When we think of what drives the frequencies of the sounds we make, we often think of small things making high frequencies and big things making low frequencies. For example a petite female usually has a squeaky voice, while a tall muscular man is expected to have a deep booming voice. In the animal kingdom, a mouse squeaks, and a lion roars. But there are exceptions to the rule.
I compared the vocalisation frequencies of 193 mammals from the terrestrial, semi-aquatic, and aquatic environments, investigating the influence of various drivers. I found that environment explained up to 25% of variation in vocalisation frequency limits, while body mass contributed up to 18%.
I found that semi-aquatic mammals could produce higher frequencies than terrestrial species of the same size, and fully aquatic mammals could produce frequencies even higher still. Imagine again that tall muscular man... now imagine that when he opens his mouth to talk, a tiny squeak comes out instead of the booming voice you expected. That is exactly what we found with aquatic mammals.
Martin, K., Tucker, M., Rogers, T.L. (2017). Does size matter? Examining the drivers of mammalian vocalisations. Evolution, 71(2), 249-260.
Image Below: from Martin et al. (2017)
A: Minimum frequency limit B: Maximum frequency limit