Brain and behavioural asymmetries in non-human species

Papers published in this Special Issue report various aspects of asymmetrical brain function and its expression in the behaviour of vertebrate and invertebrate species. Since the discovery made four decades ago, that lateralized brain function and behaviour is not a unique feature of humans, evidence of its presence in a broad range of non-human species has accumulated (discussed by Rogers, Vallortigara, & Andrew, 2013). Indeed, research on laterality in non-human species has provided insights to understanding laterality in humans (Güntürkün, Ströckens, & Ocklenburg, 2020; Ocklenburg & Güntürkün, 2012). It is present in motor behaviour and perception and it has been reported in a broad range of avian, amphibian, reptilian and mammalian species. More recently, laterality has been found even in the small brains of bees and some other invertebrates (Frasnelli, Vallortigara, & Rogers, 2012, and see contributed paper by Waite and Frasnelli on turning behaviour in bumblebees).

Today, there are few scientists who would argue against the evidence of laterality in non-human species (see contributed paper by Corballis). Nevertheless, persistent adherence to the idea of human superiority is still manifested in other ways. One paper in this special issue (by Lindell and Lindell) shows how the dominant belief in the superiority of humans and apes compared to other primates is manifested even in the lateralized depiction of primates in photographs.

Comparative evidence, however, supports in part the hypothesis that laterality is stronger and more consistent in humans than in other vertebrate species, based especially on the evidence of the strength of limb preference in non-human species compared to humans (e.g., Strökens, Güntürkün, & Ocklenburg, 2013). There are exceptions to this general pattern; examples include footedness in some species of parrots, with a population bias of 90% (e.g., left footedness in sulphur-crested cockatoos, Rogers, 1980) and asymmetry in invertebrates of a similar strength (e.g., laterality of olfactory memory in bees, Rogers & Vallortigara, 2008). More research on strength of laterality in different species and in the performance of different responses and tasks will prove or disprove the postulated stronger laterality in humans.

Some research has begun to reveal the function of lateralization. Within a species, individuals with strong neural laterality perform some tasks more efficiently than do those with weak laterality (Magat & Brown, 2009), especially seen in performance of a task requiring simultaneous attention to two pertinent stimuli (Rogers, Zucca, & Vallortigara, 2004). Theories that try to account for the alignment of laterality at the population level have also been put forward (see Vallortigara & Rogers, 2020).

It could be maintained that humans display laterality for a greater number of functions than do other vertebrates but that seems unlikely, at least for perception and processing, if we consider the wide range of lateral asymmetries now known be present in chicks (Vallortigara, Chiandetti, & Sovrano, 2011, and see the contribution by Loconsole et al. discussing flexibility and reversal learning in chicks) and in pigeons (Güntürkün et al., 2020), both species that have become models for studying the nature, function, development and cellular correlates of asymmetry (see contributions by Xiao and Güntürkün and by Morandi-Raikova et al.).

Generally, it was not our aim for this special issue to simply add evidence for or against any debate singling out humans from other species but to present a broad picture showing the complexity and function of laterality in a range of species. Therefore, as editors, we encouraged the contribution of papers that did not simply report the existence of laterality in yet another non-human species but, instead, considered laterality from a comparative perspective, thereby discussing evolutionary aspects of laterality, the function of laterality or the development of laterality. Hence, papers dealing with behavioural, structural, physiological and cellular mechanisms of lateralization are included in this special issue. For example, sleep behaviour is lateralized and the contribution by Mascetti discusses the neural functions related to unihemispheric sleep. Interhemispheric interactions are, of course, important for brain function, as shown by the neurophysiological studies of the role of interhemispheric communication in the pigeon (see contribution by Xiao and Güntürkün).

Papers in this special issue cover a wide range of species, including flamingos, wolves, antelopes and monkeys (two papers, contributed by Regaiolli and colleagues and contributions by Fourie al. and Fu et al., respectively), and also test whether laterality is associated with curiosity in dolphins and starlings (contribution by Hausberger et al.), anxiety in zebrafish (contribution by Petrazzini) or personality in donkeys (contribution by Diaz et al).

We thank the authors for contributing their ideas and research findings.