Sex and the Single Bat (2) Sperm storage
In the October issue we left
a number of females in a
post-coital slumber. So what
happens next? Bats and
humans are both mammals
and so, if you paid attention
to sex education lessons at
school, you may well think
you the answer is obvious.
But it isn’t – as Jude Hirstwood explains.
In human reproduction sperm meets egg and fuses to
form the cell which gives rise to
the embryo. This cell promptly
divides repeatedly and embeds
itself into the uterus wall. A
placenta develops and nine
months later a human baby
emerges. Been there, done
that, bought the postcard and,
in some cases dear reader, you
will have even passed the
practical.
But our bat is in hibernation;
her metabolism has slowed to
a virtual stop. Is it really a
bright idea to have a
developing embryo in you are
this time? Obviously not, but
what alternatives are there?
Bats could leave mating until
the spring but males are too
weary after hibernation to get into all that courtship
malarkey and making sperm would further deplete their
energy reserves. Admittedly, there is more food about,
but now we hit another snag. The text books say that
the larger the mammal the longer the gestation period.
A mouse
is pregnant for 21 days, but our similarly sized bat
hasn’t read the books and has a 40 day or more
gestation period. This is because the young must be
born sufficiently well developed to hang on to mum
from the outset. If conditions are unsuitable, the
pregnant females can go into torpor, which will extend
the gestation period.
So how to solve this conundrum. Badger solve the
same problem by delaying implantation (The fertilised
egg goes into suspended animation and doesn’t
embed itself into the uterus wall until later.) This is the
tactic adopted by some tropical species such as flying
foxes. Others have a more cavalier approach;
vampires have more or less constant environmental
conditions and food availability and so have up to four
oestrus cycles a year. The Madagascan bat Tadarida
is even more efficient and can ovulate whilst still
lactating, which is very rare for a mammal. But food
and environment is more restricted for our British bat,
so she uses another strategy.
Sperm storage in the uterus of a pallid bat Antrozous pallidis (after
Hill and Smith 1984)
She simply prevents the sperm meeting the eggs until
1-3 days after hibernation has ended. Easy peasy.
Some female pipistrelles that mate early in the year
may store their sperm for 7 months. (By way of
comparison human sperm can survive in the human
female for about 5 days.)
But this brings more problems – how do the sperm
survive over winter? One factor is that the fluid levels in
the uterus have high carbon dioxide levels, which
inactivate the sperm, thus saving
energy. In additions fluids in the
uterus contain high levels of
glucose and fructose that can act
as a food supply. Better still there
are tiny projections (microvilli) in
the uterus wall. It is now thought
that some species’ sperm attach
themselves to these and can
absorb nutrients from the female.
And for now, that is where we will
leave them, snoozing through the
winter, and should a marauding
Daubenton's happen along, he
may well find that his sperm can’t
displace these
earlier arrival
because the
previous male has
left a chastity belt in the form of a
vaginal plug of seminal fluid and old sperm to block his
entrance; that is as long as the female hasn’t expelled
it to make room for one more inside.
Part 3 of this series will look at what happens from
fertilisation to birth
Reproductive Chiroptrivia
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The only mammals that menstruate are
primates and bats.
Male bats have the highest blood testosterone
level of any mammal.
A microlitre (a thousandth of a litre) of Noctule
sperm contains between 6 and 12 million
sperm.