Signaling With Their Heads

Sometimes you have to shout above the noise to be heard. Some songbirds, for example, sing louder or otherwise alter their tunes in noisy environments so that their territory-establishing or mate-seeking songs can still be heard.

But what about animals that use visual cues, not sound, to maintain their territory? Their habitats can be ''noisy'' too, with wind-blown vegetation that might mask any distinctive movements on their part.

Terry J. Ord of the University of California, Davis, and colleagues have found that two Puerto Rican lizards, at least, alter their displays in such visually noisy environments. The findings are being published in The Proceedings of the Royal Society B: Biological Sciences.

The researchers studied crested and yellow-chinned anoles in El Yunque, the Caribbean National Forest, southeast of San Juan. To defend their territory, these small lizards perform push-ups that cause their heads to rise and fall. The head bobbing is a signal to other lizards as much as 25 yards away to steer clear.

Both species live in trees, perching on trunks five or six feet off the ground. In such a position, background leaves and branches moving in the wind might reduce the effectiveness of the visual signal, by making it harder for other lizards to distinguish the head movement.

The researchers videotaped head-bobbing lizards under various wind conditions and compared head-bobbing speed with that of the background vegetation. They found that the two were well correlated. The faster the background movement, the more rapid the head bobbing. So just like some songbirds, these lizards can adjust their signals to conditions to get their message across.

Fish as Recyclers

A fish is a fairly efficient recycler of nutrients. It eats food and digests it, and then it excretes nitrogen, phosphorous and other nutrients in a dissolved form that other organisms can readily use. Such recycling is an important part of aquatic ecosystems, and the degree of recycling differs from species to species. So if certain fish disappear, the effects could be profound.

Just how profound has been analyzed by Peter B. McIntyre, currently at Wright State University in Ohio, and colleagues in a paper in The Proceedings of the National Academy of Sciences.

The researchers (Dr. McIntyre conducted the work while at Cornell) studied the effects of fish extinctions in two species-rich environments, a tropical river in Venezuela and Lake Tanganyika in East Africa. They compiled data on population sizes and nutrient-recycling rates for the various species in each environment and used it for computer models.

They simulated the extinction of different species chosen according to various criteria. Those most harvested by fishermen; rarest, largest or smallest; or those in certain positions in the food chain. The researchers found that the extinction of the rarest species had little effect on nutrient recycling, as might be expected. But choosing fish by size or position in the food chain led to results that were little different than if species were chosen for extinction at random.

The extinction of the most-fished species, however, had greater effects, causing the fastest declines in nutrient recycling. Yet even with this result, the simulations found that if surviving species were allowed to increase population size, the impact of the species loss was moderated.

The researchers say the study points to how complex species-rich environments can be and to another reason to be concerned about the effects of overfishing.

Storing CO2 Under the Sea

As of last Saturday, it's now officially O.K. to store carbon dioxide under the seabed.

Amendments to a 1996 United Nations protocol on the ocean dumping of wastes known as the London Convention allow the sequestration of CO2 captured from industrial processes, as a way to cut down on atmospheric emissions of this greenhouse gas. Some projects have been under way, anyway, but the change clears the way for more, provided that costs can be reduced.

The Trick Is in the Antennas

Rocket scientists know how to keep a spacecraft stable: with instruments (gyroscopes, say) that detect when the craft is rolling, pitching or yawing, and control devices (thrusters, say) to compensate.

The tobacco hawkmoth is no rocket scientist, yet it is able to keep itself stable while flying or hovering. So how does it do that? It uses its antennas like a gyroscope, researchers say.

Scientists have known for some time that houseflies and other two-winged insects have a pair of paddle-shaped vestigial wings, called halteres, that help them maintain stable flight. But moths have four wings, and ''it was always somewhat of a mystery how four-winged insects could achieve the same kind of stability,'' said Sanjay P. Sane of the University of Washington, lead author of a study on the hawkmoths in the journal Science.

Hawkmoths are popular laboratory insects because they are relatively large, with long antennas, making them easy to study. ''The antenna itself is a nice model system,'' Dr. Sane said. ''It's both an olfactory and mechanosensory organ.''

In a series of experiments, the researchers showed that the mechanosensory properties of the antennas were what helped keep the moth stable. The antennas vibrate at the same frequency of the wings (about 25 cycles per second), and when the moth rotates around any axis (rolling, pitching up or down, or spinning side to side), Coriolis, or gyroscopic, forces act on the antennas, deflecting them. (It's the same kind of effect seen, though on a larger scale, when a free-swinging Foucault pendulum precesses as the Earth rotates.)

The deflection causes sensory organs at the base of the antennas to stretch, which in turn causes neurons to fire, sending a signal to the moth's brain. The brain then instructs the wings to beat slightly differently, compensating for the rotation and keeping the moth from going into a nosedive.

The researchers found that if they amputated the antennas at the base, the moths became poor fliers, since there was nothing left to vibrate. When the antennas were glued back on, the insects regained their flying skills.

Dr. Sane said the antennas' mechanosensory action was very similar to how the halteres work on two-winged insects. ''It's some sort of convergence I suppose, evolutionarily speaking,'' he said.

Drawing (Drawing by Chris Gash)