Predicting responses to warming temperatures for early growth rate, size and age at sexual maturity and mortality in male <i>Xiphophorus multilineatus</i>

The majority of ectotherms conform to the temperature-size rule, a reaction norm where individuals reared in colder environments grow slower, take longer to reach sexual maturity and attain a larger adult size than conspecifics from warmer environments. In relation to the temperature-size rule which influences growth rates, tradeoffs often exist between growth rate and survival across taxa, namely increased growth rate frequently leads to a higher probability of reaching sexual maturity but a shorter adult life span. We examined whether the temperature-size rule applies to one of the two male alternative reproductive tactics (ARTs) in Xiphophorus multilineatus . Sibling males from the courter ART were reared at one of two temperatures (25 °C and 20 °C) and monitored through sexual maturity. Males followed the temperature-size rule of faster growth and earlier maturation at a smaller size in the warm environment. While early growth rates and mortality were higher in the warm treatment, we did not detect an influence of growth rate on mortality when controlling for temperature and dam. We did detect, however, evidence of dam-associated variation in size at sexual maturity, but not age, with potential implications for the mechanisms impacting the onset to puberty. Finally, depending on whether the smaller sneaker males also conform to the temperature-size rule, we discuss how tradeoffs that help to maintain the two male ARTs in X. multilineatus (age and size at sexual maturity, growth-mortality rate tradeoff) might be altered by warmer temperatures.