Gorillas in zoos can get pregnant from (about) the age of six. Ideal is for them to get pregnant when they're almost 10. The pregnancy of a gorilla is about 8.5 months (comparable with the pregnancy of a human baby). In general one baby is born. Twins are very unique.
Gorillas are born throughout the year. With some animals there's only a short breeding season, resulting in births during an x number of months. It's like that with squirrel monkeys (± November till January), barbary macaques (± August till December) and ring tailed lemurs (± April till June). But not with gorillas (and other apes).
Births happen at night or early in the morning most of the time. In the wild that is because the group has to move when they wake up. For some reason also in zoos the births are in the night and early in the morning. But there are exceptions. Two of them I remember: Dalila -back then- in Apenheul and Sindy in Artis. Dalila gave birth to her daughter Kisiwa on the island, in daytime. That's why she's named Kisiwa: it means island. Sindy gave birth in 2016 in the (late) afternoon.
When the twins in Burgers' Zoo were born (mom: N'Gayla), they were the 6th twins in Europe; the first ones fully raised by mom. Also outside Europe 6 twins were born (that is till 2013). Also there only one set of twins was fully raised by the mom. In the other cases one or both twins didn't survive, or one or both twins where hand reared. Two recent cases of twins where it went wrong was in Germany. N'Gayla's half-sister (!!) Dian in Frankfurt gave birth to twins on 15 September 2015. Unfortunately the boy died after 2 days/ the girl died after 4 months (19 January 2016) To everyone's surprise Dian gave birth to another set of twins on 2 March 2017. Unfortunately the boy also died after 2 days. The other twin, a girl named Xetsa, is still alive. Since she's almost one her chance of staying alive is very high. The first few months of a gorilla baby's life are hard.
On 2 March 2013 (before the Burgers' Zoo twins, so they're actually not '6th set of twins' but 7th) a set of twins were born in Duisburg. They were conjoined twins and they were dead at birth. Officially they were twin 6 in Europe, but their birth wasn't mentioned by the zoo to the public.
For as long as a gorilla kid drinks milk, the mom's cycle is absent or suppressed. Even though gorilla babies stat eating normal food from when they're around 4 months old, they keep drinking milk till they're around 3 to 4 years. So all this time the mom can't get pregnant. When the kid stops drinking, the cycle starts again. Most of the time she's pregnant soon after. That's why in general a gorilla females has a baby each 3 to 5 years (with some exceptions, female Tamani in Blijdorp is known for getting pregnant very quickly every time).
Since it's going well with the population in European zoos, the studbook keeper decided to put females on birth control around the period that she could get pregnant again, so less babies are born in zoos nowadays. Because it's a big problem to find new places for the kids when they reach the age of moving. For girls it's still okay, but it's a big problem for boys.
I'd like to give an example of times between births, when mother nature isn't stopped. I choose Mandji, living in Apenheul. She's wild born in/around 1975. Here are the dates of birth of her kids:
- 10 December 1982
- 16 October 1986
- 5 October 1989
- 28 April 1993 (died 27 September 1993)
- 19 July 1994*
- 17 April 1998
- 23 August 2001
- 15 October 2007**
- 10 January 2011
* Since the baby born on 28 April 1993 died, she had another baby one year later.
** The silverback, Bongo, she lived with after coming to Apenheul, suddenly died on 14 August 2005. They were without a silverback for a while. A new silverback, not related to Bongo, had to be found. They finally found one in Zoo Krefeld, Jambo (at that moment 13 years old). End 2006 Jambo moved to Apenheul. That's why there was a long time between the kids from 2001 and 2007.
The gender of a baby isn't always visible soon/quick. The mom carries hare baby on her belly, supported by her hand. So it's difficult to 'look between the baby's legs' Sometimes it takes days, weeks or even months, but sometimes it's easy to see the gender quickly. Some mothers are so relaxed and they are lying in front of a window with the baby on her belly. She's not supporting it with her hand most of the time. Depending on how long it takes for the gender to be determined, the baby gets a name gets picked soon or not so soon
Motherhood is not instinctive for gorillas. Neither for us, humans by the way. We also have to learn about it. So do gorillas. But they can't learn through reading about it or through stories of their own mom or other related experienced mothers
How do they learn? By looking how other gorilla moms do it. Among others with their own mom when the girl gets her first sibling, but also by other females in the group who delivers and raises a baby. If females didn't have the opportunity to look at examples, most of the time they don't know what to do when they gave birth.
A good example of this is in Artis. For many years females Sindy and Dafina live there. Both are hand reared and never saw examples. Because of that they didn't know what to do after giving birth. That resulted in Sindy's babies dying immediately or soon after giving birth (the first one immediately, the 2nd and 3rd after 6 days).
Dafina wasn't able to raise her kids either. All her first 3 kids moved to 'the gorilla kindergarten' in Stuttgart. That's were for a long time babies went to when their mom wasn't able to raise them. All three of them are still alive.
After all these 'failures' they put both females on birth control and they searched for a female who grew up in a 'decent family group'. They found that female in Binti, born in Apenheul on 19 November 1992. She was barely 7 when she moved to Amsterdam. A lot of gorillas were born and raised in Apenheul. Binti saw all these births and the babies growing up from very close. As all bit sisters (and sometimes big brothers) she also was able to baby sit now and then. It's awesome to see a little gorilla walking around with a baby on their belly or on their back. But that's how they learn.
I looked it up and Binti has witnessed 12 births and she witnessed 10 of them being raised. Two babies died ad the age of 5 months and 6 months. So she really had many examples and she has been able to do a lot of babysitting. Among those babies was one full brother. He was around 8 months when Binti moved.
When Binti was old enough, she cold get pregnant. She should be the first one that got pregnant. When Binti arrived Kumba was the silverback. He moved in April 2002 and almost 2 months later Ivo arrived; Ivo then moved to Berlin on 21 September 2005. Both Kumba and Ivo were infertile. On 23 September 2005 Akili arrived.
Like I said: Binti was the first one who was the first one who was allowed to get pregnant. When they saw enough mating and a test showed that she was pregnant, they took the other 2 ladies off birth control (for your information: back then I didn't visit Artis; everything I tell now is what I heard from people, I'm writing it down in my own words). Akili wasn't fertile
Binti was the first one who gave birth, on 13 March 2007. Shortly after that, on 12 July 2007, Sindy gave birth to her 4th kid (the first one with Akili). She has had about 4 months to see what Binti was doing with a baby and yessss: she picked the baby up and put it to her breast. She did everything what she had to do! I heard from visitors who already visited Artis back then that she wasn't 100% perfect, but fact is that that boy (my most favourite gorilla boy) is a big healthy boy, living in a bachelor group in Pairi Daiza, Belgium since 22 July 2016. After Sindy also Dafina gave birth, on 18 June 2008. This was her 4th. Also she learned a lot from Binti and Sindy and she was finally a perfect mom.
This seems to be a successful method!
Unfortunately not all hand reared females who have the opportunity to see others do a good job, do a good job their selves when they give birth to a baby.