EXTREME SPERM The most oversized male sexual feature yet documented may be a fruit fly’s giant sperm (right, one sperm at 200 times magnification from Drosophila bifurca; left, same magnification of male surrounded by a male reproductive tract).
S. Lüpold et al/Nature 2016
Forget it, peacocks. Nice try, elk. Sure, sexy feathers and antlers are showy, but the sperm of a fruit fly could be the most over-the-top, exaggerated male ornamentation of all.
In certain fruit fly species, such as Drosophila bifurca, males measuring just a few millimeters produce sperm with a tail as long as 5.8-centimeters, researchers report May 26 in Nature. Adjusted for body size, the disproportionately supersized sperm outdoes such exuberant body parts as pheasant display feathers, deer antlers, scarab beetle horns and the forward-grasping forceps of earwigs.
Fruit flies’ giant sperm have been challenging to explain, says study coauthor Scott Pitnick of Syracuse University in New York.
Now he and his colleagues propose that a complex interplay of male and female benefits has accelerated sperm length in a runaway-train scenario.
Males with longer sperm deliver fewer sperm, bucking a more-is-better trend. Yet, they still manage to transfer a few dozen to a few hundred per mating. And as newly arrived sperm compete to displace those already waiting in a female’s storage organ, longer is better. Fewer sperm per mating means females tend to mate more often, intensifying the sperm-vs.-sperm competition. Females that have the longest storage organs, which favor the longest sperm, benefit too: Males producing greater numbers of megasperm, the researchers found, tend to be the ones with good genes likely to produce robust offspring. “Sex,” says Pitnick, “is a powerful force.”
Index of male excess
Among courtship-oriented body ornaments and weapons (red), the giant sperm of fruit flies (Drosophila) are the most disproportionately exaggerated, according to an index adjusted for body size. Higher numbers (bottom axis) indicate greater exaggeration.
Editor’s note: This story was updated June 7, 2016, to clarify that greater numbers of megasperm signal good genes and to correct the publication date.
