Data from: 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 similar or larger size. Together, these results suggest that size-assortative mating is explained by a mutual preference by both males and females for larger mates. We suggest that monogamous pairing occurs in cardinalfish because mouthbrooding restricts multiple mating by males. Size-assortative pairing follows as larger males likely prefer the more fecund larger females, and larger females prefer larger males because they can successfully brood all of their eggs. Mutual mate choice will likely explain size-assortative pairing in other fish species with paternal care.

诸多动物均存在体型交配偏好（size-assortative mating）现象，匹配理论（matching theory）推测其成因在于雌雄个体均倾向于选择体型更大的配偶。尽管已有研究在珊瑚礁鱼类中记录到单配制（monogamy）与体型匹配配对现象，但其背后的行为机制尚未得到验证。本研究通过长期观测与实验探究，旨在阐明父育口孵（paternal mouthbrooding）型珊瑚礁鱼类——巨齿天竺鲷（Sphaeramia nematoptera）的体型配对偏好成因。在为期23个月的观测周期内，共计记录65组配对个体，其雌雄体型间呈现显著正相关。该体型交配偏好并非源于幼年时期的配对绑定，因重新配对现象极为普遍：仅7%的幼年配对个体在8个月后仍保持结伴状态。对于此期间更换配偶的成年个体而言，其自身体型与新配偶体型间同样存在显著相关性。在实验移除原有配偶后，雌雄个体均会迅速与体型相近或更大的个体组建新配对。综合上述结果可知，体型交配偏好可归因于雌雄双方对大体型配偶的共同偏好。我们推测天竺鲷之所以采用单配制配对，是因为口孵行为限制了雄性的多次交配行为。体型匹配配对之所以出现，是因为大体型雄性更倾向于选择繁殖力更强的大体型雌性，而大体型雌性则偏好大体型雄性——后者可成功孵化其全部鱼卵。这类双向配偶选择机制，或许同样可以解释其他存在父育行为的鱼类的体型交配偏好现象。