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Long Croakers Make Better Daddies

by Donald L. Blanchard
CHS Editor

In gray tree frogs (Hyla versicolor), male calls consist of a series of pulses that last from one half to two seconds in duration. Female frogs have been shown to preferentially seek out and mate with males which produce the longer duration calls. In a study reported in the June 19, 1998 issue of Science magazine, Allison Welch and her colleagues at the University of Missouri in Columbus, decided to test the "good genes" hypothesis. In 1995, they collected nine pairs of male frogs, one long caller and one short caller to each pair. To minimize extraneous variables, the members of each pair had been calling within two meters of each other. In 1996, six pairs were selected from a group of wild-caught males that had been calling simultaneously in captivity. In the 1995 sample the average calling time for the long callers from each pair was 1.74 seconds, versus 1.05 seconds for the short callers. In the 1996 sample, the numbers were 1.41 seconds versus 0.68 seconds.

In 1995, eggs from nine gravid females were collected, and each fertilized by one of the nine pairs of males, half the eggs from each female by a long caller and the other by the short caller. In 1996, eggs were collected from 11 females, and were fertilized by at least one pair of males, to produce sixteen pairs of maternal half-sibling. The resulting tadpoles were raised to metamorphosis (1995) or beyond (1996) at two food levels, the higher feeding level corresponding approximately to conditions found in the wild. All of the tadpole/froglet groups were evaluated for larval growth, time to metamorphosis, larval survival, body mass at metamorphosis, and, in 1996, post-metamorphic growth. In slightly over half of the comparisons (10/18), no significant difference was noted between the offspring of long calling males as compared with those of short calling males. However, in all cases where a significant difference was noted, the offspring of long calling males outperformed those of short callers.

Frogs are an ideal study subject to test the "good genes" hypothesis, because aside from fertilization, males contribute nothing to the survival of the offspring. In this study, extraneous factors that might influence tadpole survival and growth were kept at an absolute minimum; the only difference between test samples was the paternity of the offspring. Thus, it provides a convincing demonstration that frogs with long calls are more genetically fit than short callers. Apparently those finicky females do know how to tell a good male when they hear one.

References:
Welch, Allison M., Raymond D. Semlitsch, and H. Carl Gerhardt. Call Duration as an Indicator of Genetic Quality in Male Gray Tree Frogs. Science, Vol.280 (1928-1930), June 19, 1998.