Going the distance: the barnacle (Balanus nubilus and others)
It’s not the size that matters, they say, but it is hard not to be impressed by the scale of the barnacle penis. Indeed, biologists since Charles Darwin have marvelled at its amazing member, which measures in some species up to a whopping nine times the length of its body.
Why is the barnacle thus endowed? As adults, these marine crustaceans cement themselves to rocks in intertidal zones, hulls of boats and the bodies of turtles and whales. This lifestyle makes sexual reproduction somewhat challenging. Although barnacles are hermaphrodites, carrying both male and female genitalia, they rarely have sex with themselves. Instead, they use their extraordinarily long penises to reach out and find a mate within striking distance.
Barnacle penises resemble muscular telescopes. They have a series of folded rings that allow it to stretch to great lengths. It is also covered with chemical-sensing bristles that “sniff out” receptive mates. Outside of the breeding season, the penis shrinks, and in some species falls off completely, only to be regrown the next time it’s needed.
Spot the difference: the spotted hyena (Crocuta crocuta)
To the untrained eye it is hard to pick the females in a clan of spotted hyenas. That’s because females not only ‘wear the pants’, with even the lowliest female dominating the highest-ranking male, but also sport a pendulous clitoris that looks very much like a penis and a fleshy pseudo-scrotum.
Like the penis, the clitoris – or ‘pseudo-penis’ – is used to urinate and is capable of erection. Females also have no vagina, which means that mating can be a hit-and-miss affair where the males (with their actual penis) must inseminate the females through the narrow clitoral opening. Birth for first-time mums is even more eye-watering, as the pups exit through this same narrow canal, splitting it open. More than half the pups suffocate on their way out.
The biology behind the masculine-looking clitoris is not yet fully understood. It appears to develop without the influence of male sex hormones. One suggestion is that the cellular receptors that detect male sex hormones could be faulty in spotted hyenas, so that the receptors are permanently switched on. Other hyena species – brown ones, striped ones and aardwolves – have escaped this evolutionary quirk.
Screwing around: the Argentine blue-billed duck (Oxyura vittata)
The Argentine blue-billed duck is another animal that rates highly in the penis-to-body length stakes. With a penis that can stretch to be as long as its body, it boasts the longest of any vertebrate (even if it’s not as impressive as the barnacle’s).
But size is far from the only remarkable thing about the blue-bill’s penis. For a start, the fact it even has one is unusual. Just 3% of birds – including ducks, ostriches, emus and kiwis – have penises. The rest mate by males and females touching their respective ‘cloacae’ – all-in-one orifices for both reproductive and digestive functions – in a move called a “cloacal kiss”.
Duck penises are twisted like corkscrews, and grow anew each year. Duck vaginas corkscrew in the opposite direction. The twisted organs are a classic case of an evolutionary battle of the sexes. In this case, the females have the upper hand. Their vaginas, which also have numerous ‘dead-end’ pockets, are a physical barrier against forced mating by randy males. When a female is receptive to a male’s advances, however, her vagina relaxes, allowing access to her labyrinthine reproductive tract and a greater chance of fertilising her eggs.
Till death do they partake: the antechinus (Antechinus stuartii and others)
Death by sex might not sound like much of a strategy, but for the antechinus, a small Australian marsupial, it’s a way of life. In all but one of more than a dozen antechinus species, males live for a single frenzied season of fornication and die soon after.
It is not uncommon for insects and fish such as Pacific salmon to put their resources into a single bout of mating before they cark it. In mammals, however, suicidal reproduction, or ‘semelparity’, has evolved in only four groups – three small mouse-like, insect-munching marsupials like the antechinus, and the northern quoll (Dasyurus hallucatus), which is the size of a small cat.
Food supply is the most likely culprit. In places where insect availability is highly variable across seasons, females squeeze their mating into a short window so their offspring will be born when food is plentiful. This is especially important for marsupials, whose pouched young need to be suckled for several months.
The result is that a male antechinus is a kamikaze Casanova, staking all his resources on frantic mating marathons lasting up to 14 hours. This goes on for two to three weeks, by the end of which he is on death’s door. His fur falls out, his stress hormones are through the roof and his immune system collapses. Then he dies, though hopefully his genes will live on in the next generation.
Together forever: the anglerfish (Melanocetus johnsonii and others)
Anglerfish, with their bobbly lures hanging off their heads, are weird creatures at the best of times. But how the males lure females into sex is stranger still. Other male fish impress prospective mates with brightly coloured scales or mating dances. Not so for some species of anglerfish. A young male, which is significantly smaller than a female, will hunt down a companion and sink his teeth into her flesh. Enzymes in the suitor’s mouth then melt away his own mouth and his mate’s skin, fusing the two together. Thus attached, the male becomes a tumour-like parasite, relying on the female for food, delivered directly via their now melded blood vessels.
Nutrition aside, the parasitic male is also afforded prime real estate from which to release sperm when the female releases her eggs. That’s not to say there is no competition; in some species, females have been observed with up to eight males attached.
Perfect couples: marsupials
Female marsupials – mammals that carry their young in pouches – aren’t plumbed like other mammals. Whereas humans and most other mammals have a single vagina, leading to a single uterus, marsupials – such as koalas, kangaroos, wombats, opossums and sugar-gliders – have three vaginas and two uteri. The left and right vaginas carry sperm from the penis – which is two-pronged, so as to inseminate both vaginas at once – while the central vagina serves as a birth canal for the jelly-bean-sized joeys that, once born, make their way to the mother’s pouch to continue their development. It’s perhaps not a coincidence that pouches and multiple vaginas developed together. The narrow size of the central passage probably makes early birth followed by development of the young in a pouch a necessity.
Dyani Lewis is a freelance science journalist based in Melbourne, Australia.
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