Page 66 - Summer 2020
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ADAPTED EARS OF TERRESTRIAL MAMMALS
a hearing organ (Figure 1) equipped with signal-amplify- ing sensory cells sensitive to displacements measured on a nanoscale ruler (Clack et al., 2016).
The story that we tell here focuses on one well-known outcome of evolution through natural selection and one outcome that is just emerging.
Golden Moles and Their Remarkable Middle Ears
The golden mole subfamily Chrysochlorinae is home to
11 species of highly specialized mammals (Bronner, 2020). Members of this taxon distinguish themselves from the only other golden mole subfamily, Amblysominae, by virtue of middle ear specializations thought to augment subterranean auditory performance. Golden moles as a group are small, insectivorous, burrowing mammals inhabiting wide-ranging climates, altitudes, and floral sys- tems of sub-Saharan Africa. All species live a subterranean
lifestyle, save one, the Namib golden mole (Eremitalpa granti namibensis) that we highlight in this article (Figure 2, photograph). External ear openings of golden moles are tiny and covered with dense, iridescent hair, and they have vestigial eyes covered by tough, thick skin that render them image blind. These phenotypic features are clear indica- tors of their unusual, but certainly not unique, mammalian lifestyle. Their fusiform body shapes combined with specialized cranial features and appendages adapted for digging suit them ideally for a subterranean lifestyle.
The specialization of interest here, however, has nothing to do with digging, but everything to do with the detec- tion of subterranean “sounds” originating in the form of seismic waves in an ancient desert known as the Namib erg that extends along the Atlantic coast of Africa from
Angola in the north to the northern tip of South Africa. These soilborne waves almost certainly influenced the evolution of the auditory periphery of at least some mem- bers of this taxon. The chief evolutionary outcome of this process in Chrysochlorinae species was hypertrophy of the malleus, the middle ear bone commonly referred to
as the “hammer,” that is set into motion by the sound- induced vibration of the tympanic membrane, commonly known as the eardrum (Figure 1).
In some species of Chrysochlorinae, malleus size can be remarkable (Figure 2). To put the incredible nature of this adaptation in perspective, consider the species Amblyso- mus hottentotus, the Hottentot golden mole. Its average body mass is over 2.5 times the mass of the Namib golden mole, the smallest of the golden mole species. Although the malleus mass of the Hottentot variety scales propor- tionally with body mass, as with the majority of mammals, the mass of the Namib golden mole’s malleus is more than 60 times that of the Hottentot variant, which unambigu- ously justifies its designation as an evolutionary adaptation. Accordingly, it is reasonable to presume that a significant amount, if not the great bulk, of the malleus size difference between Hottentot and Namib varieties is the product of environmental modification, an adaptation shaped by the force of natural selection.
The Namib Golden Mole
The Namib golden mole has abandoned the inflexible underground lifestyle of its relatives; it is celebrated instead for its sand-swimming skills, so much so that they are known in colloquial terms as “sand sharks.”
 Figure 2. Bottom: relationship between body mass and malleus mass is shown for numerous mammals (purple area) and for Chrysochlorinae species (red area). Data obtained from Nummela, 1995; von Mayer et al., 1995; Mason, 2001, 2003; Mason et al., 2018; and Coleman and Colbert, 2010. Top: scaled reconstructions of middle ear ossicular chains (blue, malleus; light blue, incus; yellow, stapes) and inner ears reproduced from Crumpton et al., 2015, with permission. Inset: photograph by G. B. Rathun, reproduced with permission.
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