Influences of body protein and body fat on oestrus of yearling caribou

Abstract

Abstract Age of primiparity is a critical demographic parameter that influences lifetime reproductive success and fitness. Caribou (Rangifer tarandus) exhibit variation in age of primiparity but generally are considered sexually immature (i.e. do not ovulate or achieve pregnancy) until they are ≥2.5 years old, unless nutrition is exceptional. Using captive, female, yearling caribou, I tested the exceptional nutrition hypothesis, focusing on ovulation because caribou were not permitted to breed. I quantified faecal progesterone concentrations throughout the breeding season and compared those to estimates of body mass, body fat and body protein of caribou. Body mass of yearling caribou ranged from 90.9 to 122.7 kg, body fat varied almost 5-fold (6.3–31.1%) and body protein ranged from 13.7 to 21.2 kg. Faecal progesterone ranged from 89 to 3334 ng/mg (673 ± 13; mean ± SE). Despite highly variable nutritional condition, all n = 11 yearlings exhibited oestrus. Mean date of first oestrus among yearling females was 22 October (±3 days) and caribou initiated oestrus 3.8 days earlier for every 1 kg increase in body protein. Yearling caribou experienced 3–9 (5.6 ± 0.7) oestrous cycles and for every 1% point increase in ingesta-free body fat, number of oestrous cycles between October and March increased by 0.3. Yearlings ovulated even in relatively marginal nutritional condition, indicating exceptional nutrition was not required for ovulation. Population assessments assuming yearling caribou are largely sexually immature may require additional scrutiny or contextualization, particularly if they are used to gauge nutritional status of populations to inform alternative conservation and management strategies.