The bat that came out of the dark

Matt Walker, BBC News 14 Sep 09;

A tiny bat living in central Italy has emerged from the dark and started hunting by day.

This switch in hunting strategy is highly unusual among insectivorous bats, which routinely hunt at twilight or by night to avoid predators.

Yet a small group of soprano pipistrelles has been spotted brazenly flying by day in a mountain canyon within an Italian beech forest.

Only one other species of insectivorous bat frequently flies during daylight.

A research team lead by Dr Danilo Russo, a bat expert from the University of Bristol, UK and the University of Naples Federico II in Italy report the discovery in the journal Mammalian Biology.

Together with colleagues from both institutions, Dr Russo initially set out to find the roosts of another species, the barbastelle bat ( Barbastella barbastellus ), in the beech forests within a mountain canyon near the village of Villavallelonga in the Abruzzo, Lazio and Molise National Park in central Italy.

"One late afternoon, walking in the woodland, we spotted some bats flying unusually early," Dr Russo explains.

"We thought the phenomenon might be occasional, as sometimes happens, so we came back at the same time on the following days and the bats were there."

The scientists established that the bats are soprano pipistrelles ( Pipistrellus pygmaeus) , a species with a high-pitched call that is closely related to the common pipistrelle ( Pipistrellus pipistrellus ).

The bats routinely come out to forage well before sunset, commonly feeding on gnats, wasps and bugs.

Deadly trade-off

Such behaviour is extremely rare for insectivorous bats.

These bats face a trade-off. During the day there are more insects around to feed on. But the bats themselves are very vulnerable to being caught and eaten by predatory birds, which fly by sight.

So instead, the bats have evolved to fly in the dark, only emerging during the hours of twilight or darkness. That keeps the bats safe, and their echolocating ability allows them to navigate and hunt those insects that are still about.

Yet over many evenings spread over two summers, the researchers continually saw this one population of soprano pipistrelles hunting insects by day.

The bats only do so at the bottom of the canyon, where local conditions seem to provide a safe haven for day flying.

The forest canopy lining the canyon protects the bats from predators, while offering a bountiful supply of insect food.

The researchers suspect that other local populations of bats may also have become day-hunters.

But they have not been recorded before by scientists as "bat researchers seldom look for bats in daytime", says Dr Russo.

"In my career I have rarely come across a fully occasional daytime flight," he adds.

Individual bats are sometimes seen flying during the day. But they usually do so soon after hibernation, when they are starving and need to replace fat reserves depleted during the winter.

"But this behaviour, performed systematically, was absolutely new to us."

Only one other bat, the Azores noctule ( Nyctalus azoreum ), is known to regularly hunt by day.

This species hunts insects in the dry forests of the Azores, where there are no predatory birds that fly by day.