FERTILITY OF FEMALE MOOSE (ALCES ALCES) IN RELATION TO AGE AND BODY COMPOSITION

Abstract

We determined the fertility of female moose in central British Columbia, Canada, by examining a sample of 1198 reproductive tracts (183 of those with associated kidney mass and kidney fat mass measurements) from moose that had been shot by hunters between 22 Nov and 10 Dec, 1977 to 1995. we determined body composition by assuming kidney fat mass was an index of body fat and kidney mass was an index of body mass. Logistic regression indicated that the probability of becoming pregnant was related to kidney fat mass and age but not to kidney mass. Pregnancy rate was 19% for yearlings and greater than 73% for cows 2 to 10. The probability of having twins was related to kidney fat mass, age and kidney mass. The twinning rate was 14% in 2-year-old or older females. Fertility declined after age 13. Cows required 1.68 kg of kidney fat to achieve a 50 % probability of conceiving twins but only 0.257 kg to achieve a 50 % probability of conceiving a single fetus. Young females required greater fat reserves for pregnancy than did older females and the pregnancy threshold was inversely related to moose size. A higher fat threshold for pregnancy in young female moose may be an adaptation to increase their probability of surviving through the winter and producing a viable offspring. Variation in fertility among populations was more likely related to variation in the fat-fertility threshold than to variation in fat reserves.