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Abstract Eye size varies notably across taxa. Much work suggests that this variation is driven by contrasting ecological selective pressures. However, evaluations of the relationship between ecological factors and shifts in eye size have largely occurred at the macroevolutionary scale. Experimental tests in nature are conspicuously absent.Trinidadian killifish,Rivulus hartii, are found across fish communities that differ in predation intensity. We recently showed that increased predation is associated with the evolution of a smaller eye. Here, we test how divergent predatory regimes alter the trajectory of eye size evolution using comparative mark–recapture experiments in multiple streams.We found that increases in eye size are associated with enhanced survival, irrespective of predation intensity. More importantly, eye size is associated with enhanced growth in communities that lack predators, while this trend is absent when predators are present.Such results argue that increased competition for food in sites that lack predators is the key driver of eye size evolution. Aplain language summaryis available for this article.
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This content will become publicly available on February 1, 2026
Experimental transplants demonstrate shifts in predation favour evolution of aggressive behaviours in Trinidadian killifish
Abstract Theory asserts larger brains facilitate behaviours that enhance fitness. Research has demonstrated that increased brain size improves cognition and survival. However, the majority of research has focused on cross‐species comparisons. Experiments that manipulate selection to investigate the connection between brain size, behaviour and fitness are needed.Trinidadian killifish (Anablepsoides hartii) live in communities with (high predation: HP) and without (killifish only: KO) predators. Predator absence is associated with high population densities, increased intraspecific competition and evolved larger brain sizes.We tested for evolutionary shifts in behaviour by subjecting second‐generation lab‐reared killifish to a mirror aggression assay. We also quantify selection on brain size and behaviour by transplanting wild HP killifish to KO sites and tracking individual fitness (growth rates) with a mark‐recapture design.Lab‐reared killifish from KO sites—specifically males—exhibited higher levels of aggression than HP killifish. In the transplant experiment, HP killifish exhibited strong increases in aggression following the introduction to KO sites. Increased brain size was correlated with increased growth in transplanted HP killifish, yet there was no association between brain size, aggression and growth.Our results indicate that declines in predation and increased competition favour increases in aggression but further research is needed to determine if and how brain size and behaviour are linked through natural selection. Read the freePlain Language Summaryfor this article on the Journal blog.
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- PAR ID:
- 10572440
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Functional Ecology
- Volume:
- 39
- Issue:
- 2
- ISSN:
- 0269-8463
- Page Range / eLocation ID:
- 520 to 530
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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