Browsing by Author "Kenagy, GJ"

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    Activity and space use by degus: A trade-off between thermal conditions and food availability?
    (2003) Bacigalupe, LD; Rezende, EL; Kenagy, GJ; Bozinovic, F
    We analyzed the influence of environmental temperature (open versus shaded habitat) and experimental food availability on surface activity of the degu (Octodon degus), a day-active herbivorous rodent that inhabits central Chile. We manipulated food availability and compared open and shaded study plots to determine the influence of thermal conditions on aboveground activity. Degus displayed a bimodal pattern of daily activity during relatively warmer thermal conditions in the austral autumn, whereas activity was unimodal during colder conditions in winter. However, they had a unimodal activity pattern under warm conditions in the shade when food availability was artificially enhanced in autumn. We observed more animals active in the plots where food was supplemented under all conditions. Our results illustrate spatial and temporal shifts in activity of degus involving a trade-off based on avoidance of exposure to heat and the search for food.
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    Cost of living in free-ranging degus (Octodon degus)
    (2004) Bozinovic, F; Bacigalupe, LD; Vásquez, RA; Visser, GH; Veloso, C; Kenagy, GJ
    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. (C) 2003 Elsevier Inc. All rights reserved.
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    Energy expenditure and testosterone in free-living male yellow-pine chipmunks
    (2002) Place, NJ; Veloso, C; Visser, GH; Kenagy, GJ
    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 M. 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. J. Exp. Zool. 292:460-467, 2002. (C) 2002 Wiley-Liss, Inc.
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    Milk composition of free-living yellow-pine chipmunks (Tamias amoenus)
    (2003) Veloso, C; Place, NJ; Kenagy, GJ
    Milk is the sole food source of mammals during early postnatal development, and its composition may be modified to meet changing nutritional and energetic demands from birth to weaning. These demands are especially acute in small mammals that breed in highly seasonal environments. We investigated the temporal course of milk composition during lactation in free-living yellow-pine chipmunks, Tamias amoenus, a small altricial omnivorous rodent that produces a single annual litter immediately after emerging from hibernation. Over the course of lactation the total energy concentration of milk increased more than two-fold, to approximately 13 kJ ml(-1). The main component of the milk was lipids, which increased from 10 to 30% of total milk content by wet mass. Proteins increased from approximately 5 to 10%, whereas carbohydrates remained low, at 4-5%. The progressive augmentation of milk energy content during lactation contributes to the increased demands of rapid growth in this altricial species. (C) 2002 Elsevier Science Inc. All rights reserved.
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    Relation of glucocorticosteroids and testosterone to the annual cycle of free-living degus in semiarid Central Chile
    (1999) Kenagy, GJ; Place, NJ; Veloso, C
    We investigated seasonal patterns of plasma glucocorticosteroids (GCs) in both sexes and testosterone (T) in males in relation to the annual cycle in central Chile of a natural population of the degu (Octodon degus), a caviomorph rodent. We wanted to find out which GCs are present in degus, whether their seasonal variation suggests suppressive or synergistic interrelationships with T, and whether seasonal variation in GC levels indicates a relationship with energy mobilization and demands of reproduction. Degus mated in late autumn, and female body mass increased in pregnancy and remained high during lactation and throughout spring. Over the subsequent period of summer drought both sexes declined to a minimal body mass before the next mating season. Cortisol appears to be the principal GC in degus. In fact cortisol levels were so high that the extremely low levels of corticosterone measured were probably largely due to the cross-reactivity of our corticosterone antiserum with cortisol. Titers of cortisol in females exceeded 1000 ng/ml at lactation in the spring of 2 years; cortisol declined greatly following lactation and during the summer and reached its lowest mean level of about 500 ng/ml at mating. Males were more difficult to capture than females and thus our sampling was limited, but male cortisol levels were similar to those of females during the times of year when we measured them. Male T levels remained within a low range all year, but at mating, when mean T was highest (0.16 ng/ml) and when most males had detectable T, degus showed their lowest cortisol levels. The minimal cortisol level of males during mating;represents a possible suppressive effect of T,as described in other mammals. At the time of their spring emergence, 60% of juvenile males had detectable T levels comparable to those of adults, suggesting important organizational effects of T at that time in their maturation. Peak cortisol titers in both sexes were associated with lactation in females, when energy mobilization, production, and body mass were at their greatest. (C) 1999 Academic Press.
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    Time and energy use under thermoregulatory constraints in a diurnal rodent
    (2000) Bozinovic, F; Lagos, JA; Vásquez, RA; Kenagy, GJ
    (1) This study was designed to understand how thermoregulatory constraints affect the timing of foraging bouts and amount of food obtained from a feeding station that was subject to an intense heat load. (2) The degu, Octodon degus, is a diurnal re-dent inhabiting hot arid environments in Chile. Two-hour trials were run in a laboratory experimental arena to determine time allocation between a food station located under a heavy radiant heat source (2500 W/m(2)) and the rest of the arena, including a cool burrow. A video camera recorded the animal's movements during the trial, and tapes were analyzed afterwards for seven degus with a heated food site and seven degus with a normal, unheated food station. (3) Degus with a food patch exposed to heat spent less time foraging and harvested less food than animals with no heat load. Over a two hour period time use of the food patch was significantly less when heated than when not heated (0.65 +/- 0.59 vs 4.90 +/- 1.73 min, respectively). Thus the degus subjected to heat above their food source minimized time exposure to a physiologically stressful condition by decreasing their visits to the heated food source, thereby reducing food (= energy) intake. (4) Oxygen consumption (VO2), evaporative water loss (EWL) and body temperature (T-b) at air temperatures (T-air) of 33-34, 36-37, and 39-40 degrees C were also determined. The metabolic response to rising T-air amounted to a doubling of VO2 and a five-fold increase of EWL at T-air 39-40 degrees C, compared with 25 degrees C; and T-b rose above 42 degrees C. (5) Using data from these metabolic measurements together with a theoretical heating model, heating curves were obtained that suggested that degus can exploit patches of food for only about 3 min at 60 degrees C and 8 min at 50 degrees C. These simulations are in agreement with our observations that degus reduced their time spent feeding when there was a thermal constraint on foraging. (C) 2000 Elsevier Science Ltd. All rights reserved.