Learning allows an animal to adjust its behavior in an adaptive way in a changing environment, where fitness consequences of a given action vary from generation to generation, or even within the lifetime. While fitness benefits of learning are relatively well understood, we know little about fitness costs of learning ability, constraints on its evolution, and the nature of heritable variation on which natural selection can act. As with any fitness-related trait, knowledge of these aspects is essential to understanding why, how, and when learning ability evolves under natural selection

The existence of lines that evolved a higher learning ability (High-learning lines) and poorly learning lines of the same origin (Low-learning lines) opens an opportunity to study the fitness costs of an ability to learn. Such costs have been postulated - neural structures and processes are energetically expensive, which should have consequences for survival, reproduction or offspring quality, especially under food shortage. Yet, there is little evidence for such costs. To address these costs, we compared larval competitive ability of populations that evolved improved learning with the competitive ability of the control populations. When food was scarce, the flies from the High-learning lines showed a lower competitive ability than the Low-learning lines. Thus, the evolution of a better learning ability in the High-learning lines was associated with a reduction of larval competitive ability. This is the first direct evidence of an evolutionary trade-off between the ability to learn and another ecologically important trait.

The cost described above is a constitutive cost of learning ability, paid by individuals with a genetically high learning ability, whether or not they actually use this ability. However, the act of learning is itself likely to be costly in terms of energy used for processing and storing information. Thus, individuals that are forced to exercise their learning ability repeatedly over a long time should eventually start showing some decline in fertility or survival compared to individuals of the same genotype that are not forced to learn. >from *Evolutionary biology of learning ability in Drosophila*

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