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

extant mammals at the genus level, where the frequency of
gigantism was marginally lower than expected (P = 0.1).
Mammals show a general tendency to follow the island rule
at the genus level. The observed slope of mean insular clade
mass versus mean continental clade mass is significantly less
than 1 for genera both including and excluding extinct species
(extant only: SMA slope = 0.96, 95% CI = 0.93–0.99, r =
)0.24, P < 0.05; Prandomization < 0.05, Fig. 4a; extant + extinct:
slope = 0.92, 95% CI = 0.89–0.96, r = )0.41, P < 0.01;
P randomization < 0.05, Fig. 4d; see also Fig. 1c). The slope for
families is not significantly different from 1 (slope = 1.01, 95%
CI = 0.95–1.08, and 0.95, 95% CI = 0.89–1.01, with and
without extinct species, respectively, Fig. 4b,e). The slope for
orders likewise is not significantly different from 1
(slope = 1.01, 95% CI = 0.74–1.37, and 0.87, 95% CI =
0.76–1.05, with and without extinct species, respectively;
Fig. 4c,f).
When analyses were conducted using the mammalian
phylogeny, regressing mean body sizes within insular clades
on the mean size within their mainland sister clades returned a
slope significantly shallower than 1 using SMA (n = 91,
slope = 0.931, 95% CI = 0.877–0.987, r = )0.25, P < 0.01),
Insular mean mass (log 10 scale)
but not using randomizations (P = 0.155; Fig. 5a). Restricting
the analysis to insular versus mainland sister species, we get a
slope consistent with the island rule using both tests (n = 57,
slope = 0.870, 95% CI = 0.81–0.93, r = )0.49, P < 0.01;
Prandomization < 0.05, Fig. 5b).
DISCUSSION
(a) (b) (c)
(d) (e) (f)
The largest species in bird and lizard taxa tend to be insular
more frequently than expected were insular status assigned to
species at random. Interestingly, however, this holds within
bird genera (but not when species maximum rather than mean
masses are used), whereas lizards only show gigantism among
families. Furthermore, the lizard giants are often the result of
insular radiations (e.g. Hoplodactylus, Gallotia, Cyclura) on
islands lacking mammalian carnivores (here New Zealand, the
Canaries and the Antilles, respectively). Such absence of
mammalian predation has been hypothesized to promote
lizard gigantism by allowing for more foraging, as less time is
spent hiding from predators, and by enabling lizards to evolve
the role of top predators (Case, 1982; Meiri, 2008). In birds,
the largest members of genera that are insular seem to be quite
Continental mean mass (log 10 scale)
Island vertebrates and body size extremes
Figure 4 Standardized major axis regression of mean body size (log 10-transformed mass, in g) on islands versus continents for species in
mammalian genera (a, d), families (b, e) and orders (c, f). Plots (a)–(c) are for extant taxa only and (d)–(f) include extinct taxa (see
Materials and Methods). The dashed line has a slope of 1 and an intercept of 0. The solid line represents the standardized major axis
regression slope estimate.
Journal of Biogeography 7
ª 2010 Blackwell Publishing Ltd