- Trent University
Summary
Conspicuous ornamentation has been linked to immunological and physiological condition in males of many species. In species where both sexes are ornamented, it is unclear whether the signal content of ornaments differs between males and females. We examined the immunological and physiological correlates of carotenoid-based bill and plumage ornamentation in American goldfinches Spinus tristis, a species in which bright orange bills are sexually monomorphic but yellow plumage is sexually dimorphic during the breeding season. Because bill color is dynamic over short periods while plumage color is static over longer time frames, we tested whether these signals have the potential to provide temporal information about immunity and condition. In both sexes, bill color (but not plumage color) was negatively related to leukocyte differential, a measure of recent stress, while plumage color (but not bill color) was positively related to resting metabolic rate. In females, bill color also positively correlated with immunoglobulin Y, a component of acquired immunity, while plumage color positively predicted natural antibody levels, a component of innate immunity. In males, neither bill color nor plumage color predicted immune function, suggesting that the mechanisms underlying these signals vary with sex. Our results demonstrate that dynamic signals such as bill coloration do not merely reflect the same information provided by static signals but that these two classes of signal provide information about different temporal aspects of phenotypic quality. Furthermore, our results indicate that a signal expressed in both sexes has the potential to provide different information depending on the sex of the bearer.
Methodology
We captured 41 male and 28 female American goldfinches in July 2008 at Queens University Biological Station in Ontario, Canada (44°33' N, 76°19' W). Goldfinches were captured with mist nets placed around feeders at three different sites within a 15-km radius. On capture, we categorized sex and age class (in their second calendar year [SY]—i.e., hatched the previous summer—and after their second year [ASY]) based on plumage (Pyle 1997), measured mass (±0.1 g), and morphological features (±0.1 mm) and collected ca. 100 mL of blood. Blood was collected from the brachial wing vein into two heparinized capillary tubes, sealed with clay, and stored up to 4 h on ice until processing. An additional drop of blood was collected into a nonheparinized capillary tube and used to make two blood smears following Davis (2005). Slides were air-dried and stored until staining took place. Blood samples were centrifuged (Hemata STAT-II, Separation Technology) for three 60-s intervals. Plasma was removed, placed into microcentrifuge tubes, and frozen at -20°C for up to 4 wk. Samples were then transferred to a -80°C freezer until analysis.
Because bill color is dynamic and changes rapidly in this species in response to stress of capture (Rosen and Tarvin 2006; M. F. Rosenthal, T. G. Murphy, N. Darling, and K. A. Tarvin, unpublished data), we measured color reflectance (R) of the bill within 1 h of capture (mean ± SE = 32.1 ± 2.7 min), which is a few hours before stress-induced color change has been detected spectrometrically (M. F. Rosenthal, T. G. Murphy, N. Darling, and K. A. Tarvin, unpublished data). Individuals were then transported to the biological station and placed in visually isolated housing units (1.2 m3 ) for ca. 6 h, where they received water and Nyjer seed ad lib. On the same evening of capture, up to three birds were placed into individual metabolic chambers where oxygen consumption was measured overnight as an index of RMR. After RMR was measured, birds were used in a dominance experiment the next morning (see Murphy et al. 2009) and released at their capture site. Only adult (ASY) birds were used for this experiment. All research was conducted under Queen’s University animal care protocol 2005-044 and Trent University protocol 08046.