in this developmental transition. At 25-30 days, subjects were implanted subcutaneously with Silastic tubes that were empty (controls), filled with testosterone propionate, filled with estradiol benzoate, or filled with a combination of ATD (an aromatization inhibitor) and flutamide (an anti-androgen). Once a week between ages 30 and 90 days, they were given three-choice tests where the three stimulus types were the family members, unpaired males, or unpaired females. The preferred category was defined as the one adjacent to the proximity zone in which the subject spent the most time. Control males were more likely to prefer females and less likely to prefer the family as they got older, and control females were increasingly likely to prefer males. Males treated with testosterone or estradiol showed a premature increase in preferences for females. Females treated with ATD plus flutamide failed to show the normal increase in preferences for males shown by controls. These results indicate an involvement of sex steroids in the maturation of sexual preferences in a socially monogamous species that relies on visual and auditory, rather than olfactory, cues for sexual or other social behavior.