My father's heart rate has been oddly low, no history of problems. what can it be?

Bradycardia: See a dr. For a multitude orf possible causes and to asure it is not sometthing that needs addressing.

Probably nothing: While the normal resting heart beat is usually around 60, some people run a lower heart beat, especially if they are athletic. We generally don't treat a number, we treat a patient. If he has had no symptoms (dizziness, fainting, easy fatigue-ability, shortness of breath with exertion) and is longstanding it is likely nothing, but he should see a doctor.

How low?: Is he having symptoms of fatigue, dizziness or passing out? If yes, he needs further evaluation.