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Evolution, 60(8), 2006, pp. 1731–1742

THE ISLAND RULE IN LARGE MAMMALS: PALEONTOLOGY MEETS ECOLOGY

1,2 3,4
PASQUALE RAIA AND SHAI MEIRI
1 Universita` degli Studi del Molise, Dipartimento di Scienze e Tecnologie per l’Ambiente e il Territorio, Via Mazzini 8,
86170 Isernia, Italy
2 E-mail: pasquale.raia@libero.it
3 Natural Environment Research Council Centre for Population Biology, Imperial College London, Silwood Park Campus, Ascot,
Berkshire, SL5 7PY, United Kingdom
4 E-mail: s.meiri@imperial.ac.uk

Abstract. The island rule is the phenomenon of the miniaturization of large animals and the gigantism of small
animals on islands, with mammals providing the classic case studies. Several explanations for this pattern have been
suggested, and departures from the predictions of this rule are common among mammals of differing body size, trophic
habits, and phylogenetic affinities. Here we offer a new explanation for the evolution of body size of large insular
mammals, using evidence from both living and fossil island faunal assemblages. We demonstrate that the extent of
dwarfism in ungulates depends on the existence of competitors and, to a lesser extent, on the presence of predators.
In contrast, competition and predation have little or no effect on insular carnivore body size, which is influenced by
the nature of the resource base. We suggest dwarfism in large herbivores is an outcome of the fitness increase resulting
from the acceleration of reproduction in low-mortality environments. Carnivore size is dependent on the abundance
and size of their prey. Size evolution of large mammals in different trophic levels has different underlying mechanisms,
resulting in different patterns. Absolute body size may be only an indirect predictor of size evolution, with ecological
interactions playing a major role.

Key words. Body size, character displacement, competition, island rule, predation, resource base, sexual size di-
morphism.

Received November 30, 2005. Accepted June 4, 2006.

Vertebrates on islands undergo considerable body size body size alteration via life-history traits (Melton 1982;
changes over short evolutionary times (Lister 1989). In mam- Brown et al. 1993; Marquet and Taper 1998; Palkovacs 2003;
mals, large species seem to dwarf on islands while small Raia et al. 2003).
species increase in size. This well-known phenomenon (Fos- The above hypotheses are not mutually exclusive. Many
ter 1964; Lomolino 1985) was named ‘‘the Island Rule’’ (Van factors probably govern body size evolution on different is-
Valen 1973). The rule has many exceptions, with some large lands (Dayan and Simberloff 1998). In a recent review, Lom-
mammals getting even larger on islands (e.g., the Kodiak olino (2005) claimed that the island rule is a general emergent
bear Ursus arctos middendorffi, and the extinct Crete deer attribute at different phylogenetic scales and advanced the
Cervus major) and some small mammals getting smaller (e.g., existence of taxon-specific ‘‘fundamental’’ body sizes
Mastomys huberti [Ganem et al. 1995], Sundamys muelleri (Brown et al. 1993) functioning as evolutionary attractors.
[Nor 1996], Parantechinus apicalis [Mills et al. 2004]). Fur- Here we offer a new explanation for the island rule that
ther, Meiri et al. (2004, 2006) found no predictable trend for includes both ecological interactions and resources as factors,
carnivores. Nevertheless, the island rule continues to be a and we show that different mechanisms affect the sizes of
major theme in island biogeography (Clegg and Owens 2002; herbivores and carnivores.
Boback and Guyer 2003; Lomolino 2005; Lomolino et al. Many spectacular examples of body size evolution on is-
2005). lands come from fossil faunas in the Mediterranean (Am-
Many hypotheses have been advanced to explain this rule brosetti 1968; Malatesta 1980; Lister 1996; Sondaar 1977;
(Hooijer 1949; Foster 1964; Case 1978; Heaney 1978; Lom- Bover and Alcover 2000; Vos 2000), where there appear to
olino 1985; Roth 1992). Reduced insular species richness can have been several forms of minute elephants and hippopot-
drive the pattern through rarity of predators and competitors amuses (Caloi and Palombo 1983; Lister 1993). No wild
(Dayan and Simberloff 1998). Accordingly, Boekschoten and ungulates survived human colonization on any of these is-
Sondaar (1966) and Sondaar (1977) suggested that the re- lands. However, most of these faunas have been intensively
duced (often absent) predation pressure on islands allows studied for more than a century, and detailed stratigraphic
large species to attain smaller sizes because large size is a and paleoecologic reconstructions are available for some
means to counteract predation (e.g., Sinclair et al. 2003). (Vos 2000; Abbazzi et al. 2004; Marra 2005). These examples
Smith (1992) argued that in the absence of mammalian pred- afford the unique opportunity to study the island rule in large
ators small herbivores grow large to facilitate more effective ungulates. Size changes in insular carnivores are usually not
digestion. Other authors suggested that smaller size is a way as drastic as they are in herbivores. This may be because
of coping with resource shortages on small islands (Heaney other selective forces affect the sizes of animals in those
1978; Lomolino 1985; Angerbjo¨rn 1986; Roth 1992; Burness trophic levels. Although data on fossil carnivores are insuf-
et al. 2001). However, Meiri et al. (2005a,b; 2006) have ficient to allow statistical testing, a large, global dataset of
shown that neither island area nor isolation have a consistent recent insular carnivores, compiled by Meiri et al. (2004,
effect on carnivore body size. A third group of hypotheses 2005b) offers the possibility of comparing patterns and pro-
explains the island rule in terms of selective advantage of cesses in these two trophic groups.
1731

2006 The Society for the Study of Evolution. All rights reserved.