There has been wide disagreement as to whether sperm competition among animals can produce a soldier class of sperm to fight against other males. Utilizing mathematical models, we analyze the appropriate conditions for the evolution and maintenance of a soldier sperm class. We conclude that: (1) soldier sperm evolve even if one soldier sperm can kill or block less than one competing sperm; (2) soldier sperm evolve faster when there is a large variance in the number of competing sperm; (3) soldier ratio increases until reproductive sperm are too scarce to fertilize all ova or a sperm intensely refuses to become a soldier; and (4) soldier sperm are more likely to be smaller than reproductive sperm. Our models suggest that the conditions for the evolution of a soldier sperm class are not stringent.

Corresponding Editor: T. Markow