Joaquin Phoenix, who won an Oscar this year for his role in the film Joker, has reportedly welcomed his first child with fiancee Rooney Mara. They named the baby boy after Phoenix’s late brother, River, who died in 1993.

The director Victor Kossakovsky revealed the news at the 2020 Zurich film festival after a screening of the black-and-white documentary Gunda. Joaquin was an executive producer on the film.

Asking about Phoenix’s involvement in the film, Kossakovsky said: “He just got a baby by the way … A beautiful son called River.”

River Phoenix, the star of films such as Stand By Me and My Own Private Idaho, died of an accidental drug overdose aged 23. Speaking recently to Anderson Cooper on the TV programme 60 Minutes, Joaquin Phoenix reflected on his bond with his brother and how media attention after his death affected their family.

“River was a really substantial actor and movie star, and we didn’t really know it,” he said. “So during that time in which you’re most vulnerable, there are helicopters flying over. There are people that are trying to sneak on to your land. Certainly, for me, it felt like it impeded on the mourning process.”

Phoenix and Mara co-starred in the films Don’t Worry, He Won’t Get Far on Foot and Mary Magdalene. They are prominent campaigners for animal rights, with Phoenix devoting much of his awards acceptance speeches to advocating for a vegan lifestyle.

Two days after his Oscar win, Phoenix rescued a cow and a calf from an abbatoir. In July, a hostage siege in Ukraine ended after the president ceded to kidnappers’ demands to urge people to watch Earthlings, a 2005 eco-themed film narrated by Phoenix.