• Monkeys With Smaller Testicles Scream Louder to Compensate, Study Finds
    http://nextshark.com/howler-monkey-testicles-study

    A new study finds that Howler monkeys scream louder when they have smaller testicles.

    The study published yesterday in the journal Current Biology said that howler monkeys with deep roars — used to attract females, deter love rivals and scare off predators — tend to also have smaller testicals and a shortage of sperm compared to their peers.

    Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys: Current Biology
    http://www.cell.com/current-biology/abstract/S0960-9822(15)01109-4

    •Howler monkey hyoid volume varies significantly between sexes and among species
    •Hyoid volume negatively correlates with number of males per group and testes volume
    •Larger hyoids lower formant spacing, increasing the acoustic impression of body size
    •Results provide the first evidence of a trade-off between vocal investment and testes

    Summary

    Males often face a trade-off between investments in precopulatory and postcopulatory traits [ 1 ], particularly when male-male contest competition determines access to mates [ 2 ]. To date, studies of precopulatory strategies have largely focused on visual ornaments (e.g., coloration) or weapon morphology (e.g., antlers, horns, and canines). However, vocalizations can also play an important role in both male competition and female choice [ 3–5 ]. We investigated variation in vocal tract dimensions among male howler monkeys (Alouatta spp.), which produce loud roars using a highly specialized and greatly enlarged hyoid bone and larynx [ 6 ]. We examined the relative male investment in hyoids and testes among howler monkey species in relation to the level of male-male competition and analyzed the acoustic consequences of variation in hyoid morphology. Species characterized by single-male groups have large hyoids and small testes, suggesting high levels of vocally mediated competition. Larger hyoids lower formant frequencies, probably increasing the acoustic impression of male body size and playing a role analogous to investment in large body size or weaponry. Across species, as the number of males per group increases, testes volume also increases, indicating higher levels of postcopulatory sperm competition, while hyoid volume decreases. These results provide the first evidence of an evolutionary trade-off between investment in precopulatory vocal characteristics and postcopulatory sperm production.