Decision rules diverge between sympatric Featherfin cichlids despite shared preferences, ecology, and evolutionary history.

It has been hypothesized that divergence in animal decision processes can be attributed to differences in information processing mechanisms. However, the required comparative tests of this idea are hindered by ecological, perceptual, or motivational divergence among species that confound a direct analysis of decision processes. To overcome these limitations, we study two closely related, sympatric cichlid fishes (Aulonocranus dewindti and Cyathopharynx furcifer) that share visual capacities, ecological context and object-removal motivations, and we perform a systematic assessment of decisional strategy divergence in wild animals, using established paradigms from human psychology research. In a series of in situ field cognitive experiments in Lake Tanganyika, comprising over 5,000 trials, we first find that the two species make similar decisions under low perceptual load, with identical size and color preferences and comparable oddity effects. Yet, when task complexity increased and choices required integrating conflicting features, the species' decisions diverged. C. furcifer made rapid, consistent decisions based on a single dominant feature, whereas A. dewindti took longer to decide and integrated all features of the options. A decoy paradigm confirmed that A. dewindti's primarily relied on available information. These findings show that closely related species can exhibit markedly different decision outcomes despite sharing sensory abilities, ecological context, and baseline preferences. The observed divergence is consistent with differences in prioritization and integration of information under complex conditions.