Mismatched partners that achieve postpairing behavioral similarity improve their reproductive success

Abstract

Behavioral similarity between partners is likely to promote within-pair compatibility and to result in better reproductive success. Therefore, individuals are expected to choose a partner that is alike in behavioral type. However, mate searching is very costly and does not guarantee finding a matching partner. If mismatched individuals pair, they may benefit from increasing their similarity after pairing. We show in a monogamous fish species-the convict cichlid-that the behavioral similarity between mismatched partners can increase after pairing. This increase resulted from asymmetrical adjustment because only the reactive individual became more alike its proactive partner, whereas the latter did not change its behavior. The mismatched pairs that increased their similarity not only improved their reproductive success but also raised it up to the level of matched pairs. While most studies assume that assortative mating results from mate choice, our study suggests that postpairing adjustment could be an alternative explanation for the high behavioral similarity between partners observed in the field. It also explains why interindividual behavioral differences can be maintained within a given population.

Keywords: assortative mating; behavioral convergence; convict cichlid; mate choice; monogamy; parental care; partner compatibility; personality; reproductive success.

Fig. 1. Change in within-pair behavioral similarity between contexts.

The mean behavioral similarity index (± bootstrapped 95% CI) for matched pairs (open circles; n = 15) and mismatched pairs (solid circles; n = 13) as a function of the context: isolated individuals and pairing context. A significant interaction between the nature of the pair (matched or mismatched) and the context was observed (mixed-effects linear model: χ²₁ = 6.88, P = 0.0087). The similarity index significantly decreased for mismatched pairs (mixed-effects generalized linear model: χ²₁ = 9.07, P = 0.0026), whereas there was no significant difference between contexts for matched pairs (χ²₁ = 0.59, P = 0.443). Between the contexts, only the significant post hoc comparisons were included in the figure (***P < 0.001). Mismatched pairs were significantly less similar than matched pairs in the isolated context (F_{1, 27} = 21.39, P < 0.0001), whereas there was no difference in the similarity index once individuals were paired (F_1,23 = 0.0028, P = 0.96). Considering the continuous difference between partners instead of discrete categories (proactive or reactive) leads to consistent results: a larger initial difference in the behavioral score between partners significantly relates to a larger change in similarity (r = 0.60, P = 0.0025). n.s., not significant.

Fig. 2. Reproductive benefits of convergence.

Reproductive success was assessed as the mean number of fry (± bootstrapped 95% CI) for matched pairs (open circles) and mismatched pairs (solid circles). To allow for comparison with matched pairs, we dichotomized mismatched pairs into converging pairs (the 50% most similar in the pairing context) and nonconverging pairs (the 50% least similar in the pairing context). Nonconverging mismatched pairs had significantly fewer fry than converging mismatched pairs (permutation test: P = 0.04) and significantly fewer fry than matched pairs (P = 0.04). Only the significant comparisons were included in the figure (*P < 0.05 after correcting for multiple comparisons).

Fig. 3. Proactive versus reactive flexibility in mismatched pairs.

The mean frequency of agonistic behaviors (± bootstrapped 95% CI) toward the intruder for reactive partners (triangles) and proactive partners (diamonds) in mismatched pairs as a function of the context: isolated context and pairing context. A significant interaction between behavioral type (reactive or proactive) and context was observed (mixed-effects linear model: χ²₁ = 9.52, P = 0.0020). For reactive fish, aggressiveness significantly increased between contexts (χ²₁ = 11.93, P = 0.0006), whereas it was consistent for proactive fish (χ²₁ = 0.04, P = 0.85). For each given behavioral type, only the post hoc tests between the contexts were included in the figure (n.s.: P > 0.10; ***P < 0.001). Reactive and proactive fish significantly differed in the isolated context only (χ²₁ = 39.59, P < 10⁻⁵), whereas there was no difference once they were paired (χ²₁ = 0.29, P = 0.60).