Review of sensory modalities of sirenians and the other extant Paenungulata clade

Abstract

Extant members of Paenungulata (sirenians, proboscideans, and hyracoideans) form a monophyletic clade which originated in Africa. While paenungulates are all herbivorous, they differ greatly in size, life history, and habitat. Therefore, we would expect both phylogenetically related similarities and ecologically driven differences in their use and specializations of sensory systems, especially in adaptations in sirenians related to their fully aquatic habitat. Here we review what is known about the sensory modalities of this clade in an attempt to better elucidate their sensory adaptations. Manatees have a higher frequency range for hearing than elephants, who have the best low‐frequency hearing range known to mammals, while the hearing range of hyraxes is unknown. All paenungulates have vibrissae assisting in tactile abilities such as feeding and navigating the environment and share relatively small eyes and dichromatic vision. Taste buds are present in varying quantities in all three orders. While the olfactory abilities of manatees and hyraxes are unknown, elephants have an excellent sense of smell which is reflected by having the relatively largest cranial nerve related to olfaction among the three lineages. Manatees have the relatively largest trigeminal nerve—the nerve responsible for, among other things, mystacial vibrissae—while hyraxes have the relatively largest optic nerve (and therefore, presumably, the best vision) among the Paenungulata. All three orders have diverged significantly; however, they still retain some anatomical and physiological adaptations in common with regard to sensory abilities.

Trigeminal ganglion and sensory nerves suggest tactile specialization of elephants

“…22 Similarly, a recent comparative review of the sensory abilities of the Proboscidae devotes more space to elephant hearing than to elephant touch. 23 We note that the infraorbital nerve is more than 6-times thicker than the vestibulo-cochlear nerve (an $37 times larger cross-section). Thus, our data entertain the possibility that elephant touch is truly extraordinary.…”

Vision-related convergent gene losses revealSERPINE3’s unknown role in the eye

“…S3). A splice site mutation shared between dugong and manatee as well as patterns of relaxed selection indicate an ancestral loss of SERPINE3 in the ancestor of sirenians, which mostly use their tactile sense for navigation through murky water (45) (Fig. S1, Tab.…”

Convergent evolution in Afrotheria and non-afrotherians demonstrates high evolvability of the mammalian inner ear

“…All aquatic and semi-aquatic mammals had a cochlea with a limited number of turns, and hence a short cochlear canal, which is linked to a reduced sensitivity to low-frequency sounds in terrestrial mammals (West 1985;Manoussaki et al 2008). Most of them also had a broad basal cochlear turn that is not tightly coiled with the next one, a feature associated with highfrequency hearing in aquatic mammals (Fleischer 1976;Ekdale & Racicot 2015;Moore et al 2021). Arboreal mammals, especially the scansorial taxa, were characterised by a short common crus, and hence reduced mechanical coupling between the two vertical semicircular canals (Muller & Verhagen, 2002).…”