As in other species with harem-like mating systems, the voice organs of reindeer differ according to gender. Adult males have a much larger air sac than females and the young.
In early life, growth of the air sac seems to be comparable in male and female reindeer until they reach the age of two–three years, at which point the female's air sac stops growing, whilst the male's continues to develop.
The authors of the research paper further explain that the rutting calls appear to be understood by rival males as an indicator of the caller's fighting ability.
By contrast to male red deer, who are renowned for roaring with the head elevated, male reindeer emit a hoarse rattling with the head kept low. In this posture the inflated air sac expands the neck region. Both acoustic and optical display serve to deter rival males and thereby prevent fighting, allowing to conserve energy. Simultaneously, the performance serves to attract female mating partners.
Males vocalize almost exclusively during the autumn rutting season whereas females are mostly silent during that period. Instead, vocal communication of females tends to mainly occur with their young, in the first months after the birth in summer. In mother-young communication the air sac individualizes the calls and contributes to individual recognition.