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Inside JEB |
ZEBRA FINCH'S CHEAP CHIRRUPS
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When an ardent male zebra finch starts chirruping, he isn't whistling just for pleasure. With each rendition of the song that he learned as a fledgling, he advertises his fitness, hoping to attract a mate. But how much effort is a male putting into his courtship recital? Franz Goller explains that measuring the metabolic cost of song in a respirometry chamber isn't straightforward, especially if the bird is strutting around at the same time. So he designed a zebra finch-sized respirometry mask, to collect each bird's exhaled breath. With the new system, Goller could resolve individual gasps as each male repeated his characteristic song, and directly measured the serenade's metabolic cost. Even though the bird's metabolic rate only rose by 10% from the resting rate, he was amazed to see that some of the birds sang so hard, that by the end of the serenade they were hyperventilating (p. 967).
Most metabolic measurements are made by placing small animals inside a respirometery chamber, but the volume of air inside the chamber is too large to resolve individual breaths when a small bird exhales, so Goller decided to build a tiny mask to fit over a birds beak to collect each breath as the bird sang. Goller knew that if he was going to get any species of bird to cooperate with him, it would be zebra finches. Working with Michele Franz, he adapted a zebra finch-sized helmet to fit over the bird's beaks, with inlet and outlet tubes, to measure the amount of oxygen the birds consumed as they sang. As well as wearing the helmet, the birds were fitted with a tiny backpack to measure the pressure pulses driving air through the birds' syrinxes as they sang. Then they tempted the males to perform with a glimpse of a female and recorded the song.
The resolution improved enormously. Not only could Goller see each breath as the birds chirruped, but he also realised that the pressure signature from each song cycle was as distinctive as the song itself, and it took less than 1.5 s for the respirometer to register the oxygen levels as the bird exhaled. He could correlate oxygen consumption with each breath.
The first surprise came when Goller looked at the way the bird breathed when it prepared to launch into its song cycle with a couple of preliminary chirrups; it breathed in and out but hardly consumed any oxygen at all. As soon as the bird launched into a song cycle, its oxygen consumption rose, but fell with each successive repetition. Goller also compared the bird's metabolic rates before and during singing, and found that the metabolic rate only rose by 0.1 ml g-1 min-1 during a song bout, less than the presong metabolic rates varied between individuals.
The biggest surprise came when Goller realised that some of the males exhaled so enthusiastically while singing, that by the end of the recital, they had to stop breathing for a second to recover! Could the ability to sing hard enough to hyperventilate be the vital fitness indicator that the females are searching for? Goller isn't sure, but having convinced the males to sing into the mask, he hopes to learn how the song helps a female select her mate.
References
Franz, M. and Goller, F. (2003). Respiratory
patterns and oxygen consumption in singing zebra finches. J. Exp.
Biol. 206,967
-978.
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