Re: Why quite so many sperm (humans), are they in competition with each other?

A great series of questions, Pete. I'll start with the last, as it's the
most straightforward, before moving on to sperm competition, which remains
a highly active and debated field of research to this day.

Diploid individuals (which possess two copies of each chromosome) undergo
meiosis to produce haploid sex cells (containing one copy of each
chromosome) such as sperm and eggs. Meiotic cell division mixes the
combination of genes that were received from the individual's father and
mother via independent chromosomal assortment and recombination. As a
result, the sperm cells (or egg cells) produced by a single individual
differ from one another in genetic composition, and will therefore produce
different offspring if they happen to fertilize an egg. If this weren't the
case, then the variation between siblings in a family, with which we are
all familiar from every day experience, would not be present!

Most basic biology textbooks provide good introductions to genetics and add
additional detail to the explanation I've provided above. The following
would be starting point:

Purves, WK, Sadava, D, Orians, GH, and Heller, HC. (2004). "Life: the
Science of Biology" (7th ed., Chapters 9-11). Sinauer Associates:
Sunderland, MA.


The answer to why so many sperm are produced breaks into two main
components, which I will address in turn.

1) Following insemination, the sperm cells of many species must travel a
long distance relative to their microscopic size in order to reach an egg.
Along the way, sperm cells may run out of energy stores or die, and overall
they each have a relatively low chance of successfully fertilizing an egg.
The odds against a single sperm cell may be offset by producing many
millions of sperm, out which just a tiny proportion will ever reach and
perhaps fertilize an egg.

2) Sperm production is dictated in part by body size (A whale produces more
sperm than a mouse), although quite a lot more variation remains, some of
which can be explained by differences in reproductive strategy. For species
in which males directly deposit sperm into females, sperm production is
relatively lower than it is among males that disperse their sperm into an
open environment and rely mostly upon chance for a few of their sex cells
to reach a female. Most vertebrates provide good examples of the former
reproductive strategy, while some invertebrates, such as bivalves (clams,
mussels, oysters), are good examples of the latter.

The picture gets even more complicated when we consider species in which
females mate with multiple males. If a male's chances of reproductive
success following insemination are proportional to the amount of sperm he
deposits in a female, then selection will favor males that deposit the
largest amount of sperm possible. In such circumstances, one could most
certainly say that sperm from different males within the same female are in
competition with one another, and sperm production relative to body size is
high. In support of this idea, it was found that testis size in primates
correlates strongly with the degree of polyandry; that is, males tend to
produce more sperm the greater the degree to which multiple males
inseminate single females. The following reference provides a nice review. 

Harcourt, AH. (1997). "Sperm competition in primates". American Naturalist,
vol. 149, pgs 189-194

In a similar vein, the males of some insect species have evolved
specialized penises with reverse-oriented hooks that are designed to remove
the sperm of other males and increase the odds that their own sperm will
fertilize a female's eggs. Pretty subversive! 

An excellent popular account of sperm competition can be found in:

Birkhead, T. (2002). "Promiscuity: An Evolutionary History of Sperm
Competition". Harvard University Press: Cambridge, MA.


Best regards,
Carey