How winding a sperm's path is depends on the degree to which the concentration of calcium ions in the sperm changes.

Eggs attract sperm by sending out chemical cues that activate calcium-ion release in the sperm's tail, or flagellum. When Luis Alvarez and Benjamin Kaupp at the Center of Advanced European Studies and Research in Bonn, Germany, and their co-workers stimulated calcium surges in the flagella of sea-urchin sperm, the sperm switched from swimming in circles to bouts of near-straight runs interlaced with turns. The curvature of the sperms' movement correlated with how quickly the concentration of calcium changed, regardless of the total concentration.

The researchers propose that two different calcium-ion-binding reactions could allow sperm to sense the rate of change in calcium levels.

J. Cell Biol. http://dx.doi.org/10.1083/jcb.201106096 (2012)