Mice: tiny invaders with a lot of power, and they can cause quite a bit of havoc outdoors too
House mouse on the Farallon Islands NWR. Picture: Jim Tietz, Point Blue
‘Of all other living creatures, elephants cannot abide a mouse or a rat’, declared Pliny the Elder in his History of Nature (published in 77AD). Is this an ancient ‘fake news’ story?
Researchers at the University of Melbourne examined reliably-documented cases of elephants appearing to run from mice. They concluded that the great beasts were not responding to the tiny rodents as such, but to the sudden movement of a creature, in their immediate vicinity, which the poor-sighted elephants couldn’t see.
When shown mice held in the hand, up close and personal, elephants don’t respond at all. The world’s largest land animal is afraid of bees, and with good reason, but aversion to mice seems to be a myth.
‘Musophobia’ however is widespread. It is extraordinary that harmless creatures, weighing just 20 grams, can cause people 30,000 times heavier than them, to shriek and climb in panic onto chairs. But being able to inspire such fear is just one of this little animal’s accomplishments, as a report just published shows.
The Farallon Islands are situated 50km from the coast of California. The Southeast Farallon, about the size of Croke Park, supports a huge seabird breeding colony. A century or two ago, house mice were accidentally introduced there. To say that the invaders thrived in their new surroundings would be a gross understatement.
Scientists, led by Michael Polito of Louisiana State University, have studied the progress of the mice on the Farallons.
Mouse densities on islands elsewhere seldom exceed 10 individuals per hectare. Although rats are a major problem at some Irish seabird colonies, the presence of mice is not even mentioned. Their density on Southeast Farallon, however, reaches a staggering 1,300 per hectare in autumn.
How is such a population explosion even possible?
The food requirements of this multitude are gigantic. Mice, opportunistic feeders, will eat almost anything available, whether animal or vegetable. They are, it is thought, too small to break open seabird eggs or kill nestlings, although they will scavenge on broken eggs or the carcasses of dead chicks. Such food, however, is available only during the bird breeding season.
What do the mice eat during the rest of the year? The researchers used stable isotope analysis to answer this question. The technique identifies unique chemical signatures in animal tissue, indicating the types of food a creature has consumed.
The results showed that the Farallon mice change their diet with the seasons. In spring, when their population density is lowest, they take seeds and leaves. In early summer, insects are targeted. Detritus created by nesting seabirds becomes increasingly available as summer progresses.
When mouse numbers peak in autumn, the focus once again is on insects. This change has brought the mice into conflict with the park authorities.
The islands are home to a unique sub-species of amphibian; the Farallon arboreal salamander — a creature depending on insect food for its survival. In autumn, unfortunately, the mice ‘eat it out of house and home’.
The seabirds also suffer. Burrowing owls and other birds of prey, attracted by the mouse bonanza, converge on the Farallons, where they also target seabird chicks.
A day of reckoning is coming.
CONNECT WITH US TODAY
Be the first to know the latest news and updates
