Attack of the killer mice

On the desolate shores of Gough Island in the deep South Atlantic, a natural history horror story is unfolding. Giant mice evolved from animals aboard ships that stopped at the island in the 19th century are attacking local seabird chicks and devouring them alive. The RSPB has warned that the monster mice - up to three times heavier than those in the UK - threaten to chew rare species such as the albatross into extinction.

The chicks stand about a metre high and weigh up to 10kg; even at triple the size of their British cousins, the mice are 45g in their boots. "It is like a tabby cat attacking a hippopotamus," says Geoff Hilton, an RSPB biologist who proves not all scientists are unwilling to summarise complex predator-prey interactions in a snappy soundbite.

Snappy bites, it turns out, are exactly how the carnivorous rodents bring down the hapless young birds. "They're not actually attacking the birds," says the RSPB's Richard Cuthbert. "It's not like that scene in Monty Python where the rabbit kills people by leaping at the jugular. They just nibble away around the back of the bird. Imagine them chewing a pumpkin - that's what they're doing." (Another soundbite: hurrah for the RSPB's media training.)

Too slow to react, the birds suffer a death of a thousand bites as they succumb to blood loss and infected wounds over a couple of days.

But just how did these mutant killer mice grow so big? Robert Whittaker, a biogeographer at Oxford University and an expert on island biodiversity puts it down to two factors. The first is a natural reaction to bulk up in cold climates - and the South Atlantic answers the description. The second is the well-known "island effect": dump small mammals on an island with no predators and they invariably get bigger.

"The thinking is that islands release them from the normal predator pressure," he says. Bigger animals may usually make easier prey, take longer to reach sexual maturity or just live in tunnels wide enough to let a hungry weasel follow them home. Take away the weasel and the bigger mice will suddenly thrive to pass on their bulky dimensions to their children.

"There is a theory that there is an optimum body size," Dr Whittaker says. "The intriguing thing is that big animals stranded on islands get smaller."