Rotifers live in a male-free society

Sex 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. According to their DNA, they haven’t had sex in perhaps 40 million years.


Without mixing up their genes through sex, 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.


Pandas are good at sex

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 sex.

Even though their territories can be enormous, males and females locate one another at exactly the right time for ovulation, primarily by monitoring chemical sex 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.

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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 ‘panda porn’ or ‘panda Viagra’ is much more controversial.


Some girls 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 of sex into new places.


Animals have had sex on the moon

The diversity of mites’ sexual behaviour is staggering. There are mate guarders, harem keepers, warring males, macho show-offs and incidences of 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 Demodex spp. are having sex on your face right now. It’s probably the only animal to have had sex on the moon, carried by the 12 men who have walked on it.


Genitalia can sing

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 (sea slugs), lobes (turtles), hooks (mosquitofish), finger-like extensions (barnacles) and a detachable swimming penis (the Argonaut octopus).

Some penises have become adapted for other sexual purposes. The lesser water boatman frantically rubs its penis 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.


The value of DIY

One of sex’s 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 significant.


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.


Homosexual acts are widespread

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 poor-quality 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 UK birds.

Though animals rarely eschew sex totally with the opposite sex, observations of individuals partaking in homosexual activities throughout their lives are wonderfully common, from 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.

According to the experts, bottlenose dolphins indulge in homosexual activities as much as heterosexual activities. One of their favourite activities is ‘goosing’, when dolphins of the same sex nudge each other’s genital slits with their beaks. Other bottlenoses indulge in ‘socio-sexual petting’, when homosexual and heterosexual pairs stroke one another’s undersides with their outstretched flippers.

Only in recent years have scientists begun to lift the lid on the evolutionary causes that may be responsible. Though homosexual 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.


Duck dramas

Being largely internalised soft structures, female genitalia can be tricky to study. Among the best understood are those of ducks. Intense competition between male ducks has done remarkable things. They have evolved a long corkscrew penis 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 anti-corkscrew, 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 sexually transmitted diseases.


Fatal attractions

Episodes of sex 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 sex 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.


Main image: Leopard slugs mating. © Jasius/Getty


Jules Howard is a zoology correspondent, naturalist and author of more than 10 books including The Wildlife Pond Handbook. He writes for a number of publications including The Guardian, Science Focus and BBC Wildlife Magazine.