As the spring sun crests the frosty horizon in southeastern Norway, it brings with it the promise of warmer temperatures and longer days. Mountain hares (Lepus timidus) don’t seem to mind the prolonged winter nights; they are, after all, nocturnal. But come spring, the hares ramp up their daytime activity and exchange their white coat for a brown one that blends in with the thawing landscape.

With the approach of mating season, interactions between the normally solitary creatures can become hostile. As is the case throughout much of the animal kingdom, male hares battle for access to potential mates, rearing up on their hind legs and swinging punches with their forelimbs. “The first one to land a direct hit against the head or body of his rival is usually the winner,” says photographer Erlend Haarberg, who has endured temperatures of -25 degrees Celsius (-13 Fahrenheit) every spring for the past quarter century to photograph the hares. But unlike the role played by females in many other species, the prized female hares are hardly passive spectators in this competition. They have a say in who partners with whom, and when. And they make their preferences known through sparring.

Despite the storied reproductive rates of most rabbits and hares, mountain hare populations in Norway appear to be dropping fast. In the absence of comprehensive census data, hunters’ harvests are used to approximate population trends: In 1990, hunters shot 125,000 mountain hares; today the total is only 15 percent of that—about 20,000 hares per year.

Many scientists think climate change bears much of the blame. Norway’s snowpack is becoming less reliable. In Hedmark County, just south of where this photo was taken, there was snow on the ground for 165 days in 2006; in 2014, the number of days with snow cover dropped to just 99. This shift eliminates the hares’ primary line of defense against predators: camouflage. It appears the costume change they’ve evolved over millennia can’t adjust quickly enough to keep up with this sort of trend, so the hares find themselves mismatched against their background for increasing periods of time. This forces the vulnerable animals to make tough trade-offs. As a 2016 study showed, having white fur when the ground is bare may deter hares from leaving their burrows to search for food. And it may put them at a higher risk of predation when they do.

While mountain hares are now considered Near Threatened in Norway, they are still fighting for survival across the country’s picturesque landscape. So Haarberg will continue to spend cold, sleepless nights in his blind, waiting for the prizefighters—and the sun—to appear. “Just to experience the spring months in the mountains, when nature awakens after the dark winter, is something I look forward to every year.”