In conclusion, this field study demonstrates the largely overlooked importance of dietary intake of n-3 LC-PUFA for brain development of wild vertebrate consumers, which have so far been recognized just under laboratory conditions (Pilecky et al. 2021). We have shown the link between n-3 LC-PUFA deprived diets and consumer’s brain size in the context of competition with an invasive species. However, the ongoing global change is predicted to limit availability of n-3 LC-PUFA for vertebrate consumers on a large spatial scale (Hixson & Arts, 2016; Heilpern et al. 2021) and it is still poorly understood how wild vertebrate consumers respond to this dietary deprivation of n-3 LC-PUFA. Our results indicate that it can have negative impacts even on species which are pre-adapted to dietary scarcity of these important biomolecules (i.e., salmonids often consume high proportion of low-quality terrestrial prey, Syrjänen et al. 2011; Evangelista et al. 2014). Dietary intake of n-3 LC-PUFA have a high potential to affect fitness of vertebrate consumers (Twining et al. 2021; Pilecky et al. 2021), and therefore further studies are needed to understand how the availability n-3 LC-PUFA in natural diets affects development of brain, behaviour, physiology and life-history of vertebrates.