Studies of genetic variation in metabolic traits have so far not focused on birds. In our study population of captive zebra finches we found evidence for a significant heritable genetic component in basal metabolic rate (BMR). Heritability of all morphological traits investigated (body mass, head length, tars length and wing length) was significantly larger than zero. All traits were positively phenotypically correlated. Eight of 10 genetic correlations presented in this study differed significantly from zero, all being positive, suggesting the possibility of correlated responses to any selection acting on the traits. When conditioned on the genetic variance in body mass, the heritability of BMR was reduced from 25% to 4%. Hence, our results indicate that genetic changes in BMR through directional selection are possible, but the potential for adaptation independent of body mass may be limited.

SUMMARY: Basal metabolic rate (BMR) is a physiological trait believed to show adaptational changes. Few studies have tested whether BMR shows stable between-individual variations. Repeatability indicates that the trait might be heritable and therefore a possible target for natural selection. We tested whether BMR was repeatable over a considerable time of the lifespan of a small passerine bird: the zebra finch Taeniopygia guttata. BMR was measured six times over a 2.5 year period in captive zebra finches. BMR residuals showed significant repeatabilities over a short (1.5 months) and a long (2.5 years) period for each sex as well as for both sexes pooled. In contrast to earlier studies on metabolism, our calculated repeatability (R) did not change significantly from the short to the long period in either males(R from 0.501 to 0.465), females (R from 0.413 to 0.522) or the pooled data (R from 0.571 to 0.567). Our results show that there are consistent between-individual variations in BMR on which natural selection can work, provided that this trait is heritable.

Inbreeding may increase the extinction risk of small populations. Yet, studies using modern genomic tools to investigate inbreeding depression in nature have been limited to single populations, and little is known about the dynamics of inbreeding depression in subdivided populations over time. Natural populations often experience different environmental conditions and differ in demographic history and genetic composition, characteristics that can affect the severity of inbreeding depression. We utilized extensive long-term data on more than 3,100 individuals from eight islands in an insular house sparrow metapopulation to examine the generality of inbreeding effects. Using genomic estimates of realized inbreeding, we discovered that inbred individuals had lower survival probabilities and produced fewer recruiting offspring than noninbred individuals. Inbreeding depression, measured as the decline in fitness-related traits per unit inbreeding, did not vary appreciably among populations or with time. As a consequence, populations with more resident inbreeding (due to their demographic history) paid a higher total fitness cost, evidenced by a larger variance in fitness explained by inbreeding within these populations. Our results are in contrast to the idea that effects of inbreeding generally depend on ecological factors and genetic differences among populations, and expand the understanding of inbreeding depression in natural subdivided populations.

Young birds, in their post-natal growth period, may reduce their growth and metabolism when facing a food shortage. To examine how such responses can be mediated by endocrine-related factors, we exposed Japanese quail chicks to food restriction for either 2 days (age 6–8 days) or 5 days (age 6–11 days). We then measured growth and resting metabolic rate (RMR), and circulating 3,3′,5-triiodo-l-thyronine (T3) and 3,5,3′,5′-tetraiodothyronine (T4) levels as well as expression patterns of genes involved in growth (insulin-like growth factor-I: IGF-I) and thyroid hormone signalling (thyroid-stimulating hormone-β: TSHβ, type II iodothyronine deiodinase: D2, thyroid hormone receptors isoforms: TRα and TRβ). The food-restricted chicks receiving a weight-maintenance diet showed reductions in structural growth and RMR. Plasma levels of both T3 and T4 were reduced in the food-restricted birds, and within the 5 days food-restricted group there was a positive correlation between RMR and T3. IGF-I mRNA showed significantly higher abundance in the liver of ad libitum fed birds at day 8 compared with food-restricted birds. In the brain, TSHβ mRNA level tended to be lower in food-restricted quails on day 8 compared with controls. Furthermore, TRα expression was lower in the brain of food-restricted birds at day 8 compared with birds fed ad libitum. Interestingly, brain D2 mRNA was negatively correlated with plasma T3 levels, tending to increase with the length of food restriction. Overall, our results show that food restriction produced significant effects on circulating thyroid hormones and differentially affected mRNA species in the thyroid hormone signalling pathway. Thus, we conclude that the effects of food restriction observed on growth and metabolism were partly mediated by changes in the endocrine-related factors investigated.