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434 J.R. Michaux et al.
In contrast, predator pressure is clearly lower on small
islands compared to large islands, where it is similar to the
continental regions. The relationship between a lower pres-
sure of predators and insular gigantism could be explained
by the following scenario. On large islands or on the continent,
small mammals have a greater advantage to be as small as
possible to escape from predators. On small islands, with
fewer predators but also less habitat diversity and low food
reserves (Lawlor, 1982; Angerbjörn, 1986; Cirer & Martinez-
Rica, 1990), a larger size will have a selective advantage in
terms of mechanisms relating to food foraging, territory
defence and access to reproduction (Case, 1978; Angerbjörn,
1986). This kind of advantageous character will become fixed
if the population size is small and populations are isolated in
small areas (Michaux et al., 1996b).
The relationships and the interaction strength between the
insular Apodemus and particular predator species are likely
to differ. Indeed, only one predator species, the weasel
Mustela nivalis, is absent exclusively on the islands where giant Recherche et de la Technologie fellowship. This study was
individuals of Apodemus are observed. This suggests that
the interaction strength between these two species is very
considerable and that the absence of the weasel may be a key
determinant in the appearance of giant woodmice.
As A. sylvaticus colonized the studied Mediterranean
islands at least 2000 years before R. rattus and M. musculus
(Reumer & Sanders, 1984; Vigne, 1992; Audouin-Rouzeau,
1994), an additional hypothesis to explain the woodmouse
insular gigantism would be that: firstly, its body size may have
evolved in the absence of its principal competitors (through
the mechanism of decrease of interspecific competition), and
secondly, that the current high population levels of Mus and
Rattus may have caused ‘unnatural’ and very recent depres-
sion in the population levels of Apodemus. However, prior
to the arrival of Rattus and Mus on the islands, the number
of competitor species observed on small islands with large-
bodied Apodemus was already of the same order as that observed
on the large islands where no gigantism is observed (Reumer
& Sanders, 1984; Vigne, 1992). We therefore suggest that a
decrease of interspecific competition probably had a low role
in the appearance of the insular gigantism in the woodmouse
during this period and that our predator pressure scenario
retains relevance to the period when Apodemus was not in
competition with Rattus and Mus.
Finally, our results also highlight the importance of genetic
distances, i.e. genetic isolation, as a significant correlation
was found using independent contrasts (taking into account
the phylogeographical confounding effects), whereas a lack
of significant correlation was found using the raw data. A
decrease in gene flow seems to be required for selection for
increasing body size to occur.
As the number of predator and competitor species are of
the same orders in the North Sea islands where giant wood-
mice are also observed (St Kilda, Rhum and Fair islands)
(Delany & Healy, 1964; Delany, 1970), we can postulate that
our hypothesis may be generalized to this region. The same
reasoning may also be applied for the garden dormouse
(Eliomys quercinus) populations from the Mediterranean
islands. Indeed, the two islands where giant forms are observed
(Lipari and Formentera) are characterized by a significantly
lower number of predators, while the number of competitor
species is of the same order as on the large Mediterranean
islands where no gigantism is observed. Nevertheless, it
would be dangerous to extrapolate our hypothesis to other
insectivorous or carnivorous small mammals because their
predators and competitors are very different.
ACKNOWLEDGMENTS
This research was supported by a Belgian FNRS fellowship to
J. Michaux (mandat ‘Chargé de Recherches’) and by grants of
the FNRS covering field collections in the Mediterranean
islands. J. Goüy de Bellocq is supported by a Ministère de la
also supported by the Institut Français de la Biodiversité.
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