There be carnivorous rats on those islands. Sixty-two species, to be exact, across the broader Indo-Australian Archipelago. Among them are small- and large-bodied rats, worm-eaters with elongated snouts (“vermivores”), and even amphibious forms (Fig. 1), and they are all the subjects of a recent Evolution paper by Kevin Rowe and colleagues on the ecological and dietary evolution in this group.
Among orders of mammals, rodents are the most diverse. Like, its not even close. Their diversification in continental Southeast Asia and Indonesia has been spurred even further by island biogeographic dynamics, resulting in highly endemic faunal assemblages across different parts of the archipelego. The pattern appears in many other taxa as well. If you have so much as glanced at a biogeography book, you know the boundaries of these assemblages are demarcated in memorium: Wallace’s line, Huxley’s line, Lydekker’s line.
Many species of rodents worldwide are omnivorous. And even more will consume animal material opportunistically. But dedicated carnivory like that observed in so many species of the Indo-Australian Archipelago is relatively rare. Rowe et al. used a multilocus phylogeny representing a significant portion of the murid rodent diversity in this region to reconstruct at least 5 independent transitions to carnivory over the past 7 million years.
Even more interesting is that at least 1 transition to carnivory has occurred on each major unit of the archipelago (Sahul, Phillipines, Sulawesi, and the Sunda Shelf). This is the kind of replication that many evolutionary biologists dream (drool?) about, because it provides compelling evidence for natural selection and the topography of the underlying adaptive landscape in this tropical region.
Yet another salient result of the paper speaks to a larger debate on how adaptive radiations proceed. Some models of radiation predict that, early in their history, taxonomic and ecological diversification should be tightly coupled. However, empirical support for this (often deduced from morphological characters) remains mixed. Similarly, Rowe and colleagues found that carnivory in Indonesian rodents did not evolve instantly upon colonization. Instead, transitions sometimes took several million years.
As more species-level molecular data are generated for Indonesian floras and faunas, it will be important to continue asking similar questions about ecological diversification in light of phylogeny.
Now that’s a research program worth sinking your teeth into. Unless you’re a vermivore.
Harmon, L. J., Losos, J. B., Jonathan Davies, T., Gillespie, R. G., Gittleman, J. L., et al. (2010). Early bursts of body size and shape evolution are rare in comparative data. Evolution, 64: 2385-2396. DOI: 10.1111/j.1558-5646.2010.01025.x
Rowe, K. C., Achmadi, A. S., & Esselstyn, J. A. (accepted). Repeated evolution of carnivory among Indo‐Australian rodents. Evolution. DOI: 10.1111/evo.12871