Photos from open sources All that, at first glance, different males and females, males and females, is the result anisogamy. If you conduct a survey on this topic, most most likely, it will not be difficult to answer. As admitted once on his trouble woodpecker from the tales of Saltykov-Shchedrin, to distinguish one sex from another is easy even at night. And there is nothing strange, because we learn this from early childhood. Respondents would indicate mainly anatomical differences (first of all, of course, on the genitals), and some, one would think, would remember about the difference in roles in procreation (only one gender out of two gives birth, breastfeeds, and babies usually also take care of the same gender – the most beautiful in the world). These signs are enough for sex differences in humans, but in the animal kingdom everything is much more difficult. For example, most birds do not have obvious differences in the genital area. Can you dear reader, determine the sex of the sea urchin (see below) or bivalve mollusk from a photograph? 
A photo from open sources What gender is this sweet creature? (A photo alessandro_eye.) Moreover, the males of many animals devoid of what even remotely resembles a fertilizing organ. Many species are known whose males and females do not subscribe to what seems to us to be the traditional role of the sexes. For example, in some fish males look after caviar, and in marine fish males literally bear offspring. It turns out anatomical differences are not the main thing. What then? Chromosomes? Indeed, the XY system is found not only in humans, but also in many other vertebrates, as well as insects: males are present two varieties of sex chromosomes (X and Y), and females carry two instance of the same sex chromosome X. But in birds, some reptiles and butterflies are the opposite: ZW in females, ZZ in males. Genital chromosomes play the role of the first violin in the process of controlling hormones, which contribute to fundamental changes in the body, visible, usually with the naked eye. There is no doubt that these indicators help us distinguish women from men every day, especially in people (for example, in men, thanks to these hormones more facial hair). However, this factor cannot be recognized. defining. For many reptiles, gender is dependent on temperature at which embryos develop. Moreover, there are species representatives of which at one point change their gender. Not forget about hermaphrodites, that is, individuals who embody both sex (e.g. garden snails that harm every spring our cottages). So what really separates the males and females? The size of gametes – germ cells, the fusion of which leads to the birth of a new generation. Individuals producing relatively small cells, usually called males, and all the rest are females. The emergence of sexes was a transition from isogamy, when the gametes of all individuals have the same size, to anisogamy, in which reproduction involves fusion cells of various sizes.
Photo from open sources Warning! Гермафродит! (Фото Charles &Clint.) But then what about all these external signs by which we distinguish between men and women? Do they not play any role? Of course, they play, but compared with the above factor, they secondary. Let’s see what follows from the asymmetry of the genital cells. The first one. You can bake from a given amount of flour several large cupcakes or many small ones. That’s nature “decided” that it makes sense to produce an astronomical amount tiny gametes (sperm) at a time. On the other hand, all that such a small cage can do for posterity – this half supply it with genetic material. Therefore need more cells – with all the “machinery” and nutrients, necessary for fetal development. Remember the chicken egg. Continuation of this asymmetry can be considered pregnancy and lactation. mammals. Thirdly, the asymmetry has led to the horde small germ cells have to fight for the right to merge with large and few. This led, in particular, to a broad the spread in nature of polyandry, i.e. mating of a female with several males during one reproductive cycle. Вin such cases, not so many gametes of one male, how many sperm of several potential fathers. AND last thing. The production of small cells requires less energy, and it proceeds constantly, while females are born with a fixed the number of oocytes. Obviously, males are in this sense in privileged position: if it was not possible to impregnate someone Now, you can try again for several days (hours, minutes) later. Females have to carefully bear offspring and take care of him after birth. At the same time, the males are paying increased competition for access to the female. It leads to sexual selection, that is, the development in males of traits that allow either fertilize as many females as possible, or charm them (recall peacock), or more often defeat other males (hence branched horns and fast sperm). Therefore, in males of many animals either extravagant appearance or large sizes develop or special forms of behavior. And that’s why it’s accepted in many species, so that females choose a male, and not vice versa. All this has long been known and called the Bateman principle: females invest in childbearing more resources, and therefore they themselves become a limited resource, for possession of which there is a fierce war among representatives opposite sex. But speaking quite strictly, then gametic asymmetry is a consequence, not a cause. At the heart of everything – that ecological theater, on the stage of which history unfolds evolution. Prepared from The Conversation.
Bird Time
