April 4, 2016
Isaro’s twins: Three months and counting
The birth of twins is a very rare occasion among the mountain gorillas monitored by the Dian Fossey Gorilla Fund, so our trackers were surprised and delighted on Jan. 18, when they found mother gorilla Isaro holding two newborn infants. This is only the third set of surviving twins among the known groups of mountain gorillas. Since there can be special challenges faced by a mother gorilla in caring for twins, our staff was nervous about what the future might hold for Isaro’s new twins, yet very excited and hopeful.
Now, some three months after their birth, the twins and mother Isaro are doing well, even though their group has been ranging in very high-altitude areas and has also been weathering the rainy season. Right around the time of the birth, this group had migrated into the alpine zone of Mount Karisimbi, at about 3,500 meters high. Here, the group members spend their days under dense thorny bushes and on the slopes of deep ravines, which makes them extremely hard to observe closely. It also means that our trackers need extra time to reach the group each day, leaving less time for observation. Luckily, we’ve had enough time to see that the twins, mother and other group members all appear to remain in good health!
In fact, the twins seem to have grown at the same rate, meaning that Isaro is doing a good job managing the feeding needs of both infants equally. They are suckling regularly and developing the dense hair coat and typical dense crown on the tops of their heads that are the norm for infant mountain gorillas at this early age. Our field staff have not yet been able to identify the gender of the twins, but this is not surprising since normally it requires several months, a good dose of luck, and perfect visibility to be able to do so.
Our field staff correctly predicted that the group would move downhill to their normal home range around late March, which will give a better chance for detailed and accurate observations, as well as increased data collection.
Twins are a “blessing”
Research assistant Didier Abavandimwe was the first of our staff members to see the twins. “I couldn't believe my eyes because since I started working with the Fossey Fund in 2009, I had never experienced such an amazing event in the gorilla groups we monitor,” he says. “When I saw them, the first thing that came to my mind was that the birth of Isaro's twins is a blessing for everyone who works so hard for the survival of mountain gorillas in general, and Isabukuru's group particularly. It’s almost like a reward, an encouragement, an excitement…” Of the more than 270 births that have occurred in the Karisoke gorilla groups over the last 50 years, only three have involved twins. The first set of twins we observed was born in 1986 and only survived nine days, and those born in 2008 died the day they were born as a result of infanticide. Among the groups the Rwanda Development Board (RDB) monitors, there have also been three sets of twins born and two sets that have survived.
Caring for twins is challenging
Gorilla mothers are completely responsible for infant care at this stage and even despite the increased energetic demands of nursing twins, mothers will not allow other group members to hold or assist in any other way. As the infants age, other group members or siblings might help carry them, but this likely will not happen until after they reach one year of age. One of mother Isaro’s biggest challenges in the first few weeks after the twins’ birth was in keeping up with the group, as mothers usually use their arms to support infants until they are strong enough to grasp on tightly by themselves. So, with twins, Isaro sometimes needed to use both arms to make sure the infants are secure, which could make walking difficult. We don’t have enough data to know if there is a statistically different rate of survival between twins and single-infant births. However, we do know that roughly 25 percent of single infants die during their first year of life and we would expect the rate for twins to be just as high if not higher. Luckily, Isaro is a successful mother who has previously raised four offspring, and so we have high hopes, which so far have been accurate.
Mother Isaro has experience
Mother Isaro, who is 16 years old, still has a 6-year-old and a 3-1/2 year old in the group. This is the fourth group she has lived in and now counts 21 gorillas among its members, including the newborn twins. Isaro was born in Beetsme’s group, and her father was the legendary silverback Titus. In 2007 she left her natal group, following silverback Bwenge, then moved to Pablo’s group where she stayed just a few months, before leaving with silverback Isabukuru, as he formed his own group in June of that year. Isabukuru’s group grew rapidly, becoming the second-largest gorilla group monitored by the Fossey Fund. Many females and infants are in the group, giving the infant twins a great opportunity for play and to develop independence from their mother, once they are old enough.
“Isaro is lucky and can count on a lot of support in rising the twins by the dad, Isabukuru, who is one of the most actively involved in infant caring of all silverbacks, and from her older daughter, Keza, who is seeking any opportunity to gain maternal skills,” says Winnie Eckardt, Ph.D., our research manager at Karisoke.
“The birth of the twins is a gift in many ways,” says Gorilla Program Manager Veronica Vecellio. “First for the pure beauty of nature, but also to remind us how vulnerable and precious gorillas are by just looking at the dedication of Isaro, thriving in such a severe environment with two babies to take care of. It’s a rare opportunity to observe closely the maternal investment and to compare with single-born and the other surviving twins. For Isaro everyday will be a challenge to face and we really hope the twins will do well in this delicate period of their lives!"