Katrina G. Salvante

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.

Effects of corticosterone on the proportion of breeding females, reproductive output and yolk precursor levels

In this study we investigated the role of corticosterone (B) in regulating the proportion of laying females, timing of breeding, reproductive output (egg size and number), and yolk precursor levels in chronically B-treated female zebra finches (Taeniopygia guttata). Corticosterone treatment via silastic implant elevated plasma B to high physiological (stress-induced) levels (24.1 +/- 5.3 ng/ml at 7-days post-implantation). B-treated females had high plasma levels of very-low density lipoprotein (VLDL) but low levels of plasma vitellogenin 7-days post-implantation, suggesting that corticosterone inhibited yolk precusor production and perhaps shifted lipid metabolism away from production of yolk VLDL and towards production of generic (non-yolk) VLDL. Only 56% of B-treated females (n = 32) initiated laying, compared with 100% of sham-implanted females (n = 18). In females that did breed, corticosterone administration delayed the onset of egg laying: B-treated females initiated laying on average 14.5 +/- 0.5 days after pairing compared to 6.4 +/- 0.5 days in sham-implanted females. B-treated females that laid eggs had significantly higher plasma B levels at the 1st-egg stage (45.9+/-9.0 ng/ml) than did sham-implanted females (7.9+/-6.8 ng/ml). Despite this there was no difference in mean egg mass, clutch size, or egg composition in B-treated and sham-implanted females. These results are consistent with the idea that elevated corticosterone levels inhibit reproduction, but contrast with studies of other oviparous vertebrates (e.g., lizards) in relation to the role of corticosterone in regulating egg and clutch size.

TECHNIQUES FOR STUDYING INTEGRATED IMMUNE FUNCTION IN BIRDS

EVOLUTIONARY PHYSIOLOGISTS AND ecologists seek to understand the mechanisms that underlie trade-offs involving life-history traits. These trade-offs arise when resources are limited, and allocation of resources to certain traits limits the amount of resources available for other traits (Williams 1966, Stearns 1992). Recent studies have found that maintaining or activating immune function can be resource-dependent (Tsiagbe et al. 1987; Saino et al. 1997b, 2003; Alonso-Alvarez and Tella 2001) and metabolically costly (Demas et al. 1997, Lochmiller and Deerenberg 2000, Ots et al. 2001, Martin et al. 2002) and, therefore, may compete with lifehistory traits for nutrient or energetic resources (for reviews see Sheldon and Verhulst 1996, Lochmiller and Deerenberg 2000). Therefore, the study of immunocompetence (i.e. the ability of a host to prevent or control infection by pathogens and parasites) has become the focus of many studies on fitness-related trade-offs in free-living birds (Saino et al. 1997a; Horak et al. 2000; Norris and Evans 2000; Hanssen et al. 2003, 2004). In their seminal review, Norris and Evans (2000) contrasted the techniques currently used by ecologists to measure immunocompetence with the techniques used by immunologists. Reviewing examples of trade-offs between immune-system maintenance and resource allocation to life-history traits, they concluded that future studies need to (1) assess multiple components of the immune system to make conclusions about how these components interact and

Sex‐Specific Variability in the Immune System across Life‐History Stages

Organisms theoretically manage their immune systems optimally across their life spans to maximize fitness. However, we lack information on (1) how the immune system is managed across life‐history stages, (2) whether the sexes manage immunity differentially, and (3) whether immunity is repeatable within an individual. We present a within‐individual, repeated‐measures experiment examining life‐history stage variation in the inflammatory immune response in the zebra finch (Taeniopygia guttata). In juveniles, age‐dependent variation in immune response differed in a sex‐ and context‐specific manner, resulting in no repeatability across stages. In adults, females displayed little stage‐dependent variation in immune response when laying while receiving a high‐quality (HQ) diet; however, laying while receiving a low‐quality (LQ) diet significantly reduced both immune responses and reproductive outputs in a manner consistent with a facultative (resource‐driven) effect of reproduction on immunity. Moreover, a reduced immune response in females who were raising offspring while receiving an HQ diet suggests a residual effect of the energetic costs of reproduction. Conversely, adult males displayed no variation in immune responses across stages, with high repeatability from the nonbreeding stage to the egg‐laying stage, regardless of diet quality (HQ diet, ndocumentclass{aastex}nusepackage{amsbsy}nusepackage{amsfonts}nusepackage{amssymb}nusepackage{bm}nusepackage{mathrsfs}nusepackage{pifont}nusepackage{stmaryrd}nusepackage{textcomp}nusepackage{portland,xspace}nusepackage{amsmath,amsxtra}nusepackage[OT2,OT1]{fontenc}nnewcommandcyr{nrenewcommandrmdefault{wncyr}nrenewcommandsfdefault{wncyss}nrenewcommandencodingdefault{OT2}nnormalfontnselectfont}nDeclareTextFontCommand{textcyr}{cyr}npagestyle{empty}nDeclareMathSizes{10}{9}{7}{6}nbegin{document}nlandscapen

What comes first, the zebra finch or the egg: temperature-dependent reproductive, physiological and behavioural plasticity in egg-laying zebra finches

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

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The metabolic cost of avian egg formation: possible impact of yolk precursor production?

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