DIFFERENTIAL ATRESIA OF OVARIAN FOLLICLES AND ITS EFFECT ON THE CLUTCH SIZE OF 2 POPULATIONS OF THE VIVIPAROUS LIZARD SCELOPORUS-MUCRONATUS

1. The phenomenon of follicular atresia, defined as the death of ovarian follicles prior to ovulation, has seldom been noted in the ecological literature as a major factor in the proximate control of clutch size, but it has been demonstrated that it influences clutch size in many lizard species. 2. Follicular atresia was examined by analysing the difference between the number of vitellogenic follicles recruited at the onset of reproductive activity and the number of in utero embryos in females of the viviparous lizard Sceloporus mucronatus; with female body size factored out, the number of vitellogenic follicles recruited from the germinal beds was not different between populations whereas clutch size was significantly different between the populations. 3. The difference in clutch size is due to varying rates of atresia, with 52% observed in the montane population but only 2% in the lowland population prior to correction for female body size; size corrected values are 54-6% for the montane population and 17.0% for lowland females. 4. These data suggest that maximal clutch size, based on the number of follicles recruited for vitellogenesis, is species specific and determined by body size. 5. Clutch size, however, is determined by the rate of atresia which could be influenced by the milieu of endocrine factors created by proximate (environmental) stimuli.