If animals have a way of sensing imminent danger from radioactivity in the air, the boar at the time may have fled the area and avoided intense radioactivity at the moment, but they continued to EAT contaminated mushrooms, etc.
Maybe INGESTED radioactive particles has a lesser effect after coming into contact with digestive juices, enzymes, etc.
Fully grown healthy humans also seem to stand a better chance of survival and even remaining in good health (what kinds of EFFECTIVE protective gear was the Soviet government providing in 1986? and what protocols/supplements were available at that time for recovery?)....