In this task, you're given passages that contain mentions of a time duration related quesry and we are supposed to write answer to a question that involves event "frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to saturday market" usually happens every few weeks/months, etc.
Note that a lot of the questions could have more than one correct answers. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.
Q: Sentence: Despite their close genetic relationship and the ability to inter-breed, there are a number of diagnostic features to distinguish the gray wolves from domestic dogs. Domesticated dogs are clearly distinguishable from wolves by starch gel electrophoresis of red blood cell acid phosphatase. The tympanic bullae are large, convex and almost spherical in gray wolves, while the bullae of dogs are smaller, compressed and slightly crumpled. Compared to equally sized wolves, dogs tend to have 20% smaller skulls and 30% smaller brains.:35 The teeth of gray wolves are also proportionately larger than those of dogs; the premolars and molars of wolves are much less crowded and have more complex cusp patterns. Wolves do not have dewclaws on their back legs, unless there has been admixture with dogs. Dogs lack a functioning pre-caudal gland, and most enter estrus twice yearly, unlike gray wolves which only do so once a year. Dogs require fewer calories to function than wolves. The dog's limp ears may be the result of atrophy of the jaw muscles. The skin of domestic dogs tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather.
Question: How often do female wolves enter estrus?
A:
once a year.