Reconstructing the reproductive history of female polar bears using cementum patterns of premolar teeth |
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Authors: | Sarah Medill Andrew E Derocher Ian Stirling Nick Lunn |
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Institution: | (1) Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada;(2) Wildlife Research Division, Science and Technology, Environment Canada, 5320-122 Street, Edmonton, AB, T6H 3S5, Canada;(3) Present address: Department of Environment, Government of Nunavut, Box 209, Igloolik, NU, X0A 0L0, Canada |
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Abstract: | Premolar teeth collected from 220 adult female polar bears (Ursus maritimus) from western Hudson Bay, Canada, were examined to determine whether past reproductive events are recorded in cementum. The
widths of annular cementum growth layer groups (GLGs) were measured and compared as proportional width index (PWI) values
to correct for age and body size bias. Known reproductive states (pregnant, with cubs, or with yearlings) were used to confirm
and calibrate cementum annuli. Significant differences in PWI were observed between GLGs formed the year females were pregnant
versus when accompanied by cubs or yearlings. The probability of a female having produced a cub in adulthood was determined
by fitting a logistic regression model between the ΔPWI of females when pregnant and with their cubs. Logistic regression
of ΔPWI (β0 = −0.229, β1 = −13.465, G
2 = 46.55, df = 1, P < 0.001) correctly classified the presence or absence of cubs in 72% of GLGs. Cementum width did not vary between different
litter sizes. Observations of females with early litter loss suggests that longer periods of lactation contributes to decreased
cementum width and therefore cementum may record a minimum age of litter survival. Predictions of litter production rate (0.43 litters/female/year)
derived from cementum were similar to field observations; however, age at first parturition was underestimated by 1 year.
We conclude that patterns of cementum deposition may be useful to determine individual reproductive histories and establish
course estimates of reproductive parameters when regular field observations are not feasible. We also conclude that reproductive
parameters derived from cementum are not adequate on their own for monitoring populations which are in decline or under stress
and field observation should not be replaced under these conditions. |
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