Fast life in the slow lane : life history and energetics of a basal placental mammal Setifer setosus (Schreber, 1778).

Abstract

There is increasing evidence that homeothermy (the maintenance of a high and stable body temperature-Tb) as observed in modern mammals was derived from an ancestral heterothermic (flexible Tb regulation) state. One of main hypotheses for why this occurred is that homeothermy benefits parental care. As such the study of the thermoregulatory physiology during reproduction in an otherwise heterothermic mammal can provide insights into the evolution of homeothermic endothermy in mammals. This thesis presents data collected over three reproductive seasons from one such mammal, the greater hedgehog tenrec (Setifer setosus, Tenrecidae). Flow-through respirometry was used to measure resting metabolism over a range of ambient temperatures (Ta). At low Ta S. setosus demonstrated a high propensity for torpor and highly labile Tb. This high degree of heterothermy was abandoned during reproduction; pregnant and lactating females maintained higher Tb and metabolic rates. Tb obtained from free-ranging animals showed similar trends. Reproductive females had less variability in Tb, whereas non-reproductive females and males had a higher propensity for torpor as well as higher overall Tb lability. These data indicate a larger degree of homeothermy during reproduction. Concurrent with the collection of physiological data, the use of radio-transmitters, implanted into the peritoneal cavity along with the temperature data-loggers, allowed for novel observations on the life-history of this little-studied species. The most striking finding was that S. setosus demonstrates an exceptional capacity to assimilate energy. In the short active season males showed high levels of activity and occupied home ranges larger than predicted for their body size. Females, in addition to maintaining a higher degree of homeothermy, can have up to three litters per year. Over this same time period individuals of both sex double their body mass in preparation for hibernation. Such high energetic outputs are thought to be incompatible with the low basal metabolic rates which this species displays. An explanation of this incongruity can be found in the high Ta at the study site, which negated most thermoregulatory costs. In reproductive females, the fitness benefits of small increments in homeothermy seem to be offset by the relatively low fitness costs involved in minimal thermoregulatory energy demands. Homeothermy during reproduction is therefore likely to have been a first step in the progressive evolution from heterothermic to homeothermic endothermy in mammals.