Fellow pregnant chimp Zombi, who was there at Sonna's side when she died, took the newborn under her wing.
Zombi, who is expected to give birth later this month, immediately adopted him -- caring for the "healthy" and "strong" infant.
"From the minute she first cradled the newborn, she's been amazing -- grooming, supporting and nursing the little man as though he was her own."
Laura Hanley, Monarto Zoo Senior Primate Inmate Keeper
Montarto Zoo's senior primate keeper told the AFP that she's never seen anything like that happen before.
"I'm humbled by what we've seen from the chimpanzee troop over the last few days -- it's unheard of to see a heavily pregnant chimpanzee adopt an orphaned newborn infant."
Laura Hanley, Monarto Zoo Senior Primate Keeper
Hanley said in a statement that she doesn't know of anywhere else in the world where a pregnant chimp has acted like a surrogate to another infant, especially one who is so close to giving birth, herself.
The primate keeper was optimistic that Zombi would continue to care for Boon after the birth of her own child. Hanley said the zoo would let "nature take its course" and only intervene if they see the baby chimp struggling.
Adorable, right? Check out this gallery for photos of baby animals:
Pregnant female chimp adopts adorable baby orphan in 'unheard of' act of kindness
(Photo Credit: Corbis)
(Photo Credit: Corbis)
Two weeks old baby tiger 'Schuna' is pictured on September 5, 2010 at the zoo in Wuppertal, western Germany. As 'Schuna' was not accepted by its mother 'Mymosa', the animal is bottle-fed and brought up by keepers. (Photo credit: HORST OSSINGER/AFP/Getty Images)
Giraffe mother 'Jujis' (L) looks after her giraffe baby 'Thabo' in their enclosure at the zoo of Hanover, northern Germany on August 12, 2011. The 1.90 metre tall and 80 kilogramme heavy Rothschild giraffe baby will be fed by his mother for th next15 months. The Rothschild giraffe is among the most endangered giraffe subspecies with only a few hundred members in the wild. (Photo credit: HOLGER HOLLEMANN/AFP/Getty Images)
NARA, JAPAN - MAY 11: A newly born deerlet is seen at Rokuen, a deer shelter on May 11, 2014 in Nara, Japan. (Photo by The Asahi Shimbun via Getty Images)
(Photo credit: Getty Images)
TOKYO - JUNE 24: A baby Fennec is seen at Sunshine International Aquarium on June 24, 2009 in Tokyo, Japan. The small nocturnal fox babies were born on May 17 and opened to the public from June 20 attracting many visitors to the Aquarium. (Photo by Junko Kimura/Getty Images)
XI AN, CHINA - SEPTEMBER 02: A newborn female giant panda is seen at Shaanxi Province Rare Wildlife Rescue and Breeding Research Center on September 2, 2013 in Xi An, China. 8-year-old giant panda Xin Xin gave birth to the baby on July 18 after being artificially inseminated. (Photo by F.Kingi/ChinaFotoPress via Getty Images)
GREAT BRITAIN - APRIL 17: Lambs in the Cotswolds, Oxfordshire, United Kingdom, UK. (Photo by Tim Graham/Getty Images)
(Photo credit: Shutterstock/Natalia Melnychuk)
(Photo credit: Shutterstock)
FUKUOKA, JAPAN - NOVEMBER 05: Newly born Asian short-clawed otters are seen at the Fukuoka Municipal Zoo on November 5, 2013 in Fukuoka, Japan. The otters were born on August 5. (Photo by The Asahi Shimbun via Getty Images)
A five-month-old still nameless bear explores its enclosure with its mother Huanca on February 12, 2014 at the zoo in Duisburg. (Photo credit: ROLAND WEIHRAUCH/AFP/Getty Images)
(photo credit: Maureen P. Sullivan)
A one-month-old jaguar cub looks out from a bucket at the Leningradsky Zoo in Saint Petersburg on April 15, 2015. (Photo credit: OLGA MALTSEVA/AFP/Getty Images)
Nine day old baby chimpanzee Deyo sits on the back of it's mother Lady at the 'ZOOM' Zoo in Gelsenkirchen, western Germany, on March 19, 2015. (Photo credit: ROLAND WEIHRAUCH/AFP/Getty Images)
This photo taken on December 28, 2014 shows a red-faced macaque baby monkey playing in its box at a zoo in Hangzhou, in eastern China's Hangzhou province. The baby monkey, born some two months ago, quickly became popular for its restless movements that resembled a gymnast, local media reported. (Photo credit: STR/AFP/Getty Images)