Tue, March 17, 2020

Female Gorillas Affect Group Success

Although gorilla groups are led by a dominant silverback (adult male), who determines the group’s daily travels and certain other activities, adult females play crucial roles in the success of every group.

Of course, having numerous females is critical to the stability and growth of the mountain gorilla population. Adult females don’t give birth while they are nursing their young, which takes up to four years. With more females, the better it is for all mountain gorillas!

And even though they are largely peaceful, female gorillas do have hierarchies within their groups, with dominant females having preferred access to feeding spots and proximity to the dominant male. Hierarchy among females is regulated by several factors, such as number of offspring and relationships with other group members, especially the dominant silverback. All of this helps shape the groups in many ways.

Matriarch Effie, right, in 1976, with one of her early offspring, Puck. Effie’s “clan”
became the largest wild mountain gorilla family ever recorded.

Matriarch Effie

Ever since Dian Fossey launched our in-depth studies of mountain gorillas in the 1960s, our field staff has observed many historically significant females, but none perhaps as important as matriarch Effie, who was first seen by Dian Fossey in 1967. Effie lived in Fossey’s study group 5, which later transformed into Pablo’s group and is still monitored today.

Fossey was greatly impressed by the strong maternal instincts of the female gorillas she observed and none more so than Effie. One example occurred when Effie swiftly rescued her daughter Poppy from a tree, where the youngster had fallen and become entrapped in a stranglehold between branches. Fossey also noted Effie’s “dependable temperament,” which not only provided security to her offspring but helped her maintain top rank in her group.

Effie already had two offspring when Fossey first saw her, soon to be followed by more daughters, which eventually created an important extended family. Many of her daughters became strong leaders and productive mothers in their own groups, including Puck, Tuck, Poppy, Maggie and Mahane.

Eventually Effie’s offspring and subsequent generations came to be known by Fossey and our subsequent researchers as the Effie clan, or more formally as the Effie matriline. It is the largest wild mountain gorilla family we have ever recorded and one which is still going strong 53 years after Effie was first seen.

Many of Effie’s female descendants are now in groups monitored by the Rwanda park authorities, such as the Susa group. In the groups currently monitored by the Fossey Fund, we have 22 members of Effie’s matrilineal family. And that’s without counting all the males who are also related to her!

Effie’s grandson, the late Cantsbee, became the most successful silverback of our time, leading the largest group ever and siring the most offspring. Another grandson — Mafunzo — is still a leading silverback today. And Effie lived until 1994.

Current Effie descendants

The Fossey Fund monitors around 40 adult females currently in the groups that we protect and study every day. The number varies slightly due to normal events, such as births, deaths, and transfers between groups.

For example, last year, two Effie granddaughters — 26-year-old Ukuri and 14-year-old Ishyaka — transferred groups several times, and in the process helped to establish two new groups, which we now monitor every day (Mutobo group and Urugwiro group).

Ukuri is with the newest group, Urugwiro, after making several transfers. She is a mother of three offspring, who live in her former groups. Ukuri has always had high status, following in the footsteps of grandmother Effie, and so we expect that she will become dominant in Urugwiro’s group as well.

Ishyaka was born to mother Poppy in Susa’s group and then transferred to Pablo’s group, before she had offspring. She then had one youngster, Akariza, who moved with her to Kureba’s group as they split from Pablo’s group after leader Cantsbee died. Ishyaka now is with the new Mutobo group, after her many moves, though Akariza remained behind in Kureba’s group.

The best female event of the year was when we saw Effie granddaughter Makuba appear in Kuryama’s group. Makuba had transferred to an unmonitored group in 2007, and we had not seen her since then. But during the 2010 and 2016-2018 mountain gorilla censuses, we received DNA reports showing that she was with an unknown group ranging outside of our area, in the Democratic Republic of Congo. Then in March 2019 we saw her with Kuryama’s group, which also ranges in Congo but paid a brief visit to our area. And the good news was that she had a new infant with her.

Other leading females

The historic Pablo group currently has eight adult females, with the two eldest — Gutangara, age 36, and Mukecuru, age 39 — in the dominant positions at this time, after two other important females left the group in 2018. Both Gutangara and Mukecuru had moved to Pablo’s group in 1995 from their original group (Susa).

Gutangara is currently the most successful female we monitor, with seven offspring, all of whom are thriving. Her success is at least in part due to her family bonds. Four of her offspring still live with her in Pablo’s group and they spend a lot of time together. She is also the mother of the two silverbacks who recently formed their own new groups: Mutobo and Urugwiro.

Also exciting is that both Gutangara and her daughter Shishikara gave birth in 2019, only a month apart. This allows us to observe how close family bonds may help and influence the raising and development of their infants. We assume that the support of family members will speed up the youngsters’ maturation and weaning, helping them to become independent more quickly than usual.

Facilitating genetic diversity

Female gorillas also have a major responsibility in helping to keep the gene pool as diverse as possible, which they do by transferring among groups as they become adults, so as to find new, unrelated males with whom to mate.

Last year we observed an unusually high number of female transfers — 35 in total, involving 16 females. Twenty-one of these moves were related to the formation of the two new groups formed by Mutobo and Urugwiro, as well as to the ending of one group (led by Kubona, who became a lone silverback again).

This highlights both the independence and the crucial importance of females as they move from group to group, thus facilitating genetic diversity in the population.

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