Independence among physiological traits suggests flexibility in the face of ecological demands on phenotypes.

Phenotypic flexibility allows animals to adjust their physiology to diverse environmental conditions encountered over the year. Examining how these varying traits covary gives insights into potential constraints or freedoms that may shape evolutionary trajectories. In this study, we examined relationships among haematocrit, baseline corticosterone concentration, constitutive immune function and basal metabolic rate in red knot Calidris canutus islandica individuals subjected to experimentally manipulated temperature treatments over an entire annual cycle. If covariation among traits is constrained, we predict consistent covariation within and among individuals. We further predict consistent correlations between physiological and metabolic traits if constraints underlie species-level patterns found along the slow-fast pace-of-life continuum. We found no consistent correlations among haematocrit, baseline corticosterone concentration, immune function and basal metabolic rate either within or among individuals. This provides no evidence for constraints limiting relationships among these measures of the cardiovascular, endocrine, immune and metabolic systems in individual red knots. Rather, our data suggest that knots are free to adjust individual parts of their physiology independently. This makes good sense if one places the animal within its ecological context where different aspects of the environment might put different pressures on different aspects of physiology.

Individual variation in plasma estradiol-17beta and androgen levels during egg formation in the European starling Sturnus vulgaris: implications for regulation of yolk steroids.

While it is clear that maternal transfer of steroids to egg yolk can have significant effects on offspring phenotype, an unresolved question is whether females can facultatively adjust yolk hormone levels independently of their own plasma levels or whether yolk steroid levels are simply a direct consequence of temporal variation in the female's hormonal status. In part, this is because we lack detailed information about the day-to-day pattern of changes in plasma hormone levels during the laying cycle for non-domesticated birds. Here, we describe changes in plasma estradiol-17beta (E2) and androgens, throughout laying in relation to specific stages of ovarian follicular development in the European starling (Sturnus vulgaris). Plasma E2 levels increased rapidly from the onset of rapid yolk development (RYD) to reach maximum levels in birds with a complete follicle hierarchy (> or = 4 yolky follicles). However, levels decreased linearly throughout the later stages of follicle development returning to pre-breeding values before the final yolky follicle was ovulated. In females with > or = 4 yolky follicles there was 10-fold variation in plasma E2 levels among individual females, but this was not related to plasma levels of the main yolk precursor vitellogenin or to the total mass of yolky follicles developing at the time of blood sampling. In contrast to E2, plasma androgen levels showed only a very gradual linear decline throughout the laying cycle from pre-RYD to clutch completion. Furthermore, androgen levels showed less individual variability: 4-fold variation among females with > or = 4 yolky follicles, although this was also independent of our measures of reproductive function. Data on inter- and intra-individual variation in female hormone levels are important to set-up a priori predictions for, and interpretation of, studies of yolk hormone levels.

Follicular development and plasma yolk precursor dynamics through the laying cycle in the European starling (Sturnus vulgaris).

We investigated the quantitative matching of plasma yolk precursor supply (the plasma pool) to follicle demand during yolk formation in European starlings (Sturnus vulgaris). Plasma concentrations of the two yolk precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL), were only elevated coincident with rapid yolk development (RYD) and matched variation in total yolky follicle mass. VTG and VLDL were low (<0.4 microg/mL and <4.2 mg/mL, respectively) in nonbreeders and prebreeders with no yolky follicles, and at clutch completion. They increased to 4.02 microg/mL and 19.4 mg/mL in birds with a full follicle hierarchy (F1-F4), and concentrations then remained high and actually increased up to the point where only a single, yolky (F1) follicle remained. However, there was some evidence for mismatching of supply and demand: (a) precursor concentrations increased throughout the laying cycle even though the number of developing follicles decreased. We suggest that this is because of a requirement to maintain a large precursor pool to maintain high uptake rates; and (b) in birds with a full follicle hierarchy, precursor concentrations were negatively correlated with total follicle mass. This suggests that high uptake rates in large follicles can actually deplete circulating precursor concentrations. Plasma concentrations of both yolk precursors increased rapidly in the early morning with (predicted) time after ovulation, consistent with a lack of fine control of precursor concentrations. However, mean plasma VTG concentrations did not differ between morning or evening samples. In contrast, plasma VLDL concentrations were lower in the morning (16.8 mg/mL) than in the evening (22.9 mg/mL). Although there is marked individual variation in plasma VTG and VLDL (four- to eightfold), both precursors were repeatable in the short term (24 h), and plasma VTG was repeatable over a 14-d interval between successive breeding attempts.

Social status does not affect resting metabolic rate in wintering dark-eyed junco (Junco hyemalis).

Studies of wintering birds have demonstrated a correlation between social rank and energy expenditures. It is assumed that dominance is energetically costly because of increased activity, possibly caused by elevated androgen levels. As winter acclimatization leads to an increase in metabolic rate, maintaining dominance status in a cold climate can be a substantial challenge. We measured resting metabolic rates in dominant and subordinate dark-eyed juncos (Junco hyemalis) living in small groups in a controlled winter environment. We found no significant effect of social rank when controlling for body size. It has been shown previously that high testosterone levels during the nonbreeding season can lead to higher body conductance, fat loss, and higher nocturnal body temperature. A hypothesis explaining our result is that for juncos it is preferable to maintain low androgen levels during winter and to maintain social rank using a mechanism other than higher agonistic activity.