AceShowbiz - Rooney Mara was left "devastated" after witnessing the horrific conditions inside British animal farms in an undercover investigation for campaign group Animal Equality.

The 34-year-old actress went undercover at several farms in the U.K. over the summer for the investigation entitled "With My Own Eyes", and saw some terrible scenes such as mother pigs crushing their infants to death due to being kept in such tiny, confined spaces.

Reflecting on the experience, Rooney said it had made her more than a little emotional, especially as she'd just become an auntie after sister Kate Mara welcomed a daughter back in May.

"Nothing prepares you for looking into the eyes of a mother pig whose life is to be impregnated and left in a cage until she's slaughtered," she said. "I kept thinking about my sister who just had a baby, and how beautiful that instant desire to nurture and protect your child is."

"I can't imagine how awful it must be to be literally trapped and crushing your own babies and not be able to do anything about it."

Animal Equality claimed one farm had a staggering 3,000 pigs confined in tiny crates, while another housed around 50,000 chickens. Some of the chickens were found injured and unable to move - resulting in death from starvation and thirst - while others were found to have been bred to be "abnormally large." This resulted in their legs and organs being unable to function.

Rooney added she hopes her investigation will help "impact other people who want to change their minds as well."

Sharon Nunez, President of Animal Equality, said in a statement, "Animal Equality's undercover investigators face great personal risk to expose what the meat industry doesn't want you to see, and what the public has every right to know."

"We are deeply grateful to Rooney for bravely looking into the eyes of these animals, and joining us in our promise to them to share their cries with anyone who will listen. Her courage is truly inspiring."