ITHACA, N.Y. -- The northern tip of the Chihuahuan Desert juts fromnorthern Mexico into the southern parts of Arizona, New Mexico and Texas.It is where black-tailed jackrabbits scurry, roadrunners sprint anddiamondbacks slither. For the continent's largest desert, punctuated bymountains, the monsoon rains end sometime in late summer. Then the flowersbloom, and the desert bees come out. And like the desert's floral seedswaiting for their season in the sun, scientists have learned that thedesert bees have adopted a patient reproductive strategy. Desert beelarvae patiently lie in wait in the desert soil for a chance to emerge,usually after a year or more. Scientists call this process "diapause," abet-hedging strategy that results in the survival of the best-rested.
But according to new research by Cornell University entomologist Bryan N.Danforth, not all the viable larvae emerge in any one year of diapause, andtheir "coming out" is triggered by rain. Writing in the October issue of"The Proceedings of the Royal Society of London", Danforth notes that byspreading reproduction over several years, desert bees can keepcatastrophic losses of their kin to a minimum in very bad drought years.
"This is evidence of a bet-hedging life history," says Danforth, anassistant professor at Cornell. "The bees are giving up reproduction inyear one to reproduce in year two, and this is the first study that showsthat rainfall triggers emergence in bees. That the bees haverainfall-induced emergence is remarkable and helps further explain why beesare so diverse in arid regions."
According to Danforth, "This past year was a good year for the bees. Ashumans we do a lot of bet-hedging, too. We use a bet-hedging strategy whenwe buy mutual funds for investments. As humans, we learn to spread out ourrisks all the time. Well, so have plants, and now we learn so have thedesert bees."
Bet-hedging also can have ecological effects, in that the sheer variety ofpollinating desert bees might be enhanced by such a life history.
"What's remarkable about the results is the similarity between the bees anddesert plants under the same conditions," says Danforth. He explains thatthe seeds of desert annual plants and the overwintering desert bee larvaemature and reproduce over a short period of time following the monsoonrains. Most of the rainfall occurs during July, August and September, withthunderstorms sometimes so dramatic that roads get washed out.
But the rains trigger the flowering and signal the bees to emerge. Likeseeds, the bee larvae are buried in the soil, exposed to high temperatureand low humidity and subject to attack by foraging ants, fungal pathogensand desiccation.
"That desert doesn't get a lot of rain; it's a pretty harsh environment.What's amazing is that animals -- in this case the bees -- and plants haveevolved such similar mechanisms for survival," says Danforth. "Thisstrategy allows the bees to survive the 10 months or more between the rainyseasons."
Desert bees, slightly smaller than houseflies, flit from flower to flowerpollinating the desert world, specifically visiting "sphaeralcea"(pronounced: sfer AL see a), mallow plants related to cotton. "These arevery egalitarian bees," says Danforth. Unlike their honey-making cousins,there are no queens and no workers. "Everyone is a queen, everyone is aworker. They are the soccer moms of the insect world," he says.
His research shows that rain triggers the bees' emergence from the larvalstate into pupae, and then finally into adult desert bees. The lower thelarval weight, he found, the better the chance for the bees to emerge.
Interestingly, the bee larvae lose weight during diapause. And the moreweight they lose, the better their chance of coming out in the next rainyseason. Male larvae lose as much as 14 percent of their body weight, whichwould be the equivalent of a 200-pound man losing 28 pounds. The femalelarvae lose only about 7 percent of their body weight during diapause.
This published study, "Emergence dynamics and bet hedging in a desert bee,"Perdita portalis,"" was funded by two Theodore Roosevelt memorial grantsfrom the American Museum of Natural History in New York City, a predoctoralfellowship from the Smithsonian Institution, a National Science Foundation(NSF) postdoctoral research fellowship in environmental biology and an NSFresearch grant in systemic biology.
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