Sperm heteromorphism

The sperm types might differ in size, shape and/or chromosome complement.

The non-fertilising morph(s) are a means in which males can provide nutrition to the female, her eggs or the fertilising sperm.

This might be offensive (e.g. displacing or even killing rival fertilising sperm) or defensive (e.g. by blocking areas of the female tract or creating a hostile pre-fertilisation environment).

Alternatively, the sperm may transfer chemicals similar to sex peptide, a chemical carried on the sperm of Drosophila melanogaster that makes females less likely to accept mates (i.e. it is an anti-aphrodesiac).

This theory was also tested in the fruit-fly Drosophila pseudoobscura, but the results suggested that "cheap filler" was not important in that species.

Scanning electron microscopic image of immature parasperm lancet (infertile sperm morph) of Fusitriton oregonensis showing the tail brush still present, which later develops into part of the body of the parasperm. It is producing when sperm competition occurs.