And about the bone – it seemed

Monogamy has deprived men of the penis bone

Sofia Dolotovskaya, N+1

British researchers, analyzing the evolution of the baculum – the penis bone – in mammals, showed that its length is determined by the level of competition between males: the stronger the competition, the longer the baculum. The article was published in the journal Proceedings of the Royal Society B (Brindle, Opie: Postcopulatory sexual selection influences baculum evolution in primates and carnivores).

The baculum (os baculum, os penis) is a penis bone. It is formed from the connective tissue of the cavernous bodies of the penis and is present in many placental mammals. So, all representatives of insectivores have a baculum (they include, for example, moles, hedgehogs and shrews), bats, wool-wings and rodents. At the same time, the hares – a rodent–related order that includes hares, rabbits and squeakers - do not have it (although, as it turned out recently, the American pika still has a small baculum). Also, many, but not all, representatives of the predatory order have a penis bone: bears, cats, dogs, raccoons, martens and pinnipeds (seals and walruses; previously they were separated into separate detachments, but recently included in the predatory order). Most primates also have a baculum–but not all. In addition to humans, it is also absent in tarsiers and in two genera of the South American family of spider monkeys: coates (Ateles) and woolly monkeys (Lagothrix).

Bakulum of the walrus (Wikimedia Commons)

The size of the baculum varies greatly even in related animals and does not depend on the size of the body. For example, in marmoset monkeys, which weigh about 500 grams, the length of the baculum is only a couple of millimeters – and in tiny galagos with a body weight of no more than 70 grams, this bone reaches a length of 13 millimeters. In great apes, despite their large size, the penis bone is very short (in bonobo chimpanzees, for example, only 6-8 millimeters) – and a person does not have it at all. The largest baculum has a walrus: up to 60-70 centimeters.

Bakulum of the Japanese Macaque (Wikimedia Commons)

Because of all this, many zoologists call the baculum "the most diverse bone" among all mammalian bones. The authors of the new article decided to find out why this bone is so unevenly distributed among mammals and varies so much in size and shape.

To do this, they conducted a phylogenetic analysis of the evolution of the length of the baculum (including its presence or absence) in mammals. In particular, scientists focused on primates and carnivores – groups in which both species with and without a baculum are found. The analysis also included parameters such as the mass of the testicles, the reproduction system (polygamy or monogamy), the seasonality of reproduction and the duration of intromission (penetration) during sexual intercourse.

Firstly, the analysis showed that the ancestral mammal did not have a baculum – but ancestral primates and ancestral carnivores already had one. Apparently, this bone first appeared in mammals between 145 million years ago (when the branches of placentals and marsupials separated) and 95 million years ago (when the last common ancestor of primates and carnivores lived).

Secondly, it turned out that among both primates and predatory animals, the baculum is most often found in animals with a long intromission (longer than three minutes), and the longer the intromission, the longer the baculum. This is quite an expected result, because the main purpose of the baculum is just to maintain the hardness of the penis during sexual intercourse. In addition, the baculum turned out to be longer in polygamous animals and animals with seasonal reproduction. The size of the testicles, on the contrary, turned out to be in no way related to the length of the baculum.

Summarizing the results, the authors concluded that the length (and in general the presence) of the baculum is determined by the post-copulatory competition of males (that is, competition that occurs not before, but after mating with a female). In polygamous animals with time-limited reproduction, competition between males becomes very high during the breeding season. In such conditions, it is advantageous for a male to mate with a female for as long as possible: not only because more sperm will get into her this way, but also because this will prevent her from mating with another male competitor for some time.

This also explains why a person does not have a baculum. Human communities are usually characterized by monogamy or, less often, polygyny (mating of one male with several females), and there is no seasonality of reproduction – which means that competition between males is usually low. Because of this, the duration of intromission in humans is also small (on average about five minutes) – therefore, he simply does not need a bakulum.

Portal "Eternal youth" http://vechnayamolodost.ru  14.12.2016