RAPID CITY, S.D. (AP) — Motorists are being urged to keep an eye out for wandering bison in Wind Cave National Park in South Dakota after rangers noticed an uptick in the number of crashes between vehicles and the animals.

There have been six such crashes this year, with half resulting in fatal injuries to the bison. Park officials say there were no bison crashes last year and only two the year before, neither of which killed the bison. The park, in southwestern South Dakota, has 500 bison.

The latest crash happened Saturday night when a woman driving on U.S. 385 struck and killed a bison, park spokesman Tom Farrell said. The woman had no visible injuries, but her car was totaled and had to be towed, Farrell said.

Farrell told the Rapid City Journal that he’s not sure what’s behind the increase this year, but he said drivers need to watch their speeds and look out for the animals, especially at night when they’re hard to see. Bison can often weigh as much as a ton.

“It’s almost like looking at a black hole, they’re very hard to see” at night, Farrell aid.

Kobee Stalder, visitor services manager at Custer State Park, just north of Wind Cave, said bison are “very difficult” to see in the dark due to their dark-brown fur and the location of their eyes in the middle of their head. The eyes usually don’t reflect until a car is close.

“By the time your lights connect with the eyes to make them shine, it’s too late,” Stalder said. Bison may stand in the middle of the road day or night and don’t tend to jump out of the way like other wildlife do, he said.

None of Custer State Park’s 1,350 bison have been hit this year, but two were hit in 2018 and one each was hit in 2017 and in 2016. One died soon after the crash, while the other three had to be euthanized, Stalder said.

Both Stalder and Farrell agreed that speeding is a major cause of bison crashes.