Reproductive strategy predicts thermal biology and microhabitat use in two lizards along a riparian-upland gradient

Life history traits may constrain how species respond to environmental challenges. In squamates, viviparity is often associated with adaptation to cold climates. However, sympatry between viviparous and oviparous squamates is not uncommon, suggesting that flexibility in other traits (e.g., behavior, physiology) may provide alternative strategies for overcoming the challenges associated with reproduction in cold climates. We compared microhabitat associations and thermal physiology of the two lizards that occupy higher latitudes than any other lizard species in North America, the viviparous northern alligator lizard (Elgaria coerulea) and oviparous western skink (Plestiodon skiltonianus). Our study populations were distributed among thermally and structurally different habitats in the Bitterroot National Forest (Montana, USA). Northern alligator lizards selected microhabitats that were colder and closer to their thermal set range than western skinks. They also had a colder preferred body temperature, thermal set range, and critical thermal limits than western skinks. Northern alligator lizards thermoregulated less precisely and were almost exclusive to riparian habitats, which provide colder and more humid conditions than upland habitats. In contrast, western skinks were common in riparian and upland habitats. These results align with some but not all expectations regarding the evolution of squamate viviparity. We discuss our findings in regard to these expectations and propose mechanisms that may explain the patterns observed here. Our results also lend insight into the habitat requirements of these species in an understudied, but rapidly developing portion of their ranges.