The male beetles were given a few days to recover, and then they were allowed to mate with females again. The longest penis spines are most important They then used a microscope to help them burn off spines individually with a precision laser. The researchers used a penis pump – a thin pipette tip with a small air hose – to inflate the penis. The latter is a complex procedure in which the male beetle is anaesthetised with carbon dioxide. They have bred males with shorter penis spines, and on some of the beetles, they shortened the spines themselves using lasers. In the new study, the biologists have therefore manipulated the penises directly. They found that the males with the longest spines on their penises fertilise the most eggs and get the most offspring.īut it remained unclear why this was so, and there could theoretically be other hidden factors at play. In 2009, Arnqvist and his colleague Cosima Hotzy anaesthetised male beetles from 13 countries and measured the length of their penis spines. This is where the bruchid beetle enters the picture. When you think about it, it makes good sense that the penis is longer, so that the sperm is delivered nearer to the eggs, or that the amount of sperm is high.īut this phenomenon is actually very difficult to get hard experimental facts on – because how do you isolate characteristics in for instance a penis and study their effect individually? ”Only in the past 10-20 years have we started to realise that it’s probably due to sexual selection in the male sex organs.” ”It’s a truly basic, widespread and important evolutionary pattern,” says Arnqvist. Say what you will about animal penises but they’re a perfect example of how fascinating, diverse, and maybe a bit weird, nature truly is.Many closely-related species can only be distinguished by the shape of the penis.īut how come teeth, limbs, brains or the body shape don’t evolve as quickly? The more complex penises belong to primates like bushbabies and lemurs, who mate with many partners in their lifetime. Primates with simpler penises tend to be monogamous (only have one partner) or polygynous (mate with only a few partners). We are the only primate without a bone in the penis and have a relatively more simple structure. The size and shape of penises in primates vary too and compared to other animals, the human penis is actually rather dull. In fact, humans have a proportionately large penis size compared to their body size – but length is not everything (am I right?). The largest gorilla will only have a penis that is two and half inches long when erect. However, the penis size of many primates is much compared to the testes. To put that into perspective, a human’s testis is only 50 grams, so chimps have a testis that is 3 times that! This is a third of the size of its brain. To accommodate this increased level of sperm production, the chimpanzee’s testis is relatively large, at a weight of 150 to 170 grams. The more sperm produced, the more likely it is that the female’s offspring will have that chimp’s DNA (which is the main goal in all primates, and indeed many animals). This results in sperm competition whereby each of the males need to produce lots of sperm to outcompete the other males. What’s more, each of these females are only fertile about once or twice every four years! So, a large penis is not exactly necessary.Ĭhimpanzees are pretty promiscuous creatures with several males mating with one female whilst she is fertile. Silverback males own a group of three-four females, known as a harem, who are constantly either pregnant or breastfeeding. Whilst this may seem surprising, gorillas don’t need massive genitals given the sexual dynamics of this species. Did you know that male silverback gorillas weigh around 250 kilograms, but their penis and testes are only 10 grams? That’s equivalent to a vending machine having a testis the weight of 10 paperclips.
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