Postnatal development of the largest subterranean mammal (Bathyergus suillus): Morphology, osteogenesis, and modularity of the appendicular skeleton.

Abstract

Subterranean mammals show a suite of musculoskeletal adaptations that enables efficient digging. However, little is known about their development. We assessed ontogenetic changes in functionally relevant skeletal traits, and ossification patterns (periosteal and endochondral bone modules) in a truly subterranean scratch-digging rodent, Bathyergus. We studied 52 individuals (202 long bones) from a wild population by using a multiscale approach involving internal and external morphology. Multivariate analysis showed significant morphological changes during ontogeny. A specialized phenotype is expressed perinatally (eg, greater external robustness and developed olecranon, teres major, and deltoid processes), whereas adults presented slender bones with significantly thicker cross-sections. Ossification modules scaled mostly isometrically with body size parameters. Periosteal modules showed high variability and tended to grow faster than endochondral modules. Scratch-digging adaptations appear at perinatal age and then specialize in subadults. Early development of agonistic and digging behaviors and onset of sexual maturation seems to contribute to its development, although genetic factors also seem to play an important role. Ontogenetic differences are probably a trade-off to counteract weaker cortical bone properties and poor muscle development in juveniles, whereas slender but thicker cortical bones maximize bone resistance during burrow construction without compromising locomotor performance in adults.