Effect of pitfall trap design on internal trap temperature and the implications for live-trapped lizards

Abstract

The capture of animals in live traps poses inherent risks of heat stress and mortality to trapped individuals. Despite a long history of pitfall trap use in New Zealand for monitoring small lizards, the design of traps and their covers often varies; however, the effects that this has on the internal temperature of the traps is unknown. Poor trap design may increase the risk of stress and mortality if internal temperatures exceed thermal limits. We tested the influence of three aspects of trap design (cover material, cover colour, and internal trap size) on the maximum and mean internal trap temperatures at Kaitorete Spit, New Zealand. Temperatures were recorded with dataloggers across 24 days during midsummer (December 2020 to January 2021). Internal temperatures reached a maximum of 38.2 °C (on a day with a maximum air temperature around 31 °C). Those trap temperatures are above predicted harmful thermal limits of some New Zealand gecko species and levels that induce avoidance behaviour in some skinks. Maximum temperatures were lower under plywood covers than those made of plastic or Onduline, brown covers than black, and in 4 L traps rather than 1 L. The best trap design had thermal maxima 3.9 °C lower than the worst design, averaging 4.9 °C above air maximum temperatures in the best design compared with 8.6 °C above for the worst. As climate change increases temperatures in some areas, the risk of heat stress and mortality rises for lizards constrained in pitfall traps. We recommend the use of plywood covers and larger internal trap sizes to reduce this risk.