Darwin first set eyes on the Galapagos tortoises in the 1830s, during his
famous voyage on the Beagle, noting how each group had adapted to the
environment around it. But his visit also shows how hard it can be for the
dwarfs and giants created by the island rule to cope when their ecosystem is
disrupted, especially when humans are involved. Almost immediately on
landing on the islands, the Beagle’s crew killed 18 tortoises for their
fresh meat.
With no predators, the creatures were tragically easy game. They were so
large, it could take six men to carry one, and their meat became a staple
food. Not surprisingly, their numbers rapidly dwindled – their survival
threatened between hunters and collectors, keen for a trophy giant tortoise
to show off. Even modern attempts to ensure that Lonesome George’s species
survived were doomed: a female tortoise was imported for him, and lived with
him for 15 years, but while they did mate, the eggs were infertile.
The Galapagos offers the most famous examples of the island rule, but there
are equally bizarre examples closer to home. The fossil record of the
Mediterranean islands is particularly rich: many of the species (including
the mouse-goat and the species of giant shrew and dormouse in the Balearics)
were discovered in the early 20th century by the British scientist
Dorothea Bate, the first woman to be employed at the Natural History Museum
as a scientist. As well as the Balearic fauna, her finds included extinct
dwarf hippos and elephants on Cyprus and Crete.
Now, Dr Victoria Herridge and Professor Adrian Lister, palaeontologists at the
Natural History Museum, are re-examining Dorothea’s discoveries. They
revealed last month that one of her Cretan species was not an elephant, as
had been thought, but was a mini-mammoth – the smallest ever discovered. At
just 40in (one metre) tall, it was the size of a large dog.
The more you look, the more such strange adaptations can be found. On Malta
there was a swan too big to fly; the gigantic and terrifying ostrich-like
moa in New Zealand was at least 10 ft tall, while on Mauritius a species of
pigeon evolved into the flightless, hapless dodo. Madagascar was home to
three different species of miniature hippo, and on Jersey, the deer halved
in size in 6,000 years.
On Crete there were at least eight species of deer, which ranged in size from
dwarf to gigantic. Minorca had a giant rabbit, Nuralagus rex, which
weighed in at 22lb (10kg). Even humans are not immune: in Indonesia, fossil
remains no more than 3ft tall have been found. When Homo floresiensis
was discovered in 2003 on the island of Flores, it was immediately nicknamed
the Hobbit. A puzzling combination of primitive and human characteristics,
it did not become extinct until about 17,000 years ago, long after Homo
sapiens had become the dominant human species. But why it should have
evolved like this, and survived so late, is not understood.
Why this should happen to animals on islands raises fascinating questions.
What global changes forced species now found only in Asia and Africa to
migrate? Why should some animals swim or float to islands, and not others?
Why should some survive or evolve in dwarf and giant forms – and then become
extinct?
The subject is of great importance today, particularly in the context of how
mammals might respond to climate change. For what the island rule shows is
just how ably animals – even tortoises – can adapt to new and very different
environments.
During the Pleistocene period – the most recent series of ice ages –
normal-sized hippos and elephants lived round the Mediterranean. In the
colder phases, as the ice sheets spread as far as Germany and southern
Britain, sea levels dropped dramatically. Still, to reach Cyprus, for
example, animals would have had to swim at least 30 miles from the nearest
mainland: no land bridge emerged from the retreating sea.
On large land masses, great size is necessary for hippos and elephants as a
defence against predators. For some reason, those predators did not make the
journey to the islands. As the islands were so mountainous, and food less
plentiful, the animals’ shape seems to have gradually adapted. Successive
generations became smaller and stockier; with hippos, the preferred method
of movement changed from swimming to walking, and on the tips of their toes
rather than their footpads, presumably to cope with the rocky terrain.
Why these island giants died out is not certain, but it could have coincided
with the arrival of humans. As for the tiny creatures, it’s possible that
they drifted to islands on natural rafts of matted grass. Once isolated –
and again, with an absence of predators – shrews and dormice no longer
needed to be so tiny; with few enemies, mammals and birds had less need to
escape into burrows or holes, or take flight.
And, despite Lonesome George’s death, giganticism is still alive and well
elsewhere. Gough Island, a remote seabird colony in the South Atlantic, was
invaded by house mice thought to have escaped from visiting fishing vessels
more than a century ago. Once there, the island rule applied. With no
predators, the mouse adapted into a monster that can be 10in (25cm) long,
capable of killing a 22lb (10kg) albatross chick. Research into decreasing
numbers of the Atlantic petrel on the island, published a few weeks ago by
the Zoological Society of London, showed infrared film footage of mice
attacking the babies in their nests. The researchers estimate that, of 1.6
million chicks that hatch yearly, 1.25 million are eaten by the mice.
The Atlantic petrels are listed as an endangered species. Not surprisingly,
the researchers are calling for the mice to be eradicated – but whether the
bizarre rules of island evolution can summon up an appropriately monstrous
cat in time to save them is open to doubt.
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