The large fetal brain consumes a lot of resources
The human brain consumes up to 25% of the total energy metabolism of the body, having a mass of only 2% of the body weight. The metabolic rate of a rapidly growing fetal brain is comparable to that of an adult. All this requires a high rate of transfer of glucose and oxygen from the mother to the fetus through the placenta, which in turn depends on the degree of penetration (invasiveness) of its tissues into the uterine tissue. In accordance with the level of fetal metabolism in placental mammals, four types of placentas have evolved with varying degrees of invasiveness.
The highly invasive placenta, characteristic of great primates, involves immersion of the chorionic villi directly into the mother’s blood, which ensures optimal transport of glucose and oxygen into the fetal blood and the rapid development of its brain. But do vampires with jumpers, who have also been found to have highly invasive placenta, have large brains? It turned out that in these animals the ratio of brain mass to body mass (0.03) is even higher than in humans (0.02). The large size of the human brain is due to the development of the associative cortex (intellect), while in the vampire with the jumper, the development of sensory zones responsible for hearing, smell and touch.
At the same time, if the evolution of a person went to encephalization as a result of an increase in brain mass against the background of a stable body weight, then in a jumper and a vampire encephalization increased due to a decrease in body weight. A light body is necessary for a vampire to fly, and for a jumper, body size is critical in terms of thermoregulation in conditions of underground life in a climate with sharp fluctuations in daily temperature.
Thus, humans and these animals have practically reached the limit of encephalization – even smaller body sizes are not able to provide sufficient metabolism for the large brain, and bearing a “second” brain in case of pregnancy requires an extreme metabolic rate and high placenta permeability.
However, why so far the majority of placentals have retained more primitive minimally invasive types of placentas, while the highly invasive placenta makes it possible to accelerate fetal development and shorten pregnancy?
For example, dolphins with an average encephalization (0.01), bypass only a weakly invasive epitheliochorial placenta, due to the low permeability of which the duration of pregnancy increases (12-16 months). Dolphins, like most mammals, preferred low-invasive placentas, because close contact of the fetal tissue with the mother’s blood practically guarantees an immunological conflict. But it was the higher primates, vampires and jumpers who developed the mechanism of the mother’s immunity to the father’s antigens on the fetal tissues, which made the appearance of highly invasive placentas possible. This mechanism consists in high sexual activity!Поділитися цим: