Digging Up Convergence in Fossorial Rodents: Insights into Burrowing Activity and Morpho-Functional Specializations of the Masticatory Apparatus

Abstract

Fossorial habits are tightly related to digging abilities in vertebrates and the most extreme fossorial specialization is being restricted to conducting the entire life underground.Many mammals, especially rodents, show behavioural, morphological and physiological adaptations to fossorial life, mainly for gaining access to sources of food and escaping predators and extreme climatic fluctuations. Adaptations to fossorial life are found in more than ten families of extant and extinct rodents, on most continents. Examples are Eurasian mole voles (Cricetidae), African mole-rats (Bathyergidae) and root-rats, Asian zokors and bamboo rats (Spalacidae), North American pocket gophers (Geomyidae) and mountain beavers (Aplodontidae), and South American tuco-tucos (Ctenomyidae) and cururos (Octodontidae).The constraints imposed by digging and living underground have led to strong behavioural and morphological convergences, notably involving the functioning of the rodent masticatory apparatus. Whereas most mammals use their claws for digging, rodents are unique in that some species use their ever-growing incisors for this purpose, with most subterranean species having become chisel-tooth diggers. Here, we review examples of convergence found in the main morphological and functional components of the rodent digging apparatus in relation to burrowing activity. We first present the different modes of digging in rodents, focusing on the chisel-tooth digging mechanisms and their associated burrowing behaviours. Following this, several morphological specializations of the skull and the main jaw adductor muscles are described in relation to their associated contribution to biting efficiency. Specialized incisors allow subterranean rodents to dig in hard soil and to consume hard subterranean parts of plants, and their morphological and structural characteristics are considered in the last part of this chapter. Data on incisor bite force of fossorial rodents are also compiled to highlight the enhanced efficacy of the masticatory apparatus of chisel-tooth digging species. Despite the different cranial and muscular morphotypes in rodents, we underscore the fact that multiple 3 modifications of the different components of the masticatory apparatus have led to similar overall morphologies and functions, overcoming phylogenetic inheritance. This remarkable example of convergence needs further scrutiny at both the micro-and macroevolutionary level to more fully understand how different rodent families evolved to deal with such external constraints.