The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra, Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography

Abstract

The pink fairy armadillo Chlamyphorus truncatus is the smallest extant armadillo and one of the least-known fossorial mammals. The aim of this study was to establish if its middle ear is specially adapted to the subterranean environment, through comparison with more epigeic relatives of the groups Euphractinae (Chaetophractus villosus, Chaetophractus vellerosus, Zaedyus pichiy) and Dasypodinae (Dasypus hybridus). We examined the middle ears using micro-computed tomography and subsequent three-dimensional reconstructions. D. hybridus has a relatively small middle ear cavity, an incomplete bulla and 'ancestral' ossicular morphology. The other species, including Chlamyphorus, have fully ossified bullae and middle ear ossicles, with a morphology between 'transitional' and 'freely mobile', but in all armadillos the malleus retains a long anterior process. Unusual features of armadillo ears include the lack of a pedicellate lenticular apophysis and the presence, in some species, of an element of Paaw within the stapedius muscle. In common with many subterranean mammals, Chlamyphorus has a relatively flattened malleo-incudal articulation and appears to lack a functional tensor tympani muscle. Its middle ear cavity is not unusually enlarged, and its middle ear ossicles seem less robust than those of the other armadillos studied. In comparison with the euphractines, there is no reason to believe that the middle ear of this species is specially adapted to the subterranean environment; some aspects may even be indicative of degeneration. The screaming hairy armadillo, Chaetophractus vellerosus, has the most voluminous middle ear in both relative and absolute terms. Its hypertrophied middle ear cavity likely represents an adaptation to low-frequency hearing in arid rather than subterranean conditions.

Keeping an ear out: size relationship of the tympanic bullae and pinnae in bandicoots and bilbies (Marsupialia: Peramelemorphia)

“…( P. eremiana, P. myosuros , and P. papillon ). Larger bullae have been similarly noted in a number of species residing in low precipitation environments, such as rodents ( Alhajeri et al 2015 ; Alhajeri 2016 ; Khajeh et al 2019 ), armadillos (Cingulata; Basso et al 2020 ), and some leporid lagomorphs ( Kraatz et al 2015 ). Habitat aridity and complexity impact the distance different sound frequencies travel and therefore which frequencies carry the most useful information for sound localization.…”

“…This would make low-frequency sound detection more useful in arid conditions or where sound carries (i.e., for open conditions rather than closed environments). As a result, arid-adapted species tend to have larger, better-developed bullae compared with species from more mesic environments ( Keen and Grobbelaar 1940 ; Lay 1972 ; Hunt 1974 ; Basso et al 2020 ). Conversely, wetter environments, often typified by dense vegetation, pose a different kind of auditory environment: acoustic signals degrade rapidly as a result of obstacles blocking, refracting, and possibly altering the frequency of the signals ( Blumenrath and Dabelsteen 2004 ; Boncoraglio and Saino 2007 ).…”

PolyQ length-based molecular encoding of vocalization frequency in FOXP2

“…Vocalization and hearing abilities are scarcely characterized in most Xenarthra species ( Figure 3 C). Interestingly, however, the fossorial Dasypus novemcinctus , thought to have low-frequency hearing, 84 , 85 occupies the upper part of the scatterplot (ratio >4.5) while the insectivoran Tamandua tetrapoda is in the lower part, together with other non-xenarthran insectivorans.…”

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

“…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.…”