Thermal influences on oviposition in the montane butterfly Euphydryas gillettii

Euphydryas gillettii in a montane meadow in Wyoming oviposits in clusters on the highest large leaves of its larval host, the shrub Lonicera involucrata. Egg masses are found on the underside of leaves which predominantly face the southeast and thereby intercept morning sun. Measurements of leaf temperatures show that, during the cool, clear mornings, these leaves are warmer than leaves of the opposite orientation. Manipulative experiments demonstrate that eggs on these slightly warmer microsites hatch significantly faster than eggs on leaves of other orientations. Because of the rapid onset of winter, an increased developmental rate is likely to correlate directly with increased fitness. The proximal mechanism yielding such a result may be that females choose oviposition sites in the shade.