Abstract
The Gray-faced Sengi (Rhynchocyon udzungwensis) is a newly-discovered species of sengi (elephant-shrew) and is the largest known extant representative of the order Macroscelidea. The discovery of R. udzungwensis provides an opportunity to investigate the scaling relationship between brain size and body size within Macroscelidea, and to compare this allometry among insectivorous species of Afrotheria and other eutherian insectivores. We performed a spin-echo magnetic resonance imaging (MRI) scan on a preserved adult specimen of R. udzungwensis using a 7-Tesla high-field MR imaging system. The brain was manually segmented and its volume was compiled into a dataset containing previously-published allometric data on 56 other species of insectivore-grade mammals including representatives of Afrotheria, Soricomorpha and Erinaceomorpha. Results of log-linear regression indicate that R. udzungwensis exhibits a brain size that is consistent with the allometric trend described by other members of its order. Inter-specific comparisons indicate that macroscelideans as a group have relatively large brains when compared with similarly-sized terrestrial mammals that also share a similar diet. This high degree of encephalization within sengis remains robust whether sengis are compared with closely-related insectivorous afrotheres, or with more-distantly-related insectivorous laurasiatheres.
Highlights
The Macroscelidea – or sengis – are small-bodied insectivorous mammals notable for their well-developed proboscis and robust hindlimb musculature
When the brain size of R. udzungwensis is compared with the four other species of Macroscelidea in the dataset, the R. udzungwensis datapoint falls on the allometric trend line defined by the two smaller-bodied Elephantulus species and the two larger-bodied Rhynchocyon species (Figure 3)
The sample size is small, this indicates that the brain mass of R. udzungwensis is consistent with what would be expected in a sengi of its body mass
Summary
The Macroscelidea – or sengis – are small-bodied insectivorous mammals notable for their well-developed proboscis and robust hindlimb musculature Their unique combination of physical, behavioral, and life history traits have been described as a ‘microcursorial adaptive syndrome’ [1] which includes small body size (,1 kg), a unique, highly cursorial locomotion, primarily myrmecophagous insectivory, relatively exposed sheltering habits, social monogamy, precocial litters, and female absentee neonatal care [2,3]. Morphological studies have traditionally included sengis in the polyphyletic group ‘Insectivora’ (with shrews, hedgehogs, moles, golden moles, tenrecs, and solenodons). Such morphological studies highlight the adaptive similarities among small-bodied insectivoran mammals irrespective of phylogeny. Sengis are relatively well- understood in terms of taxonomic position, behavioral ecology, and general morphology, but very little is known about their neuroanatomy
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