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.

Evolutionary and Biomechanical Basis of Drumming Behavior in Woodpeckers

Understanding how and why behavioral traits diversify during the course of evolution is a longstanding goal of organismal biologists. Historically, this topic is examined from an ecological perspective, where behavioral evolution is thought to occur in response to selection pressures that arise through different social and environmental factors. Yet organismal physiology and biomechanics also play a role in this process by defining the types of behavioral traits that are more or less likely to arise. Our paper explores the interplay between ecological, physiological, and mechanical factors that shape the evolution of an elaborate display in woodpeckers called the drum. Individuals produce this behavior by rapidly hammering their bill on trees in their habitat, and it serves as an aggressive signal during territorial encounters. We describe how different components of the display—namely, speed (bill strikes/beats sec–1), length (total number of beats), and rhythm—differentially evolve likely in response to sexual selection by male-male competition, whereas other components of the display appear more evolutionarily static, possibly due to morphological or physiological constraints. We synthesize research related to principles of avian muscle physiology and ecology to guide inferences about the biomechanical basis of woodpecker drumming. Our aim is to introduce the woodpecker as an ideal study system to study the physiological basis of behavioral evolution and how it relates to selection born through different ecological factors.

Put some muscle behind it: Understanding movement capacity of tropical birds

The capacity for flight varies widely among bird species and influences their ecology, evolution, and conservation. Variation in vagility is influenced by behavioral responses to the nature of gaps between habitat elements as well as intrinsic characteristics of the species, particularly physiological traits influencing the physical capacity for sustained flight. Here, we briefly summarize the current state of knowledge revealing the wide variety of movement capacities of Neotropical birds. We then review current knowledge of avian muscle physiology and the role that muscle characteristics may play in influencing movement behavior. We argue that fundamental shifts in our understanding of avian muscle physiology and the influence of physiology on movement behavior remain to be elucidated, in part because knowledge from other vertebrates is being inappropriately applied to birds. In particular, critical evaluation of assumptions applied to birds from detailed studies of mammals is needed. Moving away from simple binary categorizations of avian flight muscles as “red vs. white” or “fast vs. slow” to characterize the cellular mechanisms and specific isoforms active at various life stages or seasons is also needed. An increasingly large number of avian species with a wide array of flight styles from hummingbirds to soaring raptors are appearing in GenBank, facilitating detailed physiological and evolutionary comparisons among species. Properly assessing the muscle physiological characteristics of Neotropical bird species with a wide array of movement capacities may improve our abilities to predict which species are most sensitive to landscape fragmentation and other factors that influence dispersal and migration.

Structural plasticity of the avian pectoralis: a case for geometry and the forgotten organelle

ABSTRACTThe avian pectoralis muscle demonstrates incredible plasticity. This muscle is the sole thermogenic organ of small passerine birds, and many temperate small passerines increase pectoralis mass in winter, potentially to increase heat production. Similarly, this organ can double in size prior to migration in migratory birds. In this Commentary, following the August Krogh principle, I argue that the avian pectoralis is the perfect tissue to reveal general features of muscle physiology. For example, in both mammals and birds, skeletal muscle fiber diameter is generally accepted to be within 10–100 µm. This size constraint is assumed to include reaction-diffusion limitations, coupled with metabolic cost savings associated with fiber geometry. However, avian muscle fiber structure has been largely ignored in this field, and the extensive remodeling of the avian pectoralis provides a system with which to investigate this. In addition, fiber diameter has been linked to whole-animal metabolic rates, although this has only been addressed in a handful of bird studies, some of which demonstrate previously unreported levels of plasticity and flexibility. Similarly, myonuclei, which are responsible for protein turnover within the fiber, have been forgotten in the avian literature. The few studies that have addressed myonuclear domain (MND) changes in avian muscle have found rates of change not previously seen in mammals. Both fiber diameter and MND have strong implications for aging rates; most aging mammals demonstrate muscular atrophy (a decrease in fiber diameter) and changes in MND. As I discuss here, these features are likely to differ in birds.

Glutamine synthetase in avian muscle contributes to a positive myogenic response to ammonia compared with mammalian muscle

In mammalian models of cirrhosis, plasma ammonia concentration increases, having numerous adverse effects, including sarcopenia. The objective of this study was to identify differences between avian and mammalian myogenic response to applied ammonia and glutamine. Primary chicken breast and thigh, primary rat, and C2C12 myotubes were treated with ammonium acetate (AA, 10 mM) or glutamine (10 mM) for 24 h and compared with sodium acetate (10 mM) and untreated controls. Myostatin mRNA was significantly higher in C2C12 and rat myotubes treated with AA compared with glutamine and controls ( P < 0.01), whereas myostatin was unchanged in chicken myotubes. AA-treated C2C12 myotubes had significantly higher glutamine synthetase (GS) mRNA expression compared with controls, but GS protein expression was unchanged. In contrast, GS mRNA expression was unchanged in thigh myotubes, but GS protein expression was significantly higher in AA-treated thigh myotubes ( P < 0.05). In both breast and thigh myotubes, intracellular glutamine concentration was significantly increased in AA- and glutamine-treated myotubes compared with controls but was only increased in glutamine-treated C2C12 and rat myotubes ( P < 0.05). Glutamine concentration was significantly higher in all treatment media collected from avian myotube cultures compared with both C2C12 and rat media ( P < 0.01). Myotube diameter was significantly larger in avian myotubes after treatment with both AA and glutamine ( P < 0.05). C2C12 and rat myotubes had a significantly smaller myotube diameter after AA treatment ( P < 0.001). Altogether, these data support species differences in skeletal muscle ammonia metabolism and suggest that glutamine synthesis is a mechanism of ammonia utilization in avian muscle.