Differences in inner ear asymmetry levels between slow-moving and fast-moving primates

The vestibular system of the inner ear, highly involved in maintaining balance during activities and positional behaviour of animals, has been deeply studied in order to link its morphology with the specific type of locomotion. Previous studies (e.g. Perier et al. 2016, Gonzales et al. 2018) have shown that, in primates, the amount of morphological variation is higher in slow-moving species than in fast-moving ones. These results indicate a lower selective pressure and a reduced functional demand for postural adjustments in the former group, and a higher selective pressure for balance and postural capabilities in the latter. In this study, we assessed if the difference in selective pressure are also linked to the fuctuating asymmetry levels of the vestibular system, and to which extent. Fluctuating asymmetry is defined as a non-directional variation between left and right sides of a bilateral character, and it may rise as a result of an inability to control development under genetic or environmental stress. There are very few analyses of the interspecific variation of the asymmetry levels of this organ in mammals in general (Berlin et al. 2013), and to our knowledge none so far at the intraspecific level. Here we analyzed the intraspecific variation of the bony labyrinth (inner ear) morphology and of the fuctuating asymmetry levels of left and right labyrinths among six species of lorisiform primates differing in their activities and positional behaviours of their locomotor repertoires: three being slow-moving (Loris tardigradus, Nycticebus coucang, and Perodicticus potto), and three being fast-moving taxa (Paragalago granti, Galago moholi, and Otolemur crassicaudatus). Our results highlight the difference in amount of intraspecific variation between slow-moving and fast-moving taxa. Furthermore, fuctuating asymmetry levels of slow-moving taxa also tend to be higher than in fast-moving taxa. Based on these results, it may be expected that relaxation of the selective pressure applied to the morphology of the bony labyrinth is the likely reason for this higher amount of intraspecific variation and asymmetry levels in slow-moving taxa, and that it may be related to a decreased demand for rapid postural changes.