Reproductive timing and aseasonality in the sun bear (Helarctos malayanus)

• Published in: Journal of mammalogy 2012-04, Vol.93 (2), p.522-531
• Main Authors: Frederick, Cheryl, Hunt, Kathleen E., Kyes, Randall, Collins, Darin, Wasser, Samuel K.
• Format: Article
• Language: English
• Subjects: Animal reproduction; Aquariums; Bears; Biological and medical sciences; Black bears; Breeding seasons; delayed implantation; estrous cycle; Estrus; FEATURE ARTICLES; fecal hormones; Females; Fundamental and applied biological sciences. Psychology; Hormones; Mammalia; Mating behavior; Nature conservation; postpartum estrus; Pregnancy; pseudopregnancy; Reproduction; reproductive hormones; seasonality; Ursidae; Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution; Zoos
• ISSN: 0022-2372; 1545-1542
• DOI: 10.1644/11-MAMM-A-108.1

Most species of bears exhibit a reproductive strategy that includes a highly defined breeding season, delayed implantation, pseudopregnancy, and no postpartum estrus. Sun bears (Helarctos malayanus), a tropical lowland species, represent a potential departure from the typical ursid reproductive pattern. We studied reproductive timing in captive sun bears by examining fecal hormones and behavior of 13 female and 5 male sun bears, the entire North American population of H. m. euryspilus at the time of the study. Fecal estrogens and progestins in females did not vary by season or month, nor did fecal androgens in males. Estrus occurred in 11 of 12 months; breeding and births occurred year-round. However, birth rates were lower in the spring and higher in the fall and winter. Our study documented 1 complete pregnancy, in which delayed implantation did not occur; the delay between estrus and the progesterone rise (i.e., implantation) in the pregnant bear was similar to that seen in nonpregnant bears. All bears, whether nonpregnant or pregnant, exhibited a prolonged period of high progesterone after estrus, indicating obligate pseudopregnancy. Studbook records show that estrus can occur within 6 months after parturition; these cases were typically associated with loss of the previous cub. Our findings suggest that sun bears are unique among ursids, being polyestrous, nonseasonal breeders that do not appear to exhibit delayed implantation and that are capable of initiating estrus in the event of early loss of a cub.