The human test candidates recognised the buttocks just as quickly whether the photos were upright or inverted. Shortcut in the brainĪs has been shown previously, humans recognise faces in an upright position faster than in an inverted position. 'This is a good indication that this category has priority over other categories of objects.' The study with apes took place in a primate institute in Japan where Kret worked for almost a year. The chimpanzees were faster in clicking on the buttocks when they were upright rather than inverted. They could indicate on a touchscreen which faces and buttocks they recognised. She had test candidates - humans and chimpanzees - look at photos of faces and buttocks both upright and inverted. Kret discovered that there is also a buttocks inversion effect in chimpanzees. When people see an object in a photo, for example a house, they recognise it just as rapidly (or slowly) whether or not the object is inverted. Faces are recognised by the brain faster than other objects, but this does not apply if the faces are inverted. Recognising a person at a glance translates in psychology to the well-known 'face inversion effect' (see image). The images were shown upright (left-hand column), or upside down (right-hand column). Examples of the stimuli used in the experiment (faces, buttocks and, as a control, feet of both humans and chimpanzees). For chimpanzees, the buttocks are therefore very important in recognising one another at a single glance. When females are in their monthly fertile period, the parts around the anus and vagina dilate and become dark pink in colour. Like the face in humans, chimpanzees derive important information about identity, attractiveness and health from the buttocks. The findings can be found in PLOS ONE on 30 November.Ĭredits video: Primate Research Institute, Kyoto University Information about attractiveness and health The key question that Kret explored was whether chimpanzees process rear ends just as efficiently as we process faces. Geneticist Alberto Civetta of the University of Winnipeg, Canada, who studies sperm competition in insects and other invertebrates, is impressed with the new study, which he calls "very convincing." He looks forward to Kingan's yet unpublished study on another primate ejaculate gene, which she says also shows rapid evolution in chimpanzees.To date little is known about how humans and chimpanzees process information about faces and buttocks. That's not surprising, says Kingan, as gorilla males monopolise their females and their sperm rarely, if ever, have to compete. In fact, some of the gorilla genes are so garbled that they may not be functional anymore. Humans had slightly more variation, whereas each of the seven gorillas carried a unique version. The results, published in this month's Journal of Molecular Evolution, indeed show a much lower variability of the gene in chimpanzees, with all individuals carrying the same version. To test if such an evolutionary sweep has actually happened, Kingan and her faculty advisors determined the DNA sequences of the gene in 12 humans, 10 chimps, and seven gorillas. This would lead to reduced variability in the gene, as more effective versions sweep through the population again and again, each time replacing outdated ones. That kind of reproductive head-to-head means that new and improved versions of the gene for semenogelin ought to constantly appear and spread as they give their owners the edge in sperm competition, says evolutionary biologist Sarah Kingan, then an undergraduate at Brown University in Providence, Rhode Island. But another ingredient, an enzyme that rides in the front part of the ejaculate, can breach the plug. One of those proteins, called semenogelin, coagulates into a kind of vaginal plug (gelatinous in humans, solid in chimps) that keeps out sperm from the female's subsequent suitors. In response, males have evolved large testes that make voluminous ejaculates of sperm and cocktails of proteins. It's not unusual for female primates to mate with more than one male in rapid succession. The research uncovered the genetic footprint of rapid evolution in a male protein that helps block the sperm of subsequent flings from entering the vagina. Now a study finds that the effects of promiscuity extend inside the testes as well. Their wanton sex lives, in which males compete to impregnate females, have led males to evolve huge testicles, three times the size of humans'. Chimps are the most notorious swingers among the great apes.
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