Impressive size did not help the female mountain gorillas give birth to more children

Reproductive success of female mountain gorillas affected by their position in the group, but not body size, reported in PLoS ONE. In addition, dominant females can be quite miniature. They differ from males of their own species, in which the number of descendants depends on the grade — the ability to stand up for themselves, and she, in turn, from the dimensions (the bigger they are, the better).

The Eastern mountain gorilla (Gorilla beringei beringei) live in groups that consist of one or several males and many females. The leader of the group — the strong, experienced (often the oldest) buck. It protects all family members in the event of danger, which usually comes from outside. Conflicts within groups are rare enough, but to become the main, the male will in any case need to be bigger and more experienced than the rest of the members of their own sex.

Among female mountain gorillas, too, has its own hierarchy, but in the past scientists have not investigated how it affects the body size — by default, it was assumed that the position of females on the social ladder also depends on the size, i.e., larger females are more aggressive, and therefore dominate. It was also unclear what determines the reproductive success of females gorillas — their size, social status, or both. After all, to leave more offspring over a lifetime, the female should start to breed as early as possible (probably when it has not reached its maximum size), but the body withstood the pregnancy happened to give birth to a healthy baby.

American, Spanish, and German primatologia under the direction of Edward Wright (Edward Wright) from the Institute of evolutionary anthropology max Planck checked, which determines the number of offspring of females of the Eastern mountain gorillas in the national Park of Virunga in Rwanda. They constructed the frame, which allowed to remotely estimate body length and width of the back at the 34 females, and analyzed the results of daily observations of the gorillas, who were with 2000. Scientists have calculated how many times and through what periods of time each gave birth to a gorilla and what percentage of cubs survived to the age when you start to eat adult food.

It turned out that the size of the female gorilla (the average width of the back at them and 48.9 cm, and body length is 71.1 cm) does not correlate nor her rank, nor how often appear in the light of her cubs. However, the social status of the monkeys was associated with the number of its descendants. From high-ranking females, the birth intervals were significantly shorter than low-ranking. On average, they accounted for 44.5 per month, but as you get closer to the top of the social ladder by one degree declined by about 2.6 months. The survival rate of young gorillas is not dependent on the size and status of their mothers. It was estimated at 65.1 per cent, from 64 cubs 41 has reached the age of transition from mother’s milk to adult food.

These results can be explained by the fact that female mountain gorillas rarely conflict with each other and almost never use physical force — so they do not have to be larger to dominate. It is also possible that the size of female gorillas do not reflect her actual ability to fight.

Interestingly, scientists have found correlations between number of offspring and their survival to how much energy is consumed by the female. Larger organisms need to absorb more calories, but reproductive success in gorillas, as it turned out, not depend on their size. And though high-ranking females is available more variety of food, they consume not more and not less energy than others.

It turns out that while it is unclear why dominant female mountain gorillas give birth more often. The authors note that it would be interesting to know how the dimensions gorillas correlate with the duration of her life. It is known that high-ranking females live longer, so they leave more offspring over a lifetime than low-ranking.

It is believed that the intelligence of gorillas is not as developed as in bonobos and chimpanzees. However, one also finds complex forms of behavior — those who do not pursue momentary utilitarian purposes. So, it is very likely that even adults mountain gorillas on their own initiative play — splashing in the water for fun.

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