These are the testicles of a male Qinling male snub-nosed monkey. Such over-sized testicles may be a sign of intense sexual competition among males. In this species, a single male controls a harem of females, and other males try to displace him.
These are the testicles of a male Cape ground squirrel. Each female copulates with multiple males, so in order to have the best chance of fertilising her eggs, a male needs to produce a lot of sperm – hence the oversized testicles. Curiously, males regularly masturbate, which at first glance sounds like a waste of precious sperm. However, a 2010 study found that the males generally do it after mating with a female, suggesting that it is a way to groom their genitals and reduce the risk of contracting a sexually-transmitted infection.
These are the genitals of a male whitetip reef shark. They are known as "claspers". During mating the male tries to insert one of them into the female's genital tract. The females often resist by twisting their bodies, and males must hold onto them with their mouths.
You might have been thinking "bird", but it is actually a mammal: a female short-beaked echidna. Echidnas are one of the few remaining "monotremes": mammals that retain some of the characteristics of their reptilian ancestors. In particular, echidnas lay eggs rather than giving birth to live young. The female does so through an opening called the cloaca. Male short-beaked echidnas have four-headed penises, but only ejaculate from one half at a time.
This was a tricky one. It is actually the mating organs of a pair of leopard slugs, which are among the largest slugs alive – reaching lengths of over 4in (10cm). Leopard slugs are simultaneous hermaphrodites, meaning each slug has both male and female sexual organs. They court each other for hours by circling and licking each other. Then the pair dangle themselves from a thread of mucus, extrude their "penises" and entwine them, and exchange sperm. Later both will lay eggs.
You might not have got this exactly right, but we expect you were at least in the right ballpark. It is the penis of a male Sumatran rhinoceros. Like most rhinos, it is critically endangered. As a result, scientists are urgently studying their breeding habits in order to help breed them in captivity.
This is one of the most familiar animals on Earth: the common earthworm. A 1997 study revealed that earthworms court each other by visiting each other's burrows, sometimes up to 17 times. The actual copulation lasts for 69-200 minutes.
This is the penis of a male grey seal. In this species males must compete aggressively for females, and the competition is tough. In one study, researchers "estimated that 275 cows participated in mating, compared with 37 bulls, during the entire breeding season". Even mating is violent. The researchers noted that in one case: "as the bull attempted to insert his penis, he made several bites at the cow's neck, and the cow attempted to return the bites, although this was not possible from her position."
This is a black sea cucumber. It is also known as a lolly fish, which is misleading because it is not a fish. Sea cucumbers are echinoderms, meaning they belong to the same group as starfish. This one is squirting its sex cells into the water. However, it has been reported that this species can also reproduce asexually by dividing in two.
Feast your eyes on the genitals of a male Wellington tree weta. These huge insects are related to grasshoppers and are only found in New Zealand. A 1992 study of captive wetas recorded a complex mating process: "On locating the female the male will attempt to pacify her by touching her body repeatedly with his palps. During this operation he will pause momentarily to carry out a curious shuddering motion and then resume the touching. The shuddering encourages the female to be more receptive, which she indicates by remaining still. Continuing his attentions, the male feels for the end of the female's abdomen with his own. He then draws back until he is facing in almost the opposite direction to the female and hanging from her and the branch. Connection then takes place and continues for about one and a half minutes. It is broken by either animal. When mating is terminated by the male he is often aggressive and will threaten to bite."
These are nudibranchs or sea slugs, called lined nembrothas. Like all nudibranchs, they are simultaneous hermaphrodites, meaning each one has both male and female sex organs. During mating, each nudibranch supplies sperm to fertilise the other's eggs.
This is the ovipositor of a female scaly cricket. The females use this dagger-like organ to lay their eggs.
This is a pair of common cuttlefish mating. These squid-like molluscs mate face-to-face. According to a paper published in 1999, mating lasts 10 minutes on average. "For the first 6 min (on average 63% of the mating duration), the male flushed strong jets of water directly at the female's buccal membrane" – apparently to dislodge sperm placed there by other males. "Then, in a single discrete movement that lasted an average of only 14 s, the male's modified fourth left arm – the hectocotylus – wrapped around a single large bundle of spermatophores and transferred them to the female's buccal membrane." From then on, the male broke open his spermatophores, releasing the sperm and attaching them to the female "in rows 3 to 5 deep".
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