David L. Swanson: Research Interests

• Evolution of metabolic patterns in birds. A major research focus in my laboratory is the evolution of metabolic patterns in birds, principally focusing on the passerine birds or songbirds. Small birds that are resident in cold climates generally show marked winter increases in thermogenic capacity (overall capacity for heat production) that are accompanied by winter increases in cold hardiness. Thus, high thermogenic capacity appears to be a feature typical of birds wintering in cold climates. Our lab is interested in how selection for cold hardiness may have influenced whole-organism metabolic patterns in birds and how such past selection may affect present-day bird distributions. By investigating variation in thermogenic capacity and cold hardiness in birds with different lifestyles (e.g., migrants vs. residents) and among different avian taxa, and incorporating data on avian and climatic evolution, we hope to elucidate fundamental relationships among cold, metabolic physiology and biogeography in birds.

          Current Projects:

• To address the question of whether thermogenic capacity varies with overwintering climate or phylogeny we are conducting comparative analyses of summit metabolic rates (M_sum, maximum cold-induced metabolic rates) in migrant and cold-climate resident birds from a diversity of avian taxa. Results from these studies should elucidate important factors in the evolution of thermogenic capacity in birds and this should, in turn, inform us of whether the evolution of high thermogenic capacity and cold tolerance in birds played an important role in the evolution of current biogeographic distributions.

• We are also currently examining the relationships among exercise maximum metabolic rates (MMR, using a hop-flutter wheel), summit metabolic rates (M_sum), and basal metabolic rates (BMR) in individual birds. Both sustained exercise and prolonged shivering are forms of endurance muscular activity, so physiological adjustments promoting endurance may enhance both exercise and shivering performance. If so, then MMR and M_sum should show correlated phenotypic variation among individual birds. In addition, if increases in exercise and shivering capacities incur support and maintenance costs, then BMR should also show correlated phenotypic variation with MMR and M_sum, as suggested by the aerobic capacity model for the evolution of endothermy.

• Mechanisms of seasonal acclimatization of metabolism in small birds. Small birds wintering in cold climates generally show substantial improvements in cold hardiness in winter relative to summer and this improvement in cold hardiness is largely metabolic in nature. Another major focus of my laboratory is to examine the mechanistic basis of these seasonal improvements in cold hardiness. We take an integrative approach to addressing these mechanistic questions by looking at seasonal changes occurring over multiple levels of organization. Examples include thermogenic capacity and basal metabolic rate at the organismal level; mass changes in organs and tissues; and mobilization, transport and catabolic capacities for metabolic substrates in storage tissues, plasma and muscle cells. By studying winter acclimatization at these various levels we hope to discern both fundamental and variable responses promoting cold adjustment in small birds.

          Current Projects:

• One of the typical seasonal changes associated with winter acclimatization in small birds is an increase in pectoralis muscle mass, which is associated with increased thermogenic capacity  and cold tolerance. Winter increases in pectoralis mass generally range from about 10-30% in small birds, which rather closely parallels winter increases in M_sum, which generally range from about 10-40%. We are currently beginning investigations into what factors might regulate this muscle growth, and a candidate for such regulation is myostatin, a potent autocrine/paracrine inhibitor of muscle growth in mammals and birds. If myostatin plays a role in seasonal muscle mass regulation in birds, we hypothesized that myostatin gene expression and protein levels should be lower in winter, which would free up the muscle to grow. Gene expression data from house sparrows, a species showing winter increases in muscle mass and M_sum, are consistent with our hypothesis, as myostatin gene expression was lower in winter than in summer birds. We are currently exploring further a role for myostatin in regulating muscle mass in other cold-climate wintering birds, as well as in migrants, which also show increases in muscle mass prior to migration.

• A second general seasonal adjustment in small birds is an increased reliance on fats to fuel sustained shivering in winter relative to summer. Because birds rely on fatty acids from adipose stores to fuel prolonged shivering, adjustments in fat mobilization, transport and oxidation may be sites for seasonal adjustments. Previous data have shown that fat oxidation capacities often are increased in winter in small birds and trans-membrane and intracellular transport capacities are also potentially limiting factors for fat metabolism. In collaboration with Chris Guglielmo's lab, we have shown that intracellular fat transport capacities, mediated by fatty acid binding protein, increase in winter in some species of small birds. We plan to further investigate seasonal variation in trans-membrane and intracellular transport capacities for fatty acids in small birds.

• Physiology of freeze-tolerance and supercooling in overwintering anurans. Another aspect of my interest in cold tolerance physiology involves the overwintering strategies of terrestrial anurans. My laboratory has conducted studies of the physiological capacities underlying overwintering strategies in four species of terrestrially wintering anurans, the Plains Spadefoot (Spea bombifrons), Great Plains (Bufo cognatus) and Woodhouse's Toads (B. woodhousii) and the Chorus Frog (Pseudacris triseriata). The spadefoot and toads are freeze-intolerant burrowers, and the chorus frog is a freeze tolerant species that overwinters in shallow terrestrial hibernacula. Studies in my lab have shown that the chorus frog mobilizes glucose as a cryoprotectant upon freezing, but the burrowing anurans do not.  We are also investigating other physiological parameters that influence freezing survival in chorus frogs such as regulation of hepatic glycogen stores and a role for aquaporins in water transport during freezing. Through these studies, we hope to elucidate mechanisms promoting freezing tolerance in chorus frogs and how these frogs differ physiologically from freezing intolerant species.

          Current Projects:

• We are currently investigating the biochemical regulation of the glucose accumulation upon freezing in the chorus frog, which differs somewhat between the chorus frog and other freeze-tolerant species of anurans in that glycogen phosphorylase activity (the enzyme that breaks down liver glycogen stores to glucose) does not increase upon freezing, but does increase in winter relative to summer.

• We are also investigating other physiological parameters that influence freezing survival in chorus frogs such as regulation of liver glycogen stores seasonally and during the pre-hibernation period and a role for aquaporins in mediating water transport during freezing.

• We have also recently initiated studies on Northern Cricket Frogs (Acris crepitans), which reach the northwestern-most extent of their range in southeastern South Dakota. Cricket frogs have a unique overwintering strategy in that the use cracks in the mud of stream banks or crayfish burrows as hibernacula, largely avoid freezing in these thermally buffered hibernacula, and show poor freezing tolerance in more southern portions of their range. Cricket frogs use similar sites for hibernacula in South Dakota, but the colder winters characteristic of South Dakota compared to more southern regions of the cricket frog range, may produce colder temperatures within these hibernacula. We have been investigating whether freezing tolerance capacity varies geographically in this species, hypothesizing that northern populations might show greater freezing tolerance if geographic variation in freezing tolerance exists. Our data suggest that South Dakota cricket frogs also show only poor freezing tolerance, tolerating short freezing bouts (6-hours) fairly well (80% survival), but not tolerating longer freezing bouts (24-hour, 0% survival). Temperature data from hibernacula suggest that in some hibernation sites, temperature varies somewhat on a daily basis, dropping below the freezing point of the frog's body fluids for short periods of time (hours) before warming again. Thus, the limited freezing tolerance may be ecologically relevant in the species, but it appears that selection of appropriate hibernaculum sites is critical for overwintering survival in cricket frogs. We are currently planning experiments to carefully document hibernaculum conditions used by cricket frogs in this area to determine what features are important to hibernacula site selection by cricket frogs.

• Use of woodland and wetland habitats by migrating and breeding birds in the northern prairie region. A final research interest in my laboratory is the use of woodland and wetland habitats for stopover biology and breeding in migrant birds. Most of our work to date has investigated the influence of woodland landscape patterns (i.e., large native riparian woodlands vs. small human-planted farmstead woodlots) on migration and breeding of passerines in South Dakota. Native riparian woodlands in the northern prairie region have been greatly reduced in extent over the past century by conversion to agriculture and flooding under reservoirs, but smaller woodland habitats surrounding farmsteads have appeared during this time. We are interested in determining if the farmstead woodlots can substitute for lost native woodland habitat for migrating and breeding woodland birds. Our data suggest that both woodland habitats serve as effective stopover habitat for migrating birds and that both provide suitable breeding habitat for those birds nesting in both types of woodland. However, several breeding species occur only in native riparian habitats and do not occur, or occur at very low abundance, in woodlot habitats. Thus, farmstead woodlots appear to partially substitute for lost native woodland habitat, but preservation of native woodland habitats are necessary for maintaining overall bird diversity in the northern prairie region. Examples of specific aspects of these studies include relative abundance and diversity of migrants at natural and artificial woodland habitats, stopover duration and mass gains of migrants, the influence of woodland orientation on migrant use, and orientation of nocturnal migratory flights through the region. Results of these studies could have significant importance for conservation of these birds, many of which are exhibiting regional or continental population declines, because the migration period is a very energetically demanding phase of the annual cycle.

          Current Projects:

• Our lab is currently studying shorebird stopover biology and energetics in order to compare the efficacy of natural and managed wetland sites in the northern prairie region for successful stopover in shorebirds. We are also interested in how mid-continental shorebird migration patterns compare to patterns of shorebird migration and stopover along the coasts of North America. Our broad research goal is to understand what features of the landscape, and habitats within the landscape, are important to migration and how effectively shorebird migrants can meet the energetic demands of migration at these stopover sites.

• We are also currently investigating the breeding ecology of American Three-toed Woodpeckers (Picoides dorsalis) in spruce habitats in the Black Hills of South Dakota. Aspects of this study include (1) finding nests and measuring nesting productivity and vegetative characteristics important to nest site selection, (2) measuring foraging substrate selection, and (3) measuring home range size and overlap with spruce habitats by radio-tracking and GIS. The goal of these studies is to identify habitat features that are important to successful nesting in three-toed woodpeckers and to determine how tightly tied these birds are to spruce habitats in the Black Hills. We are also using molecular genetic methods (mitochondrial DNA and microsatellites) to determine whether the isolated population of American Three-toed Woodpeckers in the Black Hills is genetically distinct from other more contiguous populations of this species.