Conditional reciprocity in invertebrates is robust to disturbances

Reciprocity, one of the evolutionary explanations for cooperation, evolves between individuals when helping is conditional upon receiving help in return (“A helps B because B helped A before”). Often tested in controlled laboratory conditions, its robustness to external disturbances has rarely been evaluated. Here, we investigated whether reciprocity is robust when disturbances occur in between the donation by one individual and its return by a partner. As model system, we used a simultaneously hermaphroditic polychaete worm (Ophryotrocha diadema), where partners in isolated dyads engage in iterated exchanges of eggs for sperm (eggs are a costly donation relatively to sperm). If a partner donates eggs and the other fertilizes them, the latter is likely to donate eggs a couple of days later. (Eggs and larvae are then cared by the parents which sit at the nest and only leave it for foraging.) We removed the eggs from the nests daily or weekly and found that reciprocity decisions were not substantially altered; focal worms were more likely to lay if the partner laid before. We also found that, irrespective of treatment, worms exchanged comparable number of eggs, and were stimulated to lay significantly sooner if the partners laid a second clutch before they reciprocated. These results show that reciprocity is robust to disturbances which may happen in the 2 days’ time gap between egg donation and egg reciprocation.