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Deadly for bugs, perfect for bat naps

Interior of carnivorous pitcher plant is a daytime refuge for nighttime hunters

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7:10pm, January 25, 2011
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Batman’s not afraid of some little old meat-eating plant. He naps in its death traps.

For a daytime roost, both male and female Hardwicke’s woolly bats wedge themselves partway down the throats of vase-shaped leaves that capture insects for a vining species of carnivorous pitcher plant, says tropical ecologist Ulmar Grafe of University Brunei Darussalam in Gadong. The bats fold up to be smaller than a cell phone but are still too big to slip all the way down to the narrowest part of the leaf’s tapered bottom, where a pool of digestive liquid drowns insects that fall in.

What the leaf trap does catch from visiting bats of the species Kerivoula hardwickii hardwickii are their nitrogen-rich excretions, Grafe says. The bat and the plant, an elongated variety of Nepenthes rafflesiana, may have evolved a nitrogen-for-naps trade in a loose mutualism, the researchers propose in a Biology Letters paper posted online the week of January 24.

“To my knowledge, this is the first report of bats roosting inside pitcher plants,” says bat scientist Thomas Kunz of Boston University.

Bats need places to rest where a daytime predator won’t scoop them up, explains Nathan Fuller, also of the Boston University bat lab. Other species have evolved ways to bend over leaves into green tents or shape leaf stems into a cage that can shelter an entire bat harem. But for the second most species-rich order of mammals (after rodents), the world’s 1,250-plus kinds of bats are still full of surprises.

Stray anecdotes and one line in a field guide about finding bats inside a pitcher plant “were always put off as a coincidence,” Grafe says. After one of his students found a death-trap napper, however, Grafe and his colleagues decided to take a more rigorous look.

The researchers spent part of June and July 2009 in a forested peat swamp in Brunei Darussalam, on the northwestern coast of Borneo, seeing where the elusive bats sleep. Affixing tiny, temporary electronic tags on 17 bats, the team radio-tracked the animals for an average of six days. The forest offered plenty of furled leaves, tree cavities or even other kinds of pitcher plants, but researchers found their bats roosting only in the upper leaves of one particular pitcher plant species.

Monitoring 223 pitcher plants, researchers found that nearly 29 percent had a bat visit during the course of six weeks. Hosting a napping bat gave the plants a modest boost in nitrogen. And tissues from bat-cuddling pitchers contained more of the nitrogen-15 isotope, a signature of more higher-food-chain, bat-excreted nitrogen supplies, than did comparable pitchers that dined on just ants and other insects, the researchers report.

Pitchers that dangle off the upper vines of the bat-friendly variety of the plants are more elongated than those of the typical variety and aren't as successful at catching insects, Grafe says. Yet the roomy, elongated pitchers do have bat appeal. The plants may be able to get along without bats, but Grafe says the relationship still looks like a mutually beneficial one to him.

A mutualism would be plausible, says carnivorous-plant specialist Barry Rice of Sierra College’s Rocklin campus in California. In general though, he’s cautious about using the term. “I've seen people slap the phrase on the most casual of relationships,” he says. “Sorting out the relationships among life forms can be more complicated than a teenager's Facebook account.”

The bat-plant claim is the second pitcher plant–mammal link proposed in recent years. In 2009, another research team noted that a different pitcher plant has evolved an unusual giant leaf shape. It just fits a tree shrew, perching on the pitcher rim while licking buttery white secretions from the leaf lid and defecating into the pitcher.

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