2/19/2009
Sperm
Sperm Morphology
• Sperm are popularly thought to have but a
single purpose -- fertilization of the egg.
This is not so!
Sperm
• "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.”
• Human sperm, moreover, come in
different "morphs," or shapes, designed for
different functions. Most common are the
"egg
egg getters
getters," the standard government
governmentissue sperm with conical heads and
sinewy tails designed for swimming speed
— the Mark Spitzes of the sperm world.
But a substantial minority of sperm have
coiled tails.
kamikaze sperm
• These so-called kamikaze sperm are
poorly designed for swimming speed. But
that's not their function. When the sperm
from two different men are mixed in the
laboratory, kamikaze sperm wrap
themselves around the egg getters and
destroy them, committing suicide in the
process.
• These physiological clues reveal a long
evolutionary history in which men battled with
other men, literally within the woman's
reproductive tract, for access to the vital egg
needed for transporting their genes into the next
generation. Without a long history of sperm
competition, evolution would have favored
neither the magnitude of human sperm volume
nor the specialized sperm shapes designed for
battle.
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2/19/2009
Robin Baker
• Noticing that sperm in a mixed sample
tends to clump together--making it less
mobile--and to have a high mortality rate,
reproductive biologist Robin Baker,
formerly of the University of Manchester,
proposed about a decade ago that some
mammals, including humans, manufacture
"killer" sperm whose only function is to
attack foreign spermatozoa, destroying
themselves in the process.
Alternative View
• 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.
Relative sperm volume
• Research on sperm competition reveals
that men's sperm volume, relative to their
body weight, is twice that which occurs in
primate species known to be
monogamous, a clue that hints at a long
evolutionary history of human sperm
competition.
Alternative View
• To test this idea, reproductive biologist Harry Moore and
evolutionary ecologist Tim Birkhead of the University of
Sheffield in the U.K. mixed sperm samples from 15 men
in various combinations and checked for how the cells
moved, clumped together, or developed abnormal
shapes "These
shapes.
These are very simple experiments
experiments, but we
tried to mimic what goes on in the reproductive tract,"
Moore says. The team found no excess casualties from
any particular donor or other evidence of warring sperm,
they report in the 7 December Proceedings of the Royal
Society. "The kamikaze sperm hypothesis is probably
not a mechanism in human sperm competition," says
Birkhead.
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