Small within‐clutch variance in spiderling body size as a mechanism for avoiding sibling cannibalism in the wolf spider Pardosa pseudoannulata (Araneae: Lycosidae)

Abstract This study investigated why sibling cannibalism is so rare in the second instar of the wolf spider Pardosa pseudoannulata (Araneae: Lycosidae), despite the fact that the spider is cannibalistic and does not discriminate kin from non‐kin. Laboratory experiments revealed that relative size difference between the cannibal and its victim affected the occurrence of nymphal cannibalism, as in other spiders. The degree of the size difference, measured as the coefficient of variation (CV), was smaller within a brood than among broods, but was greater in smaller clutches that might have been produced by less well fed females. Statistical analysis confirmed that intraclutch cannibalism would occur in less than about 1% of encounters among siblings that were randomly selected from the same clutch, because about 99% of intrabrood encounters fell within a 10% size difference, which is the marginal value above which frequent cannibalism occurs. Thus, interclutch cannibalism was expected to occur much more frequently than intraclutch cannibalism. Considering the potential benefit of cannibalism under food stress and high nymphal mortality, even with the sacrifice of their full sibs, the small intraclutch variance in spiderling body size may be understood as a tactic adopted by mother spiders to avoid sibling cannibalism.