It’s only been two weeks into deer season in Michigan and already three hunters have been found dead.

And now we know why.

According to a Fox News report, the men were 57, 65 and 83 years old and were found in Arenac and Tuscola counties in the state. And according to the report, officers believe they suffered a heart attack while dragging heavy deer back to their vehicles after the kill.

“Deer hunting typically involves hiking to a stand of trees or a blind spot, waiting in the cold for hours, and then dealing with a sudden rush of adrenaline when a deer is spotted,” says Dr. Bradley Serwer, cardiologist and chief medical officer at VitalSolutin, said Fox.

“Once a hunter hits the deer, he often has to pursue the deer a long distance over rough terrain,” he continued. “If the deer is found, it must be transported back to a vehicle. This typically involves pulling a 100- to 200-pound animal over varied terrain.”

Serwer told Fox that “cold exposure increases the risk of cardiac arrhythmias, particularly ventricular arrhythmias, which can cause sudden cardiac arrest.”

Add to that the rush of adrenaline, which Serwer told Fox can “increase the risk of ventricular arrhythmias and sudden cardiac arrest,” as well as the walking back and forth to belay sites, which can put “a strain on the cardiovascular system.”

And if you have to make the return trip with a heavy deer behind you, it can be really tiring.

Serwer said fox hunters can reduce the risk of such an incident by trying to “stay in shape.”

He also recommended staying hydrated, avoiding alcohol and dressing appropriately. He also suggested keeping a phone with you in case you need to call for help.

You can read more about the deaths and suggestions for preventing local tragedies here.