Do simultaneous hermaphrodites choose their mates? Effects of body size in a planarian flatworm

1. We tested whether or not the simultaneous hermaphrodite Dugesia polychroa mates randomly with respect to partner size, a strong indicator of female fecundity. 2. The number and duration of copulations were recorded in forty‐eight pairs over 5 days. To minimize effects of individual differences in copulatory activity, all animals were each tested twice: once with an equally large partner ( n = 24 equal‐sized pairs) and once with a partner of different size (smaller or larger, respectively, ratio 1 : 1.5, n = 24 unequal‐sized pairs). 3. The animals copulated frequently. No significant differences were observed between treatments in the number of pairs that copulated. Mating behaviour of equal‐sized and unequal‐sized pairs was compared in a pair‐wise analysis. Latency to the first copulation and the average number of copulations did not differ when mating with an equal‐ or unequal‐sized partner. Average copulation duration increased with increasing size in equal‐sized pairs, but not in unequal‐sized pairs. 4. In an additional experiment, where three large and three small animals were combined (three replicates) and observed for 1–3 days, the mating pattern did not differ from random mating by size. 5. Our data indicate that in D. polychroa the decision to mate is essentially random with respect to size. We argue that this is not caused by, for example, high costs of mate searching, but that simultaneous hermaphrodites with reciprocal insemination mate indiscriminately when (i) matings are easy to obtain and (ii) the cost of mating is low or compensated for by sperm digestion.