Corticosterone profiles in northern cardinals (Cardinalis cardinalis): Do levels vary through life history stages?

As animals move through life history stages, energy requirements for each stage will vary. Both daily and annual variation in the glucocoriticoid hormones (specifically corticosterone, or CORT, in birds) helps provide the variable energy needed through life history stages. In many bird species, CORT is higher in the breeding season when energy demands can be quite high and is often lower in the non-breeding season. Additionally, CORT has a role to play in the response to stressful stimuli and the level to which CORT is elevated following stressful events can vary through the annual cycle as well. Here we report on baseline and stress-induced CORT levels in both sexes of northern cardinals, Cardinalis cardinalis, a non-migrating, year-round territorial species across life history stages. Corticosterone is overall higher in the non-breeding season than the breeding season in both sexes. Males tend to have higher levels of stress-induced CORT than females, although the observed patterns are complex. Our findings differ from one of the more common profile reported in songbirds where breeding season CORT tends to be higher than non-breeding levels. A strong influence may be the prolonged breeding season seen in cardinals; lower levels of CORT during breeding may guard against adverse maternal effects, interruptions in breeding behaviors, or egg production. Additional investigation of species with similar ecologies to northern cardinals, and more populations of cardinals, may show that annual glucocorticoid profiles are more labile than previously appreciated.

Sex-specific hypothalamic-pituitary-gonadal axis sensitivity in migrating songbirds.

In seasonally migratory species, the overlap between the migratory and breeding life history stages is a balance between the physiological and behavioral requirements of each stage. Previous studies investigating the degree to which songbirds prepare for breeding during spring migration have focused on either circulating hormone levels or direct measures of gonadal recrudescence. In this study, we evaluated the phenology of breeding preparation in a long-distance migratory songbird, the Swainson's Thrush (Catharus ustulatus), by assessing hypothalamic-pituitary-gonadal (HPG) axis sensitivity with gonadotropin-releasing hormone (GnRH) bioassays throughout the migratory period. During spring migration both males and females had a significant response to GnRH injections as reflected in elevated testosterone levels. The magnitude of response to GnRH injections, Rpotential, in females stayed consistent throughout spring migration; however, Rpotential in males increased as the migratory season progressed. It is clear that at least some degree of endocrinological breeding development occurs either before or during spring migration in both sexes, however the phenology appears to be sex specific. In males this breeding development continues at a relatively steady pace throughout the migratory period while in females, relatively little endocrine breeding development occurs during migration. These sex-specific differences in the phenology of the endocrine breeding development warrant future investigations for both male and female songbirds. Moreover, research focused on how physiological breeding development is balanced with the expression of migratory traits in long-distance songbird migrants is needed.

Cognition, personality, and stress in budgerigars, Melopsittacus undulatus.

To study the fitness effects of individual variation in cognitive traits, it is paramount to understand whether traits such as personality and physiological stress influence cognitive performance. We first tested whether budgerigars showed both consistent personalities and cognitive performance across time and tasks. We tested object and food neophobia, and exploratory behavior. We measured cognitive performance in habituation, ability to solve foraging problems, spatial memory, and seed discrimination tasks. Budgerigars showed consistency in their neophobic tendencies and these tendencies were associated with their exploratory behavior. Birds were also consistent in how they performed in most of the cognitive tasks (temporal consistency), but were not consistent in their performance across tasks (context consistency). Neither corticosterone levels (baseline and stress-induced) showed a significant relationship with either cognitive or personality measures. Neophobic and exploratory tendencies determined the willingness of birds to engage only in the seed discrimination task. Such tendencies also had a significant effect on problem-solving ability. Our results suggest that consistent individual differences in cognitive performance along with consistent differences in personality could determine response to environmental change and therefore have important fitness consequences.

Testosterone elevation and response to gonadotropin-releasing hormone challenge by male northern cardinals (Cardinalis cardinalis) following aggressive behavior.

There is much discrepancy about the relationship between testosterone (T) and male aggressive behavior. For example, in birds, males of many species significantly elevate T levels during inter-male conflict. However, this is not universal, and in species where males typically do not elevate T during aggressive interactions, concentrations of the hormone are often assumed to be circulating at maximum levels. We examined if male northern cardinals (Cardinalis cardinalis) significantly elevated T during simulated territorial intrusions (STIs). We also examined if individuals had the capacity to further elevate T levels in response to gonadotropin-releasing hormone (GnRH) injections immediately after an aggressive encounter. Our results indicate that male cardinals do not significantly elevate T levels in response to STIs, but have the physiological capacity to significantly elevate T in response to GnRH injections following aggressive interactions. This implies that T levels of individuals captured during STIs were not at maximum concentrations. However, additional findings in this study also suggest the possibility that prolonged social instability could elicit significant elevations in T in males of this species, warranting further investigation.

Non-breeding gonadal testosterone production of male and female northern cardinals (Cardinalis cardinalis) following GnRH challenge.

Yearly, testosterone (T) levels fluctuate as many vertebrates cycle through reproductive and non-reproductive periods. Among many temperate birds, it is well established that levels of T peak as gonads recrudesce for breeding and then fall as gonads regress prior to the non-breeding season. While the tissues producing breeding season T are well studied, the tissues responsible for non-breeding T have received less investigative attention. We examined the ability of male and female Northern Cardinals (Cardinalis cardinalis) to elevate gonadal T following standardized injections of gonadotropin-releasing hormone (GnRH) across three non-breeding seasons. Males and females were capable of significantly elevating gonadal T production following GnRH injections during periods of reproductive quiescence. The magnitude of T elevation varied across the non-breeding season, but not between sexes. To our knowledge, this is the first report of a significant increase in gonadal T production following GnRH injections administered in the non-breeding season.

Natural selection on testosterone production in a wild songbird population.

Because of their role in mediating life-history trade-offs, hormones are expected to be strongly associated with components of fitness; however, few studies have examined how natural selection acts on hormonal variation in the wild. In a songbird, the dark-eyed junco (Junco hyemalis), field experiments have shown that exogenous testosterone alters individuals' resolution of the survival-reproduction trade-off, enhancing reproduction at the expense of survival. Here we used standardized injections of gonadotropin-releasing hormone (GnRH) to assay variation in the testosterone production of males. Using measurements of annual survival and reproduction, we found evidence of strong natural selection acting on GnRH-induced increases in testosterone. Opposite to what would be predicted from the survival-reproduction trade-off, patterns of selection via survival and reproduction were remarkably similar. Males with GnRH-induced testosterone production levels that were slightly above the population mean were more likely to survive and also produced more offspring, leading to strong stabilizing selection. Partitioning reproduction into separate components revealed positive directional selection via within-pair siring success and stabilizing selection via extrapair mating success. Our data represent the most complete demonstration of natural selection on hormones via multiple fitness components, and they complement previous experiments to illuminate testosterone's role in the evolution of life-history trade-offs.

Seasonal and sex-related variation in song control nuclei in a species with near-monomorphic song, the northern cardinal.

Studies concerning the song control system (SCS) in songbirds generally focus on males due to their prodigious song production. Both seasonal and age related differences have been found in the size of male SCS regions. Among those studies that have addressed females some level of sexual size dimorphism has been found, with females generally having smaller SCS area than males. Among those species where female song has been studied, typically females either sing much less than males, or they duet with their mates, but in general do not produce independent song. Here we present information on seasonal and sex differences in SCS in the northern cardinal (Cardinalis cardinalis) a species where both sexes sing, females sing independent of their mates, and song is produced by males over a prolonged period of time (7-8 months). We collected brains from free-living adult cardinals, both in the late non-breeding season and during the early breeding season, and measured three song control nuclei; HVC, Area X and RA. There were sex differences in all three areas assessed at the two time points considered. Additionally, there was a seasonal difference in both sexes for all areas assessed. In both time points male SCS nuclei were 1.5-2.0 times larger than female SCS nuclei. These data show that even in those species with independent female song there may still exist sex differences in the SCS nuclei. Similarity in song between the sexes could be related to differences in hormone receptors or hormone levels in the brain, while the small-observed changes in SCS area in males may allow for early breeding season song production and song production outside of the breeding season.

Natural variation in a testosterone-mediated trade-off between mating effort and parental effort.

Male birds frequently face a trade-off between acquiring mates and caring for offspring. Hormone manipulation studies indicate that testosterone often mediates this trade-off, increasing mating effort while decreasing parental effort. Little is known, however, about individual covariation between testosterone and relevant behavior on which selection might act. Using wild, male dark-eyed juncos (Junco hyemalis), we measured individual variation in testosterone levels before and after standardized injections of gonadotropin-releasing hormone (GnRH). The GnRH challenges have been shown to produce short-term testosterone increases that are similar to those produced naturally in response to social stimuli, repeatable in magnitude, and greater in males with more attractive ornaments. We correlated these testosterone increases with behavioral measures of mating and parental effort (aggressive response to a simulated territorial intrusion and nestling feeding, respectively). Males that showed higher postchallenge testosterone displayed more territorial behavior, and males that produced higher testosterone increases above initial levels displayed reduced parental behavior. Initial testosterone levels were positively but nonsignificantly correlated with aggression but did not predict parental behavior. These relationships suggest that natural variation in testosterone, specifically the production of short-term increases, may underlie individual variation in the mating effort/parental effort trade-off. We discuss the implications of these results for the evolution of hormonally mediated trade-offs.

Seasonal and individual variation in response to GnRH challenge in male dark-eyed juncos (Junco hyemalis).

Concentrations of gonadal steroids such as testosterone (T) often vary widely in natural populations, but the causes and particularly the consistency of this variation is relatively unexplored. In breeding males of a wild population of the dark-eyed junco (Junco hyemalis), we investigated seasonal and individual variation in circulating T during two breeding seasons by measuring the responsiveness of the HPG axis to a standardized injection of gonadotropin-releasing hormone (GnRH). Individuals were bled prior to and 30min after injection. Pre- and post-challenge levels of T were measured using EIA. Many subjects were sampled repeatedly across multiple breeding stages. Plasma T concentrations nearly doubled in response to GnRH during early spring, but showed significantly smaller increases in later breeding stages. When controlling for seasonal variation in response to challenge, we also found repeatable differences among individuals, indicating individual consistency in the release of T in response to a standardized stimulus. These seasonal and individual differences may arise from comparable variation in responsiveness of the pituitary or a decline in gonadal sensitivity to downstream gonadotropins. In contrast, pre-challenge T showed almost no seasonal changes and did not differ consistently among individuals. To our knowledge, this is the first demonstration of individual repeatability of short-term hormonal changes in a wild population. Such repeatability suggests that hormonal plasticity might evolve in response to changing selection pressures.

Females competing to reproduce: dominance matters but testosterone may not.

The associations among aggression, testosterone (T), and reproductive success have been well studied, particularly in male birds. In many species, males challenged with simulated or real territorial intrusions increase T and levels of aggression, outcomes linked to higher dominance status and greater reproductive success. For females, the patterns are less clear. Females behave aggressively towards one another, and in some species, females respond to a social challenge with increases in T, but in other species they do not. Prior work on female dark-eyed juncos (Junco hyemalis) had shown that experimental elevation of T increases social status and intrasexual aggression. Here, we conducted two experiments designed to answer three questions: Are endogenous concentrations of T associated with dominance status in captive female juncos? Does dominance status influence readiness to breed in female juncos? And do captive females increase T in response to a challenge? In the first experiment, we introduced two females to a breeding aviary, allowed them to form a dominance relationship and then introduced a male. We found that dominant females were more likely to breed than subordinates, but that dominance status was not predicted by circulating T. In the second experiment, we allowed a resident male and female to establish ownership of a breeding aviary (territory) then introduced a second, intruder female. We found that resident females were aggressive towards and dominant over intruders, but T did not increase during aggressive interactions. We suggest that during the breeding season, intrasexual aggression between females may influence reproductive success, but not be dependent upon fluctuations in T. Selection may have favored independence of aggression from T because high concentrations of T could interfere with normal ovulation or produce detrimental maternal effects.