by Greg Mayer
Australia is a zoogeographer’s dream world—it’s the most spectacularly distinctive place on Earth, and we know why. Around 250 million years ago, most of the world’s continental plates amalgamated into a single super-continent—Pangaea. During the Mesozoic (the “Age of Reptiles”), Pangaea began breaking up, with many of today’s southern continents (South America, Africa, Antarctica, Australia, and India) pulling away to form the somewhat smaller super-continent of Gondwanaland.
The continental breakup continued, with the various parts of Gondwana separating from one another (hence the traditional rallying cry of irredentist geologists, “Reunite Gondwanaland!”). Africa, India, and, most recently, South America eventually bumped into the northern continental masses, making for interesting geology, and—of the greatest importance for zoogeography—allowing large land animals to move between the major land masses. Such animals are not, in general, susceptible to “occasional means of transport”, as Darwin called them, that allow birds, bats, and smaller animals to traverse greater or lesser expanses of the sea, and thus large land animals require a dryshod path to disperse.
But, unlike those other Gondwanan remnants, Australia has not—yet, anyway—bumped into the northern land masses, and thus has undergone a long and and continuing period of splendiferous isolation, during which time many unique endemic forms have arisen, and radiated into the great variety of ecological niches occupied by different lineages in the rest of the world. Most famous of these are the Australian marsupials, which have undergone a continental-scale adaptive radiation, which Jerry highlighted in chapter 4 of WEIT.
This radiation has brought marsupials into most of the ecological niches inhabited by placental mammals in the rest of the world—predators, herbivores, gnawers, burrowers, insectivores, gliders, etc. Two things are evident in the radiation of Australian marsupials. First, that convergence can lead to remarkable similarity when distantly related lineages adapt to similar environments—Jerry highlights this in the figure above; but, second, that sometimes forms inhabiting the same ecological niches can be quite different.
We can see both of these by thinking about which animals are the big, dominant, mammalian carnivores and herbivores. Everywhere on Earth but Australia, these animals are cats, dogs, cattle, and deer (taking cattle in the sense of the family Bovidae, including antelope, goats, etc.). In Australia, the dominant carnivores are the thylacine (or Tasmanian wolf or tiger), native cats (hence, placental cats in the post title, to make clear who I meant), and devils. The skull of the thylacine is remarkably wolf-like, showing a close convergence in shape and dentition to the placental wolf. Native cats have the name, but are less similar to cats; and devils are pretty much sui generis. The dominant big herbivores in Australia are kangaroos: instead of plains full of buffalo and antelope, Australia is full of kangaroos. Although they eat similar types of plants, the modes of locomotion are startlingly distinct, showing that close convergence is not inevitable. (There were some more ungulate-like marsupials in the past, but they are now extinct.)
All this was brought to mind by a new article in BMC Evolutionary Biology by Katrin Koch and colleagues on the placental cats of Australia. For some millennia now, in addition to Darwin’s occasional means of transport, another factor has allowed animals to cross the seas—human transport. And for thousands of years, man has broken Australia’s tens of millions of years of isolation by bringing in a diversity of placental mammals. Most famous is the dingo, the feral descendants of dogs brought from southeast Asia about 4,500 years ago. Since European settlement, the number of mammalian imports—both wild and domestic—has increased dramatically.
In a careful review of historical records (which includes this memorable sentence in his methods section: “Incidentally, I discovered that indexers of books rarely index ‘cat’.”), Ian Abbott (2002) showed that, despite the potential for cats to have been brought to Australia earlier by Aborigines, Malay trepangers (sea cucumber fishermen—it’s great that there’s a word for that!), or shipwrecks, the first cats seem to have been brought in by the earliest European settlers in the late 18th century. The map below shows places where cats were presumed to have been introduced (arrows) and dated records of cat appearance (dots):
Koch and colleagues looked at microsatellite and mitochondrial DNA—both rapidly evolving parts of the genome, and thus good for studying infraspecific phylogeny—in over 200 feral cats from Western Australia and a number of islands round Australia, including the outlying territories of the Cocos (Keeling) Islands (an island group where cat control is an issue, due to their depredations on wildlife: Algar et al., 2003) and Christmas Island.
What they found was that the cats of mainland Western Australia, and the big island of Tasmania, were all genetically fairly similar (the red areas in the figure), while the smaller islands were more distinct genetically, including an island very close to the mainland (Dirk Hartog Island, DHI–green), as well as the two outlying islands of Cocos (yellow) and Christmas (blue). Curiously, two smaller islands off southeastern Australia grouped with Christmas. Small island populations can diverge due to random genetic effects (founder effect and genetic drift), historic phenomena (founded from different sources), and selective differences (distinct environments on islands). But it’s hard to tease these apart from these data alone.
This paper has made a minor splash in the media (see Jerry’s mention here about NY Times coverage), with most places proclaiming that it shows that Australian cats came from Europe as opposed to southeast Asia. Now, the paper does show that of the 63 mitochondrial haplotypes that they found, 25 are also present in Europe (the European data are from a paper by another group). But to distinguish sources of colonization, you need to have samples from all the potential sources, find out if the source populations have any diagnostic or characteristic alleles or mutations, and then see if these are found in the colonized (i.e., Australian) populations. But Koch et al. had only three non-Australian cats—apparently one from Sulawesi, and two from Borneo. (Curiously, they refer to these as “Malaysian”, and also use that term for 17th century Malay trepangers. But Malaysia is a 20th century political construct, and Sulawesi is in Indonesia, not Malaysia, so Malay would be a better term that covers the cultural/linguistic/geographic region.)
The three Malay cats group with Australian mainland cats. But on such a slim basis, we can make no conclusions about the relative importance of the two potential source areas. While concluding their data “indicate a mainly European origin of feral cats in Australia”, the authors do allow that, “However, caution is needed in inferring the involvement of Asian cats in the history of cat colonization in Australia due to the small number of Asian samples.” We can, in fact, be sure that a significant, if not the greatest, part of Australian cat ancestry is European, but that is because of the historical researches of Abbott (2002). The genetic work of Koch et al. lays a basis for further studies of genetic variation in Australian cats and their relation to cats from other regions, but it does not, on its own, really speak to the latter question.
Abbott, I. 2002. Origin and spread of the cat, Felis catus, on mainland Australia, with a discussion of the magnitude of its early impact on native fauna. Wildlife Research 29:51-74. abstract
Algar, D., G. J. Angus, R.I. Brazell, C. Gilbert and D.J. Tonkin. 2003. Feral cats in paradise: focus on Cocos. Atoll Research Bulletin 505. (Actually published in 2004.) pdf
K. Koch, K., D. Algar, J. B. Searle, M. Pfenninger and K. Schwenk. 2015. A voyage to Terra Australis: human-mediated dispersal of cats. BMC Evolutionary Biology 15 (262), 10 pp. pdf