Size matters: male and female mate choice leads to size-assortative pairing in a coral reef cardinalfish

Many animals exhibit size-assortative mating, and matching theory predicts this occurs because both males and females prefer bigger mates. Monogamy and size-assortative pairing have been described for coral reef fishes, but the underlying behavioral mechanism has not been tested. Here, we took a long-term observational and experimental study to resolve the causes of size-based pairing in the paternal mouthbrooding coral reef cardinalfish Sphaeramia nematoptera. For 65 pairs observed over a 23-month period, there was a strong size-correlation between paired males and females. This size-assortative mating was not a consequence of pairing at a young age as re-pairing was common, with only 7% juvenile pairs still found together after 8 months. For adults that changed partners over this period, there was a strong correlation between the size of individuals and the size of their new partners. Following experimental removal of partners, both males and females quickly repaired with partners of ...