Brain miniaturization and its implications for cognition: evidence from Salticidae and Hymenoptera

Scientists have always been fascinated and puzzled by the marvel that is the human brain. This organ seems to be of an exaggerated size for our body, demanding an enormous amount of energy and resources to be maintained. For evolution to favour such an enormous expenditure of energy, the benefits must outweigh the cost. During the history of neuroscience different authors have attempted to correlate the size of a species nervous system with its cognitive abilities, in the search for an explanation on why we have such a big brain: if possessing an oversized brain is how a species can produce outstanding cognitive abilities, then the survival advantage outweighs the costs. However, brain size correlates first of all with body size, having nothing to do with the cognitive capabilities of a species. To solve this problem, different measures have been adopted, like brain-to- body weight ratio, encephalization quotient, the raw number of neurons in the cortical areas. When confronted with empirical evidence, however, all these measures fail to predict the presence of complex cognition, especially for species phylogenetically distant from us. In particular, miniaturized organisms, like insects or spiders, exhibit outstanding behaviours, products of complex cognition, with brains multiple orders of magnitude smaller than ours. It has been proposed that our premise is misguided. Cognition does not need a big brain to manifest, quite the opposite: a higher number of neurons increase the memory buffer and becomes more robust against noise, while cognitive processes only require a handful of cells well organized in complex circuits. The process of brain miniaturization during evolution should have favoured the birth of small but complex neural circuits, capable of dealing with multiple situations. In this framework, in this thesis, I have presented some of the studies carried out during my PhD project on miniature organisms. Firstly, the ants are described. As these insects are phylogenetically similar to bees and bumblebees, which have been extensively studied in the last three decades and have been found capable of outstanding cognitive processes, they represent the first candidate to understand if complex cognition is widespread in invertebrates With two different studies, we tested the ability of ants to perceive and register information from the environment. It appears that the process of miniaturization during evolution has favoured the development of clever circuitry, that let the ants process a great variety of information with only a handful of neurons, and register those with a load-independent memory process, suggesting the presence of complex cognitive abilities. Secondly, as the main topic of my project, the jumping spiders are presented. These arachnids have recently caught the interest of scientists for their unique hunting strategies, that involve detouring perspective taking, categorization and other cognitive skills. I have tested their visual perception to understand if it is guided by the same rules that govern the human’s one (e.g., Gestalt principles). However, I failed to design a methodology capable to consistently train the spiders, and as such the results were inconclusive. To overcome this problem, I designed an automated training system. This proved to be an effective way to train jumping spiders, opening future possibilities for the study of this species’ cognitive abilities.