Coming in from the dark: Mosaic Evolution of Subterranean Mammals. Regression, Progression and Global Convergence by Eviatar Nevo

Abstract

Oxford University Press, 1999. £95.00 hbk (xxvi + 413 pages)ISBN 0 19 857572 6‘Out of sight, out of mind’ might explain why the depth of our knowledge concerning the biology of subterranean mammal species is superficial relative to that of their aboveground counterparts. This disparity has been partially rectified with a dramatic increase in the number of publications over the past decade, but new species of underground dwellers are still being discovered and even their basic biology is unknown. In response to this information deficit, no less than three books, dealing solely with subterranean species, have recently been written. The first of these to surface is that of Eviatar Nevo’s monumental overview of the morphology, physiology, behaviour, genetics and ecology of all subterranean mammals. There is no disputing Nevo’s qualifications for this challenging task, he has spent a lifetime of work on the subject and has published over 100 senior-authored scientific papers. This book draws together information from all subterranean species and therefore provides a database for what Nevo terms a ‘global experiment in the comparative method of evolutionary biology’. What makes subterranean species a suitable model for such an endeavour necessitates a closer look at both the organismal and molecular adaptations demanded by the niche they have occupied.Dark, humid, poorly ventilated and cramped. This is ‘home sweet home’ to approximately 6% of mammal species, which spend all, or part, of their lives belowground. Relatively safe from predators, and the fickle extremes of temperature and humidity, the underground ecotope provides a vast, simple and stable niche – however, it comes with a handsome evolutionary price-tag. The energetic cost of foraging underground might be as much as 3400 times higher than for a similar sized animal aboveground. This daunting workload often has to be performed under oppressive conditions of low oxygen (hypoxia) and high carbon dioxide (hypercapnia). Add to this that the search for food is often ‘blind’ and that belowground productivity is low, and the stage is set for production of some of nature’s phenotypic eccentrics.The commonality of the challenges that life underground poses has led to remarkable examples of adaptive convergent evolution in subterranean species across the globe. A trademark of this convergence has been the marked regression (e.g. eyes) and progression (e.g. forelimbs) of key physical characteristics in response to the strong selective pressures of the underground niche. Thus, geographically disparate species, such as marsupial moles of Australia (Nototyctes typhlops) and golden moles of Africa (Chrysospalax spp.), are remarkably similar in physical appearance, with small tubular bodies, short robustly clawed forefeet, underskin eyes, no ear pinnae and a blunt hairless rostrum.Similarly, physiological adaptations have converged and common energy saving features of subterranean mammals include reduced metabolic rates, low body temperatures and high thermal conductance. To cope with the often poorly ventilated burrow systems, most belowground-dwelling species have enhanced oxygen carrying capacity of the blood and a tolerance to the high carbon dioxide concentrations typical of nests and sealed burrows.By contrast, life history patterns and behaviour do not appear to have converged to the same degree as the morphology and physiology of subterranean mammals. Except for the brief period of mating, most subterranean species are solitary and aggressive towards conspecifics. However, within certain families, such as the Bathyergidae, there is a range of behaviours from strictly solitary (e.g. the Cape dune mole-rat, Bathyergus suillus) to social cooperatively breeding species (e.g. naked mole-rats, Heterocephalus glaber). Naked mole-rats are an excellent example of an underground specialist – having all but lost the ability to thermoregulate, these poikilothermic, hairless, inbreeding socialites are wonderful examples of adaptation to a fossorial existence.This book excels in providing an in-depth look at the specifics of molecular and organismal evolutionary biology by using subterranean mammals as model organisms. It further serves as a wonderful reference source for initiating a literature search on a particular aspect of mammalian subterranean biology. Unfortunately, much of the information pertaining to the final comparative synthesis incorporating all subterranean mammals (section VI) was based on a data set compiled more than a decade ago. Thus, many of the recent advances in this field, with the exception of the author’s principal study animal – the blind mole-rat Spalax ehrenbergii, are not included and, therefore, its overall usefulness to the specialist reader is limited. To the student of evolutionary biology this broad overview of subterranean species will provide a wealth of information against which to critically examine aspects of the synthetic theory of evolution. It serves as one of the most striking examples of how life evolves through natural selection, in response to the challenges of a specialized environment.