Reciprocal hybrids between X. fraseri and X. muelleri show a strongly marked matrocline expression of several characters, e.g., time of apparition of melanophores on various body parts, ground-colour, pigment pattern, size and shape of the eyes. On the other hand, the hybrids tend to be intermediate in certain characters of body proportion, or may even show signs of luxuriance, like in the length of the subocular tentacle and of posterior extremities. A striking feature is the absence of the fourth claw on the prehallux in the hybrid combination X. muelleri x X. fraseri. A clawed prehallux is typical for X. fraseri and proved to be dominant in hybrids between X. fraseri and several other Xenopus species, which lack an armed prehallux.