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

S. Meiri et al.
Insular mean mass (log 10 scale)
(a) (b)
Continental mean mass (log 10 scale)
evenly distributed between continental shelf, volcanic and
continental plate islands (e.g. the Philippines, New Guinea,
Tasmania, Hawaii and New Zealand). This suggests that release
from predation on islands has often promoted gigantism in
lizards and perhaps gigantism associated with flightlessness in
birds (e.g. the New Zealand moas, Sylviornis; Russell, 1877;
McNab, 1994), but perhaps a different mechanism drove
gigantism among birds that retained their power of flight (e.g.
high population density; Blondel, 2000).
Size extremes in insular mammals show no departure from
null expectations. However, mammals conform to the island
rule at the genus level, especially when fossil species are
included. This pattern also emerges when we compare sister
species, a better test of size evolution than comparing clade
members ignoring their phylogenetic affinities.
In all three taxa, deviations from a slope of 1 seem to stem
from small (mammals and birds) or large (lizards) members of
generally large-bodied forms (e.g. small elephants, artiodactyls,
ratites and ducks, large iguanas). Extinct mammals show the
most drastic cases of dwarfism. However, many of the recently
extinct insular lizards and birds are extremely large (Pregill,
1986; Blondel, 2000) and extinction in these taxa was probably
much more prevalent on islands than on mainlands, whereas
extinctions of large mammals were common in mainland
settings (Barnosky et al., 2004). Thus in early Holocene times
there were giant insular birds in orders now exhibiting an
overall tendency for small sizes on islands. Indeed very large,
recently extinct, insular birds include members of the Falconiformes
(e.g. Amplibuteo, Harpagornis moorei, Circus eylesi;
Worthy et al., 2002; Suárez & Olson, 2007) and Strigiformes
(e.g. Tyto riveroi, Ornimegalonyx oteroi; Alcover & McMinn,
1994), ratites (Dinornis, Aepyornis; Worthy et al., 2002),
Anseriformes (e.g. Cygnus falconeri and the ‘very large Hawaii
goose’; Milberg & Tyrberg, 1993; Paxinos et al., 1999),
Ciconiiformes (Threskiornis solitarius, perhaps a Pelecaniform;
Mourer-Chauvire et al., 1995), Gruiformes (Diaphorapteryx
hawkinsii, perhaps Aptornis; Holdaway, 1989; Raia, 2009),
Galliformes (e.g. Megavitiornis, perhaps Sylviornis; Steadman,
2006) and Columbiformes (Raphus cucullatus, Pezophaps
solitaria, Natunaornis gigoura; Steadman, 2006; Worthy,
Figure 5 Standardized major axis regression
of mean body size (log 10-transformed mass,
in g) on islands versus continents within (a)
mammalian sister clades and (b) a subset of
the data in (a): only sister species [only clades
in (a) where both mainland and insular
sample sizes are 1]. The dashed line has a
slope of 1 and an intercept of 0. The solid line
represents the standardized major axis
regression slope estimate.
2000). Thus, the apparent tendency for smaller mean size in
insular members of large-bodied avian orders may be a result
of human-mediated extinction (Steadman, 2006; Pavia, 2008)
rather than a feature of natural insular evolution.
The finding that bird genera harbour more insular giants
than expected by chance is surprising, given that the mean size
of species within bird genera does not seem to differ between
islands and mainlands (Fig. 2a). A thorough study of the avian
subfossil record may even reveal that when extinct taxa are
included a similar pattern will be revealed within families and
orders.
A common explanation for insular gigantism in birds and
reptiles (e.g. elephant birds, moas, Komodo dragons and the
giant skinks of Cape Verde and New Caledonia) is that they
have evolved large size on islands with no mammalian
competitors or predators (Russell, 1877; Case, 1978; McNab,
2002; Meiri, 2008). While we view this as a highly likely
explanation, we are not sure it can explain our finding that
island birds tend to be the largest members of their genera
more often than is expected by chance. Our impression from
the data is that these largest members of avian genera are
mostly found on large islands, rich in bird, reptile and often
mammal species. Being classified as congenerics of mainland
forms, insular giants seldom occupy niches vacated by
mainland mammals, and usually differ relatively little from
the size of their mainland relatives. Currently we are unable to
sufficiently explain why this pattern prevails, or why it holds
only for birds, and only within genera, and note that species
maximum sizes [probably an inferior size measure because it is
more sensitive to sample size (Meiri, 2007) but representing
about 7% more species (Dunning, 2008)] do not show the
same trend.
The evidence we find for the island rule in mammals
emerges primarily via insular dwarfism in large taxa. Curiously,
the tendency of large mammals to dwarf on islands (see
also Raia et al., 2010), which is corroborated by our phylogenetic
tests, and when fossils are included, is also linked to the
absence of predators and competitors, and seems more
prevalent in herbivores than in carnivores (Raia & Meiri,
2006). McNab (2002) has claimed that gigantism in insular
8 Journal of Biogeography
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