The effect of transdermal corticosterone application on plasma corticosterone levels in pregnant Lacerta vivipara.

Relationships between hormones and behaviour can be explored by altering endogenous hormone levels, often through implantation of silastic tubing or osmotic pumps filled with a hormone or its agonists or antagonists. However, organisms in sensitive life-history stages (such as pregnancy) may be adversely affected by the surgical procedures associated with these manipulations, necessitating use of non-invasive techniques. We demonstrate that the application of a sesame oil-corticosterone mixture to the skin of pregnant female common lizards (Lacerta vivipara) elevates plasma levels of the hormone. Pregnant female L. vivipara were captured and treated daily for 1-20 days with the sesame oil-corticosterone mixture (experimental group) or with vehicle only (control group). Blood samples were collected and analyzed for corticosterone by radioimmunoassay. Baseline plasma corticosterone levels were elevated within 1 h in the experimental group. Similar levels ( approximately 145 ng/ml) were found over the subsequent 2 days, and by day 5 had risen significantly higher ( approximately 281.9 ng/ml), where they remained for the duration of the experiment. These increases are comparable to those found in other species using related techniques. No significant changes in plasma corticosterone levels occurred in the control group. Finally, corticosterone levels also were determined for untreated females that were captured, held overnight, sampled, and released to access to the natural range of basal corticosterone levels. Basal plasma levels of corticosterone in pregnant females varied among individuals independently of female body size or corpulence.

Body condition and the adrenal stress response in captive American kestrel juveniles.

We examined the adrenal response to handling stress of birds in different body conditions. In order to affect the birds' body condition, young (73-d old) female American kestrels (Falco sparverius) were maintained for 6 wk on one of three diets: a control diet (fed ad lib.) and two calorically restricted diets. To invoke a stress response, we removed birds from their cages and took repeated blood samples over the course of an hour. All birds responded to handling stress with an increase in plasma corticosterone, but control birds (in good body condition) showed a more rapid increase to maximum corticosterone levels, followed by a decrease. Both groups of food-restricted birds had a slower rate of increase to maximum corticosterone levels and then maintained high corticosterone levels through 60 min. These results suggest that birds in good physical condition respond more quickly to stressors and adapt physiologically to stressful situations more rapidly than do birds in poor physical condition. This difference may reflect the ability of birds in good condition to mobilize fat for energy, while birds in poor condition must mobilize protein (i.e., muscle).

Corticosterone and the stress response in young western screech-owls: effects of captivity, gender, and activity period.

We used a standard handling protocol to examine the stress response of captive young western screech-owls during their active (nighttime) and inactive (daytime) periods and to compare the stress responses of captive and free-living owls. Circulating corticosterone levels were significantly higher during the inactive period than in the active period in this nocturnal species. This suggests that the daily pattern of corticosterone secretion is reversed in nocturnal birds and is correlated with activity period rather than with the light/dark cycle. Young (ca. 4-5 mo old) screech-owls of both sexes showed increases in plasma corticosterone up to 30 min after capture, followed by significant decreases at 60 min. This pattern is similar to those of other species of birds examined previously, except that decreases in corticosterone at 60 min rarely have been observed. Such decreases may be the result of physiological differences between adult and young birds, habituation to handling in captive birds, or the effects of body condition. Corticosterone levels and the response to capture and handling were comparable in captive and free-living owls, which suggests that the captive owls were not subjected to chronically high levels of stress.

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.

Testosterone and survival: a cost of aggressiveness?

Free-living male brown-headed cowbirds were captured during the breeding season, implanted with testosterone-filled Silastic tubing, and released. Radioimmunoassay verified that the implants maintained circulating plasma testosterone values at maximal breeding season levels well beyond the normal time of decline. Survival to the following year of these implanted males was compared with survival of unimplanted birds captured in other years, and also with survival of males given empty implants. Androgen-implanted male cowbirds exhibited significantly reduced survival to the following year compared with either of the control groups, and also exhibited severe injuries not seen in other years. It is suggested that the increased risks associated with prolonged high testosterone levels act as a selective force to maintain reduced androgen levels except during the period of aggressive intrasexual interactions that characterizes the reproductive season. Further, it is proposed that the nature and importance of the different risks vary with a species' mating system.

The role of sex steroids in the acquisition and production of birdsong.

Male birdsong is generally regarded as a secondary sexual characteristic under the control of gonadal steroids. Song typically waxes and wanes with the seasonal cycle of testicular growth and regression and decreases after adult castration. Testosterone therapy reinstates song, induces it in females, augments it in intact males, and spring testosterone profiles correlate with seasonal song production. Thus, testosterone has been viewed as a major factor in song acquisition and production acting either directly, or after aromatization within the brain. We show here, however, that song learning and early phases of the development of singing both take place in castrated male birds with no significant levels of testosterone in their blood plasma. Testosterone seems to be required for song crystallization, however. Oestradiol was unexpectedly still present after castration, evidently from a non-testicular source, throughout the period of male song acquisition.

Alarm signals in birds: the role of testosterone.

In the laboratory and under semi-naturalistic conditions, male domestic fowl (Gallus gallus) produce more alarm calls in response to predatory birds flying overhead than females. This study tests the hypothesis that testosterone is a factor in the control of aerial alarm call production. Birds were castrated early in life and tested as adults in the laboratory. Aerial predators were simulated by cardboard silhouettes drawn overhead. Blood samples were taken at intervals and plasma testosterone levels were measured by radioimmunoassay. Each subject was tested in four conditions: with an empty Silastic implant, with a testosterone-filled implant, with an additional testosterone implant, and after removal of the implants. Results show that the presence of testosterone increases the production of aerial alarm calls, and that removal of the hormone supply results in a drop in calling rate. As a control a second vocal system, food calling, was considered. In contrast with alarm calling, there was no correlation between testosterone levels and variation in food-call production. We conclude that testosterone plays a specific role in activation of the production by cockerels of vocalizations that signal the presence of aerial predators.

Identity of gonadotropin-releasing hormone in passerine birds: comparison of GnRH in song sparrow (Melospiza melodia) and starling (Sturnus vulgaris) with five vertebrate GnRHs.

Gonadotropin-releasing hormone (GnRH) was detected in the brains of passerine birds, a recently evolved and diverse avian group. The molecular forms of GnRH in two species of birds under breeding conditions were deduced using methods of HPLC and immunology. The brain extracts of song sparrows (Melospiza melodia) contained a form of GnRH identified as chicken I GnRH-like peptide by its HPLC elution pattern and cross-reactivity with four antisera. In contrast, starling (Sturnus vulgaris) brain extracts showed molecular heterogeneity of GnRH forms; equal amounts of chicken I and chicken II GnRH-like peptides were present. Neither bird contained GnRH that could be identified as mammalian, salmon, or lamprey GnRH. Chicken II GnRH-like peptide may not have evolved after the separation of the song sparrow and starling as both peptides are found in chicken, a more primitive bird. The possibility remains that different stages of the life cycle are associated with the expression of these GnRH-like peptides or their ratio. Only determination of the primary structure will establish whether our chromatographic and immunological evidence is correct that chicken I and II GnRH are present in passerine birds and have been conserved in representatives throughout the reptiles and birds. Starlings can be added now to the growing list of submammalian species that express multiple forms of GnRH in their brains.

The influence of social cues on the reproductive endocrinology of male brown-headed cowbirds: field and laboratory studies.

Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Free-living paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females.