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example, the hearing and echolocation systems of bats and toothed whales are both based on the use of ultrasonics (see Simmons, 2017).
This, however, is frequently confused (e.g., Lorimer et al., 2015) with parallel evolution, which utilizes mechanisms and structures that were already present in the common ancestor. Strictly speaking, convergence, as defined by Fu- tuyma (2008), is the evolution of similar features indepen- dently in different evolutionary lineages, usually from dif- ferent antecedent features and/or by different developmental pathways. With respect to the evolution of mammalian, or even vertebrate, hearing, this is certainly not the case. In mammals, the shared sensory architecture and physiology arose from a common ancestry and not by convergent evo- lutionary processes. The traits involved show a high degree of conservation across the group, with specific adaptations arising from the modification of these preexisting cochlear structures and hearing processes.
For example, there is remarkable parallel evolution of the co- chleae in old world (Rhinolophidae) bats and a single known New World bat species (Pteronotus parnellii parnelli) that use constant frequency echolocation calls to detect the Doppler shift in insect wing beats in acoustic clutter (Schnitzler and Denzinger, 2011). In both Rhinolophidae and Pteronotus, the basilar membrane of the cochlea, which performs frequency analysis of acoustic signals, is greatly extended and thick- ened in the region sensitive to the call echoes (Vater and Kössl, 2011). In P. parnellii at least, it forms an acoustic fovea that behaves as an acoustic laser with exceptional frequency selectivity that is over an order of magnitude sharper than that of any other mammal (Russell et al., 2004).
Two consequences arise from common ancestry. First, it establishes that the origin of the general uniformity of the structure of the mammalian organ of Corti is the result of evolutionary conservation, not of convergence. This by it- self does not destroy the idea that physics guided the evolu- tion of the mammalian hearing sensor’s structure. As noted in the Introduction, the evolution of any sensory structure will be greatly influenced by the physics of the modality, the medium, and the structures involved. However, evolu- tionary conservatism stresses the importance of biological constraints that steer the evolution of these traits based on the historical contingencies that support them. In a way, the mere existence of a functional trait is itself a biological con- straint for the evolution de novo of a trait that could eventu- ally outperform the ancestral one. Second, the optimization of existing structures is evolutionarily more economical and,
due to the existence of well-adapted genetic “toolboxes,” far more likely than the production from scratch of a new one. Although physics often constrains what evolution can do to optimize hearing, biological constraints arising from evolu- tionary contingencies also limit the nature and degree of the physical process involved in hearing optimization.
The kaleidoscope of structural configurations seen in mod- ern groups of reptiles, birds, and mammals and their respec- tive subgroups is the result of the unique histories of each group (Manley and Köppl, 1998). At every stage, evolution- ary selection pressures worked on the physiological result of the input from the ear to the brain. If the input is adequate, in that it provides the sensory basis for survival and repro- duction, the particular construction principles of each type of ear are not only unimportant but they are not even “seen” by evolution. Thus, it is of no surprise that in many very im- portant respects, such as auditory sensitivity and frequency selectivity, the functional differences between the ears of mammals, birds, and reptiles are, despite large differences in structure, quite small (Manley, 2017). Hence, there is no single “optimal” solution for the realization of “ideal” hear- ing organs simply because the solutions are so massively, and usually irreversibly, influenced by history.
How Uniform Is the Construction
of the Mammalian Cochlea?
The idea of convergence toward a physics-guided “optimal” design of the hearing periphery is not supported by the di- versity of hearing organ structure that serves the enormous diversity of lifestyles across the animal kingdom. Even mam- malian cochleae (Figure 1) are far from uniform (West, 1985). For example, the number of turns in the cochlear coils (1.7-4.5) and sensory cell rows (3-4) vary even within single taxonomic families such as rodents (West, 1985).
The mammalian cochlea shows, as do the inner ears of rep- tiles and birds, a diversity concomitant with the variety of life- styles and thus the variety of selection pressures of different families, indeed of different individual species. Prey animals such as golden moles (Chrysochloridae) and mole rats (Het- erocephalus glaber) use infrasound to detect predators (Narins et al., 1997). At the other extreme, predators such as the fish- ing bat Noctilio and some toothed whales use sounds in excess of 200 kHz to locate their prey (Thomas et al., 2003).
Bats provide another fine example of selective pressure driven by predator-prey relationships that shaped their hearing organs but also shaped the ultrasonic detection of bats by some noc-
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