Age at sexual maturity of male Xiphophorus nigrensis in nature

" Male Xiphophorus nigrensis drastically reduce growth when they reach sexual maturity and exhibit a genetically determined polymorphism in size. We determined that the growth increments on the otoliths in X. nigrensis indicate true age in days for fish that have not reached sexual maturity. After maturity, these rings provide an estimate of the age at which sexual maturity was reached. Age at sexual maturation was significantly different across geneotypically determined size classes for field caught animals. On average, males from the smallest size class matured 46.6 d sooner than did males from the largest size class. The variance in age at sexual maturity was much greater for laboratory-reared individuals than for individuals reared in either semi-natural conditions or from the field. This is consistent with the hypothesis that the environment can influence the age at which sexual maturity is reached, and suggests that for life history studies of poeciliid fishes age at sexual maturation needs to be measured in the field. This study also demonstrates that otoliths provide an accurate estimate of age at sexual maturity but not of true age of mature males "