Associated tympanic bullar and cochlear hypertrophy define adaptations to true deserts in African gerbils and laminate‐toothed rats (Muridae: Gerbillinae and Murinae)

Abstract

Hearing capabilities in desert rodents such as gerbils and heteromyids have been inferred from both anatomical and ecological aspects and tested with experiments and theoretical models. However, very few studies have focused on other desert‐adapted species. In this study, a refined three‐dimensional morphometric approach was used on three African rodent tribes (Otomyini, Taterillini and Gerbillini) to describe the cochlear and tympanic bullar morphology, and to explore the role of phylogeny, allometry and ecology to better understand the underlying mechanism of any observed trends of hypertrophy in the bulla and associated changes in the cochlea. As a result, desert‐adapted species could be distinguished from mesic and semi‐arid taxa by the gross cochlear dimensions, particularly the oval window, which is larger in desert species. Bullar and cochlear modifications between species could be explained by environment (bulla and oval window), phylogeny (cochlear curvature gradient) and/or allometry (cochlear relative length, oval window and bulla) with some exceptions. Based on their ear anatomy, we predict that Desmodillus auricularis and Parotomys brantsii should be sensitive to low‐frequency sounds, with D. auricularis sensitive to high‐frequency sounds, too. This study concludes that in both arid and semi‐arid adapted laminate‐toothed rats and gerbils there is bulla and associated cochlea hypertrophy, particularly in true desert species. Gerbils also show tightly coiled cochlea but the significance of this is debatable and may have nothing to do with adaptations to any specific acoustics in the desert environment.

Morphological variations of auditory bullae in otomyine rodents (Rodentia: Otomyini) in southern African biomes

“…It was found that those otomyine species inhabiting regions with lower annual rainfall exhibited auditory bullae with larger volumes. This matched what was found by Nengovhela et al (2019) and we build on that study with the additional finding that this trend of inflated bullae in more arid regions is also found within a single, widely‐distributed species.…”

“…These small mammals cannot hide amongst dense grass in these dry, vegetation‐sparse regions, and thus communicating to their burrow‐mates of potential dangers is crucial in this environment (Walsberg, 2000). Specific tests for auditory ability in detecting particular frequencies have not been done for the otomyine rodents; however, studies of other rodents have indicated that bullar hypertrophy is linked with the detection of low frequency sounds (Manoussaki et al, 2008; Nengovhela et al, 2019; Vater & Kössl, 2011; West, 1985).…”

“…In arid regions, mammalian populations are generally less dense and acoustic signals would need to travel longer distances (Petter, 1953(Petter, , 1962. In comparisons of mammals and marsupials from different families, auditory bullae were found to be enlarged in desertdwelling species (Alhajeri et al, 2015;Basso et al, 2020;Mason, 2016;Nengovhela et al, 2019;Taylor et al, 2022). However, such morphological adaptations may be due to ancestral character retention, and may be limited by the ancestral phenotype.…”

Adaptation in brain structure and respiratory and olfactory structures across environmental gradients in African and North American muroid rodents

“…This study compiled data from various analyses of 3D composite images of 174 crania of 16 rodent species from two muroid families (Muridae, Cricetidae) assem-bled from two published studies (cochlea: Nengovhela et al 2019;endocranial volume: Nengovhela et al 2023) and hitherto unpublished data from our research group (brain structure and turbinate bones). A deeper insight into the role of ecological adaptations requires a narrow phylogenetic context and correction for the effects of body size.…”

“…2015; Nengovhela et al. 2019). This trend of increasing bulla size in desert rodents is usually explained by a perceived greater auditory sensitivity to low‐frequency (LF) sounds in open environments generated either by prey or by conspecifics (Lay 1972, Alhajeri et al.…”