Abstract
Knowledge of factors influencing the timing of reproduction is important for animal conservation and management. Brown bears (Ursus arctos) are able to vary the birth date of their cubs in response to their fat stores, but little information is available about the timing of implantation and parturition in free-ranging brown bears. Body temperature and activity of pregnant brown bears is higher during the gestation period than during the rest of hibernation and drops at parturition. We compared mean daily body temperature and activity levels of pregnant and nonpregnant females during preimplantation, gestation, and lactation. Additionally we tested whether age, litter size, primiparity, environmental conditions, and the start of hibernation influence the timing of parturition. The mean date of implantation was 1 December (SD = 12), the mean date of parturition was 26 January (SD = 12), and the mean duration of the gestation period was 56 days (SD = 2). The body temperature of pregnant females was higher during the gestation and lactation periods than that of nonpregnant bears. The body temperature of pregnant females decreased during the gestation period. Activity recordings were also used to determine the date of parturition. The parturition dates calculated with activity and body temperature data did not differ significantly and were the same in 50% of the females. Older females started hibernation earlier. The start of hibernation was earlier during years with favorable environmental conditions. Dates of parturition were later during years with good environmental conditions which was unexpected. We suggest that free-ranging pregnant brown bears in areas with high levels of human activities at the beginning of the denning period, as in our study area, might prioritize investing energy in early denning than in early parturition during years with favorable environmental conditions, as a strategy to prevent disturbances caused by human.
Highlights
Embryonic diapause, a widespread strategy to ensure and optimize successful reproduction, is common in plants, insects, fish, birds, and mammals [1,2]
Mean body temperature was significantly higher during the gestation period than during both the preimplantation and the lactation periods (Fig. 2A)
Age was not related to the length of duration of hibernation prior to parturition (b = 0.051, t = 0.334, P = 0.740, Fig. 5C). Body temperature This is the first time the timing of gestation has been documented in free-ranging brown bears
Summary
A widespread strategy to ensure and optimize successful reproduction, is common in plants, insects, fish, birds, and mammals [1,2]. Bears are the only mammals with delayed implantation, gestation, parturition, and lactation during hibernation, when they do not eat, drink, urinate, or defecate for several months. During this period they survive solely on their stored energy resources [4,5,6]. Gestation in ursids lasts approximately 60 days [7,8,9] This short period limits the energetic costs of reproduction by truncating embryonic development, which in turn reduces the size of offspring and the initial costs of lactation [8,10]. Quest [9] determined a
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