ABSTRACTS

Natural History, Dec, 1999 by Richard Milner

HOW BIG IS THAT SNAKE? For many millennia rattlesnakes have preyed on ground squirrels, seeking them in their underground burrows. California ground squirrels often fight back, frequently kicking sand at the reptiles and boldly advancing within a few inches of them, then Leaping sideways to avoid their strikes. When aboveground in the presence of rattlers, ground squirrels sit up bipedally, chatter, and flick their tails rapidly, warning their kin away (see "Rattler Battlers," Natural History, May 1989).

Now researchers have shown that California ground squirrels are also capable of judging precisely how dangerous an unseen snake may be solely by the sound of its rattle. In general, larger snakes are more Lethal than smaller ones, and snakes with higher body temperatures move faster and are more likely to cause harm. Large, warm rattlers are therefore the most formidable. By playing recorded rattles to wild ground squirrels at an abandoned walnut grove fifty miles south of San Ronald R. Swaisgood and his colleagues at the University of California, Davis, and Appalachian State University in North Carolina were able to show that the rodents could consistently gauge the level of danger from sounds alone, as evidenced by the squirrels' tail flicks and body movements. The researchers concluded that a rattle's loudness and pitch enable ground squirrels to assess accurately both a snake's size and its metabolic activity.

A previous study showed that toads refrained from challenging unseen members of their own species that produced louder, lower-pitched vocalizations (indicating larger, more powerful individuals), The new work on ground squirrels, however, represents the first documented example of a prey animal's using acoustic information to "size up" a stealthy predator. ("Assessment of rattlesnake dangerousness by California ground squirrels: exploitation of cues from rattling sounds," Animal Behavior 57, 1999)

EGG-TIMING COWBIRDS Cowbirds, like cuckoos, dupe other birds into hatching their eggs and raising their chicks--usually at great cost to the host's own brood. Often a cowbird kills several of the host's offspring by puncturing its eggs within a day of laying its own in the host's nest. However, cowbirds also destroy eggs at nests they do not parasitize.

Shiny cowbirds must try to synchronize their egg laying with that of their hosts, which is why they take a keen interest in any birds that are building nests nearby, Argentine ornithologists Viviana Massoni and Juan Carlos Reboreda studied 200 nests of yellow-winged blackbirds. They concluded that the cowbirds also were testing blackbirds' eggs to check their development. Egg puncturing occurred on either the same or the previous day that parasitism took place, indicating that the cowbird habitually hammers its bill into a host's eggs just prior to laying its own. If the cowbird finds the host's clutch in an advanced state of development, it abstains from using that nest, because its own eggs would not receive sufficient incubation time there.

Exactly how the birds determine an egg's development is unknown; they may simply discriminate between the softness of the white and yolk and the hardness of a well-developed embryo in an incubating egg; ("Egg puncture allows shiny cowbirds to assess host egg development and suitability for parasitism," Proceedings of the Royal Society of London B 266, 1999)

WORK FOR UNEMPLOYED BEES Honeybees work throughout the year to maintain a constant temperature within their hives. In summer, hundreds of worker bees constantly fan their wings, pausing only to spread water droplets, which creates an evaporative air-conditioning system. But what if some portions of the hive's walls become overheated anyway? In that case, as Philip T. Starks and David C. Gilley, of Cornell University, recently discovered, worker bees position themselves along the hot spots, thus collectively acting as insulation. When the researchers applied small heating pads to the glass walls surrounding captive colonies, they were able to document the swiftness of the worker bees' response as they (but not the drones or queens) gathered at the warm places on the glass.

Although worker bees do many tasks, fewer than half of them appear to be "working" at any given time, and researchers have traditionally described the apparent slackers as "unemployed." Starks and Gilley, however, believe that many of these individuals perform a vital function by being available not only to defend the colony but also to insulate the honey and brood combs against excessive heat. While workers can survive temperatures up to 122 [degrees] F, the colony's brood will be harmed if it warms beyond 96.8 [degrees] F. "Unemployed" worker bees may also spend time monitoring the colony's levels of honey and pollen reserves, for they frequently shift to active duty when stores run low. ("Heat shielding: a novel method of colonial thermoregulation in honey bees," Naturwissenschaften 86, 1999)