Image caption All sea slugs have complicated mating behaviour, but Chromodoris reticulata has surprised scientists with its ability to lose and then re-grow its penis 
 
 A sea slug that is able to detach, re-grow and then re-use its penis has surprised scientists. 
 
 Japanese researchers observed the bizarre mating behaviour in a species called Chromodoris reticulata, which is found in the Pacific Ocean. 
 
 They believe this is the first creature known that can repeatedly copulate with what they describe as a "disposable penis". 
 
 The study is published in the Royal Society's journal Biology Letters. 
 
 Male and female 
 
 The sex life of the sea slug is complicated even before detachable organs come into play. 
 
 I haven't seen anything like this before Bernard Picton, National Museums Northern Ireland 
 
 Almost all of these creatures, which are also known as nudibranchs, are thought to be "simultaneous hermaphrodites". This means they have both male and female sexual organs and can use them both at the same time. 
 
 Bernard Picton, curator of marine invertebrates at the National Museums Northern Ireland, explained: "The genital apparatus is on the right hand side of the body. So two nudibranchs come together and one faces one way and one faces the other way, with the right hand side of their bodies touching. 
 
 "The penis from one fits into the female opening of the other one, and the penis from that one fits into the female opening of the first one, if you see what I mean. 
 
 "They are both donating sperm to the other one." 
 
 But the finding that one species has added another layer of complexity to copulation surprised the sea slug expert. 
 
 "I haven't seen anything like this before," he said. 
 
 Sexual healing 
 
 Image caption The Japanese team of scientists observed the animals as they mated 
 
 The Japanese team observed sea slugs that they had collected from shallow coral reefs around Japan. They saw the animals mate 31 times. 
 
 The act took between a few seconds and a few minutes, after which the creatures would push away and shed their penises, leaving them on the floor of the tank. 
 
 However, the researchers were surprised to discover that just 24-hours later, the sea slugs had regenerated their male organs and were able to mate again. 
 
 Closer examination of the animals' anatomy revealed that the sea slugs had a large part of their penis coiled up in a spiral inside their bodies, which they would then use to replenish their missing part. 
 
 The scientists also noted that the penis was equipped with spines. 
 
 At most, the animals were able to copulate three times in succession, with each bout separated by about 24 hours. 
 
 It was not clear once the internal penis was used up after these attempts whether the animal's sex life - at least the male part of it - was effectively over or - after a few weeks or months - the organ could regrow. 
 
 No great loss 
 
 Sea slugs are not the only animals who abandon their penis. 
 
 They do have very very complicated biology Bernard Picton, National Museums Northern Ireland 
 
 Orb weaving spiders are known to lose their male organs after sex, as does a sea creature called the periwinkle and land slugs belonging to the genus Ariolimax. 
 
 However the researchers believe that Chromodoris reticulata is the first creature known that can re-grow its appendage - and its disposable penis gives it a sexual advantage. 
 
 The Japanese team says that in the first act of copulation the penis may be used to remove any sperm left by any competitors that its partner has mated with. 
 
 With the first penis and the rival sperm then abandoned, the second penis can be used to inject the sea slug with another dose of its own sperm, ensuring that their genes are the ones that are passed on. 
 
 Mr Picton said: "They do have very, very complicated biology - and a lot have awfully complicated things in terms of reproduction." ||||| Sea animal has grow-again penis 
 
 PARIS — Scientists reported Wednesday on the bizarre sex life of a sea slug that discards its penis after copulation. Then grows a new one. 
 
 "No other animal is known to repeatedly copulate using such 'disposable penes'," Japanese biologists wrote in the Royal Society journal Biology Letters, describing the behaviour as "extremely peculiar". 
 
 Dubbed Chromodoris reticulata in Latin, the red-and-white slug -- technically a shell-less mollusc -- inhabits warms waters in Southeast Asia. 
 
 The critter needs 24 hours between couplings to unroll an internally coiled and compressed juvenile penis to replace the used, external bit, the scientists found. 
 
 It can repeat this feat at least three times. 
 
 The human thumb-sized slug is an hermaphrodite, meaning it has both male and female sexual organs. 
 
 The animals perform dual sexual roles during copulation. They give sperm to a mating partner while simultaneously receiving sperm, which they store for later insemination. 
 
 The team observed copulation between sea slugs that they had captured during scuba dives and held in a tank. 
 
 After each coupling, which lasted between dozens of seconds and a few minutes, every slug discarded its penis -- a thread-like organ that it projects from its side into a partner's vagina. 
 
 The team also examined the microscopic structure and function of the male organs -- observing an internal spiral structure they believe grows into a replacement penis. 
 
 "We propose that the tissue at the spiral part of the penis is compressed and undifferentiated, gradually differentiating into the 'next penis'," the team wrote. 
 
 "It may need approximately a day for the spiral structure to be ready for copulation." 
 
 In another revelation about the sea slug's sex life, the scientists found its penis was covered with spines -- and suggested these may be used to remove the sperm of previous partners being held in store by their mate. 
 
 The spines are backward-pointing, making it difficult to withdraw the penis after copulation. This may explain the organ's disposable nature. 
 
 "Chromodoris reticulata may compensate for the short-term cost of decreased reproductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement," wrote the study authors. 
 
 Various animals are known to discard parts of their body, such as the gecko which sheds its tail. 
 
 Few, though, are willing to part with their penis, the team noted with clinical understatement. 
 
 Copyright © 2013 AFP. All rights reserved. More » ||||| Whales have especially small pelvic bones compared to their body size. A blue whale, for example, can grow up to 100 feet in length, about the size of a bus, but its pelvic bone is only around a foot long. For a long time scientists figured that the bones are so small because they are vestigial, a shrunken evolutionary remnant from an ancestor that once walked on land. 
 
 But now scientists have discovered that the tiny bones may actually play a huge role in whale reproduction. Whales belong to a group of sea-dwelling mammals called cetaceans, which also includes dolphins. Comparing hundreds of whale and dolphin pelvic bones to their genitalia and mating style, researchers found that males from more promiscuous species had larger penises and larger pelvic bones. A larger bone would theoretically give a whale more maneuverability while tackling the logistically difficult task of mating in the water, the team writes in a study accepted for publication in the journal Evolution. 
 
 “Contrary to popular belief, these are not vestigial structures. They do have a function. That function is reproduction, and they evolved in response to sexual selection,” says Jim Dines, a co-author on the paper and collections manager at the Los Angeles County Natural History Museum. 
 
 Humans and other land-dwelling mammals need large pelvic bones to anchor their hind legs and walk around. The bones are therefore connected to the base of the spine and the tops of the leg bones. In whales, the pelvic bones aren’t attached to anything in the skeleton—they’re just embedded in muscle. That led most researchers to assume that the isolated bones served no evolutionary purpose. 
 
 “Just because we can’t conceive of a function doesn’t mean it has no use. That’s a little short-sighted on the part of our imagination,” points out Nick Pyenson, a paleobiologist at the Smithsonian’s National Museum of Natural History in Washington, D.C. who was not affiliated with the study. 
 
 Dines and his team set out in search of a better answer. Cetacean pelvic bones attach to a muscle group called the ischiocavernosus muscles, which in most mammals also attach to genitals. Studies in rodents have shown that if you clip these muscles, a male will lose control of his penis. There’s also a lot of pelvic bone variation among cetacean species, and that piqued the team's interest. “There were so many differences between even closely related species, it was clear something really cool was going on,” Dines says. 
 
 Drawing heavily from the museum collections in L.A. and the Smithsonian in D.C., Dines tracked down pelvic bones from 130 cetaceans across 29 species. He estimates that roughly 90 percent of the specimens came from beached cetaceans and bycatch—whales and dolphins that got stuck in fishing nets. 
 
 Simply measuring the bones wouldn’t reveal enough of the intricate differences, though. “You could put [the bone] on a ruler, but these bones are curvy and weirdly shaped. So we would lose massive amounts of information if we did that,” says study co-author Matt Dean, an evolutionary biologist at the University of Southern California in Los Angeles. Instead the scientists used a laser scanner to create three-dimensional models of all the bones and then statistically measure their differences. 
 
 Mating style is obviously harder to put a number on. Studies have shown that males with larger testes come from species that mate with more partners. The more females they mate with, the more sperm they need. Thanks to records from researchers and the whaling industry, a lot of data on whale testes size, penis size and body size was available across different species, some of it dating back to the 1920s. 
 
 Comparing all of this data, the researchers spotted some trends, including the link between promiscuity (based on testes size) and pelvic bone size. To make sure this wasn’t a false positive, they performed the same laser scanning and analysis on rib bones—which unsurprisingly showed no correlation with testes or penis size. And looking at the cetacean family tree, when species diverged and became more promiscuous, the shape of their pelvic bones changed accordingly. 
 
 To feel the effects of sexual selection, cetaceans with larger or more uniquely shaped pelvic bones had to have a clear advantage in the wild. “Male cetaceans are very dexterous with their penises. I guess they have to be, because they’re mating in water,” says Dean. More promiscuous species also had longer penises, relative to their body size. The larger the penis, the larger the connecting muscle and thus, the larger the pelvic bone needed to control it. 
 
 At one end of the spectrum there’s the franciscana (Pontoporia blainvillei), a river dolphin that’s monogamous. It has the smallest testes and the smallest pelvic bone among the cetaceans they examined. The most promiscuous species, the right whale (Eubalaena glacialis), has the largest pelvic bones and the largest testes, weighing in at around a ton put together. “During breeding season, male right whales are swimming around with the equivalent of a Volkswagen bug inside of their body,” says Dean. 
 
 Both genetic studies and rare observations in the field suggest that female right whales mate with multiple males, sometimes at the same time. In a situation like that, it’s easy to imagine how greater dexterity would come in handy. 
 
 "Lots of different kinds of selection are likely operating on all organisms at all times, and so this is great to know that there are clear signals of sexual selection in the pelvic anatomy of all whales,” says Pyenson. 
 
 The researchers found a similar trend in females, which also had larger pelvic bones in more promiscuous species. Dean hazards two possible explanations: it could be a carry-over from sexual selection in males, or something could be going on with female genitalia, which are controlled by the same muscles. “A clitoris and a penis are the same thing,” says Dean. “It could be that these females are moving their clitorises around in weird ways that we don’t understand to possibly sample males while they’re mating. Who knows what they’re doing.” 
 
 Testing any of this in the field is near impossible—for now, there’s no way to x-ray whales having sex underwater to reveal the inner workings of their anatomy. All the researchers can do at this point is speculate. In the meantime, the techniques employed in the study have implications for how to study whale specimens going forward. Using the same laser scanning techniques, scientists could create digital copies for analysis or even 3D print a whale pelvic bone. 
 
 “To me it’s a more expedient way of making specimens like this that might be really rare or valuable accessible to other researchers across the world,” says Dines.
– The Chromodoris reticulata has a peculiar way of mating: After the deed is done, the sea slug sheds its penis ... and then grows a new one within 24 hours and does the whole thing again. "I haven't seen anything like this before," says one expert. A team of Japanese researchers discovered the surprising behavior in the species, which lives in the Pacific Ocean, the BBC reports. Chromodoris reticulata is the first creature known to have what they're calling a "disposable penis." Other creatures discard their sex organ post-copulation, but they don't re-grow it. The researchers called the slug's behavior "extremely peculiar." A large part of the slug's penis is actually an internal one that's coiled inside its body; this is what regenerates the detached portion after sex. The studied slugs were seen to mate three times in a row, 24 hours apart each time; it's not clear if the penis could regenerate once again, perhaps months later. The evolutionary purpose? Scientists aren't quite sure, but they say the purpose of the first sex act may be to remove any sperm left by a previous partner. The disposable penis is far from the slugs' only sexual quirk: The penises are equipped with spines that could help with the removal of prior sperm, AFP reports. The creatures are also "simultaneous hermaphrodites," which means they have male and female sexual organs that can be used simultaneously.