NICHE SHIFTS AND SYNDROMES 1870.000.501.001.502.002.501 10 100 1 000
Body size of mainland population (g)10 000 100 000 1 000 000Didelphimorphia
Insectivora
Lagomorpha
Rodentia
Terrestrial carnivores
Carnivores with aquatic prey
ArtiodactylaSiFigure 7.4Demonstration of the island rule: body size trends for populations of insular mammals.Sirelative size of insular forms expressed
as a proportion of body mass of their mainland relative. (Redrawn from Fig. 1 of Lomolino 2005).
Box 7.1 Dwarf crocodiles and hominids
The dwarf crocodiles of Tagant (Mauritania)
Seventy years after the last reports of their
existence and six years after the IUCN listed them
as extirpated, relict populations of the Nile
crocodile (Crocodylus niloticus) have been
rediscovered in wetlands in southern Mauritania
(Shineet al. 2001). These populations appear to
have been isolated by climatic change associated
with the Pleistocene/Holocene transition, and
have persisted in association with seasonal ponds,
known as gueltas. In summer, these gueltas can
be smaller than 1 ha. Today it is thought that
there may be small populations of these dwarf
crocodiles in some 30 or so of these Mauritanian
wetlands, feeding mainly on the guelta fishes, in
competition with human settlers. In these small
‘islands’ the species has undergone dwarfism,
resulting in adults of no more than 2–3 m body
length, just half that of adult Nile crocodiles.
Dwarfism in human islanders?
Considerable press interest followed the discovery
of the remains of six individuals belonging towhat has been claimed to be an extinct species of
human (Homo floresiensis) from Liang Bua Cave
in the island of Flores, Indonesia (Brown et al.
2004; Morwood et al. 2004). The adults were
only 1 m tall, weighing just 25 kg and with brains
smaller than one third that of modern humans.
They are thought to have reached Flores
c.50 000 yearsBP, persisting alongside modern
humans until finally becoming extinct about
18 000 yearsBP.
Where did they come from? Although Homo
erectuswere considered to have reached Java as
early as 1.5 Ma, they were until fairly recently
thought not to have crossed Wallace’s line (the
25 km gap of open ocean dividing Bali from
Lombok). Finds of 0.8 Ma stone tools in Flores
might indicate that they did reach Flores, although
no bones have yet been found. Thus H. floresiensis
may have evolved from ancient H. erectus. Human
dwarfism due to insularity has not previously been
reported, but this change in body size is consistent
with the island rule, as discussed in the text.