Between Rocks and a Hard Place – Journal of Zoology Blog

Wale, C.H., McConnell, S., van Leeuwen, S., Cowan, M.A., Spencer, P.B.S., How, R.A. and Schmitt, L.H. (2025). Disruption and irruption shape genetic variation and population structure of the common rock-rat in north-western Australia. Journal of Zoology, vol. 325, pp. 334-349. https://doi.org/10.1111/jzo.13253

“I love a sunburnt country,/ A land of sweeping plains,/ Of ragged mountain ranges,/ Of droughts and flooding rains.”

Dorothea Mackellar’s classic Australian poem My Country captures the heart of Australia’s unique and unpredictable beauty, much like the resilient and adaptable common rock-rat that thrives in its varied landscapes.

Australia is quintessentially a sunburnt country of extensive plains – sandy deserts and rocky ranges, with upwards of 70% of the land mass defined as arid, characterised by extended periods of drought interspersed with infrequent episodic rain and occasional local flooding. Unsurprisingly, fauna has adapted to this harsh environment over the eons with many ecological strategies evolved to cope with discontinuous and unpredictable ecosystems in a generally challenging, nutrient poor environment.

The five extant species of Australian rock-rats form a distinctive genus (Zyzomys) of rodents whose ancestors first colonised from Asia in the early Pliocene, 5.3-3.5 mya. Rock-rats are confined to rocky ranges and outcrops – as their name implies – although in times of abundant plant growth and seeding may venture to the extensive grassy sandplains between their preferred rocky habitats. Despite these occasional short-term expansions, their geographic ranges are typically restricted, and three of the five species are listed as threatened taxa with major concerns for their long-term survival.

The common rock-rat (Zyzomys argurus) is the most widespread and abundant of the genus with a distribution transcending both the tropical monsoonal environments and much of the extensive arid interior of the continent. The monsoonal tropical biome has a predictable wet season that cycles with a hot dry season while the arid biome has an essentially dry climate punctuated by unpredictable and episodic rainfall events. This species has both adaptable demographic parameters and morphological traits that are ideally suited to survival in both of these major austral biomes. Dependent on prevailing climatic patterns, the flexibility of year-round breeding enables production of 3-4 young several times a year, bolstering populations when food resources permit. During such times of plenty, fat accumulation within its tail provides reserves for leaner times. This relatively long appendage is also essential for balance, complementing its ‘ripple-soled’ feet that facilitate grip over rocks, an attribute shared with many other saxicoline mammals.

Our study shows that this demographic flexibility allows the species to persist in both the predictable cycles of the tropical monsoonal biome and the substantially less predictable cycles of the arid biome. Responding by breeding continuously with the plentiful resources that follow major rainfall events, individuals may disperse across the mosaiced landscape, even sandplains, to either re-establish or replenish existing populations in favoured rocky habitats. This dispersionof individuals in the Pilbara leads to widespread mixing of local populations. The dominant drought conditions that follow inevitably lead to population declines, local extinctions and population fragmentation. Although not thoroughly documented across the continent, this “boom- bust” population pattern leads to marked genetic differentiation as populations are confined to their drought refuges.

This evolved demographic flexibility to variable climatic conditions encountered by the species is reflected in the genetic patterning we describe in this study. On a narrow scale there is evidence of differentiation in the genetic profile of local populations, likely a result of isolation and inbreeding during extended periods of drought that confine individuals to rocky refugia. Broadening our lens to the regional landscape, less clearly defined genetic variation is suggestive of sporadic periods of high resource availability, after cyclonic deluges, that initiate breeding, expansion and dispersal across the landscape, enabling breeding between otherwise isolated populations. At an even broader geographical level, and on islands, the differentiation in molecular diversity in the species is a consequence of major geographic barriers that prohibit or limit exchange of individuals for periods ranging from millennia to epochs because of the impermeable geographic landscape.

Lincoln Schmitt