Cost of living in free-ranging degus (Octodon degus): seasonal dynamics of energy expenditure.

Animals process and allocate energy at different seasons at variable rates, depending on their breeding season and changes in environmental conditions and resulting physiological demands. Overall total energy expenditure, in turn, should either increase in some seasons due to special added demands (e.g. reproduction) or it could simply remain at about the same level, in which case the animals must show compensatory rebalancing of other expenditures that can be reduced. To test for the alternative hypotheses of seasonal variability or compensation, we measured total daily energy expenditure (DEE) in free-living degus (Octodon degus) at four seasons and followed this with determinations of basal metabolic rate (BMR) in the laboratory in the same individuals. DEE varied seasonally but was only significantly different (lower) in summer (non-breeding season), with a DEE:BMR ratio of only 1.6, whereas autumn, winter and spring DEE values were statistically indistinguishable from one another and showed DEE:BMR ratios ranging from 1.9 to 2.2. Our values of DEE in the field fall within the broad range of allometric expectation for herbivorous mammals in general, but the ratios of DEE:BMR are lower than expected. This, together with the lack of strong major shifts in total levels of DEE, suggests that degus are showing compensatory shifts among various categories of energy expenditure that allow them to manage their overall energy balance by minimizing total expenditure.

Energy expenditure and testosterone in free-living male yellow-pine chipmunks.

The onset of mating in yellow-pine chipmunks (Tamias amoenus) follows emergence from a prolonged period of energy conservation during hibernation. Energy expenditures are greatly accelerated to meet the demands of the reproductive season. When emerging from hibernation, typical male chipmunks (breeders) have enlarged testes and a high level of plasma testosterone (T). However, certain males that do not participate in reproduction (nonbreeders) maintain small testes and low plasma T levels and emerge several weeks later than the breeders. The timing of the terminal arousal from hibernation and onset of mating are associated with increased plasma T levels. Experimental elevation of T levels in T. amoenus outside the mating season has been associated with a decrease in body mass, further suggesting an effect of T on energy balance. To test this hypothesis, we measured daily energy expenditure (DEE) in free-living, nonbreeding male chipmunks in the presence and absence of a T-implant. We also measured DEE in breeding males when endogenous T levels were high. DEE of the nonbreeders was not affected by our manipulation of plasma T, and the DEE of breeding males did not differ from that of nonbreeders. We conclude that energy expenditure on a daily basis in male yellow-pine chipmunks is not influenced by levels of T. However, on a seasonal basis, the earlier emergence from hibernation by breeding males, which appears to be influenced by T, represents an overall seasonal energy expenditure that exceeds that of nonbreeding males.

Inefficiency of lactation in primiparous rats: the costs of first reproduction.

To investigate the energetic costs of lactation in a female mammal in relation to previous reproductive history, we compared the performance of adult female Long-Evans rats that had previously bred (multiparous) with young females that had not previously given birth (primiparous). All litters were standardized to 10 +/- 1 young. We compared maternal production (growth of pups), body mass, and energy intake (food consumption) of mothers, as well as their energy expenditure (resting oxygen consumption). The mass of litters at birth and the growth of pups during lactation did not differ according to reproductive history of the mothers. The body mass of primiparous mothers was less than that of multiparous mothers, and primiparous mothers showed an increase in mass during early lactation. To accomplish the essentially identical production of offspring under these circumstances, the primiparous mothers consumed and expended more energy than the multiparous mothers. This remarkable performance of first-time mothers results in an overall efficiency of energy allocation to reproduction amounting to only 25%, compared with 38% in multiparous mothers. The energetic inefficiency of primiparous female lactation results largely from the excessive expenditures associated with physiological and behavioral performances of first-time reproduction, together with a small component of additional expenditure due to further growth by the primiparous mothers. We suggest that this inefficiency probably contributes to the observed low reproductive success of novice breeders; furthermore, active restraint of fecundity may be an evolutionary response to the constraints of the energetic inefficiency of primiparous breeding by female mammals.

Annual cycle of energy and time expenditure in a golden-mantled ground squirrel population.

We have analyzed seasonal shifts of energy and time allocation in a population of golden-mantled ground squirrels (Spermophilus saturatus) by directly measuring total daily energy expenditure (DEE) with an isotopic technique ("doubly labeled water"=dlw), and by estimating components of total DEE through an integration of field behavioral observations with laboratory-measured rates of energy expenditure (oxygen consumption) associated with major behavioral and physiological states. Hibernation laster about 7 1/2 months, and the 4 1/2-month activity season consisted of mating, a 28-d gestation of 3-5 young, 5 1/2 weeks of postnatal growth building to a peak in lactation just before the young emerged above ground, an additional 2-3-week period of maternal care before dispersal, and finally restoration of body mass preceding hibernation. Although the hibernation season comprised nearly two-thirds of the year, it involved only 13-17% of annual energy expenditure, leaving about 85% of energy expenditure for the active season. Ground squirrels were actually present on the surface for only about 11% of the year's time, and the foraging time required to obtain the total annual energy supply amounted to only about 2% of the year's time. The squirrels fed mainly on herbs in the early season and hypogeous fungi later; both were used extensively during peak lactation when female energy expenditure and demand were maximal. Average daily foraging time increased steadily throughout the season to a maximum of 28% of aboveground time as availability of greens diminished and fungus predominated in the diet; time availability did not limit foraging since the animals sat on average for 65% of the daily surface time of about 7 h. Timing of reproduction is apparently optimized such that peak reproductive energy demands are matched with maximal food availability and moderate thermal conditions that minimize energy demand. Despite the greater body mass of males, the greatest total DEE (measured by dlw) of any squirrels at any time of year was that of females during peak lactation. For production of young and lactation through above-ground emergence of an average litter of 2.7, females required a total energy increase of 24% above annual nonreproductive metabolism. Yearling females all bred and performed similarly to older females, yet some costs were greater because the yearlings began and ended hibernation at smaller mass, compensated by giving birth later, and finally showed a greater absolute increase in body mass over the active season than older females. Annual metabolic energy expenditure of breeding males was about 18% greater than that of females, due to greater male body mass. Yet the annual energy intake requirement for both sexes was essentially identical (about 42MJ) due to the greater reproductive export by females in the form of newborn and milk. During the mating season males showed wide-ranging exploratory behavior and social interactions, including aggression, that involved considerable locomotory energy expenditures. Although we did not directly account for the energetics of these specific reproductive behaviors, they are critical to male reproductive success and on a daily basis they probably involved much greater energy expenditure than sperm production. Some yearling males avoided these costs by foregoing testicular development, yet they allocated four times as much energy to growth as older males, thereby increasing somatic condition for the future.