Does internal egg carrying impair foraging ability as much as external egg carrying in a neotropical spider?

Females not only produce costly gametes, but also store the eggs until oviposition, a period called pregnancy. The volume that eggs occupy in the female abdomen may decrease female foraging ability by making females slow. Although females of all species are subjected to these potential costs, it remains an unexplored matter in invertebrates. Females of the spider Paratrechalea ornata carry their egg sac after oviposition and thus represent a unique opportunity to evaluate pregnancy costs because females carry an extra volume before and after laying eggs. We conducted foraging ability experiments using P. ornata females on different treatments regarding pregnancy and maternal care. We first hypothesized that internal egg load and egg sac carrying decrease female foraging ability. We also hypothesized that greater egg sac size decreases female foraging ability. We found that both internal egg load and egg sac carrying decreased female foraging ability, and females about to oviposit had a similar foraging ability to females carrying an egg sac. Egg sac size did not influence female foraging ability. Our results show that pregnancy can impose high costs to female foraging ability, likely increasing their mortality during this period. The little support for our second hypothesis may also suggest that the decrease in foraging ability is not due to the volume being carried per se, but possibly an associated physiological state.