Michigan deer season is less than two weeks old 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, officials believe they suffered heart attacks while dragging heavy deer back to their vehicles after the kill.

“Deer hunting usually involves walking up to a tree stand or blind, waiting in the cold for hours and then experiencing a sudden rush of adrenaline when a deer is spotted,” says Dr. Bradley Serwer, cardiologist and chief medical officer at VitalSolutin, Fox said.

“Once a hunter hits the deer, they must follow the deer, often for a long distance over rugged terrain,” he continued. “If the deer is found, it must be transported back to a vehicle. Typically this involves dragging a 100- to 200-pound animal over varied terrain.

Serwer told Fox that “exposure to cold increases the risk of abnormal heart rhythms, especially ventricular arrhythmias that can cause sudden cardiac arrest.”

Mix that with the adrenaline rush, which Serwer told Fox can “increase the risk of ventricular arrhythmias and sudden cardiac arrest,” as well as walking in and out of duty stations, and it can put “a strain on the cardiovascular system.”

And if you have to make the return trip while dragging a heavy deer, it can be quite taxing.

Serwer said Fox fighters 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 bringing a phone in case you need to call for help.

You can read more about the deaths and suggestions to prevent tragedies in the field here.