Among mammals (and other species as well), when a female is within her fertile window, she is receptive to males’ attempts to inseminate her. However, when this period ends, the female not only rejects the male, but also acts to protect herself from penetration by kicking, hitting, or fleeing. Neuroscientists theorize that these deeply linked behaviors are regulated by two distinct mechanisms in the brain. In other words, rejection is not merely passive.
A recent study by the Champalimaud Foundation in Portugal has revealed the neural circuitry that plays a critical role during sexual rejection. Its identification and subsequent inhibition in mice reduced behaviors associated with defense, but did not increase female receptivity. The result of the experiment, published in Neuronreaffirms the hypothesis that both mechanisms are independent. The brain regulates them like knobs to increase the effectiveness of mating.
“Sexual rejection is not just the absence of receptivity, it is an active behavior (…) We wanted to understand how the brain switches between these two drastically different behavioral states,” said Susana Lima, lead author of the study and head of the Neuroethology Laboratory at the Champalimaud Foundation.
Where is the sexual behavior of animals decided?
The sexual behavior of species is one of the most studied from different branches of science. It is generally linked to the ventromedial hypothalamus (VMH) region, an evolutionarily ancient brain region. This area is also responsible for regulating the feeling of satiety, the response to stress, thermoregulation and other basic functions of a body.
The Portuguese Neuroethology Laboratory suspected that the VMH should have at least a separate population of neurons specialized in rejecting a sexual partner. There were images from past investigations that provided some clues in this regard. The Lima team found them in the previous VMH. These also had the characteristic of responding to progesterone, the hormone that fundamentally prepares the female body to develop a fetus.
“It appears that progesterone-responsive neurons in the anterior VMH act as gatekeepers for sexual rejection. When a female is outside her fertile window, these neurons become very active, causing rejection. But during fertility, her activity decreases, allowing mating to occur,” said Basman Husain, co-author of the study.
Regulation and synchronization
For researchers, the study gives a new dimension to the way the brain regulates social and reproductive behaviors. Now it is possible to think that the central organ has a dual system to drive reproduction in species. In the fertile phase of females, the hormone progesterone “inhibits” the rejection circuit. The action occurs in coordination over time, although it can be nuanced depending on the complexity of the species and its ecosystem.
“This setting gives the brain two knobs to adjust. It is a more efficient and robust way for the brain to balance these behaviors, ensuring that mating occurs when conception is most likelywhile minimizing the risks and costs of mating, such as exposure to predators or diseases,” Lima said.
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