LARGER SPERM OUTCOMPETE SMALLER SPERM IN THE NEMATODE CAENORHABDITIS-ELEGANS

Sperm competition is generally thought to drive the evolution of sperm miniaturization. Males gain advantage by transferring more sperm, whi ch they produce by dividing limited resources into ever smaller cells. Here, we describe the opposite effect of size on the competitiveness of amoeboid sperm in the hermaphroditic nematode Caenorhabditis elegan s. Larger sperm crawled faster and displaced smaller sperm, taking pre cedence at fertilization. Larger sperm took longer to produce, however , and so were more costly than smaller sperm. Our results provide evid ence of a mechanism to support recent theoretical and comparative stud ies that suggest sperm competition can favour not small, but large spe rm.