Haruka Wada

Latitudinal variation in plasma testosterone levels in birds of the genus Zonotrichia

Birds breeding in northern latitudes generally have elevated plasma testosterone levels throughout the breeding season with a peak at the onset of the breeding season. In contrast, tropical birds tend to have extremely low plasma testosterone levels year round with only a slight increase during breeding. While these patterns have been consistent in the species investigated, closely related species have not been investigated across a range of latitudes. Birds of the genus Zonotrichia present an ideal opportunity to investigate latitudinal variation in plasma testosterone levels as breeding populations occur from northern Alaska to southern Argentina. We studied three taxa of Zonotrichia: (1) Gambels white-crowned sparrows, Zonotrichia leucophrys gambelii, breeding at high latitude in northern Alaska, (2) Puget Sound white-crowned sparrows, Z. l. pugetensis, breeding at mid-latitude in Washington state, and (3) an equatorial population of the rufous-collared sparrow, Z. capensis, in Ecuador. To compare both baseline breeding and maximal testosterone levels, males from the three taxa were either bled immediately upon capture during the breeding season or first challenged with gonadotropin-releasing hormone (GnRH) and then bled. As a control, another group of birds experienced a saline challenge. Two-way ANCOVA, with treatment and taxa as factors and testis volume as a covariate, showed a significant effect of treatment on plasma testosterone levels. There was also a significant interaction between taxa and treatment. Contrary to expectations, the equatorial species exhibited greater plasma testosterone levels in response to the GnRH challenge than the high latitude species. There were no differences between the mid- and high-latitude species. The equatorial species had the smallest average testis size and within each taxa there were no relationships between plasma testosterone and testis size. These data suggest that latitudinal clines in plasma testosterone levels in Zonotrichia do not follow previously described patterns and that tropical birds can have levels of testosterone similar to northern latitude species.

Adrenocortical responses in zebra finches (Taeniopygia guttata): Individual variation, repeatability, and relationship to phenotypic quality

Although individual variation is a key requirement for natural selection, little is known about the magnitude and patterns of individual variation in endocrine systems or the functional significance of that variation. Here we describe (1) the extent and repeatability of inter-individual variation in adrenocortical responses and (2) its relationship to sex-specific phenotypic quality, such as song duration and frequency and timing of egg laying. We measured adrenocortical responses to a standardized stressor in zebra finches (Taeniopygia guttata) at two life history stages: approximately day 16 (nestlings) and 3 months of age (sexually mature adults). Subsequently, we assessed phenotypic (reproductive) quality of all individuals as adults. Marked inter-individual variation in the adrenocortical response was seen in both sexes and ages, e.g., stress-induced corticosterone ranged from 2.2 to 62.5 ng/mL in nestlings and 5.0-64.0 ng/mL in adults. We found sex differences in (a) inter-individual variation in the adrenocortical response, (b) repeatability, and (c) relationships between corticosterone levels and phenotypic quality. In males, variation in nestling corticosterone was weakly but positively correlated with brood size and negatively correlated with nestling mass (though this relationship was dependent on one individual). There was no significant correlation of adrenocortical responses between two stages in males and adult phenotypic quality was significantly correlated only with adult corticosterone levels. In contrast, in females there was no relationship between nestling corticosterone and brood size or mass but adrenocortical response was repeatable between two stages (r2=0.413). Phenotypic quality of adult females was correlated with nestling baseline and adrenocortical response.

Territoriality and testosterone in an equatorial population of rufous-collared sparrows, Zonotrichia capensis

Territorial aggression, displayed by male vertebrates in a reproductive context, is generally thought to be mediated by testosterone. The challenge hypothesis predicts that in socially monogamous species, territorial challenges should induce an increase in plasma testosterone concentrations, which will enhance aggressive behaviour and territory defence. This hypothesis is based on northern latitude birds and needs to be tested in tropical birds before it can be universally accepted. We tested the challenge hypothesis in an equatorial population of rufous-collared sparrows in Papallacta, Ecuador. This population shows an extended breeding period during which males aggressively guard territories. During the early breeding season, males were challenged with conspecific or heterospecific simulated territorial intrusions (STIs) lasting 10 min. Conspecific-challenged males responded more aggressively than heterospecific-challenged males. However, there was no increase in plasma testosterone in response to the conspecific STI. During the breeding season, males were challenged with conspecific STIs lasting 0, 10 or 30 min. Males behaved aggressively regardless of STI duration, and did not differ in plasma testosterone or luteinizing hormone concentrations. During the breeding season, males were implanted with testosterone-filled or empty silastic tubes and subsequently challenged with a conspecific STI. Testosterone implants significantly raised plasma testosterone concentrations, but testosterone-implanted males were not more aggressive than blank-implanted controls. Combined, these findings suggest that testosterone concentrations above breeding baseline are not related to territorial aggression in this population and therefore do not support the challenge hypothesis.

Transient elevation of corticosterone alters begging behavior and growth of white-crowned sparrow nestlings

SUMMARY Developing animals may face a cost–benefit tradeoff during growth mediated through hormones such as glucocorticoids, as the hormone is essential for development but can have detrimental consequences. To investigate potential tradeoffs caused by brief, moderate elevations of corticosterone in avian young, we artificially elevated the hormone levels in two ways: feeding corticosterone-containing worms and applying corticosterone dermal patches. The former experiment tested the effects of an acute corticosterone elevation (25 min) on begging behavior, whereas the latter explored the effects of artificially elevated corticosterone for 24 to 48 h on growth. Corticosterone altered both begging behavior and growth of white-crowned sparrow nestlings. It increased latency to beg immediately after the treatment and suppressed growth as early as 24 h after the patch application. These experiments also showed that the effects depended on the age or types of development (e.g. gaining mass or growing feathers) that the nestlings were going through.

Stress Responses in Tropical Sparrows: Comparing Tropical and Temperate Zonotrichia

Seasonal modulation of the adrenocortical response appears to be ubiquitous in mid‐ to high‐latitude vertebrates but has not been investigated in tropical vertebrates. Previous studies demonstrate that temperate passerines show seasonality in corticosterone secretion and corticosteroid binding globulin capacities. We examined seasonal and sex differences in the stress response in an equatorial population of Zonotrichia capensis, the only Zonotrichia that breeds in the tropics, and compared the results with those of northern Zonotrichia. Seasonal differences in tropical Zonotrichia would presumably be independent of photoperiod and thus directly related to such activities as reproduction and feather molt. In addition, we investigated the possible role of binding globulin as a sex steroid binding globulin, as suggested for temperate passerines. Similar to northern congeners, Z. capensis show seasonal modulation in total corticosterone and binding globulin capacity with higher levels during breeding than molt. However, unlike many temperate passerines, there are no sex differences in corticosterone secretion or binding globulin capacity. Furthermore, the seasonal differences in total corticosterone diminish when the free levels are calculated. The contrast between equatorial and northern congeners indicates factors such as breeding environment and life‐history strategy may play important roles in shaping stress response in these species.

How acute is the acute stress response? Baseline corticosterone and corticosteroid-binding globulin levels change 24 h after an acute stressor in Japanese quail.

Changes in plasma corticosteroid-binding globulin (CBG) capacity can alter free plasma concentration and tissue availability of glucocorticoids (GC) and hence alter the organismal response to stress. However, CBG change in response to stress has not been extensively studied. While it is clear that chronic stress can causes CBG decline and in some species acute stressors can reduce CBG during the 30-60 min of the stressor, more long-term changes in CBG following an acute stressor has received less attention. Here we investigated corticosterone (CORT: the primary GC in birds) and CBG levels 24h after an acute stressor in a unique study system: Japanese quail divergently selected for CORT reactivity to acute stress. Using this model, we examined the interaction of selected CORT reactivity with CBG response to determine if CBG shows a delayed decline in response to an acute stressor and if that decline varies by selected genetic background. We found lowered CBG capacity, elevated total CORT and free CORT 24h after acute stress in all three quail groups. These results demonstrate for the first time in an avian species that exposure to an acute stressor can affect CBG and CORT 24h later.

Ontogeny and Individual Variation in the Adrenocortical Response of Zebra Finch (Taeniopygia guttata) Nestlings

Numerous studies indicate interspecies variation in the ontogeny of the adrenocortical response in birds; however, little is known about the extent of interindividual variation in avian young. Toward this end, we examined the ontogeny and interindividual variation in the adrenocortical response in zebra finch (Taeniopygia guttata) nestlings. We measured baseline and stress‐induced total (bound and free) corticosterone, corticosteroid binding globulin capacity, and resulting estimated free corticosterone levels in nestlings of four different ages (days 5, 10, 16, and 21). In addition, we investigated the potential correlates of interindividual variation (brood size and mass). Nestlings at days 5 and 10 post‐hatching showed no significant increase in total or free corticosterone levels in response to a standardized handling stress, whereas an adult‐like stress response was seen by day 16 post‐hatching. There was large interindividual (fivefold) variation in both baseline and stress‐induced corticosterone among individual nestlings at any age. We estimate that half of this individual variation in the adrenocortical response could be explained by between‐clutch variation (e.g., genetics), while the other half could be explained by other factors such as rearing environment (based on estimated intraclass correlation coefficients). Total baseline corticosterone, but not stress‐induced corticosterone, was negatively correlated with fledging mass in this species.

Juvenile Nutritional Stress Affects Growth Rate, Adult Organ Mass, and Innate Immune Function in Zebra Finches (Taeniopygia guttata)

Developmental conditions may influence many aspects of adult phenotype, including growth and immune function. Whether poor developmental environments impair both growth and immune function or induce a trade-off between the two processes is inconclusive, and the impact of the timing of stress in determining this relationship has so far been overlooked. We tested the hypothesis that the long-term effects of nutritional stress on growth, body composition, and immune function in zebra finches (Taeniopygia guttata) are different depending on whether stress is experienced during an early or a juvenile phase (i.e., before or after nutritional independence, respectively). We raised birds on high (H) or low (L) food conditions until posthatch day (PHD) 35 and switched treatments for half of the birds in each of the H and L groups from PHD 36 to 61. We found that unfavorable juvenile conditions (PHD 36–61) increased somatic growth rates and liver mass, body fat, and some aspects of immune function. We also observed a positive relationship between growth and immune function, as individuals that grew faster as juveniles also had better innate immune responses as adults. There was no effect of treatment on basal metabolic rate. These findings demonstrate the importance of juvenile developmental conditions in shaping multiple aspects of the adult phenotype.

Counterbalancing effects of maternal mercury exposure during different stages of early ontogeny in American toads.

Maternal transfer of environmental contaminants is a disadvantageous parental effect which can have long-lasting implications for offspring fitness. We investigated the effects of mercury (Hg) on the reproductive success of female amphibians and the subsequent effects of maternal transfer on the development of their offspring. American toads (Bufo americanus) maternally transferred Hg to their eggs, and there was a negative relationship between Hg concentrations and the percentage of viable hatchlings produced in clutches. However, when we continued to monitor larvae that successfully hatched, we found 21% greater metamorphic success in larvae from Hg-exposed mothers compared to reference larvae. The negative effect in the embryonic stage and positive effect in the larval stage counterbalanced one another, ultimately resulting in no difference in predicted terrestrial recruitment, regardless of maternal Hg exposure. Our findings demonstrate that maternal effects on survival manifesting at different stages in ontogeny have the potential to produce complicated outcomes.

Developmental changes in neural corticosteroid receptor binding capacity in altricial nestlings

Altricial nestlings typically do not show an adrenocortical response during the early post‐hatch period. This may be a result of an immature hypothalamic‐pituitary‐adrenal axis, or an enhanced control of the axis by negative feedback. To examine whether the dampened adrenocortical response is due to higher receptor densities in hypothalamus and hippocampus, the major sites for negative feedback and tonic inhibition, we explored the ontogenetic changes in glucocorticoid (GR) and mineralocorticoid receptor (MR) binding capacities in the brain of white‐crowned sparrow nestlings. During the 10‐day nestling period, MR binding capacity decreased with age, whereas GR capacity was not affected. In addition, this overall decline in MR levels was driven entirely by a decline in cerebellar MR. No age‐related changes were observed in hippocampal or hypothalamic areas. Our findings suggest that enhanced negative feedback does not play a major role in the attenuated adrenocortical responses seen in white‐crowned sparrow nestlings.