A tail of two newts: Aquatic tail size carries over but does not impair terrestrial locomotion in eastern newts (Notopthalamus v. viridescens)

Background/Question/Methods

Individual organisms encounter regular changes in the environment throughout their lifetime, resulting in fluctuations in phenotypic optima. Consequently, many species have evolved phenotypic flexibility to adjust to these seasonal changes. The ability of an organism to produce seasonal traits in preparation for a breeding season is known to impact its reproductive success, but less is known about how these flexible sexual traits might carry-over to impact fitness in a non-breeding season. We used the eastern newt (Notopthalmus viridescens) to determine if a trait that exhibits phenotypic flexibility would carries over between seasons with divergent optimal phenotypes. In many populations, all newts increase their tail size during the aquatic (breeding) season and decrease tail size for the terrestrial (non-breeding) season. In order to determine if this trait has an associated cost across seasons, we first determined whether tail size was correlated across environments.  We then tested newt locomotor performance in both the aquatic and terrestrial phases, and analyzed performance as a function of tail size. We expected that newts with larger tails in the aquatic environment would also exhibit larger tails in the terrestrial environment, resulting in reduced locomotor performance.

Results/Conclusions

We confirmed that aquatic (breeding) newts possess larger tails than terrestrial (non-breeding) newts, with males undergoing a more dramatic reduction in tail size when transitioning between phases. There was a strong correlation between tail size in the aquatic and terrestrial phases, suggesting that this trait carries-over between the aquatic and terrestrial environments. Tail size was positively related to locomotor speed in the aquatic phase, as expected, but not in the terrestrial phase. There was no relationship between maximum locomotor speeds in the aquatic and terrestrial phases, suggesting no evidence of a general trade-off in locomotor capacity between phases. We maintain that there are different phenotypic optima between the aquatic and terrestrial phases, as evidenced by the seasonal changes in tail size. However, it does not appear to be based on terrestrial locomotor performance. Further inquiry may find that large tails are disadvantageous in the terrestrial environment for other reasons, such as water balance, or that locomotor costs are only observed in structurally complex, natural settings.