Male butterflies are driven to drink. (Zoology).

Monarch butterflies that winter in California's Pismo Beach, especially males that had a demanding day, search out dewdrops as a water source.

Butterfly watchers have long studied so-called puddling behavior, where butterflies congregate around water edges, says Dennis Frey of California Polytechnic State University in San Luis Obispo. He noticed something he suspected might be similar while he watched butterflies set off early in the morning toward a dewy meadow.

Frey and his colleagues found that after a sunny day with low humidity, more butterflies visited the dewy meadow than usual, some of them increasing their body weight by 7 percent with a long drink of water. However, Frey also noticed that after a day when the colony he was tracking had been particularly busy mating, the percentage of males going out for a drink rose. The researchers report their findings in the summer Journal of the Lepidopterists' Society.

Butterfly matings begin when a male snags a female and the two tumble to the ground. Male monarchs encase their sperm with a gel that's 90 percent water, and the biggest sperm packages can equal 10 percent of a male's weight, Frey reports. A female receiving a particularly large and wet package is more likely to resist the next male's advances than is a female receiving only a little gel.

Frey predicts that butterfly watchers in the rest of the country may be able to see more monarch drinking binges in hot spells and during mating periods.--S. M.