Today in EvoLiteracy News: Louder Vocalizations in Howler Monkeys Correlate with Small Testicles. The significance of this study is the simultaneous test of three hypotheses (vocal competition, environmental adaptation and size exaggeration) and the elegant documentation of an “evolutionary trade-off” in which vocalizations (rather than the usual morphological traits) are involved. Enjoy! – GPC

In 1871, Darwin suggested that the vocal organs of male Mycetes (Alouatta) –howler monkeys– have been sexually selected. What for? Darwin implied that, in sexually dimorphic species, females prefer and mate with males possessing conspicuous ornaments (e.g. large antlers, bright coloration, elaborate vocalizations), which convey information about fitness.

• Darwin wrote: “…The vocal organs of the American Mycetes caraya [Alouatta caraya] are one-third larger in the male than in the female, and are wonderfully powerful. These monkeys in warm weather make the forests resound at morning and evening with their overwhelming voices. The males being the dreadful concert, and often continue it during many hours, the females sometimes joining in with their less powerful voices… Whether most of the foregoing monkeys have acquired their powerful voices in order to beat their rivals and charm the females, or whether the vocal organs have been strengthened and enlarged through the inherited effects of long-continued use without any particular good being thus gained, I will not pretend to say; but the former view, at least in the case of [another primate] the Hylobates agilis [the gibbon*], seems the most probable…” (The Descent of Man and Selection In Relation to Sex, 1871).
  • [*Darwin highlights that the gibbon (H. agilis) “…is remarkable from having the power of giving a complete and correct octave of musical notes, which we may reasonably suspect serves as a sexual charm…]

A new study published in Current Biology explains that hyoid bones (crucial for roars in Alouatta) are highly sexually dimorphic, vary among species and with greater sexual dimorphism in species with larger hyoids. The study examines a phenomenon called “evolutionary trade-off;” a situation in which the enhanced expression of one trait (hyoid bone) is directly associated with the diminished expression of another (size of testicles). The assumption is that both traits are costly for an organism to express and, therefore, the trade-off resides in allocating resources to only one of them, for optimization.

Here is the story: All species of howler monkeys have a modified larynx with an enlarged hyoid bone, which works as a resonance chamber. Howls are energetically expensive to produce (link to video below). Now, what is the connection between the monkeys’ vocalizations and the size of their testicles? The study, authored by Dunn et al. (nine co-authors), explores three hypotheses: vocal competition (i.e. acoustic signals announce presence during defense of territories), environmental adaptation (i.e. hyoid bones are important for sound frequency adjustments in dense forests), and size exaggeration (i.e. voluminous hyoids work as enhancers of the acoustic impression of body size conveyed by roars).

RIGHT: Skeleton of a male Alouatta in which the hyoid bone is clearly visible –throat region. LEFT: two images of the hyoid bone showing its chamber-like structure (top) and smooth surface and elongated shape (bottom). The hyoid is very robust (made mostly of compact bone tissue).

Dunn et al. found support for the vocal competition and size exaggeration hypotheses, but not for the environmental adaptation. Here are some of the authors’ generalizations:

(1) males of various species, who live in larger groups of males, have smaller hyoid bones (panel A in figure below) and larger testicles (panel B), the latter an adaptation for sperm-sperm competition (the trade-off: because both roars and sperm production are expensive, only one trait is conspicuously expressed);

(2) males with large hyoid bones have smaller testicles (panel C in figure above; the trade-off again; these monkeys usually live in single-male-and-many-females groups);

(3) males with larger hyoid bones vocalize at low(er) frequencies (panel D in figure above; size exaggeration hypothesis); and

(4) all howler monkey species produce very low-frequency vocalizations in respect to their body mass and in contrast to other mammals or corresponding body mass (see figure below; also consistent with the size exaggeration hypothesis). – GPC

VIDEO: Take a look at the energetic movement of the thorax while this howler monkey vocalizes; the entire anatomy participates in the roaring. In the rainforest, howler monkeys’ vocalizations can heard from kilometers away.