Functionally mediated cranial allometry evidenced in a genus of rock-wallabies

In assessments of skeletal variation, allometry (disproportionate change of shape with size) is often corrected to examine size-independent variation for hypotheses relating to function. However, size-related trade-offs in functional demands may themselves be an underestimated driver of mammalian cranial diversity. Here, we use geometric morphometrics alongside dental measurements to assess craniodental allometry in the rock-wallaby genus Petrogale (all 17 species, 370 individuals). We identified functional aspects of evolutionary allometry that can be both extensions of, and correlated negatively with, static or ontogenetic allometric patterns. Regarding constraints, larger species tended to have relatively smaller braincases and more posterior orbits, the former of which might represent a constraint on jaw muscle anatomy. However, they also tended to have more anterior dentition and smaller posterior zygomatic arches, both of which support the hypothesis of relaxed bite force demands and accommodation of different selective pressures that favour facial elongation. By contrast, two dwarf species had stouter crania with divergent dental adaptations that together suggest increased relative bite force capacity. This likely allows them to feed on forage that is mechanically similar to that consumed by larger relatives. Our results highlight a need for nuanced considerations of allometric patterns in future research of mammalian cranial diversity.

An evaluation of faecal pellet counts to index rock-wallaby population size

This study assessed the efficacy of faecal pellet counts to index population size within yellow-footed rock-wallaby (Petrogale xanthopus) colonies. One hundred 1-m2 quadrats were permanently located in each of two colonies and emptied of pellets on a monthly basis between September 1996 and August 1998. The mean number of pellets (MNP) accumulated per season (quadrats as replicates) and a mean pellet presence/absence per quadrat index (MPAP) were calculated for each colony. Other population estimates and indices (direct counts, mark–recapture and aerial surveys) were also collected at these colonies or drawn from other studies, allowing comparison with the pellet data. The trends in abundance across seasons suggested by these additional estimates and indices were an overall increase at one colony with no change at the other. While these trends were reflected by the seasonal MNP trends across the eight seasons monitored, this was less often the case for the like-season MNP comparisons. The large degree of variation in seasonal MNP data may render it unsuitable for use in such short-term like-season comparisons. The MPAP seasonal data were also found to be unreliable as an index of population size for yellow-footed rock-wallaby. In Spring 1997, one pellet collection period was missed, resulting in a combined September–October 1997 count at each colony. This increased period between pellet counts appears to have resulted in increased pellet decomposition and/or loss. This disrupted sampling regime produced a marked discrepancy in the pellet indices, particularly in the colony with more individuals and thus more pellets to lose, and highlights the importance of a structured sampling regime.

What we don't know and haven't learned about cost - benefit prioritisation of rock-wallaby management

Research and translocations of brush-tailed rock-wallabies (Petrogale penicillata) in New South Wales have, in conjunction with studies in Victoria and Queensland, provided extensive insights yet also document the high variability in the species’ response to management. Nonetheless, experts are being asked to quantify predicted response for cost–benefit prioritisation models that will rank threatened species and populations worthy of future funding, with little consideration of the basic principles behind adaptive management. The weaknesses of these prioritisation models must be evaluated carefully by experts in order that appropriate advice is provided which genuinely assists decision-making. I explore the questions facing rock-wallaby ecologists as a case study of how much more we need to know and learn within adaptive approaches to conservation before our predictions are robust.

High levels of mitochondrial DNA divergence within short-eared rock-wallaby (Petrogale brachyotis) populations in northern Australia

Most population genetics studies of rock-wallabies conducted to date have examined remnant colonies of threatened species inhabiting southern Australia. In this study we examined the natural pattern of contemporary and long-term gene flow among colonies of the widespread and abundant short-eared rock-wallaby, Petrogale brachyotis, in the relatively unmodified landscapes of Australia’s tropical north. We sampled 105 wallabies from seven colonies 1.2 km to 250 km apart. Mitochondrial DNA (mtDNA) control region sequence analysis was conducted on samples from all colonies and microsatellite analysis (10 loci) on samples from the three largest colonies. The microsatellite data revealed no evidence of inbreeding within colonies, but higher levels of genetic diversity were found in the Kakadu National Park population compared with the smaller, more isolated colonies at Litchfield National Park. Both the mtDNA and microsatellite results showed that populations of P. brachyotis are naturally highly structured even within this relatively intact landscape, with only limited contemporary and long-term gene flow between colonies more than 1.2 km apart. Nine mtDNA control region haplotypes were identified within the seven colonies. There were unusually high levels of sequence divergence (up to 6.9%) within colonies at Litchfield NP. This divergence suggests that multiple taxa may exist within what is currently recognised as P. brachyotis. Alternatively, if current taxonomy is correct, the high levels of divergence raise the possibility of ancestral isolation and divergence of populations in allopatry with subsequent admixture at a secondary contact zone. The possibility that these unusually divergent haplotypes result from introgressive interspecific hybridisation with the sympatric P. concinna appears unlikely.