Lizards of the viviparous genus Phymaturus inhabit regions in which a significant proportion of the year is unsuitable for growth and reproduction. All Phymaturus studied have shown biennial female reproductive cycles. We studied the reproductive biology of Phymaturus zapalensis, which lives on rocky outcrops in cold and arid environments of Patagonia, Argentina. Male and female reproductive cycles, mean annual reproductive output, and sexual dimorphism were analyzed. Additionally, new dimorphic traits of Phymaturus antofagastensis and Phymaturus tenebrosus were compared with published data. The female reproductive cycle of P. zapalensis is annual-biennial (because of skipping a year of reproduction), and synchronous with the annual male reproductive cycle. In midspring, males show spermatozoa in epididymis, and females have enlarged follicles, characteristic of imminent ovulation. The presence of 45% of the adult females with oocytes smaller than 4 mm, in all months of capture except October, suggests a year of skipped reproduction. Female P. zapalensis had greater SVL and a relative larger interlimb length than males. Males exhibited relatively greater head width, neck width, and diameter of front and hind legs. Our data indicate that the potential for annual reproduction exists in this biennial breeding genus. The capability of P. zapalensis to perform either an annual or biennial reproductive cycle, instead of the characteristic biennial cycle of the genus, could be the result of a longer activity season, higher temperatures during spring, summer, and autumn months, together with a high-energy omnivorous diet.
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