Primate Audition: Reception, Perception, and Ecology

From masks to muscles: Mapping facial structure of Nycticebus

Facial musculature in mammals underlies mastication and nonverbal communicative facial displays. Our understanding of primate facial expression comes primarily from haplorrhines (monkeys and apes), while our understanding of strepsirrhine (lemurs and lorises) facial expression remains incomplete. We examined the facial muscles of six specimens from three Nycticebus species (Nycticebus coucang, Nycticebus javanicus, and Nycticebus menagensis) using traditional dissection methodology and novel three-dimensional facial scanning to produce a detailed facial muscle map, and compared these results to another nocturnal strepsirrhine genus, the greater bushbaby (Otolemur spp.). We observed 19 muscles with no differences among Nycticebus specimens. A total of 17 muscles were observed in both Nycticebus and Otolemur, with little difference in attachment and function but some difference in directionality of movement. In the oral region, we note the presence of the depressor anguli oris, which has been reported in other primate species but is absent in Otolemur. The remaining muscle is a previously undescribed constrictor nasalis muscle located on the lateral nasal alar region, likely responsible for constriction of the nares. We propose this newly described muscle may relate to vomeronasal organ functioning and the importance of the use of nasal musculature in olfactory communication. We discuss how this combined methodology enabled imaging of small complex muscles. We further discuss how the facial anatomy of Nycticebus spp. relates to their unique physiology and behavioral ecology.

The importance of auditory, olfactory, and visual cues for insect foraging in owl monkeys (Aotus nancymaae)

Nocturnal mammals have unique sensory adaptations to facilitate foraging at night. Owl monkeys (Aotus spp.) are pair-living nocturnal platyrrhines adept at capturing insect prey under low-light conditions. Owl monkeys use acoustic and chemical cues in intraspecific communication and use olfaction to detect fruit as they forage. We conducted an experiment to determine which cues (auditory, olfactory, and visual) Aotus nancymaae rely upon when foraging for insects. We scored the behavior of 23 captive owl monkeys during a series of trials in which monkeys were provided sensory boxes with insect cues either present (experimental box) or absent (control box). Each cue was tested alone and in combination with all other cues (multimodal cues). We used generalized linear mixed models to determine which cues elicited the greatest behavioral response. Owl monkeys approached and spent more time near experimental boxes than control boxes. Male owl monkeys were quicker than their female partners to approach the sensory boxes, suggesting that males may be less neophobic than females. The owl monkeys exhibited behaviors associated with olfaction and foraging (e.g., sneezing, trilling) during trials with multimodal cues and when only olfactory cues were present. When only visual or auditory cues were present, owl monkeys exhibited fewer foraging-related behaviors. After approaching a sensory box, however, they often touched boxes containing visual cues. A. nancymaae may rely on olfactory cues at night to detect a food source from several meters away and then rely more on visual cues once they are closer to the food source. Their use of sensory cues during insect foraging differs from nocturnal strepsirrhines, possibly reflecting physiological constraints associated with phylogeny, given that owl monkeys evolved nocturnality secondarily from a more recent diurnal ancestor.

Adding noise is a confounded nuisance

A wide variety of research and clinical assessments involve presenting speech stimuli in the presence of some kind of noise. Here, I selectively review two theoretical perspectives and discuss ways in which these perspectives may help researchers understand the consequences for listeners of adding noise to a speech signal. I argue that adding noise changes more about the listening task than merely making the signal more difficult to perceive. To fully understand the effects of an added noise on speech perception, we must consider not just how much the noise affects task difficulty, but also how it affects all of the systems involved in understanding speech: increasing message uncertainty, modifying attentional demand, altering affective response, and changing motivation to perform the task.

Hearing Sensitivity of Primates: Recurrent and Episodic Positive Selection in Hair Cells and Stereocilia Protein-Coding Genes

The large spectrum of hearing sensitivity observed in primates results from the impact of environmental and behavioral pressures to optimize sound perception and localization. Although evidence of positive selection in auditory genes has been detected in mammals including in Hominoids, selection has never been investigated in other primates. We analyzed 123 genes highly expressed in the inner ear of 27 primate species and tested to what extent positive selection may have shaped these genes in the order Primates tree. We combined both site and branch-site tests to obtain a comprehensive picture of the positively selected genes (PSGs) involved in hearing sensitivity, and drew a detailed description of the most affected branches in the tree. We chose a conservative approach, and thus focused on confounding factors potentially affecting PSG signals (alignment, GC-biased gene conversion, duplications, heterogeneous sequencing qualities). Using site tests, we showed that around 12% of these genes are PSGs, an α selection value consistent with average human genome estimates (10-15%). Using branch-site tests, we showed that the primate tree is heterogeneously affected by positive selection, with the black snub-nosed monkey, the bushbaby, and the orangutan, being the most impacted branches. A large proportion of these genes is inclined to shape hair cells and stereocilia, which are involved in the mechanotransduction process, known to influence frequency perception. Adaptive selection, and more specifically recurrent adaptive evolution, could have acted in parallel on a set of genes (ADGRV1, USH2A, PCDH15, PTPRQ, and ATP8A2) involved in stereocilia growth and the whole complex of bundle links connecting them, in species across different habitats, including high altitude and nocturnal environments.

Sensory disruption and sensory inequities in the Anthropocene

Anthropogenic disruptions to animal sensory ecology are as old as our species. But what about the effect on human sensory ecology? Human sensory dysfunction is increasing globally at great economic and health costs (mental, physical, and social). Contemporary sensory problems are directly tied to human behavioral changes and activity as well as anthropogenic pollution. The evolutionary sensory ecology and anthropogenic disruptions to three human senses (vision, audition, olfaction) are examined along with the economic and health costs of functionally reduced senses and demographic risk factors contributing to impairment. The primary goals of the paper are (a) to sew an evolutionary and ecological thread through clinical narratives on sensory dysfunction that highlights the impact of the built environment on the senses, and (b) to highlight structural, demographic, and environmental injustices that create sensory inequities in risk and that promote health disparities.