Mammalian thermoregulation is energetically costly, and mammals employ numerous strategies to ameliorate these costs, particularly at low ambient temperatures. This body of work is broadly focused on mammalian thermoregulation, with emphasis on the adaptive value and evolution of insulative fur and hibernation in rodents. Chapter 1 serves as a brief introduction to mammalian heterothermy, and general methods for this dissertation are discussed in Chapter 2.
Chapter 3 tested for an energetic tradeoff between growth and thermoregulation in juvenile Siberian hamsters (Phodopus sungorus) and the effect of an insulative pelage on intrinsic growth rate. Growth, like thermoregulation, is energetically costly, and many studies implicate an energetic tradeoff between them. Fur is known to reduce thermoregulatory costs in adult mammals, but its role in maintaining energy balance during growth is unclear. Hamsters weaned at 18 days of age and left fully furred or deprived of all dorsal fur by shaving at 20 days of age, were housed at 10°C or 23°C. Body mass, body length, and food consumption were measured until hamsters were 35 days old. Thermal challenge, whether by low ambient temperature or shaving, resulted in increased food intake and decreased efficiency at converting food into body mass - i.e., less body mass was accrued per gram of food consumed. Body mass and length were not affected by the thermal challenges. These results suggest that there is no mandatory tradeoff between growth and thermoregulation in this species, particularly when food is in abundant supply.
Chapter 4 provided a detailed descriptive account of hibernation in Turkish hamsters (Mesocricetus brandti), a common model organism for studies of hibernation. I employed continuous telemetric monitoring of body temperature in hibernating male and female Turkish hamsters at ambient temperatures of 5°C and 13°C to characterize torpor bout depth, duration, and frequency, as well as rates of entry into and arousal from torpor. Hamsters generated brief intervals of short (<12 h), shallow test bouts (Tb>20°C), followed by deep torpor bouts lasting 4-6 days at Ta=5°C and 2-3 days at Ta=13°C. Females at Ta=5°C had longer bouts than males, but maintained higher torpor Tb; there were no sex differences at Ta=13°C. Neither body mass loss nor food intake differed between the two Tas. Hamsters entered torpor primarily during the scotophase (subjective night), but timing of arousals was highly variable. Hamsters at both Tas generated short, shallow torpor bouts between deep bouts, suggesting that this species may be capable of both hibernation and daily torpor.
Chapters 5 and 6 made use of the fact that vertebrate dentin is deposited on a circadian basis, and daily layers manifest as bands on the medial surfaces of rodent incisors. Hibernation alters dentin deposition, and a distinct hibernation mark has been described on incisor surfaces of several rodent species. Chapter 5 tested the effects of day length, torpor expression, and ambient temperature on incisor dentin surface morphology in Turkish hamsters housed in one of four conditions: long days (LDs) at 22°C, short days (SDs) at 22°C, SDs at 5°C, and SDs at 13°C. Body temperature was monitored continuously with implanted radio transmitters, and teeth examined postmortem. Teeth of SD hamsters had narrower, less distinct circadian increments than those of LD hamsters, but the width of ultradian increments was similar in both photoperiods. Hibernation at both 5°C and 13°C was associated with very narrow, sharply defined dentin increments and increased tooth heterogeneity. Hamsters in SDs at 5°C that did not hibernate lacked characteristic hibernation increments. At 5°C, but not 13°C, the number and cumulative width of hibernation increments were related to number and cumulative duration of periodic arousals, suggesting that it may be possible to infer information about hibernation physiology by examining a deceased or extinct specimen's teeth.
Chapter 6 compared conventional histological preparation of incisor cross-sections to images obtained by hard X-ray micro-tomography, a technique similar to medical CT scanning. Six of the Turkish hamster specimens from Chapter 5 (2 each of LD controls, SD controls, and 5°C hibernators) were used; lower right incisors were prepared histologically and lower left incisors were scanned. Scanning was nearly as good as histological preparation, though increments were slightly but significantly wider when measured from scanned images. Several specimens of four hibernating sciurid species from the Museum of Vertebrate Zoology were also scanned in search of a putative hibernation mark; such a mark was present in only two individuals. Hard X-ray micro-tomography is a valuable tool for studying dentin microstructure when specimens are irreplaceable or otherwise unavailable for permanent alteration necessitated by conventional histology.
Two broad conclusions follow from this body of work. First, there are sex differences in thermoregulatory traits, which merit further investigation and may ultimately shed light on different selective pressures operating on males and females. Second, incremental dentin in rodent incisors is a viable tool for studying hibernation behavior and seasonal changes in growth rates in evolutionary and historical populations, provided that care is taken to account for inter- and intra-specific variation in dentin morphology.