It's a fragrance that won't be coming soon to your local perfumery. When a flower called the dead horse arum blooms, it reeks of rotting meat, not roses. Now, a team of behavioral scientists reports that the plant doesn't just stink like a corpse, but gives off heat like one too, to become even more alluring to its pollinators.
Blooms of the dead horse arum, which grows on small islands in the Mediterranean, last just 2 days each year. On the first day, the flower gives off heat and a rotting stench; on the second, both stench and heat disappear. The plant uses its meaty aroma to entice carrion blowflies down a protruding appendix in the flower's center. Spines trap the pollen-covered flies in an inner chamber where the captives pollinate the flower's female parts; imprisoned overnight while the flower's own pollen ripens, the flies escape the next morning, pushing out through wilted spines that coat them with new pollen for the next flower. Previous work had shown that the plant odors closely mimicked those given off by dead animals, in which the flies lay their eggs. But what heat did for the plant was unknown; researchers had hypothesized that it might attract beetle pollinators or help vaporize scents.
To see if heat affected the behavior of pollinators, a team of neuroethologists led by Bill Hansson of the Swedish University of Agricultural Sciences in Alnarp and Anna Marie Angioy of the University of Cagliari in Monserrato, Italy, first compared the flower's warm appendix with the rectal temperatures of a decomposing seagull. In a study published online in Biology Letters on 5 November, the team reports that the two have comparable temperatures--about 12 degrees Celsius above ambient temperature.
Next, the team studied the behavior of the flies that landed on the flower. The researchers added synthetic odor and attached a heating element to some blooms on their second day. With only odor added, the percentage of flies that found the appendix once they had landed was half that of a real flower. But adding both heat and odor boosted appendix visit rates to first-day levels.
"We've known for a while that many flowers heat up and expend a lot of energy in the process," says Robert Raguso, a biologist at the University of South Carolina in Columbia. But the study has shown for the first time what a plant may gain when it expends all that energy, he adds.