In general, larger animals have larger brains, and smaller animals have smaller brains. This trend also holds true for fish. The brain is involved in nearly every bodily function, but its primary role is information processing (Figure 1). The brain is responsible for turning incoming information, or stimuli, into information that can be used to inform decisions. Of course, some information is complicated and requires more brain power.
Is a bigger brain, a better brain?
There are costs and benefits to developing a bigger brain. The main benefit is that a bigger brain usually gives a species, or an individual, an advantage in information processing (i.e. it makes them smarter, and perhaps better at finding food or mates, and avoiding predators). However, brain tissue is costly and requires a great deal of energy to build and maintain. The Expensive Tissue Hypothesis suggests that investment into cognitive resources comes at the expense of investment into other energetically costly tissues (Aiello and Wheeler, 1995). This theory has received ample support from studies on fishes. Notably, a series of experiments conducted on guppies (Poecilia reticulata) has revealed that artificial selection for large-brained guppies confers numerous cognitive benefits, but also results in these fish developing smaller guts. Moreover, these brainiac guppies produce fewer offspring (Kotrschal et al., 2013a; Kotrschal et al., 2013b; Kotrschal et al., 2015). Therefore, while a big brain might make a fish smarter, it comes at a hefty price. This trade-off between brain development and investment into other key tissues is thought to represent the main limitation on brain size across vertebrates.
Factors that trigger cognitive investment.
With over 30,000 known species of fishes, it is no surprise that some species have bigger brains than others. Interestingly, there is also incredible variation in brain size within a species; some individuals have bigger brains than others. There are several environmental factors that have been linked with brain development, but broadly these factors can be categorized as (1) physical or (2) social in nature. For instance, fish must navigate a three-dimensional world, which is often structurally complex and highly heterogenous in the wild. Individuals that live in areas with greater physical complexity (e.g. the littoral area near the shoreline), often have larger brains than their open-water counterparts (Axelrod et al., 2018). This connection between the physical complexity of the surrounding environment and brain development is perhaps best exemplified through the comparison of fish raised in captivity versus wild fish. Wild fish reared in captivity develop smaller brains because captive conditions are structurally simplistic relative to those in the wild (Näslund et al., 2012). Even fish raised in hatcheries have been documented to have smaller brains than their wild cousins (Marchetti and Nevitt, 2002). In the laboratory, holding tanks are often quite bare and contain little structural complexity (Figure 2A). However, this affect can be counteracted through environmental enrichment. Adding rocks, vegetation, or other objects to tanks can promote brain development (Figure 2B). The social environment is also known to influence brain development of fishes. Social stress is usually a result of either isolation or overcrowding, both of which have been shown to impact learning and brain development in fish. The fish brain is most susceptible to these social factors early in development, when the brain is undergoing immense change in a short period of time. Yet, recent research has demonstrated that the fish brain remains responsive throughout adulthood, to a greater degree than most other animals (Ebbesson and Braithwaite, 2012).
Summary and conclusion.
Most people that keep fish as pets can probably get along without worrying much about the brain development of their fish. However, for scientists seeking to study animal behaviour or cognition, this is an issue of great importance. It is critical that scientists housing fish in a laboratory use environmental enrichment to ensure that the brains of their study species develop similarly to their wild counterparts. Both physical and social enrichment must be used as tools by scientists that wish to understand how fish behave in more natural conditions.
Aiello, L.C. and Wheeler, P., 1995. The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current Anthropology, 36, pp.199–221.
Axelrod, C.J., Laberge, F. and Robinson, B.W., 2018. Intraspecific brain size variation between coexisting sunfish ecotypes. Proceedings of the Royal Society B, 285, p.20181971.
Ebbesson, L.O.E. and Braithwaite, V.A., 2012. Environmental effects on fish neural plasticity and cognition. Journal of Fish Biology, 81, pp.2151–2174.
Kotrschal, A., Corral-Lopez, A., Amcoff, M., and Kolm, N., 2015. A larger brain confers a benefit in a spatial mate search learning task in male guppies. Behavioral Ecology, 26, 527–532.
Kotrschal, A., Rogell, B., Bundsen, A., Svensson, B., Zajitschek, S., Brännström, I., Immler, S., Maklakov, A.A. and Kolm, N., 2013a. Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain. Current Biology, 23, pp.168–171.
Kotrschal, A., Rogell, B., Bundsen, A., Svensson, B., Zajitschek, S., Brännström, I., Immler, S., Maklakov, A.A. and Kolm, N., 2013b. The benefit of evolving a larger brain: big-brained guppies perform better in a cognitive task. Animal Behaviour, 86, p.e4.
Marchetti, M. P., and Nevitt, G. A., 2003. Effects of hatchery rearing on brain structures of rainbow trout, Oncorhynchus mykiss. Environmental Biology of Fishes, 66, 9–14.
Näslund, J., Aarestrup, K., Thomassen, S.T. and Johnsson, J.I., 2012. Early enrichment effects on brain development in hatchery-reared Atlantic salmon (Salmo salar): no evidence for a critical period. Canadian Journal of Fisheries and Aquatic Sciences, 69, pp.1481–1490.