I used D N A fingerprinting to assess mating exclusivity in Peromyscus californicus, a species presumed to be monogamous. Putative genealogical relationships were determined in the field from transfer of fluorescent pigments. The putative father was confirmed to be the genetic father for 82 offspring from 22 complete families. An additional 17 offspring from 5 families were tested in which samples from either the mother or father were not available. The offspring within each family were most likely full siblings and hence sired by only one father. An incestuous mating between brother and sister was also confirmed with D N A fingerprinting. Thus, all offspring from 28 families resulted from exclusive matings between single male and female pairs over a 2-year period. There were no instances of multiple paternity detected, and mate switches occurred only after one member of a pair disappeared. This is the first convincing demonstration of exclusive monogamy in a mammal.

Home ranges, social organization, and nest co-occupancy of Peromyscus californicus were studied using radiotelemetry at the Hastings Natural History Reservation, California. Mated pairs were ascertained by the transfer of fluorescent pigments from lactating females to putative fathers. Mated pairs had largely overlapping home ranges that were not statistically distinguishable, whereas adjacent adults had mostly exclusive, statistically distinguishable home ranges. There was no difference in the mean home range of males and females, but mated females tended to have smaller ranges than their mate. Home range size was extremely variable (range: 150-3788m 2) and averaged 1161 m 2 across all individuals. Male home range size was inversely correlated with population density, suggestive of a social influence on home range. Putative fathers spent comparable amounts of time to females in the nest -presumably caring for the young -which supports previous laboratory reports of paternal care in this species. All data collected in this study are consistent with previous suggestions that P. californicus live in semi-permanent family groups and are monogamous.

In the laboratory rock elephant shrews (Elephantulus myurus; mean body mass 56.6 g) displayed the lowest torpor T yet recorded (ca. 5°C) in a placental daily heterotherm. It was unknown whether these low Ts were characteristic of daily heterothermy in free-ranging animals. It was also unclear how cost effective these low T s were since considerable energy is required to arouse from low Ts on a daily basis. We continuously measured body temperature once every hour for 85 days in 13 free-ranging E. myurus from May to August 2001 (winter) in Weenen Game Reserve, KwaZulu-Natal, South Africa. We recorded a total of 412 torpor bouts. Free-ranging E. myurus had a high propensity for torpor with females displaying higher torpor frequency than males. The lowest T recorded was 7.5°C at T=2.7°C and the minimum torpor T was strongly correlated with ambient temperature. Torpor arousal was tightly coupled with ambient temperature cycles. Low torpor T at low T s was therefore cost-effective because the animals offset the high cost of arousal through exogenous passive heating. Laboratory studies under constant ambient temperatures may therefore underestimate the energetic benefits of torpor in free-ranging small mammals that inhabit regions where seasonality is moderate.