Skull shape in an endangered marsupial: would northern quolls rather ship out than shape up?

Blog written by Pietro Viacava & Vera Weisbecker. Read the full paper here. Featured image: scientific illustration by Nellie Pease.

When a species is threatened by habitat loss and environmental change, it is important to understand how to best preserve its populations. This cannot be done without a solid understanding of how diverse populations are. For example, if each population is unique, it is best to manage separate conservation units. Conversely, if they are all similar, then individuals from one population can boost the numbers of other, smaller populations without losing much of the species diversity. These conservation management decisions are mostly based on the genetic diversity of populations. However, the genetic patterns used for current conservation aims do not necessarily capture the variation in form (“morphology”). Because genetic and form diversity are not necessarily the same thing, adaptations that might be essential for the survival of a morphologically distinct population might be ignored.

In Australia, marsupials (“pouched” mammals, most famously the kangaroos and koalas) are of particular conservation concern, leading the world in mammal extinctions. Members of this fascinating group give birth at very early developmental stages. Their neonates – called “pouch young” – need to climb towards the pouch and attach to the teat of their mother with strongly developed forelimbs and snouts. To survive their suckling phase, the oral apparatus needs to maintain a certain shape. It has long been suspected that this requirement might also reduce the ability of marsupials to adapt to changing environments, even in the presence of substantial genetic diversity.

We tested this suspicion using a threatened and particularly wide-ranging species of marsupial, the northern quoll. These marsupial carnivores are opportunistic foragers the size of a guinea pig. They are also the largest semelparous animal, where nearly all males die off in their first year after an intense breeding season. Until European invasion and possibly even well before, their distribution ranged across 5000 kilometres of the Northern Australian coast: from the western arid environments of the Pilbara to the humidity of the eastern tropical rainforests. Nowadays, its fragmented distribution consists of four genetically distinct mainland populations and several islands. A perfect scenario to test if these populations show matching morphological differences!

We scanned specimens from museum collections and specimens collected by the Wilson Lab from the field in Groote Eylandt.

We travelled to six museum collections in Australia and North America using a 3D surface scanner to digitally acquire the shape of 101 skulls. We covered these 3D skulls with a dense set of 900 reference points. This allowed us to test for size and shape variation among and within the four mainland populations and one island population, and additionally assess if we could discern morphological adaptations to local climatic conditions.

This portable 3D surface scanner travelled with us to the museum collections.

Unexpectedly, we found that there is little population structure in their skull shape variation and no strong evidence of discrete “morphotypes” according to either population or climate. However, we did observe that size is the strongest contributor to shape variation: smaller individuals tend to present proportionally larger braincases and narrower, less pronounced cheekbones, while larger individuals tended to have proportionally smaller braincases and wider cheekbones that are more pronounced. This results in some small variation between populations because, on average, Northern Territory and Queensland animals are bigger; Kimberley, Groote and Pilbara are smaller. However, because there were no other strong determinants of shape, similarly sized individuals even from distant populations mostly share the same shape.

Shape variation due to size variation (allometry). Spheres represent the reference points (landmarks). Vectors in black show the magnitude and direction of variation from small to big specimens. Most of the allometric shape variation concentrates on the braincase and the zygomatic arches. For example, bigger skulls display proportionally smaller braincases.

We were surprised by the lack of shape differentiation across the extensive distribution of northern quolls, particularly because other species of vertebrates with similar breaks in their distributions tend to show genetic and morphological differences. For example, other marsupials such as wallabies and kangaroos, but also other vertebrates such as lizards, amphibians and birds, reveal distinctions of lineages at the corresponding northern quoll population breaks. Thus, we asked: is there something that is holding the northern quolls back?

It is very possible that northern quoll skulls are just the perfect match for multiple situations and prey types. This “one-shape-fits-all” situation would mean that, depending on the size, the corresponding shape is well adapted to match a range of environmental scenarios and related prey items. Thus, short and rapid modifications of their environment may not require them to “shape up”.

On the other hand, the combination of a substantial effect of size and lack of strong environmental influences might also mean that northern quolls are simply not very adaptable beyond a very narrow, growth-related line of shape variation. One would expect this if a developmental constraint, arising from the need for a strongly developed oral apparatus at birth, prevented the northern quolls from adapting their shape.

There are some intriguing lessons for the diversity and conservation of northern quolls in our study. Firstly, based on our morphological results on the skull, similarly sized individuals from any population share the same shape; therefore, no adaptive variation would be lost in eventual population translocations. Secondly, distinguishing between a comfortable “one shape fits all” scenario and a scenario where northern quolls are incapable of adapting, is crucial for future research. If marsupial carnivores are locked into a particular shape, then they will need far more help to survive environmental change than we might expect. Alternatively, if the “one-shape-fits-all” holds true, we may rest more relaxed in knowing that their adaptive capabilities will equip them for a changing climate. We look forward to unravelling these questions in future work within other species of marsupial carnivores, and deciphering how fast are other species “shaping up”.

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