Tradeoff in two winter-active wolf spiders: Increased mortality for increased growth

A number of arthropods, including some spiders, forgo diapause as an overwintering strategy and are instead active at low temperatures. The higher metabolic rates and periodic foraging behavior associated with winter activity, however, can be incompatible with high levels of freeze avoidance. This is thought to be due, in part, to increased ice nucleators in the gut after feeding events and physiological complications brought on by accumulating certain cryoprotectants. To characterize the relationship between winter activity and cold hardiness, this study quantified the seasonal resistance to freezing in two common wolf spiders of hardwood forests in the Eastern United States, Schizocosa ocreata (Hentz) and Schizocosa stridulans (Stratton) (Araneae: Lycosidae). Individual spiders were collected from a deciduous forest in Kentucky from August 2012 to March 2013 and were subjected to supercooling point (SCP) determination assays. Contrary to many invertebrates with high cold hardiness, mean SCP of S. ocreata and S. stridulans remained constant throughout the study, which we subsequently determined was their lower lethal temperature. Interestingly, daily low temperatures within the leaf litter occasionally fell below the mean SCP of the spiders, subjecting them to a significant risk of freezing during winter. To determine if this high risk was a result of winter predation, we fed spiders varying quantities of prey, but found no significant association between consumption and SCP. Despite exposure to potentially lethal temperatures, Schizocosa did not seasonally augment their cold hardiness to better survive. This suggests an ecological tradeoff, where these spiders appear to assume increased mortality risk in exchange for maximized growth opportunity during a time of year when few competitors are active. We posit that the increased fitness benefits associated with early maturation and larger size in spring help to sustain these abundant spider populations.