Slaying dragons: limited evidence for unusual body size evolution ...

irds is more likely in herbivorous taxa. Additionally, in lizards
insularity is often associated with large size and herbivory
(Troyer, 1983; Meiri, 2008). Gigantism may be favoured where
resources are abundant (McClain et al., 2006), and the size of
large carnivorous vertebrates may depend on the size of
available prey; thus islands lacking large herbivorous mammals
are likely also to lack large carnivores. Because mammals can
grow much larger than either birds or lizards, one might say
that even the largest avian and reptilian predators, Haast’s
eagle and the Komodo dragon, are not large predators
compared with large mammalian carnivores. Thus low predation
and competition pressures on islands may tend to
produce both relatively small mammals and relatively large
lizards.
The nature of the islands that we study, in terms of their
area and isolation, climate, geology (e.g. whether they are part
of the continental shelf, part of a tectonic plate or volcanic)
and biogeographic settings (e.g. realm, ocean) may all affect
the mode of size evolution (Meiri et al., 2005b; Schillaci et al.,
2009). Moreover, these attributes may interact with the
ecological attributes of the different taxa themselves, such as
their functional group or guild, their diet and microhabitat
preferences, as well as their behaviour (Case, 1978; McNab,
1994; Raia & Meiri, 2006) in shaping the way that size evolves.
Such attributes of islands and taxa offer promising avenues for
research into size evolution on islands.
CONCLUSIONS
The evidence that insular conditions favour the evolution of
extreme sizes within clades is restricted to gigantism in lizard
families and, perhaps, bird genera, but is not found in these
groups at other taxonomic levels, and neither does it apply to
mammals. Furthermore, large insular lizards seem often to
result from radiations on oceanic islands with no mammalian
carnivores whereas giant insular bird species are scattered over
highly variable set of islands (S.M., unpublished). We thus
think it is unlikely that these two patterns have a common
explanation.
The island rule applies in a statistical sense to mammalian
species within genera, and between sister species. Biologically,
however, while dwarfism in large insular mammals seems
prevalent, we find no evidence for the second component of
the island rule – a general tendency for gigantism in smallbodied
mammals. Within small-sized lizard families insular
species are smaller than mainland ones, and within largebodied
families insular species are larger than mainland ones,
reversing the island rule. These findings are consistent with
intra-specific studies (Lomolino, 1985; Meiri, 2007), suggesting
that similar selection pressures may operate to produce
patterns seen both within and between species. More comprehensive
fossil data are needed to resolve the pattern of size
evolution in island birds. The different courses of size
evolution on islands taken by different taxa imply an
important role for contingency, as animals differing in their
ecology respond differently to the selective forces imposed by
agents such as resource abundance, predation and competition,
which in turn differ across different islands.
ACKNOWLEDGEMENTS
We thank Liz Butcher and Barbara Sanger from the Michael
Way Library for their enormous help in obtaining literature
sources for data used in this work. Felisa Smith kindly
provided us with the latest version of the ‘Integrating
Macroecological Pattern and Processes across Scales’ (IMMPS)
working group mammalian mass database. We thank Ian
Owens for valuable discussion and Mark Lomolino, Craig
McClain, John Welch and two anonymous referees for very
helpful comments on earlier versions of this manuscript.
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