Cohort splitting is a condition-dependent plastic life-history adaptation to variable environments in the wolf spider, <i>Pardosa agrestis</i>

Developmental plasticity enables organisms to adjust life-history strategies to environmental variability. Cohort splitting-where single cohorts divide into rapid- and slow-developing individuals resulting in facultative bivoltinism-allows us to mitigate environmental unpredictability through plastic life-history decisions. We investigated this phenomenon by documenting developmental patterns under controlled photoperiod regimes in the uniquely cohort-splitting wolf spider Pardosa agrestis and a congeneric species with typical univoltine life cycle. We hypothesized that the basis of cohort splitting is rapid growth that realizes a full growth potential, but is inhibited or uninhibited in a plastic manner. Under experimental conditions, both species possessed the capacity for rapid development but differed in regulatory mechanisms. The cohort splitter species exhibited plasticity based on photoperiod and individual condition, while the non-splitter showed a more uniform response to environmental cues, independent of body state. Sibling status minimally influenced developmental outcomes, indicating low genetic determination. Our results suggest that cohort splitting evolved through differential regulation of an ancestral uninhibited growth pattern. Cohort splitting is a bet-hedging strategy that optimizes fitness in variable environments through plastic developmental inhibition. Conversely, the unconditional inhibition of rapid growth in non-splitters probably reflects adaptation to habitats where predation pressure on late-season generations favours univoltinism.