108 Fecal metabolite monitoring as a tool to assess sexual maturation in polar bears

Abstract

Polar bears (Ursus maritimus) in managed care generally are paired for breeding starting around the age of 5 or 6 years; however, DNA analyses of the wild population indicate that males as young as 2 or 3 may sire offspring and females as young as 4 can produce cubs. There are no reports describing longitudinal reproductive hormone parameters in juvenile polar bears. The objective of the current study was to determine if seasonal shifts in testosterone and progesterone (P4) metabolites are detectable in feces of polar bears 2-3 years old as part of a greater effort to characterise reproductive metabolites in a large cohort of juveniles throughout sexual maturation. Subjects were 2-year-old male (n = 3) and female (n = 3) polar bears residing at 5 zoological institutions in the USA. Individuals were monitored for 1 (1.1) or 2 (2.2) years. Fecal samples were collected noninvasively 3-4 times/week and hormone metabolites were extracted as previously described. Testosterone was evaluated as an indicator of gonadal activity in both sexes, whereas P4 was measured in samples collected from females only. Metabolites were quantified in duplicate using established enzyme immunoassay techniques. Student’s and paired t-tests were used to compare mean metabolite concentrations between seasons [breeding (BS; January-June) and nonbreeding (NBS; July-December)] by sex and within individual, respectively. All values are reported as mean concentration (ng metabolite/g dried feces ± standard error of the means) and P-values less than 0.05 indicated statistical significance. Mean testosterone concentration of the 2-year-old males was 153.1 ± 112.5. Overall, testosterone concentrations were higher in samples collected from 2-year-old males during BS versus NBS; however, when examined within individual, this held true for only 1 of 3 males. Mean testosterone of the 3-year-old male was 170.2 ± 19.6 and values were significantly higher in BS (282.1 ± 27.2) versus NBS (74.6 ± 7.3). The overall mean testosterone and P4 concentrations of the 2-year-old females were 56.0 ± 21.7 and 57.4 ± 7.5, respectively. Two of the 3 females exhibited significantly higher testosterone concentrations during BS compared to NBS and all 3 exhibited higher P4 in the breeding versus the NBS. The 3-year-old female had significantly higher testosterone in BS (63.7 ± 4.1) versus NBS (40.5 ± 2.4) and showed evidence of regular ovarian cycles during BS. Despite no detectable differences in mean P4 between seasons (56.3 ± 7.9 and 55.1 ± 3.7), this female exhibited a distinct increase in P4 from October to November compared to the rest of NBS (85.7 ± 8 v. 43.5 ± 2.8), suggestive of pseudopregnancy. These results indicate that fecal reproductive hormone metabolite monitoring can be used to detect changes in metabolite excretion patterns associated with sexual maturation in polar bears and that males and females as young as 2 years old may exhibit seasonal variations in reproductive hormones. These data suggest bears in zoological institutions may be achieving sexual maturation earlier than believed previously and should be considered when managing bears in human care.