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 with 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. Dogs have a more domed forehead and a distinctive "stop" between forehead and nose. The temporalis muscle that closes the jaws is more robust in wolves. :p158 Wolves do not have dewclaws on their back legs, unless there has been admixture with dogs that had them. Most dogs lack a functioning pre-caudal gland and enter estrus twice yearly, unlike gray wolves which only do so once a year. So-called primitive dogs such as dingoes and Basenjis retain the yearly estrus cycle.