Beyond the prey: male spiders highly invest in silk when producing worthless gifts

In the spider Paratrechalea ornata, males have two gift-giving mating tactics, offering either a nutritive (prey) or a worthless (prey leftovers) silk wrapped gift to females. Both gift types confer similar mating success and duration and afford males a higher success rate than when they offer no gift. If this lack of difference in the reproductive benefits is true, we would expect all males to offer a gift but some males to offer a worthless gift even if prey are available. To test this, we allowed 18 males to court multiple females over five consecutive trials. In each trial, a male was able to produce a nutritive gift (a live housefly) or a worthless gift (mealworm exuviae). We found that, in line with our predictions, 20% of the males produced worthless gifts even when they had the opportunity to produce a nutritive one. However, rather than worthless gifts being a cheap tactic, they were related to a higher investment in silk wrapping. This latter result was replicated for worthless gifts produced in both the presence and absence of a live prey item. We propose that variation in gift-giving tactics likely evolved initially as a conditional strategy related to prey availability and male condition in P. ornata. Selection may then have favoured silk wrapping as a trait involved in female attraction, leading worthless gift-giving to invade.

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.