Australia, land of placentals (part I)

We are surrounded by annoying misconceptions about the diversity of animal life. For me, one of the most annoying and persistent of these is the idea that... drumroll... Australia is a 'land of marsupials' where - bar humans and introduced species like dingoes and rabbits - placental mammals have no presence. Well, it just ain't so...

In fact about 25% of Australia's mammal fauna is made up of placental mammals, and I'm not including marine mammals like sealions and cetaceans, nor bats, in this count. We are in fact talking about rodents, and specifically about murids at that (Muridae is the rodent family that includes the wide diversity of forms we refer to loosely as rats and mice). It's true that these natives aren't big-bodied animals the size of kangaroos (let alone all the recently extinct giant kangaroos, marsupial lions, wombats and diprotodontids), but they are still a significantly diverse, widespread and ecologically important component of the Australian mammal fauna. At long, long last, here is that article I've been advertising since May 2007 at least. Better late than never.

How Australian murids are related to other members of this immense group of rodents has remained uncertain. It has generally been thought that most Australian murids belong to a so-called 'old endemic' group (dubbed Hydromyinae) that is perhaps one of the most ancient lineages within Muridae (Watts & Aslin 1981), or perhaps even not part of Muridae at all (Simpson 1945). Some of these Australian murids have been regarded as distinct enough to warrant classification in their own taxonomic 'tribes' or 'subfamilies': Conilurinae was erected for the rabbit rats (Conilurus) and Pseudomyinae for the native mice (Pseudomys), for example. However, recent phylogenetic work has shown that Australo-Papuan murids form a clade with taxa from the Philippines, and that this clade shares an ancestor with another that is distributed across Africa, Europe and Asia (Steppan et al. 2005). It will be possible to use names for the various major clades within Muridae, but at the moment it's sometimes difficult to work out which name has priority, especially when the phylogenetic positions of taxonomically significant species are labile.

Australia is also inhabited by so-called 'new endemic' murids: apparently much younger than the hydromyines, all are members of the Old World genus Rattus. Some Australian rats (like R. sordidus and R. leucopus) are shared with New Guinea, and it seems that rats moved between these two landmasses several times during the Pleistocene (Taylor & Horner 1973, Taylor et al. 1983). At this time, these species probably used the terrestrial connections that then existed. To get to New Guinea in the first place however, they must have rafted across from SE Asia, and this seems to have happened more than once (Archer et al. 1999) [adjacent image shows Australia and New Guinea as they would have looked about 18000 yrs ago, from here. Note the extensive terrestrial connection between the two].

The fact that members of the Australo-Papuan + Philippine murid clade don't occur in SE Asia or Indonesia suggests to some that these murids are a relatively recent (as in, mostly post-Miocene) evolutionary development, and this is also hinted at by their fossil record. A fossil native mouse, Pseudomys vandycki, was named in 1989 from the early Pliocene Chinchilla Local Fauna of south-eastern Queensland and members of several extant genera, including stick-nest rats (Leporillus), short-tailed mice (Leggadina), rabbit rats (Conilurus), water rats (Hydromys), hopping mice (Notomys) and mosaic-tailed rats (Melomys) have fossil records extending back to the Pleistocene. A huge sample of Australian mammal fossils are now known from the Oligocene-Miocene sediments of Riversleigh in north-western Queensland, and murids are notable there for their absence: at the Pliocene Rackham's Roost site however, murids are reasonably diverse and include species of short-tailed mice, native mice and an extinct rock rat, Zyzomys rackhami (Archer et al. 1991, 1999, Godthelp 1997). Notably, bandicoots are particularly diverse and abundant in the Riversleigh sediments, leading some workers to suggest that bandicoots formerly occupied the niches that Australian murids do today. It's also been suggested that the diverse parrot fauna of Australia slowed or constrained the Australian murid radiation, given that parrots and rodents overlap somewhat in the resources they use (Rich 1991). Incidentally, Dasyuridae - the group that includes the marsupial mice and shrews, dibblers, dunnarts, planigales, phascogales, dasyures, quolls and Tasmanian devils - are only represented at Riversleigh by a couple of taxa and also seem to be a mostly post-Miocene event (Wroe 1999).

Small and plain vs big and cuddly

Most Australian murids might, by an unsympathetic person lacking in passion for the wonder of rodents, be described as nondescript and unremarkable. The native mice (Pseudomys), for example, look like standard-issue mice, though they do vary fairly considerably in size (head + body length 60-160 mm). The 25 or so species occur over most of Australia as well as New Guinea [adjacent image shows Long-tailed native mouse P. higginsi]. The two short-tailed mice (Leggadina) also don't look that special, though their anteriorly projecting upper incisors are a bit odd. Oh, and their tails are short. The five species of rock rats (Zyzomys) or thick-tailed rats look slightly less typical, with coarse fur, a delicate snout, short and broad feet, and a thickened, sparsely-haired tail. The tail is in fact particularly fragile and will easily shed its soft tissues when grabbed by a predator, the defleshed skeleton later dropping off and leaving the animal with a stumpy tail (this habit isn't unique to rock rats, but quite a widespread adaptation among murids). The two species of rabbit rats (Conilurus) also look (or looked: read on) fairly ordinary, despite their name.

However, in keeping with so many other animals on the continent, many Australian murids look weird. The two tree rat species (Mesembriomys macrurus and M. gouldii) look a bit freaky in my opinion: they're large (head + body length can be as much as 390 mm) with a long furred tail, a long but heavily built head and large-clawed furry hands and feet. Some of the other Australian murids are also arboreal like the tree rats: the prehensile-tailed rats (Pogonomys: one species is found in Australia, several others inhabit New Guinea), mosaic-tailed rats (Melomys: four species are found in Australia, many others occur in New Guinea, the Solomon Islands and on various other island groups) and giant naked-tailed rats (Uromys: two species inhabit Australia, various others are found in the Solomon Islands, New Guinea, and on surrounding islands) [adjacent image shows a prehensile-tailed rat].

The Broad-toothed rat Mastacomys fuscus looks superficially like a vole, and (like a vole) has broad molars and a narrow palate. The stick-nest rats or tillikins (Leporillus) are altogether cuter and cuddlier, and look something like long-tailed, round-eared bunny rabbits (one is shown in the composite image at the top: it's the animal in the top right corner). More on stick-nest rats later. Perhaps most atypical are the hopping mice or jerboa mice (Notomys). As the name suggests, these strongly resemble the Asian jerboas and are bipedal, long-footed, long-tailed, big-eared saltatorial murids with short forelimbs and large incisors. They all seem to possess a large sebaceous gland on the chest and are well known for the ability of some species to go entirely without drinking and to produce a hyper-concentrated urine. They dig burrows and some species form colonies [adjacent image shows Spinifex hopping mouse N. alexis, the best known species].

It's not just the appearance of some of the Australian murids that is odd: some of them also indulge in some very, very odd behaviour, and it's this we'll be looking at next. I'll post part II soon.

I went go-carting yesterday. Never, never again.

Refs - -

Archer, M., Arena, R., Bassarova, M., Black, K., Brammall, J., Cooke, B., Creaser, P., Crosby, K., Gillespie, A., Godthelp, H., Gott, M., Hand, S. J., Kear, B., Krikmann, A., Mackness, B., Muirhead, J., Musser, A., Myers, T., Pledge, N., Wang, Y. & Wroe, S. 1999. The evolutionary history and diversity of Australian mammals. Australian Mammalogy 21, 1-45.

- ., Hand, S. J. & Godthelp, H. 1991. Riversleigh: the Story of Animals in Ancient Rainforests of Inland Australia. Reed Books Australia, Kew, Victoria.

Godthelp, H., 1997. Zyzomys rackhami (Rodentia: Muridae) a new species of rockrat from the Pliocene Rackhams Roost Site, Riversleigh, northwestern Queensland. Memoirs of the Queensland Museum 41, 329-333.

Rich, T. H. 1991. Monotremes, placentals, and marsupials: their record in Australia and its biases. In Vickers-Rich, P., Monaghan, J. M., Baird, R. F. & Rich, T. H. (eds) Vertebrate Palaeontology of Australia. Pioneer Design Studio (Lilydale, Victoria), pp. 893-1070.

Simpson, G. G. 1945. The principles of classification and a classification of mammals. Bulletin of the American Museum of Natural History 85, 1-350.

Steppan, S. J., Adkins, R. M., Spinks, P. Q. & Hale, C. 2005. Multigene phylogeny of the Old World mice, Murinae, reveals distinct geographic lineages and the declining utility of mitochondrial genes compared to nuclear genes. Molecular Phylogenetics and Evolution 37, 370-388.

Taylor, J. M. & Horner, B. E. 1973. Systematics of native Australian Rattus (Rodentia, Muridae). Bulletin of the American Museum of Natural History 150, 1-130.

- ., Calaby, J. H. & Smith, S. C. 1983. Native Rattus, land bridges, and the Australian region. Journal of Mammalogy 64, 463-475.

Watts, C. H. S. & Aslin, H. J. 1981. The Rodents of Australia. Angus & Robertson, Syndey.

Wroe, S. 1999. The geologically oldest dasyurid, from the Miocene of Riversleigh, north-west Queensland. Palaeontology 42, 501-527.