“It’s usually terrifying to walk up the streams because you get a sense of carnage just around the next corner,” Armstrong says.

But in 2014: nothing. The grass was tall. The gulls were svelte. And the salmon were mostly untouched. Foxes, eagles, and wolves took their fair share, but without the bears, these lesser predators made the tiniest of dents in the salmon population. Most of the fish died naturally. “There’d be piles of dead salmon, just molding,” says Armstrong. “The bacteria were eating them instead of the bears.”

So where had the bears gone? Fortunately, at the beginning of the year, the team had collared around 15 of the animals to track their movements. As the data rolled in, the team noticed that the bears were up in the hillsides, far away from the streams. In particular, they were sticking to land that was covered in red elderberry bushes. And Bill Leacock, a team member from the U.S. Fish and Wildlife Service, noticed that all the bear droppings that summer were full of elderberry skins. Bear scat is a horrendous mess when the animals gorge on salmon, but this stuff was very different in color and consistency.

In most years, red elderberries only ripen from late August to early September, at the end of the salmon season. The two food sources don’t overlap, so the bears eat them in sequence, gorging on salmon before bingeing on berries. But, by looking at historical data, Deacy and Armstrong found that this natural timetable has changed. In Alaska, spring temperatures have increased and elderberries have been ripening earlier. In 2014, the berries ripened especially early, bringing them in sync with the spawning salmon. And it seems that whenever both items are on the menu simultaneously, the bears always choose berries.

Which makes no sense. Pound for pound, salmon contains twice as much energy as elderberries. If bears are looking to gain as much weight as possible, in anticipation of the coming winter, why would they pick the less calorific food? “All our conventional wisdom made it hard to believe that they were switching to these berries,” says Deacy.

He solved the mystery by talking to Charles Robbins, who runs the Bear Center at Washington State University. Robbins suggested that it’s not the total number of calories in these foods that matters, but the levels of different nutrients. In an earlier study, in which he offered captive bears a varied diet, he’d found that the animals mix and match their foods so they get around 17 percent of their energy from protein. That’s the level that allows them to gain weight most quickly. If they overload on protein, they actually lose weight.

Salmon are far too rich in protein—it accounts for about 84 percent of the energy in their flesh. But elderberries, by astonishing coincidence, comprise around 13 percent protein—far more than your typical berry, but almost exactly the optimal amount for a grizzly bear. By focusing on that single food, the bears can gain weight as fast as possible.