Abstract Investigations of male ornaments in the context of sexual selection have tended to focus on single ornaments, although many species of birds possess multiple ornaments. Understanding the evolution of multiple ornaments requires knowledge of correlations among ornaments in the same individual and the extent to which ornament expression reflects individual condition and behavior. Variation in four male ornaments in socially monogamous, biparental northern cardinals ( Cardinalis cardinalis ) was related to body size, indices of condition, level of paternal care, and reproductive success. Redness of breast plumage positively predicted body size and negatively predicted nestling feeding rate. Bill color predicted current body condition, with birds with redder bills in better condition. Birds with smaller black face masks had greater reproductive success. These results are consistent with the hypothesis that different ornaments in male cardinals provide information on different aspects of condition and behavior.
Summary Despite considerable recent interest in plasma and yolk testosterone (T) in female birds, relatively little is known about environmental regulation of female T, individual variation in female T or the relationship between plasma and yolk T. In breeding females of a wild population of dark‐eyed junco ( Junco hyemalis ), we assessed variation in the responsiveness of the hypothalamo‐pituitary‐gonadal (HPG) axis to a challenge with gonadotropin‐releasing hormone (GnRH) by measuring circulating T before and 30 min after a standardized injection of GnRH. We asked whether response to challenge varied seasonally or with stage of reproduction and whether it was repeatable within individuals or related to T deposited in eggs. Initial and post‐challenge levels of T were measured using enzyme immunoassay. In a subset of these females, luteinising hormone (LH) was measured using radioimmunoassay (RIA). In addition, eggs were collected from nests of 15 females that had received a GnRH challenge, and yolk T was measured using RIA. During most of the breeding season, plasma T did not increase in response to GnRH. GnRH consistently caused increases in plasma T only during the 7 days before oviposition, when females were rapidly depositing yolk in eggs but had not yet begun to lay them. Among a small subset of females we found a positive correlation between the magnitude of this increase in plasma T in response to GnRH during egg development and the amount of T deposited in the yolk of eggs collected at a later time. These results suggest that ovarian response to GnRH‐induced increases in LH is greatest when females are actively depositing yolk into eggs. Factors that stimulate the release of GnRH during egg formation may result in higher levels of plasma T which could influence adult female behaviour. Further, because plasma T was correlated with later yolk T, factors that stimulate GnRH release may also lead to higher levels of yolk T potentially influencing offspring development or behaviour.
Currently, little is known about the seasonal variation of corticosterone (CORT) levels, either baseline or stress response, within freshwater turtles. We conducted a seasonal CORT study with a species of freshwater turtle, Graptemys flavimaculata (Yellow-blotched Sawback; family Emydidae), that is endemic to the Pascagoula River system of southeastern Mississippi. Graptemys flavimaculata is commonly observed while basking on deadwood snags, with us using basking traps and dip nets as active capture methods. We caught both male (n = 60) and female (n = 49) turtles during the months of April–October during 2007 and 2008. Immediately after capture, we collected an initial blood sample, confined the turtle for 35 minutes, and then took a second blood sample. Competitive binding radioimmunoassays were done to determine CORT levels. Time zero CORT levels for both sexes were generally lower than previously reported levels for other turtle species, likely due to the trapping methods used. By time 35 min, CORT levels were significantly elevated relative to initial levels. We did not detect seasonal differences in time zero CORT levels for males or females. However, seasonal differences in time 35 CORT levels for males were detected, with higher levels observed during summer months (July, August) relative to spring (April) and fall (October) months. Seasonal differences were also detected in time 35 CORT levels of females, with lower levels observed during July (nesting season) relative to September. This study provides novel information and insight into the differences in seasonal physiological demands for this species, and aquatic turtle species in general, while also providing probable connections of physiology to seasonal life history events.
Abstract When male investment in mating varies with quality, reliable sexual signals may evolve. In many songbirds, testosterone mediates mating investment, suggesting that signals should be linked to testosterone production. However, because testosterone may change rapidly during behaviour such as territorial aggression and courtship, efforts to establish such a relationship have proved challenging. In a population of dark‐eyed juncos, we measured individual variation in the production of short‐term testosterone increases by injecting gonadotropin‐releasing hormone (GnRH). We found a positive correlation between the magnitude of these increases and the size of a plumage ornament (‘tail white’) previously shown to be important for female choice and male–male competition. We then measured naturally elevated testosterone levels produced during male–male competition and found that they covaried with those induced by GnRH. We suggest that the association between tail white and testosterone increases may allow conspecifics to assess potential mates and competitors reliably using tail white.
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.
