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Sperm flocks aid straighter swimming: groupings permit direct routes for earlier arrival at destination.

Mouse sperm shoot along straighter paths by ganging up. Yet the merits of flocks evaporate if the group becomes too large, researchers report in the Sept. 7 Proceedings of the Royal Society B.

"Sperm aggregation is one of the more enigmatic adaptations to sperm competition," says evolutionary biologist Dawn Higginson of the University of Arizona in Tucson.

Sperm competition arises when a female mates with multiples males. Cooperative behavior in sperm is rare, but examples appear across the animal kingdom--from great diving beetles to opossums. Controversy looms over whether sperm herding provides an advantage over swimming solo. In desert ants (SN: 7/26/14, p. 20) and Norway rats, sperm crews swim faster, but in species like the house mouse, packs are actually slower.

This discrepancy led evolutionary biologist Heidi Fisher and her colleagues at Harvard University to study sperm groupings in two closely related species of mouse. The researchers videotaped sperm sprayed onto a microscope slide as the squigglers gathered into parties and swam across it. The team found that sperm groups don't drive faster than lone swimmers do. But groups do travel with a straighter trajectory and therefore get to their destination more quickly.

As troop size grew, the groups' paths became straighter, peaking when groups reached seven members. By using high-powered microscopes and mathematical simulations, the team deduced that the geometry of a sperm's head restricts how many cells can face the same direction. Little hooks on sperm crowns appear to orient the cells (without physically linking them), helping a group swim less erratically than a single sperm would. When the cluster gets more than seven members, however, the group rounds into a star-shaped ball, and the whipping tails start to counteract one another, slowing the sperm down.

Promiscuity influenced sperm dynamics as well in the two species the researchers examined: the monogamous beach mouse (Peromyscus polionotus) and the promiscuous North American deer mouse (P maniculatus).

In lab tests, sperm from the promiscuous species formed the optimum size for pack travel more often than did those from monogamous mice.

Because sperm from different males face off inside the genital tracts of female P maniculatus mice, the greater tendency toward pack behavior in this species may represent an adaptation to more competition, Fisher says.

Caption: Traveling in a straight line comes easier for groups of sperm than for solo swimmers in two closely related mouse species.

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Title Annotation:LIFE & EVOLUTION
Author:Akpan, Nsikan
Publication:Science News
Geographic Code:1USA
Date:Aug 23, 2014
Words:409
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