She welcomed her son in 2020.

Just in time for Mother's Day, new mom Rooney Mara wrote a special letter for Farm Sanctuary. According to Entertainment Tonight, Rooney opened up about her son, River, and being a new mom. She and Joaquin Phoenix welcomed River back in September 2020.

Mara and Phoenix are well-known animal rights activists and Mara also used her letter to celebrate mothers of the animal kingdom.

In the message, she wrote that she's appreciative of the fact that she can raise River — who is named after Phoenix's late brother — "in all of the ways nature intended." She also urged people everywhere to nurture a "kinder and more sustainable world."

Rooney Mara
Credit: Matt Crossick/PA Images via Getty Images

"As a new mom, Mother's Day has taken on a special meaning this year. Raising our baby, River, has opened my heart to a whole new life filled with hope and more determination than ever to create a kinder and more sustainable world," she wrote. "I feel so fortunate to be able to nurture my son in all of the ways that nature intended. And I wish all mothers in the animal kingdom could experience that sacred maternal bond with their young, devoid of exploitation by humans." 

Phoenix and Mara both volunteer at Farm Sanctuary. She also noted that anyone who wanted to help animal mothers could volunteer their time with the organization or send monetary donations. 

"As I celebrate my first Mother's Day, I hope you will join me in honoring all of the inspiring moms and maternal figures who've made a difference in your life. With love, Rooney Mara," she finished.