What environmental stimulus is most likely to result in a more rapid heartbeat in an animal?

A sudden and loud noise, such as a predator's roar or an unexpected sound, is an environmental stimulus that is most likely to result in a more rapid heartbeat in an animal. This response is part of the "fight or flight" mechanism, triggering the release of adrenaline and preparing the body for quick action. Other stimuli, such as intense physical exertion or high temperatures, can also lead to an increased heart rate, but sudden threats typically elicit the most immediate response.