Discovery of sexual dimorphism of the laryngeal sac in the common minke whale Balaenoptera acutorostrata.

Mysticetes, or baleen whales, have an air sac on the ventral surface of the larynx known as the "laryngeal sac." The primary hypothesis regarding this structure's function is that it is involved in sound production. However, several other functions have been proposed, including air recycling, air storage, and even buoyancy control. In this study, we analyzed ontogenetic development and sexual dimorphism of the laryngeal sac with an aim of elucidating the function of this organ. The larynx of 61 (male: n = 40, female: n = 21) common minke whales Balaenoptera acutorostrata, collected from off the Japanese coast were used for present study. We isolated the larynx, situated between the hyoid bone and the trachea, during the flensing process. Seven linear measurements were taken using calipers, and the weight was obtained using a digital weight scale. Allometric equation and proportions to total body length or weight were used to compare laryngeal morphological differences between sexes and maturity. Measurements of laryngeal sac size were significantly larger in sexually mature males. Furthermore, examination of two male individuals of approximately the same body length but different maturities showed the sexually mature male had a larger laryngeal sac, compared to sexually immature male. The thickness of the laryngeal sac's muscle wall and the volume of the sac's lumen may be related to testes development (sexually mature whales have heavier testes). Only the width of the hyoid bone (basihyal and paired thyrohyals) was proportionally constant within all measurement sites, regardless of sex or maturity. We propose that baleen whales utilize their well muscularly developed laryngeal sac in a manner analogous to the human tongue, actively modifying its shape and volume to influence vocal production. Specifically, this structure may function as a resonance filter that creates a formant structure and contributes to the modification of phonemes generated by the U-folds of the larynx. Furthermore, the ability to produce complex vocalizations through this mechanism may have led to the enlargement of the laryngeal sac in males via sexual selection, where it also serves as a signal of their reproductive status.