Bakulum, variable and mysterious

Male copulative organs are among the most diverse structures in the kingdom of animals. Their main function is to increase the efficiency of fertilization, introducing sperm into the female sex tracks. A simple tube would have managed to do this task perfectly, but the morphology of penis is extremely complex and view -specific.

They are long and very short, smooth and prickly, single and double. And in many types of mammals, penis contain a bone called Bakulum (OS Penis). These bones are also distinguished by an extreme variety of sizes and shapes. The reasons for this diversity are incomprehensible, there is no consensus on the functions of Bakulum.

The hypothesis, of course, is enough. According to one of them, Bakulum protects the urethra (urethra) and mechanically supports the penis during copulation. It is also possible that Bakulum during mating reaches the cervix and expands it to convey more sperm, or stimulates ovulation. Or he removes the sperm of the previous male from the body of the female. It is also possible that he wounds a partner, and after that she does not let the next male to her. Finally, Bakulum can serve as an indicator of the quality of the sexual partner.

All these assumptions, however, are confirmed only in part. Say, if the task of Bakulum is to ensure the firmness of the penis during mating, then the species with longer copulation of the baculum should be longer. This pattern is carried out for primates and nastysts, but not for other predators. In the Ondatra and Greenland seals, the dimensions of the baculum correspond to the size of the body of the male manufacturer; In mice, bats and martens, no such connection was found. With other hypotheses, the situation is similar.

When none of the existing hypotheses satisfies the researchers, they put forward another. A group of American scientists led by Matthew D. Dean from the University of South California suggested that there could not be a single function for all, since baculums in different species arose several times during evolution, independently of each other [1].

Researchers approached the verification of this hypothesis extremely conscientiously. They analyzed data on 3707 types of mammals. It was possible to establish that 925 types of Bakulum have, 103 species have no, nothing is known about the rest. Many species are not described with such a detail, and OS Penis is not always easy to detect. In American cigarette, for example, he is so tiny that he was only recently discerned into a scanning electron microscope.

At the next stage, scientists chose 954 species from these mammals with well -established phylogenes and looked at how Bakulum is distributed in these phylogenetic groups. The analysis showed that during the evolution of mammals, Bakulum nine times appeared independently and was lost ten times. While this bone is found only in placental, it does not have marsupials. Therefore, the first mammals of Bakulum were absent. Now they are possessed by representatives of the five detachments - insectivorous (moles and shrews), gunpowder (bats), rodents, predators (cats, dogs, bears, sea cats and walruses) and primates.

In the primates, Bakulum during evolution arose at least twice: in lemuro -like primates and the general ancestor of the group of monkeys, including anthropoids. Then he was lost in three treasures, including a person. There is no data that some extinct types of the genus Homo , including Neanderthals, had OS Penis. Specialists of the University College of London discovered that the primates have a baculum and its size depend on the duration of intermission (penetration of the penis into the vagina). With a short intromission, Bakulum is small or completely absent [2]. The preservation of OS Penis is also facilitated by postcopulation sexual selection, which is strong in polygamous species (several males claim to be one female). Polygamous primates retained Bakulum. Hominids have a short intromission, polygamy is not peculiar to them, and they lost Bakulum.

All modern types of rodents included in the study have baculum. At the same time, these bones in different species are extremely diverse; The variety arose against the background of the selection that supports this structure.

The ancestors of bats was Bakulum, then in five treasures disappeared; Many species have been preserved.

The ancestors of modern predators had Bakulum, then they independently lost it of the common ancestor of the earthen wolf and two types of hyenas ( Proteless Cristata + Hyaena Hyaena + Crocuta Crocuta) and Binturon g arctictis binturong. It is interesting that in the case of spotted hyenas C. Crocuta, which have lost OS Penis, the level of circulating androgens is high even in females.

In addition, Bakulum independently acquired four types: the small tenrer Echinops telfairi (this beast resembling a hedgehog belongs to the Afrosoricide detachment), the American bake of Ochotona Princeps from the razher -shaped detachment, the flying mouse Thyripolor and the European Krot Talpa Europaea.

Researchers do not exclude that the data on the distribution of bakulums they obtained will change, since now, for the presence of this bone, all new types of mammals are checked. However, the main conclusion about the repeated independent occurrence of OS Penis is no doubt. In favor of this hypothesis, different types of bone bunk bone tissue, and its different localization relative to the urethra, and different relative dimensions: in some types of bakulum, it occupies up to 75% of the length of the penis, others are much less. In general, Bakulumi are not homologous structures, which do not have general evolutionary origin.

The causes of the morphological diversity of the Baklims are seen in the constant co -evolution of the male and female genital organs, however, the phenomenon of the occurrence and loss of OS Penis does not explain the hypothesis. There are cases when Bakulum arose, and then, after a long time, disappeared, without changing. On the contrary, baculums of different types of rodents are characterized by a large morphological variety.

Matthew Dean and his colleagues drew attention to the genes participating in the formation of Bakulum. There is experimental evidence that in some species that have lost Bakulum, he begins to form in the course of individual development, but remains in the embryonic state (Monkeys of the Uakari) or degrades, as in whaling. Scientists came to the conclusion that the genetic path of the development of Bakulum has not yet been destroyed, and if the baculum may be restored when the selection in the corresponding direction appears. However, further research is necessary to clarify this issue.

Most of the problems associated with Bakulum will remain unresolved until the experts find out what its functions are, and for this we need to observe the process of copulation from the inside. In people, such research was carried out on volunteers using magnetic resonance tomography [3]. However, in order for the images to be high -quality, partners must freeze at certain points, so this method is not suitable for animals. They offer a different methodology: to introduce the dissected organ and artificially strain it to females, observing what is happening using computed tomography. This process, of course, differs from the natural, but can serve as a laboratory replacement of natural copulation. In this way, scientists are now trying to find out whether Bakulum rodents really reaches the cervix when mating.

Natalia Reznik

1 Schultz ng et al., The Baculum Was Gained and Lost Multiple Times During Mammalian Evolution // Integr. Comp. Biol. 2016. 56 . 644-656. DOI: 10.1093/ICB/ICW034.

2. Brindle M., Opie C. Postcopulatory Sexual Selection Influences Baculum Evolution in Primates and Carvores // Proc. R. SOC. B. 2016. 283: 20161736. DOI: 10.1098/RSPB 2016.1736.

3. Brennan PLR Studying Genital Coevolution to Understand Intromitten Organ Morphology // Integr Compp Biol. 2016. 56. 669-681. Doi: 10.1093/ICB/ICW018.