F. Daniel Vogt

Influence of Ambient Temperature, Nest Availability, Huddling, and Daily Torpor on Energy Expenditure in the White-Footed Mouse Peromyscus Leucopus

Short-day (9 light:15 dark), cold-acclimated Peromyscus leucopus known to enter torpor (no. = 24) and P. leucopus never observed in torpor (no. = 12) were compared with and without a 5-g nest at 13, 7, and 1 C over 9 wk. Oxygen consumption and body temperature were monitored approximately hourly for 2-3 days under each treatment. A second group of short-day, cold-acclimated mice (13 torpid and 14 nontorpid) were monitored for oxygen consumption with and without a 5-g nest at 13 C, first individually and 10 days later as huddles of three. For individual mice, the time required to enter torpor (3.0-4.5 h), duration of torpor (4-5 h), and minimum body temperature during torpor (21-23 C) remained fairly constant at different ambient temperatures. The metabolic rate [cm³O₂/(g · h)] necessary to maintain minimum body temperature during torpor without a nest was 2.1 at 13, 3.0 at 7, and 3.7 at 1 C, while metabolic rates during the normothermic period at these temperatures were 4.8, 5.5, and 6.7, respectively. A similar trend, but with lower values, occurred with a nest. Without a nest, torpid mice had a mean daily metabolic rate which was 20% less at 13, 21% at 7, and 9% at 1 C than the corresponding value of nontorpid animals. Torpid mice with a 5-g nest reduced metabolic rate at those temperatures by 43%, 44%, and 34%, respectively, relative to nontorpid mice without a nest. Daily torpor and huddling together provided a 58% energetic saving at 13 C relative to individually housed, nontorpid mice. The addition of a nest fostered a 74% daily energy savings versus nontorpid, individual mice without a nest.

Seasonal Study of Spontaneous Daily Torpor in the White-Footed Mouse, Peromyscus leucopus

Thirty-five wild-caught Peromyscus leucopus were individually caged in outdoor enclosures throughout the year, and daily change in body temperature was monitored with telemetry. Spontaneous daily torpor (in the presence of food) was observed during most of the year, except during late summer, and was most frequent during winter. At this time mice became torpid on about 50% of the days that they were monitored. In order to study daily torpor in free-ranging populations, two 1.7-hectare study plots, consisting of either 72 or 60 nest boxes, were established in a Connecticut oakhickory forest. Daily torpor was observed in both mouse populations from mid-December through mid-February. The highest incidence of torpor occurred during early January when 20 out of 36 mice were found torpid. Torpor in free-ranging mice was never recorded on days when ambient temperature was above 3 C. Although abundant food was provided on one study plot, no decrease in daily torpor was observed. Free-ranging mice also exhibited considerable huddling behavior from mid-December through mid-February, and 96% of the animals in torpor were found in groups of two to six mice. In such cases, all mice in the nest box were usually torpid.

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

SummaryWhirligig beetles aggregate in the daytime into dense single-and multispecies groups (‘rafts’) of hundreds or thousands of individuals. On the 22km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same ocations.Most beetles apparently do not ‘home’ to the aggregation of their origin after dispersing at night because (a) the species composition of some aggregations changed greatly, and (b) paint-marked beetles (Dineutus horni) moved overnight from one aggregation as far as 4km, joining 11 of the 14 large (>300 beetles) D. horni groups on the lake.Throughout the night, the largest concentrations of beetles remained within 100m of the diurnal aggregation sites. Beetles reconvened into the compact rafts before daybreak, in part by following each other in sometimes long single files or ‘trains’. Their forward motion stopped after they joined large number of other beetles. We infer that following behavior enables those individuals that have dispersed from their original aggregations (during their nocturnal foraging) to find and join other aggregations before daylight.Naive fish ate the beetles despite their noxious secretions. However, fish living near rafting sites and feeding on insects on the water surface in daylight should soon learn to avoid the beetles. The rafting sites would then become ‘safe’ places. We observed fish attacking only those beetles that had been either dispersed from their rafts or released into open water away from raft sites in the daytime. We speculate that the evolutionary significance of the aggregation behavior is related to predator (fish) avoidance.

Thermoregulation in Bumblebee Colonies. I. Thermoregulatory versus Brood-Maintenance Behaviors during Acute Changes in Ambient Temperature

Colonies with 10–42 workers were subjected to ambient temperatures (Ta) of 3–38 C. Brood temperature (Tbr), the rate of oxygen consumption, the proportion of workers engaged in thermoregulatory behaviors (brood incubation and wing fanning), and the proportion of workers engaged in general brood maintenance were monitored. At Ta < 32 C Tbr was usually maintained at 28–32 C, and the percentage occurrence (incidence) of brood incubation varied inversely with Ta. At Ta > 32 C, wing fanning associated with a rise in Tbr and metabolism was prevalent. The incidence of brood maintenance was highest (40%–80%) at Ta = 25–32 C, when the combined incidence of brood incubation and wing fanning was relatively low. In contrast, the incidence of brood maintenance was <30% at near-freezing Ta and was 0%–10% at Ta > 34 C. These data suggest that the colonies attempt to regulate Tbr at a set-point temperature between 28 and 32 C and that the increased employment of workers for thermoregulation occurred at the expense of brood maintenance.

Thermoregulation in bumblebee colonies. II: Behavioral and demographic variation throughout the colony cycle

Colonies founded by wild-caught queens were reared at either 15 C with or without an insulated nest or at 25 C for the duration of the colony cycle. Brood-maintenance and brood-incubation behaviors, brood (Tbr) and nest-chamber (Tn) temperatures, and the number of bees produced were monitored. The total number of foragers was relatively fixed in young colonies. The occurrence of brood incubation varied inversely with population size and was highest at low ambient temperature (Ta). In contrast, the occurrence of brood maintenance varied directly with population and was highest at high Ta. All colonies had produced their maximum number of workers and started producing reproductives at 9-10 wk after the emergence of the first workers of the colony. The numbers of bees produced per colony at 15 C with insulation and at 25 C were similar. Uninsulated colonies at 15 C produced fewer bees and maintained lower Tbr. These findings suggest that low Ta may limit population growth but has no apparent effect on the timing of events during the colony life cycle.

Abdominal temperature regulation by arctic bumblebees

We compared the body temperature of arctic and temperate bumblebees in order to find out whether arctic bees have special thermal adaptations. The thoracic temperatures of arctic queens, drones, and workers foraging from flowers in the field were identical to those of temperate bumblebees. Arctic queens differed from temperate queens in maintaining higher abdominal temperatures (Tab). Both drones and workers also had lower Tab than queens, and in these castes Tab was indistinguishable between arctic and temperate bees. Several sets of data suggest that the high Tab of the arctic queens may be related to egg production or incubation within the body. Several observations suggest, however, that the developmental state of the ovaries, as such, is not the proximal stimulus for abdominal heating in foraging and flying queens.

Radiotelemetric assessment of diel cycles in euthermic body temperature and torpor in a free-ranging small mammal inhabiting man-made nest sites

SummaryFive free-ranging white-footed mice, peromyscus leucopus, known to inhabit man-made nest boxes were captured during winter, surgically-implanted with temperature-sensitive radiotelemeters, and released at the site of capture. The body temperatures of mice which voluntarily inhabited nest boxes were monitored from a remote location with an AM receiver on 17 days.All five mice were able to maintain stable euthermic body temperatures (approximately 35–37°C during the day; 37–39°C at night) during exposure to subfreezing ambient temperatures. Two body temperature profiles of daily torpor were obtained from one animal. On both occasions, the mouse entered torpor approximately 2 hours before sunrise, maintained a minimum body temperature of 18°C, and was fully aroused by 1500h.The influence of thermoregulation (during euthermia and torpor) on energy expenditure and survival in free-ranging mice is discussed.