Nature has come
up with many weird
and wonderful ways
of swapping genes.
Here Jules Howard
celebrates the most
surprising secrets

1. World with no males
Reproduction costs a lot of energy, so
why not evolve to bypass it altogether?
Well, one group of animals has.
Bdelloid rotifers (the ‘b’ is silent)
are tiny creatures found in bird
baths, ponds and puddles. When wet
they come to life and hoover up
micro-organisms. When conditions
become dry again they shrivel up
into a ball and are blown from place
to place. There are billions of them
on Earth, and every single one is
female. Without mixing up their
genes through reproduction with
males in perhaps 40 million years.
The rotifers should fall prey to
bacteria and viruses, their defences
outmanoeuvred. Yet they are still here.
How? It seems that drying up then
blowing from place to place may
allow them to outflank and outlast
their parasites. In their world, males
add no genetic value.

2. Pandas are good at it
Giant pandas are widely chastised
for being unable to ‘get in the
mood’ in captivity, and for having
a window of ovulation (about
36–48 hours) too tiny to be
practical. The reproductive life
of Edinburgh Zoo’s Tian Tian
and Yang Guang shows just how
difficult it can be to encourage
the species to breed normally in
captivity. But in the wild, pandas
are masters of reproduction.
Even though their territories can
be enormous, males and females
locate one another at exactly the
right time for ovulation, primarily
by monitoring chemical messages
left on trees via squirts of urine.

They also communicate vocally.
Males bleat when they approach a
reproductive female, possibly offering
an opportunity for her to assess his
size and strength. A female in oestrus
often mates with several males, so
they have evolved one of the highest
sperm counts of all bears, to better
guarantee any offspring is theirs.
As our understanding of the
animal’s wild breeding improves,
zoos adapt accordingly. For
example, keepers liberally apply
the urine of potential partners to
panda enclosures in the run-up to
breeding season. However, the use
of other panda stimulants is much
more controversial.

3. Girls who are boys
Many animals, especially fish, switch
between egg-producing (female) and
sperm-producing (male) phases during
their lives. For instance, in many reef
fish all of the juveniles are females and
become males as they grow. These are
known as ‘sequential hermaphrodites’,
a phenomenon very common across a
number of taxonomic groups.

In invertebrates, particularly slugs
and snails, things go a step further –
individuals possess male and female
genitalia at the same time. In fact many
slugs and snails even have the ability to
fertilise their own eggs.
With such flexible reproductive
equipment, it’s no surprise that a number
of invasive species are hermaphroditic.
Among the most worrying is the Spanish
slug, which has become a serious
agricultural pest across much of
Europe. A single
egg transported in
a flowerpot is all it
takes to unleash this
master and mistress
into new places.

4. Reproduction on the moon

The diversity of mites’ sexual behaviour
is staggering. There are mate guarders,
harem keepers, warring males,
macho show-offs, incest and
cannibalism. Perhaps the
most celebrated of
all is the red velvet
mite. Males create
trails of silk in their
territories that
direct females to
little packages of
their sperm, called
spermatophores. If
one approves, she
will absorb the sperm into her body.
Species of mite are everywhere – in the
noses of seals, on the legs of chickens, in
the ears of porcupines, in the middle
of a sea urchin and within the
rectums of bats. In fact
it’s likely that eyelash
mites are reproducing
on your face right
now. It’s probably the
only animal to have
reproduced on the
moon, carried by the
12 men who have
walked on it.

5. Singing genitalia
The variety of male genitalia in the animal kingdom is
jaw-dropping. There are fin-like ones (sharks), barbed
ones (cats, beetles and dragonflies), regenerative ones
(seaslugs), lobes (turtles), hooks (mosquitofish), fingerlike
extensions (barnacles) and a detachable swimming
apendage (the Argonaut octopus).
Some have become adapted for other sexual purposes.
The lesser water boatman (right) frantically rubs its
genitals against a special comb-like structure on its body
to pump out a mating call equivalent to almost 100dB.
Relative to size, it’s the loudest animal on Earth.

6. The value of DIY
One of the greatest mysteries is why so many
animals seek to pleasure themselves, rather than
find reproductive opportunities with others.
Lions, bats, walruses, warthogs, whales, dolphins
and deer are just some of those known to partake
in such ‘auto-eroticism’. Are such behaviours
evolved, or are they emergent phenomena
associated with something else, such as captivity?
The marine iguana is one species where
auto-eroticism is common – smaller males
rub themselves against rocks as they approach
reproductive females. The behaviour means that
their resultant copulations are shorter, so smaller
males are less
likely to be
interrupted by
bigger, burlier
rivals. According to
research, the strategy
is likely to increase
their chances of a
successful mating
by 41 per cent –
easily enough to
be evolutionarily

7. Monogamy is hard to find
Monogamy rarely flourishes in animal groups because fidelity
limits an individual’s reproductive potential. It only persists
among the species where the result is a higher number of
healthy offspring.
In birds, where the raising of chicks may demand care
from both parents, monogamy arises fairly frequently. But it
has popped up in other species and groups, too: antelopes,
prairie voles, some cichlid fishes and the Australian sleepy
lizard (also known as the shingleback skink). None of these
are true monogamists though – each may be inclined to
change partners between seasons.
Though many consider swans, albatrosses and emperor
penguins to be nature’s most virtuous couples, all of these
pale in significance compared with Eurasian bullfinches
and jackdaws. Bullfinches are highly monogamous, and as
a result males are modestly endowed and produce poorquality
sperm, not having any need for more sophisticated
reproductive mechanisms. On the other hand, jackdaws
remain faithful for life and stay near their partners year-round,
even within bustling and complex colonies. They are perhaps
the most monogamous of all common birds.

8. Mutual attraction
Though animals rarely eschew reproduction totally with the
opposite species, hyenas, lions, whiptail lizards, dragonflies
and bed bugs through to orcas, koalas, barn owls, king
penguins, mallards, sticklebacks and rattlesnakes, to name but
a few, do exhibit such tendencies.
According to the experts, bottlenose dolphins indulge in
them as much as heterosexual activities.
Only in recent years have scientists begun to lift the lid
on the evolutionary causes that may be responsible. Though
such animals in vertebrates obviously suffer from lower
reproductive outputs, there may be evolutionary benefits
such as kin selection, whereby non-reproductive offspring
enhance the survival and reproductive chances of their
siblings, ensuring their own family genes persist.

9. Duck dramas
Being largely internalised soft structures,
female genitalia can be tricky
to study. Among the best
understood are ducks’.
Intense competition
between male ducks has
done remarkable things.
They have evolved one
that can be ‘exploded’ into
a female’s reproductive
tract, giving a male a greater
chance than his rivals of
successful fertilisation. In
response the female reproductive
tract has evolved into an anticorkscrew,
with pockets and dead ends.
By modelling the tract of Muscovy ducks, scientists found
that she can rebuff unwanted sperm – her reproductive
passages only loosen enough to grant access to the males
that she deems worthy. They’re the ones with the brightest
bill, for those are most likely healthiest and less likely to be
infected with diseases.

10. Fatal attractions
Episodes of reproduction that are so intense the animal
dies, known as semelparity, evolve when it pays more
(in terms of offspring) for males and females to invest
everything in one act than to stay alive and breed again
next year. The Pacific salmon is a good example.
Though not strictly semelparous, frogs and toads
often live their last days during the breeding season. The
energetics of mating are arguably worse for females than
males – competition can be so intense that she drowns
under a mass of rival suitors. But when this happens in the
frog Rhinella proboscidea, death doesn’t spell the end –
the males practise ‘functional necrophilia’, squeezing eggs
from dead females which they fertilise in the water.


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