No. 78: Nov-Dec 1991
Sperm is popularly thought to have but a single purpose -- fertilization of the egg. This is not so!
"Nonfertilizing sperm with special morphologies have long been known to exist in invertebrates. Until recently, abnormal sperm in mammals were considered errors in production. Now, however, Baker and Bellis have proposed that mammalian sperm, like some invertebrate sperm are polymorphic and adapted to a variety of nonfertilizing roles in sperm competition, including prevention of passage of sperm inseminated by another male. More specifically, their 'kamikaze' hypothesis proposes that deformed mammalian sperm are adapted to fa cilitate the formation and functioning of copulatory plugs."
The author of the present paper, A. H. Harcourt, thinks that although some 20% of mammalian sperm, on the average, is abnormal (two heard, no heads, two tails, no tails, coiled tails, etc.) such sperm represents only errors on the assembly line. These abnormal sperm have no special purpose, at least in mammals. (Harcourt, A.H.; "Sperm Competition and the Evolution of Nonfertilizing Sperm in Mammals," Evolution, 45:314, 1991.)
Comment. Even if mammals haven't yet developed kamikaze sperm, some animals have; and one must wonder exactly how multipurpose sperm (and ova, too) evolved. For a copulatory plug to be effective, large numbers of mutant sperm with special plugging properties and knowing just where to go, would have to be generated all at once. That would seem to require a lot of chance mutations all at once! The "perfection" problem once again.
Reference. Kamikaze and other deviant sperms fall into category BMF18 in our catalog: Biological Anomalies: Mammals II. Ordering information here.