Lack of variation in nuclear DNA content in avian muscle

The avian pectoralis muscle demonstrates plasticity with regard to size, so that temperate birds facing winter conditions or birds enduring a migration bout tend to have significant increases in the size and mass of this tissue due to muscular hypertrophy. Myonuclear domain (MND), the volume of cytoplasm a myonuclei services, in the pectoralis muscle of birds seems to be altered during thermal stress or changing seasons. However, there is no information available regarding muscle DNA content or ploidy level within the avian pectoralis. Changes in muscle DNA content can be used in this tissue to aid in size and mass changes. Here, we hypothesized that long-distance migrants or temperate residents would use the process of endoreduplication to aid in altering muscle size. Mostly contradictory to our hypotheses, we found no differences in the mean muscle DNA content in any of the 62 species of birds examined in this study. We also found no correlations between mean muscle DNA content and other muscle structural measurements, such as the number of nuclei per millimeter of fiber, myonuclear domain, and fiber cross-sectional area. Thus, while avian muscle seems more phenotypically plastic than mammalian muscle, the biological processes surrounding myonuclear function may be more closely related to those seen in mammals.

The high-energy aerial insectivore lifestyle of swallows does not produce clear thermogenic side effects

Ecological traits related to pace of life, such as foraging strategies and activity levels, influence daily energy expenditure (DEE) and can affect fitness. A fast pace of life tends to be supported by high-energy aerobic activity and is positively correlated with high DEE and basal and maximal metabolic rates in some endotherms. Given that maximal capacities for exercise and thermogenesis are both functions of aerobic muscle output and are often positively correlated with each other, high-energy aerobic lifestyles might be associated with high aerobic capacities, which would be expected to produce high thermogenic capacities as a side effect. We tested whether the high-energy aerial insectivore lifestyle in swallows is correlated with elevated basal and maximal thermogenic metabolic rates. We measured basal (BMR) and summit (Msum = maximum cold-induced metabolic rate) metabolic rates in 6 species of swallows (Hirundinidae) and combined these data with literature data for additional swallows (n = 10 for BMR; n = 8 for Msum) and non-aerial insectivore birds (n = 215 for BMR; n = 64 for Msum) to address the hypothesis that swallows have higher BMR and Msum than non-aerial insectivores. BMR in swallows was significantly higher than for non-aerial insectivore birds for phylogenetically adjusted analyses after correcting for body mass and region of origin (tropical vs. temperate). In contrast, Msum did not differ significantly between swallows and non-aerial insectivores. Thermogenic scope (Msum – BMR), however, was lower in tropical non-aerial insectivore birds compared with tropical swallows and temperate birds. This suggests that the aerial insectivore lifestyle elevates maintenance costs, but maximum thermogenic capacities are not clearly upregulated, despite tropical swallows having higher thermogenic scope than tropical non-aerial insectivores. These data suggest that the high-energy aerial insectivore lifestyle does not produce strong thermogenic side effects in swallows.