OFFSPRING SEX-RATIO ADJUSTMENT IN REINDEER, RANGIFER-TARANDUS

Theoretically parents should be able improve their fitness by adjustin g the offspring sex ratio. Trivers & Willard (1973) predicted that in polygynous mammals mothers in superior condition should produce more m ales than weaker females. Rangifer tarandus (reindeer and caribou) is one of the most polygynous large mammals. However, we did not find any differences in body size, fat reserves or age between semi-domesticat ed reindeer (R.t. tarandus) females carrying male or female fetuses. O ur results challenge Trivers & Willard's model, and one hypothetical r eason for the lack of firm supportive evidence is the variability in w inter foraging conditions that may weaken the link between parental in vestment and offspring sex ratio.