Teiids are not territorial, and several individuals' home ranges may overlap. Males are larger than females and compete for mates. Males follow receptive females and guard them against competing males. All teiids are egg layers. The number of eggs laid by females correlates with body size both among and within species. The largest species, Tupinambis merianae and T. rufescens, may lay approximately a dozen eggs when they reach sexual maturity, but by the time a female reaches maximum size, she may lay 30 or more eggs. The nesting ecology of most teiids is simple. Females deposit their clutches in the ground or within logs or debris. Tegus in southern South America, T. merianae, T. rufescens, and T. duseni, build elaborate nests of vegetation in their underground burrows into which they deposit their eggs. Females attend the nests throughout the incubation period. T. teguixin in northern South America lays its eggs in active termite mounds in trees.
The reproductive biology of whiptails and their allies is particularly noteworthy because of the existence of unisexual species. Unisexual species have no males, and individual lizards have no sperm. Mothers lay fertile eggs that develop into identical daughters, that is, clones. This mode of asexual reproduction is called parthenogenesis, and biologists sometimes refer to parthenogenetic species as parthenoforms. Parthenogenetic teiids arise when two sexual species hybridize. Parthenogenesis has been an important mode of speciation for whiptails of the genus Cnemidophorus, which contains at least 12 unisexual species. It is known, for example, that the unisexual desert grassland whiptail (C. uniparens) originated from hybridization events between the Texas spotted whiptail (C. gularis) and the little striped whiptail (C. inornatus). Hybridization events could easily have happened multiple times, hence parthenogenetic species exist in clonal complexes, as in the Laredo striped whip-tail (C. laredoensis) complex. Some parthenoforms have the typical condition of two sets of chromosomes (diploid), whereas others have three sets (triploid). The advantage of parthenogenesis is that when a mother produces identical daughters, each of her genes doubles in frequency in each descendant generation. Because all the individuals are reproducing females, teiid populations grow more rapidly than do populations of sexual species. Within the Teiidae, there is a parthenogenetic species of Kentropyx and one of Terns; parthenogenesis also is known in seven other families of squamates.
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