What to Know The litter, found June 11, is the first documented in the Simi Hills northwest of Los Angeles

Researchers who were tracking the mother since January suspected she had given birth after watching her activity using GPS

The four kittens, all females about 4 1/2 weeks old, will help biologists better understand how the big cats survive near urban areas

Four mountain lion kittens were discovered this month inside a Simi Hills den, an important find for biologists researching how the animals navigate the varied landscape northwest of Los Angeles.

Video from the National Park Service shows the blue-eyed kittens inside the den, the first litter documented in the Simi Hills between the 101 and 118 freeways. One of the kittens hisses and takes a swipe at the camera as it enters the den while the mother, known as P-62, was away on June 11.

All four kittens -- identified as P-66, P-67, P-68, and P-69 -- are female. Biologists tracking P-62 since January suspected she had given birth, but waited until radio telemetry from a tracker told them she was away from the den before approaching the big cats' hideout nestled between the Santa Monica and Santa Susana mountain ranges.

"This is the first litter we have marked at the den in the Simi Hills, which happens to be a critical habitat linkage between the Santa Monica Mountains and larger natural areas to the north," said Jeff Sikich, biologist for Santa Monica Mountains National Recreation Area. "We are very interested to learn about how they will navigate the fragmented landscape and whether they will remain in the Simi Hills or eventually cross one or more freeways to the north or south."

During the stealthy visit to the den, they took tissue samples and checked on the kittens' health. They weighed between four and five pounds and are likely about 4 1/2 weeks old.

It's the 15th kitten litter documented by National Park Service biologists at a den site. They will help researchers, who have been studying mountain lions in the Santa Monica Mountains region since 2002, better understand how the cats survive near urban areas of Southern California.