Wierd Sex Lives of Creatures (article that will challenge your notion of sex identity)

CURIOUS LIAISONS: NATURE’S WEIRDEST SEX LIVES
by Michael Brooks
New Scientist
July 6, 2010

http://www.newscientist.com/article/mg20727671.100-curious-liaisons-natures-
weirdest-sex-lives.html

Once upon a time, sex in the animal kingdom seemed pretty simple. Flamboyant
male met coy female, male courted female, male deposited spermatozoa in the
vicinity of an ovary, then headed out to do it all again elsewhere.

Then biologists began to look more closely, at what really happens. They
found that being the biggest and brashest male doesn’t always win you mating
rights. Among weaver fish, for example, it is good fathers, the ones who
will take care of the fry, who get the girl. Females don’t always conform to
type either. The female bean weevil, for instance, would rather drink her
mate’s ejaculate than use it to fertilise her eggs. Reproduction, it turns
out, is a complex affair.

Just how complex has been emphasised anew with a slew of studies that
highlight the staggering diversity of sexual practice in the animal kingdom.
Intercourse is a bizarre and often dangerous pursuit, where sexually
transmitted infections can be desirable, living in a male harem inside your
mate can make sense, and headless lovers give you extra. Relations between
the sexes are also surprisingly convoluted. Biologists have charted virgin
births, spontaneous sex changes and, perhaps weirdest of all, males who
father their brother’s offspring. Human sexual exuberance is tame compared
with some of the things that animals get up to in the name of reproduction.

Take the male preying mantis, the poster boy of risky sex. In an ideal
world, he will jump onto a female’s back, establish a rigid grip, copulate
and jump away again, safe to repeat the process with some other female. Much
of the time, however, that grip will slip. If it does, the male slides
within reach of the female’s mandibles and he stands a very good chance of
having his head bitten off.

Being eaten by your partner during copulation is clearly not desirable.
William Brown at the State University of New York at Fredonia thinks the
males tread a delicate line. His research reveals that they approach females
with trepidation: the drive to reproduce and the drive to survive are at
loggerheads (The American Naturalist, vol 167, p 263). “Our work suggests
that males actively assess the level of risk posed by an individual female
and alter their behaviour to reduce the risk of sexual cannibalism,” Brown
says. “We expect that the level of acceptable risk to the male will depend
upon features such as the availability of safer mating opportunities, the
age of the male — and thus his expectation of future reproduction — and
perhaps even the quality of the female.”

From the female’s point of view, cannibalistic sex looks like a winner on
several fronts. Clearly, it provides a nutritious meal, making it
particularly popular among females who have not eaten for a while. But there
may also be another benefit. In some mantid species, losing your head means
that you have also lost the system of nerves that tells you to stop
copulating. Meanwhile, the nerves that keep copulation going, which are in
your abdomen, remain intact. So following decapitation, the female gets
everything the male has to offer, as it were. There’s just one downside.
“Hungrier, more cannibalistic females attract fewer males,” Brown says.

Another species in which females keep males firmly in their place is the
green spoonworm, Bonellia viridis. Found in the warm waters of the
Mediterranean Sea, B. viridis begins life as free-floating flake-like
larvae. When they settle on the sea floor, they mature over a period of
years into 10-centimetre-long females. Many, however, do not make it this
far. If a larva should settle on top of a female instead, she produces a
chemical called bonellin that turns the larva into a tiny male. This male
then creeps up her body and into her mouth, from where it migrates down to
her uterus. “Once inside the female, males assume a parasitic existence:
they depend on the female for their nourishment,” says Patrick Schembri of
the University of Malta. But there is mutual benefit. With up to 20 males
safely holed up inside her genital sac, the female can get her eggs
fertilised without expending any effort on finding a mate.

While B. viridis females keep their males captive, aphids prefer a more
detached relationship. In fact, many species only copulate once a year and
it’s not even sperm the females are after.

A female aphid can reproduce without sex. In terms of her genetic legacy, it
makes perfect sense to do this because she can produce many clones that
carry all her genes down through the generations. So why do aphids make time
for an annual bout of sex? This question puzzled Nancy Moran and Helen
Dunbar at the University of Tucson in Arizona. Their surprising discovery is
that aphids have sex to acquire sexually transmitted infections (Proceedings
of the National Academy of Sciences, vol 103, p 12803).

Infectious sex

Like you and me, aphids carry bacteria on and in their bodies, many of which
are useful. Some break down plants the insect would not be able to digest
unaided. Others confer resistance to extremes of temperature. One
particularly valuable bacterium, Hamiltonella defensa, kills the grubs of
parasitic wasps before they start growing within the aphid’s body cavity and
consume it from the inside. A female aphid can acquire such useful bacteria
by having sex with an infected male, and she can also pass them to her
future clones. “Once they are established in the clonal descendants of the
sexual female, they can be quite stable and confer longer term resistance,”
Moran says. So her female offspring will continue favouring asexual
reproduction while the males wait on the sidelines for a chance to exchange
bacteria for sex.

Aphids not withstanding, sex is an extremely popular means of procreation in
the animal kingdom. Its ubiquity is still something of a mystery but it must
offer benefits that outweigh the advantage of being able to produce numerous
exact copies of oneself by cloning. One possibility is that by shuffling
your genes and throwing your lot in with another individual, you can produce
healthier offspring that do not inherit the damaging mutations that
inevitably build up in an isolated genome. Another is that sexual
reproduction gives rise to offspring with novel genetic combinations that
increase the chances that some will survive when faced with environmental
change or disease. But wouldn’t it be more useful if, like the aphids, an
individual could hedge its bets, switching between sexual and asexual
reproduction and getting the benefit of both?

Alas, for most higher animals that is not possible. However, there are some
exceptions where cloning occurs via a process called parthenogenesis, where
the egg fuses with a by-product of egg production — known as a sister polar
body — rather than a sperm cell. This rare form of asexual reproduction has
occasionally been observed in lizards and birds: female turkeys isolated
from males for a very long time, for instance, sometimes produce young by
parthenogenesis. But it seems to come at a price. Mortality rates are high
and developmental problems abound.

Which makes a recent report from researchers at the Field Museum in Chicago
all the more intriguing. Earlier this year, Kevin Feldheim and colleagues
published the results of a genetic analysis of two white-spotted bamboo
sharks born in captivity to a mother who had never shared her tank with a
male (Journal of Heredity, vol 101, p 374). The test confirmed that they are
clones and that their mother had not experienced a close encounter of the
sperm kind. There have been a few reports of virgin births in sharks before
now, but none has been known to produce offspring that survived long term.
The bamboo sharks are now five years old and healthy, suggesting that
parthenogenesis is not an evolutionary dead end after all.

Animals that can adapt their sexual strategy to suit their situation are
clearly at an advantage. The ability to occasionally do without sperm if
there are no males available may be what has allowed sharks to stick around
for hundreds of millions of years, making them one of the most ancient
animal lineages on Earth. Some creatures have an even more impressive trick,
though — when the going gets tough they have a sex change.

Bees do it, some fish do too, but the latest creature to join the list of
transsexuals is the mushroom coral. By switching from female to male, it can
survive environmental stresses such as temperature rises that cause other
species of coral to become bleached and die. “The whole idea is to save
energy,” says Yossi Loya of Tel Aviv University in Israel, who first spotted
this behaviour in 2008 (Proceedings of the Royal Society B, vol 275, p
2335). The reason is simple — producing eggs costs more energy than
producing sperm. So a male-dominated colony is more energetically frugal,
increasing its chances of toughing out the hard times. Then, when conditions
improve, Loya has shown, some individuals flip between sexes, choosing the
one that will give them the best chance of reproducing, depending on what
their nearest neighbours are doing.

Ultimate male shirker

The mushroom coral’s pragmatic attitude to gender highlights the point that
being male is often an easier option than being female. The ultimate male
shirker, however, has to be the fire ant — but it pays a price. While the
queen and her daughters work tirelessly to keep the colony going, “males are
only spermatozoid with wings: they do nothing”, says Denis Fournier of the
Free University of Brussels (ULB) in Belgium. Even their occasional
contributions of sperm ultimately come to naught, since the eggs they
fertilise all develop into the sterile female workers. Meanwhile, the queen
produces new queens by cloning. In this way, males are cut out of the
evolutionary line.

In fact, additional males are only produced when the queen lays eggs that do
not contain any of her genetic material and a male fertilises them. This odd
situation has led David Queller of Rice University in Houston, Texas, to
suggest that the males can be considered a separate species to the females
(Nature, vol 435, p 1167).

If your sole contribution to reproduction is a single, tiny sperm, you are
always in danger of becoming expendable, so it makes sense for males to add
value. A good ploy is to help raise the kids, but if males are not to waste
time and energy caring for someone else’s offspring they need to be able to
recognise their own. That can be far from simple, as the very strange tale
of the marmoset illustrates.

These small South American monkeys are among the most attentive of fathers.
Marmosets are born as fraternal twins, developing from two distinct eggs,
but they have more in common than your average siblings. From early in
gestation they share a placenta, causing their blood to intermingle. As a
result, most are born containing cells from their twin, making them
chimeras. This has been known for half a century, but it has now become
apparent that there is a link between marmoset chimerism and doting dadhood.

In 2007, Jeffrey French and colleagues from the University of Nebraska in
Omaha reported that over half of all male marmosets have chimeric sperm,
meaning that they are in the bizarre position of being able to father their
brother’s or sister’s offspring. Some females also have chimeric eggs,
meaning they may effectively be surrogate mothers for their twin. In
addition, many marmosets also have chimeric skin and so produce odours
characteristic of both their own genetic make-up and that of their twin.
Marmosets recognise each other by these smells, and the researchers found
that fathers and uncles are more than twice as likely to look after young
with chimeric skin. Mothers and aunts, in contrast, pay less attention to
offspring with chimeric skin than to those without.

The implications are mind-boggling. Clearly the marmosets’ genes are all
mixed up — to such an extent that even the researchers are left scratching
their heads over whether these little monkeys should even be considered as
separate individuals. Still, in a world where a chimeric male can father
chimeric twins with his brother’s sperm, surely human relationships will
never look quite so complicated again.

How to survive celibacy

Animals have some very strange sexual habits, but perhaps nothing is quite
as puzzling as the bdelloid rotifer, which has survived for 80 million years
with no sex at all. This ancient line of cloners was seen by the late John
Maynard Smith as an “evolutionary scandal”. According to standard
evolutionary theory they should have become extinct long ago. Without the
gene shuffling and novel genetic combinations that sex brings, parasites or
changing environments ought to have done them in by now. However, it is now
becoming clear how bdelloid rotifers have kept their virginity for so long.

In 2008, researchers from Harvard University and Woods Hole Marine
Biological Laboratory, both in Massachusetts, found one trick the rotifers
use when they discovered that the creature’s genome is chock-full of genes
from bacteria, fungi and plants. While bdelloid rotifers may not be swapping
genes among themselves, over the millennia they have clearly had a healthy
trade via horizontal gene transfer with other organisms. This, the team
suggests, could be a satisfactory alternative to sex, giving a limited means
of shuffling genes (Science, vol 320, p 1210).

Then, earlier this year, a team led by Chris Wilson of Cornell University in
Ithaca, New York, found that the canny rotifers have another strategy that
might make sex redundant. Rather than fighting their parasites by evolving,
they evade them by allowing themselves to desiccate and be blown away on the
wind. When they reach another location, they rehydrate and get back on with
the business of not having sex (Science, vol 327, p 574).

“If parasites are indeed the problem that sex evolved to address, then
bdelloids may have a unique alternative way of solving it,” says Wilson.

………….

Michael Brooks is a consultant for New Scientist and the author of 13 Things
That Don’t Make Sense (Profile/Doubleday)