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Investigating craniodental sexual dimorphism in bandicoots and bilbies using 3D geometric morphometrics

Ménard, Josephine Mira (2018) Investigating craniodental sexual dimorphism in bandicoots and bilbies using 3D geometric morphometrics. Honours thesis, Murdoch University.

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Evolution through natural selection enables species to adapt to their surroundings and optimise themselves for reproduction and survival. Sexual selection, on the other hand, reflects modifications to improve reproductive fitness of individuals within species. Animal morphology, therefore, often represents a compromise between these two selective pressures.

Bandicoots and bilbies (Marsupialia; Peramelemorphia) are small to medium sized omnivorous marsupials, of which roughly 30 species inhabit varied habitats throughout Australia and Papua New Guinea.

This study sought to quantify craniodental variation in a range of bandicoot and bilby species to investigate the relationship between natural selection and sexual dimorphism in this clade of animals.

Micro-CT and laser scanning techniques were employed to obtain three-dimensional scans of the crania and dentition of the 25 study species and subspecies (total n=124; 63 female and 61 male). Landmark analysis was conducted on 54 landmarks placed on the cranium and upper dentition to pinpoint areas of variation within and between the genera, species and sexes.

Interspecific variation in cranial morphology between genera and species does reflect taxonomic groups and likely reflects adaptation by natural selection for different ecologies in different clades. Macrotis was the closest to the consensus shape for Peramelemorphia. Within family Peramelidae, genera formed discrete clusters, reflecting diagnostic differences in cranial shape between the two genera, separating Australian the short-nosed bandicoots (Isoodon) from the long-nosed or barred bandicoots (Perameles). The Peroryctidae clustered together, with Peroryctes nested within Echymipera, and Microperoryctes between Echymipera and Macrotis. Echymipera kalubu was the only exception and nested within Macrotis, quite distinct from the other species of Echymipera.

Intraspecific variation showed strong evidence of sexual dimorphism in the species Macrotis lagotis and Isoodon fusciventer, while most other species did not have sexual dimorphism. In the two species that did show dimorphism in skull shape, there is also significant dimorphism in body mass, with males typically being larger. Shape changes did have allometric correlations with skull size, in PC axis 1 in M. lagotis and interestingly in PC axis 2 in I. fusciventer. In P. papillon, one of the smallest species, there was a trend towards female biased sexual dimorphism both size and skull shape, though this was not significant in our sample.

This study highlights the large range of expressions of craniodental sexual dimorphism present in Peramelemorphia, and quantifies distinctions between expression within and between genera.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Warburton, Natalie and Travouillon, Kenny
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