Predation and foraging costs of variation in eggsac mass in the wolf spider Pardosa milvina (Araneae: Lycosidae).
Female Pardosa milvina wolf spiders carry an eggsac that may weight 70% of their post-reproductive weight. The fitness-related costs of such energy-intensive maternal care remain largely unknown. We measured the foraging and predation costs incurred by females that had the mass of their eggsacs experimentally manipulated. Six eggsac treatments were used consisting of either the female's own eggsac, or various black plastic beads of known weight. The treatments were as follows: 1) a bead eggsac equal in mass to that of a normal eggsac, 2) a bead twice the mass of a normal eggsac, 3) an eggsac ten times the mass of a normal eggsac, 4) a female that had her own eggsac removed and given back to serve as a control, and 5) a female that had her eggsac experimentally removed. We then measured the following for each treatment: 1) the number of females that chose to accept or reject an artificial eggsac of different weights, 2) the number of females with different eggsac weights that captured a domestic house cricket 40-50% the body mass of the female, 3) the latency to capture prey, 4) the number that avoided predation by a larger predatory wolf spider, Hogna helluo, within a 3 h period, 5) the predation latency by H. helluo, and 6) the number of females that dropped their eggsacs during prey capture attempts or predator avoidance. Females preferred their own eggsacs to plastic beads of equal weight. Results show that females presented with a bead chose based on diameter rather than mass. Females carrying an eggsac were at increased predation risk regardless of eggsac mass or whether the eggsac was real or artificial. Females carrying heavy eggsacs were less able to capture prey and more likely to drop their eggsac during foraging bouts. Eggsac mass may be constrained by both predation risk and decreased foraging efficiency, however, foraging costs appear to be incremental whereas predation costs are not.