Estradiol concentration and the regulation of locomotor activity.

We predicted that female prairie voles (Microtus ochrogaster) would not increase in locomotor activity during "induced" proestrus. We developed and tested two alternative a priori hypotheses to explain this predicted lack of activity. The Non-Response Hypothesis in which voles cannot, physiologically, increase activity in response to estradiol and, the Threshold Effect Hypothesis in which a minimal concentration of estradiol is necessary to achieve estrus, while higher concentrations influence other behavioral parameters. The non-response hypothesis predicts that hormone concentration will not affect locomotor activity, while the threshold effects hypothesis predicts that voles achieve estrus at low concentrations, while higher concentrations should trigger increasing locomotor activity. Initial results using running wheels indicated that females decreased activity during induced proestrus. Radioimmunoassay revealed that induced proestrus was achieved at relatively low concentrations of estradiol. Results from a dose response experiment allowed for rejection of the nonresponse hypothesis and supported the threshold effects hypothesis as females decreased activity at low concentrations of estradiol, showed no relative change at moderate concentrations, and increased activity at higher concentrations.

Diurnal variation in thermal and metabolic parameters of the golden hamster (Mesocricetus auratus).

The purpose of this study is to examine diurnal variation in several thermal and metabolic parameters of the golden hamster, Mesocricetus auratus. Metabolic rate, core temperature, and evaporative water loss were measured during night and day at several ambient temperatures. Wet minimal thermal conductance, dry minimal thermal conductance, basal metabolic rate, minimal net heat production and the lower critical temperature difference were estimated from these measurements. Wet and dry minimal thermal conductance, evaporative water loss, core temperature, basal metabolic rate, and lower critical temperature difference were greater during the active phase than during the resting phase. The diurnal variation in wet minimal thermal conductance was much smaller than that predicted from published allometric equations. The diurnal variation in wet minimal thermal conductance was 9% of the 24-h mean. The diurnal variation in dry minimal thermal conductance was 26% of the 24-h mean. The higher active-phase core temperature and basal metabolic rate may function to enhance peak metabolic performance during the active phase. The lower resting phase metabolism and core temperature may reduce energetic costs. The greater active-phase lower critical temperature difference may be a result of the greater active-phase basal metabolic rate. Diurnal variation in minimal thermal conductance may be caused by changes in peripheral circulation.

Energy allocation during concurrent pregnancy and lactation in Norway rats with delayed and undelayed implantation.

Despite the high cost of lactation alone, concurrent pregnancy and lactation (CPL) is widespread among rodents. Many species that exhibit CPL delay implantation of the litter in utero while nursing. The first purpose of this study was to describe the pattern of energy allocation during CPL in a species with a large degree of overlap between gestation and lactation. Resting metabolic rate, food consumption and mass changes of Norway rat dams and litters, digestive efficiency and urinary energy loss of dams, and pup tissue energy equivalents were determined for CPL dams and for dams that were only lactating (C). CPL dams had significantly higher metabolic rates than C dams. Food consumption, pup growth, tissue energy equivalents, and assimilation efficiency were similar for both groups. The energy equivalent of mass change was greater for C dams, which gained in maternal mass (lipid) during lactation, than for CPL dams, which only increased in mass because of the litter in utero. The second purpose of this study was to investigate the suggestion that delayed implantation during CPL evolved as a mechanism to lower peak energy demands during CPL. Concurrently pregnant and lactating dams were injected with estrone (ECPL dams) on days 3-16 of lactation to prevent them from delaying implantation. A group of dams that were only lactating also received estrone injections (EC dams). ECPL dams produced smaller offspring at weaning than EC dams.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of social environment on sexual maturation in female cotton rats (Sigmodon hispidus).

Age at sexual maturation among female cotton rats was measured in a variety of intraspecific social environments. Two experiments were conducted. In Experiment I, female cotton rats attained vaginal perforation and first estrus at younger ages and lighter body masses when paired from weaning with a conspecific juvenile male than when caged alone. In Experiment II, these findings were replicated and extended. Females housed with juvenile males matured at the youngest ages, while those housed alone matured at the oldest ages. Females housed with adult males matured at intermediate ages. Presence of a second juvenile female during maturation was significantly associated with early vaginal opening but not with early first estrus. The results of this study are discussed in context of similar social environmental effects on female sexual maturation that have been identified in other rodent species.

Critical radius of endotherms.

The critical radius effect for insulation, well known in the engineering literature, was used by other authors to explain the lack of insulation on newborn endotherms. If that effect existed in small animals, they would lose less heat if nude than if fur or feathers were present. We show 1) that the previous analysis, although incomplete, yields the same result as a solid insulation model with the required sophistication and 2) that a proper model of fur is a porous media model. Neither of two porous media versions yield a critical radius effect. No critical radius effect occurs because simultaneous heat transfer by conduction and radiation makes it impossible to obtain the required logarithmic increase in thermal resistance with increasing insulation radius in a porous medium.

Geographic variation in litter size in the cotton rat (Sigmodon hispidus): factors influencing ovulation rate.

Litter size in many mammalian populations varies along a gradient of latitude or altitude. This investigation tested the hypothesis that geographic variation in litter size among populations of the cotton rat is the result of differences in ovulation rate. Oviducts and uteri of virgin and recently mated lab-reared descendants of cotton rats from Kansas (KS), Texas (TX), and Tennessee (TN) were flushed on the day following the last day of estrus. Ovulation rates differ significantly among the three populations for both virgins (mean +/- SEM, KS = 5.3 +/- 0.3, TX = 4.6 +/- 0.5, and TN = 4.0 +/- 0.5) and for females that have recently copulated (KS = 6.5 +/- 0.3, TX = 5.7 +/- 0.7, and TN = 3.7 +/- 0.4; P less than 0.001). These nonvirgin females have significantly higher ovulation rates than virgins for KS and TX (P = 0.01 and 0.05, respectively), but there is no significant effect of copulation on TN. In all populations, ovulation rates of rats that release ova from both ovaries (KS = 6.1 +/- 0.2, TX = 5.9 +/- 0.4, and TN = 5.1 +/- 0.5) are significantly higher than those that use only one ovary (KS = 4.4 +/- 0.4, TX = 3.3 +/- 0.4, and TN = 3.2 +/- 0.3; P less than 0.001). The number of ovaries ovulating differs significantly between populations (P = 0.002). The effect of copulation on the number of ovaries ovulating is marginally significant (P = 0.08).(ABSTRACT TRUNCATED AT 250 WORDS)

Sex-Biased Litter Reduction in Food-Restricted Wood Rats (Neotoma floridana).

Mothers of eastern wood rats (Neotoma floridana) normally invest their lactation energy equally in male and female offspring, but alter that investment when their food is severely restricted during lactation. The effect of the altered investment is a significant bias against males in both mortality and growth.