I discussed this article earlier in the week based on what Yahoo News had to say. I have since been emailed the Biology Letters article and would like to go into a little more detail.
Bromham and Cardillo performed a literature and online database search to identify island primate populations that were somehow distinct from their mainland relatives. They restricted their search to islands less than 100,000 km2 since the literature on island dwarfing indicates that the effect is unlikely to occur on islands larger than that. For their analyses of primates, the authors examined island size, body mass, cranial measurements and head body length (or sitting height). They were not able to use encephalization quotient because such data were not reported in the literature for the primate species used. For primates they conclude:
Our analysis confirms that primates do undergo predictable shifts in body size when confined to islands. These observed changes in body size occur on islands not very distant from larger landmasses and over relatively short time-scales. Most of our comparisons are between subspecies, which in some cases may be less than 10 000 years old (Foster 1964; Smith 1992; Groves 2001), and virtually all of the islands included here were separated from the mainland after the last glacial maximum, probably less than 12 000 years ago.
There is some evidence that taxa with a greater degree of sexual dimorphism undergo a proportionally greater reduction in size on islands, possibly reflecting a role of intraspecific competition as a determinant of the island rule. Sexual dimorphism in primates has been considered an indicator of degree of intraspecific competition (Lindenfors 2002; Isaac 2005), thus may be expected to change in response to changes in the level of competition pressure on islands, as predicted under the island rule (Van Valen 1973; Lomolino 1985).
Then they looked at Homo floresiensis. As the authors point out in the supplementary material, in order to evaluate the claim that Homo floresiensis was subject to island dwarfing one has to know what its nearest mainland relative was. According to the literature their are three possibilities: Australopithecus, Homo erectus, or Homo sapiens. (Note: They took sexual dimorphism into account and only compared female specimens). They conclude that:
Secondly, the degree of size reduction observed in H. floresiensis, when compared with Homo sapiens and Homo erectus, falls within the range observed for other island primate species. For the mass dataset, the three largest island species (over 7 kg) are 52, 61 and 80% of the size of their mainland counterparts. The predicted mass of H. floresiensis is around 55% of the mass of modern Indonesian H. sapiens, around 52% of the estimated mass of Indonesian H. erectus and similar in body size to some australopithecines (see electronic supplementary material). Thirdly, although the type specimen of H. floresiensis (LB1) has an extremely small skull for a member of Homo, its skull length relative to head-body length is within the range expected for an island dwarf primate.
More important, they argue that their study can not be used to argue about the microcephaly or species identification. It seems clear to me, from reading the paper, that island did play a role in the evolution of Homo floresiensis (whatever Homo floresiensis turns out to be) as Bromham and Cardillo point out:
Examples of insular dwarf elephants and bovids have been used to argue both that hominid brains should shrink comparatively less (Martin et al. 2006a) or more (Brown et al. 2004) than their stature. We are unable to provide a direct test of these hypotheses due to lack of comparative data on brain volume for most of the primates included here. However, these results do suggest that other primate species undergo dramatic reduction in body mass, body length and skull length over comparatively short time periods when confined to islands, even relatively large islands that are not far from the mainland.
Prior to this study, most discussions about the role of island dwarfing on Homo floresiensis centered around elephants, bovids, or carnivores. The importance of this study is that we now have evidence from primates, which are a much better model than the above.