The awareness that, in most animal species, the prospects of the male's spermatozoa to reach the female's eggs are very slim in the absence of some guidance mechanism (usually chemical in nature) has been acquired gradually; first, in marine species, where both types of gametes are released into sea water (for review see Miller, 1985), and ultimately, in the last decade or so, in mammals (for review see Eisenbach, 1999). Such chemical guidance, sperm chemotaxis, is now recognized in many marine invertebrates, fish, amphibians, and a few mammals (humans included) (Eisenbach, 2004). This suggests that sperm chemotaxis is a general guidance mechanism, irrespective of whether the fertilization is external, like in most marine species, or whether it is internal, as in mammals.
In spite of this generality, there are some basic differences between sperm chemotaxis of mammals and that of marine...
