How do gorillas mate? Gorillas are the largest ground-dwelling apes still roaming earth. They live in the forests of East and Central Africa. Gorillas are divided into two main species – The western and Eastern Gorilla. Each of these species are divided into two subspecies making it a total of four gorilla subspecies species – The western lowland gorilla, cross river gorilla, eastern lowland gorilla (Grauer’s gorilla) and mountain gorillas. You can read more about the types and subspecies of gorillas. Gorillas are the closest relatives to humans after bonobos and chimpanzees. They share about 98% of the human DNA. Gorillas are generally herbivores and feed on fruits, leaves and tree shoots in the dense forests of Africa.
Gorillas live in groups of up to 30 individuals. A gorilla group usually consists of one dominant male silverback, other males, juveniles, females and their young. Among most gorilla species, interaction with other groups is very rare though it has been observed in western lowland gorillas. The dominant silverback keeps other members in check. He determines what is to be done and when. Gorilla group members are generally peaceful allowing the silverback to focus on dealing with challenges from predators (leopards), solitary males and silverbacks from other groups. If there are other silverbacks in the group, they will help him deal with intruders and other threats.
Mating and Reproduction
How do gorillas mate? In order to understand the evolution and behavior of our own species, we need to understand what goes on in the wild among our closest cousins, the great apes. Gorilla mating habits depend on several factors but we also need to understand their reproductive cycle. Females gorillas reach sexually maturity when about 10 to 12 years. The ovulation cycle occurs early (around 6 years of age) but they remain infertile till they are mature enough at 10 years.
The dominant silverback is the only one with total access to the females. Most males leave the group once they reach adulthood. When they leave their father’s group, they stay alone or gang up with other bachelor males until they are able to steal females from other groups. Most females leave their fathers group too in order to avoid inbreeding. Studies among mountain gorilla groups have shown that in cases where females stay with their original groups for long, they will tactfully avoid the dominant silverback/father to avoid inbreeding. The female would rather mate with the less subordinate males in the group than with her father. Even when she is forced too, the probability of a gorilla conceiving from the father is very low except in cross river gorilla families where inbreeding in common.
Mating in Gorillas
Mating and breeding occurs throughout the year. The dominant silverback mates with all females in the group as soon as they begin ovulation. The moment when a female gorilla is ready to mate is not evident physically like it is in chimpanzees. In most cases, it is the female who initiates the mating process as soon as she ready. When a female gorilla is ready to mate, she will approach the dominant male slowly, make sustained eye contact and purse her lips. Should the male not respond, she will attract his attention by slapping the ground as she reaches towards him. If the dominant male is too busy or doesn’t react after several attempts, she may be open to mating with other males in the group.
The dominant silverback can also initiate mating by approaching the female and touching her or making a display with a grunt. If the female is hesitant or resistant, the silverback may charge and slap the female to give in. A study in 1982 found that a silverback gorilla can become aggressive towards a particular female in order to force her to initiate the process of mating even when she wasn’t in estrus. Actual mating occurs on the ground with the silverback on top of the smaller female. The female appears to kneel down as the female completes the copulation process. It was thought that face to face sex was unique to bonobos and humans but some gorilla species (mountain and western lowland gorillas) have been observed having sex with the individuals looking directly at each other.
Sex for competition and for leisure is also not unheard of in gorillas. Female gorillas frequently use sex to gain the favor of the dominant silverback to stop him from mating with other females. There have even been cases of pregnant females mating with the silverback to reduce the chances of other fertile females from conceiving. The attention given to a fertile/ovulating female by the silverback tends to arouse competition among females in the group. In all gorilla subspecies, the dominant silverback prefers mating with older experienced mothers. Generally, western gorilla males tend to mate with every female in the group regardless of whether they are fertile or not. Mountain gorilla silverbacks prefer to mate only with a fertile female.
It is important to note that although silverbacks form strong and long lasting social bonds with females in his group, they will start leaving him for another group when he gets too old or is at the end of his reproductive years. A few will remain loyal to the dominant silverback but will join another group in case he dies. Females have been observed changing groups several times during their lifetime. They do this to protect their future offspring. If the dominant silverback dies before weaning, the infant will be killed by the male taking over.
Courtship and Reproduction
What happens after mating? If mating is successful, the female will have a gestation period of about 8.5 months. Female gorillas give birth after about every four years. When she conceives, the shape of bump 
changes and the breast increases in size but not as pronounced as in humans. On the day of birth, the female looks uncomfortable, does eat and does a lot of stretching. When she gives birth, it is normally in the morning. A female gorilla will produce about 8 babies in her lifetime but only a few will survive to adulthood. Males are not active caretakers and the infant depends entirely on the mother for their survival. The role of the silverback is to ensure that infants are accepted by other members of the group. He will protect them from any bullying from members of the group and for this reason, the mother will stay close to the dominant silverback for extra protection during the first five months.
The female carries the infant using her hands for the first four months. During the first four months, the infant suckles from the mother’s breast about every 3 hours. After the four months, the infant will ride on the mothers back and begin feeling confident enough to move a few meters away from the mother for brief periods of time. By the time it reaches 12 months, it would be confident enough to move five meters away from the mother. By the second year, the distance between mother and baby increases further and they begin spending time away from each other. The mother weans the infant at 30 months. In general, the infants will nurse until they are about four years old. Once weaned, the infant starts to make its own nest and it is at this time that the mother starts to ovulate again.
Mortality rate is high among gorillas and almost half do not make it to adulthood. One of the reasons for this low mortality is that they depend on their mothers for a very long time. Changes in group dynamics and the arrival or takeover of a new silverback is like a death sentence to all infants still nursing. The silverback will usually kill all infants in order to mate successfully with the adult females.
