For Family Survival,
Penguins Play a Game
Of 'Name That Tune'
By SHARON BEGLEY
September 9, 2005
Seventy miles is a heck of a distance to waddle to a gathering. Especially in a tuxedo. Especially on solid ice, when it's 80 degrees Fahrenheit below zero and winds blow at 100 miles per hour.
But as scientists see it, the most astonishing feat of the emperor penguins is not the multiple treks they make between the edge of the pack ice surrounding Antarctica and their inland breeding grounds. Nor is it that mom goes without food during the 63-day gestation period, nor that dad (taking over after she lays the egg so she can return to the sea to feed) fasts for four months while waiting for junior to hatch.
No, what impresses scientists is that, when mom returns to the breeding colony with food for the new chick, and when dad returns after making his own food run to the distant sea, both travelers manage to find their family in a colony that can number thousands of identical-looking birds, none of whom can be counted on to be where they were when the returning mate last saw them.
It's a risky way to live: If a female cannot find the mate she left behind with their single large egg, or if a male cannot find its chick, the chick dies. All the returnee has to go on is its mate's call -- which it has to extract from the surrounding din of beak clicks, wing flaps and other calls, and which it hasn't heard for weeks.
Yet the emperors do it. They have not only mastered "the cocktail party effect" of extracting a relevant signal from background noise, but also are wizards of information theory, the basis of modern telecommunications.
American moviegoers are thrilling to "March of the Penguins," now the second-highest-grossing documentary ever. But the film doesn't do justice to the complexity of penguins' effort to find the family left behind.
Of the 17 species of penguin, only emperors and kings breed without a nest. Because whichever parent is chick- or egg-sitting has no fixed address, the parent returning from the sea has no landmark to aim for.
Penguins may look pretty much alike even to other penguins, but they don't sound alike. To generate their unique calls, scientists have discovered, the birds use two voice boxes. That lets them emit different calls simultaneously, modulating frequency, amplitude and beat, write Thierry Aubin of the Université Paris-Sud, Orsay, and Pierre Jouventin of the Center for Functional Ecology and Evolution, Montpellier, France.
The interaction of two frequencies generates beats that penetrate solid objects such as, oh, huddled penguin bodies as dense as 10 birds per square meter. In addition, the system creates a huge variety of "vocal signatures."
Adults emit highly individual calls of four to eight syllables. A chick, which memorizes dad's call during the five weeks it spends sitting atop his feet, plays a life-or-death game of "name that tune," identifying him as he waddles through the colony like a bowling pin with feet and calls at regular intervals.
Playing recorded calls for king penguin chicks, Prof. Aubin and Prof. Jouventin find that even a syllable or two is enough for most hatchlings to recognize mom or dad (though they usually wait for at least four before leaving the crèche, apparently wanting to be sure).
From acoustics alone, the chicks should not be able to distinguish their parents' call from more than about 25 feet, beyond which the signal-to-noise ratio drops below 1. Yet, just like humans in the din of a cocktail party, they can pick out their partner's voice across the room (especially if the voice says something like, "Wow, you look terrific; have you been working out?"). Penguins can recognize a mate's or parent's call despite background noise and acoustic jamming by other calls.
"Chicks have an exceptional capacity to discriminate the correct call from extraneous calls," conclude the scientists.
Adult penguins even factor in wind conditions. In blustery weather, they increase their call's length and number of syllables, so that at 25 mph both are double what they were at 18 mph. This increases the signal-to-noise ratio, leading Prof. Aubin and Prof. Jouventin to conclude, only half in jest, that the birds "apply the mathematical theory of communication" to adjust their calls to prevailing conditions.
Unlike emperors and kings, Magellanic penguins do nest, and so have less need to perfect their name-that-tune skills. But their calls still convey information. At Punta Tombo, Argentina, Dee Boersma of the University of Washington and colleagues have banded 55,000 birds since 1983, allowing them to track who's who -- and with whom.
Her work has shown that the penguins divorce like people, and at a higher rate after reproductive failure, says Prof. Boersma. Also like people, the longer a female has been with the same mate (17 years is the record), the less enthusiastically she responds to his call at the start of the breeding season, Washington's J. Alan Clark reported last month.
"What's most important to her is to get a good nest," he says. "The female will check out the nest and only then look at the male."
Smart girl. |
