Ontogenetic allometry reveals the imprint of myrmecophagy in the skull of the numbat, Myrmecobius fasciatus Waterhouse, 1836 (Marsupialia: Myrmecobiidae)

Abstract

Ontogenetic allometry, the covariation of shape with size over the course of development, represents a fundamental component of morphological diversification that can vary across species and in association with ecological factors. Ontogenetic allometry patterns for the skull have been described for species from several marsupial lineages, associated with common dietary niches, e.g., carnivory. We here conduct a novel detailed examination of cranial ontogeny in the numbat (Myrmecobius fasciatus), unique among marsupials in being myrmecophagous and adopting a strictly diurnal habit. We investigated ontogenetic allometry using multivariate analyses in a postnatal series (n = 28) of complete numbat skulls to estimate the rate of growth of its major constituent elements, described by 15 measurements. We find positive allometry for length of the nasals and palate, which relates directly to the remarkable rostral elongation in the species relative to other marsupials, in addition to several other ontogenetic changes. These comprise negative allometry of palate breadth, elongation of upper and lower toothrows that produces diastemata between weakly developed teeth, and decreasing muzzle height, all of which contribute to the development of a long, tapering, narrow rostrum, with limited mastication ability, as seen in the small temporal space and poor development of crests and processes associated with jaw musculature. Our specific allometric results and comparisons, together with qualitative observations, reveal a distinct imprint of myrmecophagy on skull ontogeny in the numbat and help explain the development and evolution of specialized feeding function of this species.