Figure 7.

Illustrations of sperm aggregation in three species. (a) A small sperm train, in which sperm from wood mice (Apodemus sylvaticus) were joined when the hooks on their heads latched onto flagella or hooks of neighboring sperm. (b) Rouleaux in sperm from guinea pigs (Cavia porcellus), in which heads are stacked together. (c) Shows how sperm from the grey short-tailed opossum (Monodelphis domestica) form pairs and how single sperm and pairs move in non-viscous and viscous media. Paired sperm swim progressively in non-viscous and viscous media, while single sperm only swim progressively in non-viscous medium (opossum sperm illustrations used with permission from Ref. [105]. Copyright 1995 Oxford University Press.).