Dietary intake and retention of energy in form of lipids has a substantial effect on the physiological development of consumers (Arts et al. 2009). However, our results indicate, in agreement with previous laboratory studies (Speake & Wood 2005; Lund et al., 2012; Hall et al. 2014), that the availability of structural lipids, especially DHA, and not the lipid energy reserve is the key for brain development in vertebrate consumers. Our findings suggests that the specialization of dietary niche in wild consumers on resources that have similar content of energy, but differ in the content of n-3 LC-PUFA (Heissenberger et al. 2010; Twining et al. 2019; Scharnweber et a. 2021), can lead to intra-specific diversification of brain size. This is an important finding because brain size is positively correlated to cognitive capacity, especially among closely related species (Olkowicz et al. 2016) and at the intra-specific level (Møller 2010; Kotrschal et al. 2013). For example, guppies with larger brain size have shown higher learning capacity (Kotrschal et al. 2013), and large brain have been suggested to improve migration efficiency and offspring defence in barn swallows Hirundo rustica (Møller 2010). However, it should be noted that while dietary supply of omega-3 LC-PUFA is determinant for brain development of vertebrates (Pilecky et al. 2021), dietary availability of these biomolecules can also influence other phenotypic traits important for consumers fitness, such as mitochondrial efficiency (Salin et al. 2021; Závorka et al. 2021); metabolic rate (McKenzie 2001; Twining et al. 2016), growth rate (Chaguaceda et al. 2020; Závorka et al. 2021), and reproductive capacity (Brett et al. 2009; Scharnweber & Gårdmark 2020). Therefore, further carefully designed experiments in a realistic ecological context are needed to understand the trajectory of how dietary omega-3 PUFA influences fitness of consumers in the wild.