DISCUSSION
The rationale of this study was the expectation that the biogeographic
adaptive and genetic divergences between northern quolls should be
evident across their 5000-kilometre longitudinal range. Our aim was to
improve our understanding on whether the species represent morphological
or functional conservation units, which could be used alongside
population genetic approaches in the conservation management of the
species. Surprisingly, however, we found little structure in northern
quoll shape variation (~76% of shape variation remains
unexplained) and no strong evidence that any of the populations have
evolved into discrete, possibly locally adapted, morphotypes. In
particular, populations and sex differences have low effect sizes and
explain less variation than size, meaning that similarly-sized
individuals from opposite ends of the biogeographic distribution are
likely to be similar in shape, even if they are of different sex. It
also seems that most variation is evenly distributed within each of the
populations, such that more localized western populations appear just as
disparate in shape as individuals across the length of the eastern
Queensland seaboard.
There is some limited support for our concern that developmental
constraints might reduce the adaptability of northern quoll skulls, as
size explains the most shape variation and the populations seem to
differentiate by size. However, given the low amount of variation
(~16%) that size explains, and the broad overlap of
populations in size and shape, any such constraint is unlikely to be
strong. However, it is an intriguing indication that at least some of
the larger-scale evolutionary association between skull shape and size
among marsupials may be visible at the within-species level. This is
contrary to findings in other marsupials (Mitchell, Sherratt, Sansalone,
et al., 2018; Weisbecker et al., 2019), and might represent one of
several ways to shape morphological traits within the species.
Although differentiation of skull shape between populations had low
effect sizes, populations and sex are statistically distinguishable,
even when size is taken into account. This might suggest a stochastic,
possibly heritable, shallow divergence between populations which,
however, does not appear to reflect local adaptation. These effects also
demonstrate the ability of skull shape to vary independently of size
based on genetic factors related to sex or population, again
contradicting the developmental constraints hypothesis. Thus, the
population divergences do not appear to coincide with adaptive
morphological differentiations. This provides an indication that genetic
fitness benefits of outbreeding populations (Cardoso et al., 2009; Kelly
& Phillips, 2019) would not risk any adverse effect due to differential
local adaptation, although of course this would need to be further
investigated based on non-morphological (behavioural or physiological)
traits.
To better understand the functional implications of shape divergence
among the northern quolls, the displacement between landmark
configurations according to size, population, and sex can be visually
interpreted. The landmark displacement predicted by allometry identifies
two main regions of variation: larger skulls tend to have overall
smaller braincases relative to the rest of the skull, a larger sagittal
crest, a more anteriorly positioned masseteric scar and associated
dorsally-oriented zygomatic arch. Differences between sexes include
males with larger sagittal crests, smaller braincases, shorter nasals
and wider zygomatic arches.
Rearrangements of the zygomatic arch and the muzzle appear to
distinguish some northern quoll populations. This variation mirrors a
well-known evolutionary adaptation to changes in mastication (Meloro,
2011; Mitchell, Sherratt, Sansalone, et al., 2018; Mitchell & Wroe,
2019; Weisbecker et al., 2019). It is therefore possible that the
variation in muzzle length and zygomatic arch placement between some
populations could be interpreted as adaptation to a particular diet or
feeding habit within each population. For instance, populations in drier
environments, such as Pilbara or Groote Eylandt, show a shortening in
the muzzle, when compared to wetter environments, such as Northern
Territory or Queensland. Shorter faces are known to imply greater bite
forces, and thus might indicate the mastication of tougher foods
consisting of more vertebrates (Wroe & Milne, 2007). However, whether
this is truly an adaptive effect is doubtful due to the abovementioned
low effect sizes and extensive overlap in shape between populations, as
well as the lack of association between climatic factors and shape. In
addition, although precipitation is a main predictor of quoll diets
(Fancourt et al., 2015), this variable explains little variation in
shape.
Aside from hypotheses of microevolutionary adaptation, it is also
possible that much of the variation in the northern quoll skull derives
from a re-modelling process based on individual uses of the skull.
Perhaps the best example of this is the sagittal crest, which varies
widely in length among northern quoll individuals. It is common for
mammals – particularly males – to display larger sagittal crests with
age (Flores et al., 2006), but this is a purely behavioural consequence
of the pulling action of the temporalis muscle (Washburn, 1947).
Similarly, bone re-modelling during an individual’s lifetime has been
suggested for the zygomatic arch of mammals (Abdala & Giannini, 2000;
Ravosa, 1991), and is also suspected in wombats and kangaroos (Mitchell,
Sherratt, Ledogar, et al., 2018; Mitchell, Sherratt, Sansalone, et al.,
2018; Weisbecker et al., 2019). It is therefore possible that the
emphasis of shape variation on the masticatory apparatus, found along
PC1 and the allometric pattern, arise from masticatory behaviours and
possibly the “mating bite” (Braithwaite & Griffiths, 1994; Oakwood,
2000) of the larger males. This is also consistent with the weak
tendency of males to have shorter muzzles than females, regardless of
size, as shorter muzzles are related to higher bite forces (Wroe &
Milne, 2007). This again suggests that the differences between
populations are not a decisive factor to the individuals’ survival, and
rather originate from potentially non-adaptive factors such as genetic
drift or individual variation in feeding (Weisbecker et al., 2019)
between populations.
The geometric morphometric analyses of northern quoll skull shape adds
useful, quantitative, phenomic data to assessments of variation across
the distribution of an endangered marsupial. The overarching find of low
morphological differentiation, and very high levels of unexplained
variation, has two important implications. First, it suggests that
individuals of different populations are not locally adapted to the
point where a separation of population phenomes is indicated (although
it needs to be investigated if there might be behavioural or
physiological reasons to do so). On the other hand, the lack of
differentiation across the diversity of biomes, climatic conditions, or
populations is a concern because it suggests a low adaptability of the
species to environmental change. The concentration of shape variation in
the masticatory apparatus suggests individual plasticity is a major
response mechanism in the determination of northern quoll skull shape,
suggesting that there is little scope for larger-scale, heritable
variation within the species. A similar pattern of potentially high
within-species plasticity in the masticatory apparatus has also been
suggested for the living wombat species as well as kangaroos; together,
the concerning suggestion is that marsupial mammals might have a scope
for individual plasticity, but not evolve specific adaptations within
short time spans. Further research should be directed into identifying
the scope of shape variation in other threatened marsupials,
investigating other climatic variables or patterns as predictors, and
adding biomechanical and developmental studies to further dissect the
variation that exists in this clade; in addition, a comparison with
ecologically similar placental species would be useful to identify if
marsupials show less intrinsic capacity of shape variation than
placental mammals.