Egg-trading worms start reciprocation with caution, respond with confidence and care about partners’ quality

Abstract

Conditional reciprocity (help someone who helped you before) explains the evolution of cooperation among unrelated individuals who take turns helping each other. Reciprocity is vulnerable to exploitations, and players are expected to identify uncooperative partners who do not return the help they received. We tested this prediction in the simultaneously hermaphroditic worm, Ophryotrocha diadema, which engages in mutual egg donations by alternating sexual roles (one worm releases’ eggs and the other fertilizes them). We set up dyads with different cooperativeness expectations; partners were either the same or a different body size (body size predicts clutch size). Large worms offered larger clutches and did so sooner when paired with large rather than small partners. They also released smaller egg clutches when they started egg donations than when they responded to a partners’ donation, fulfilling the prediction that a players’ first move will be prudent. Finally, behavioral bodily interactions were more frequent between more size-dissimilar worms, suggesting that worms engaged in low-cost behavioral exchanges before investing in such costly moves as egg donations. These results support the hypothesis that simultaneously hermaphroditic worms follow a conditional reciprocity paradigm and solve the conflict over sexual roles by sharing the costs of reproduction via the male and the female functions.

Conditional reciprocity in invertebrates is robust to disturbances

“…These results are more reasonably interpreted as the results of a trade-off between parental and feeding/foraging: worms relieved the burden of parental care (which implies sitting at the nest whether maternal or paternal clutches are present, Picchi & Lorenzi, 2019), re-allocated resources to egg production, tended to reciprocate more often and were significantly more likely to lay even if the partner did not in its turn. In the same direction, larger worms were more likely to lay a clutch of eggs (a result already highlighted in Lorenzi et al, 2021).…”

“…For instance, rats usually help, even if at a smaller extent, individuals who did not helped before in offering food or grooming Schweinfurth & Taborsky, 2018a, 2018b. However, in contrast to rats, who diminish the amount of help offered to non-cooperative partners (Schweinfurth, 2021b), worms laid clutches of variable size when they laid a second clutch, suggesting that they do not flexibly adjust clutch size in this situation (although they do adjust clutch size to partner quality, Lorenzi et al, 2021).…”

“…For example, vampire bats are more likely to regurgitate food to individuals who offered food before than to those who did not (Carter & Wilkinson, 2013b). Hermaphroditic worms delay their egg donations when paired with less mature worms that would not reciprocate the clutch received with a size‐matched clutch (Lorenzi et al, 2021). While there exists ample evidence of reciprocal exchange of favors, it has been suggested by some authors that reciprocity only occurs under restricted conditions and is not overly robust against disturbances (e.g., Brockhurst et al, 2007; Stevens & Hauser, 2004).…”

“…For example, vampire bats are more likely to regurgitate food to individuals who offered food before than to those who did not (Carter & Wilkinson, 2013b). Hermaphroditic worms delay their egg donations when paired with less mature worms that would not reciprocate the clutch received with a size-matched clutch (Lorenzi et al, 2021).…”