Burkart, J. M. | Search bioRxiv

bioRxiv2021

The evolutionary origins of primate scleral coloration

Primate gaze following behaviors are of great interest to evolutionary scientists studying social cognition. The ability of an organism to determine a conspecifics likely intentions from their gaze direction may confer an advantage to individuals in a social group. This advantage could be cooperative and/or competitive. Humans are unusual in possessing depigmented sclerae whereas most other extant primates, including the closely related chimpanzee, possess dark scleral pigment. The origins of divergent scleral morphologies are currently unclear, though human white sclerae are often assumed to underlie our hyper-cooperative behaviors. Here, we use phylogenetic generalized least squares (PGLS) analyses with previously generated species-level scores of proactive prosociality, social tolerance (both n=15 primate species), and conspecific lethal aggression (n=108 primate species) to provide the first quantitative, comparative test of three complementary hypotheses. The cooperative eye [M. Tomasello, B. Hare, H. Lehmann, J. Call, J. Hum. Evol. 52, 314-320 (2007)] and self-domestication [B. Hare, Annu. Rev. Psychol. 68, 155-186 (2017)] explanations predict white sclerae to be associated with cooperative, rather than competitive, environments. The gaze camouflage hypothesis [H. Kobayashi, S. Kohshima, J. Hum. Evol. 40, 419-435 (2001)] predicts that dark scleral pigment functions as gaze direction camouflage in competitive social environments. We show that white sclerae in primates are associated with increased cooperative behaviors whereas dark sclerae are associated with reduced cooperative behaviors and increased intra-specific lethal aggression. Our results lend support to all three hypotheses of scleral evolution, suggesting that primate scleral morphologies evolve in relation to variation in social environment.

animal behavior and cognition↗

bioRxiv2021

No evidence for general intelligence in a fish

Differences in human general intelligence or reasoning ability can be quantified with the psychometric factor g, because individual performance across cognitive tasks is positively correlated. g also emerges in mammals and birds, is correlated with brain size and may similarly reflect general reasoning ability and behavioural flexibility in these species. To exclude the alternative that these positive cross-correlations may merely reflect the general biological quality of an organism or an inevitable by-product of having brains it is paramount to provide solid evidence for the absence of g in at least some species. Here, we show that wild-caught cleaner fish Labroides dimidiatus, a fish species otherwise known for its highly sophisticated social behaviour, completely lacks g when tested on ecologically non-relevant tasks. Moreover, performance in these experiments was not or negatively correlated with an ecologically relevant task, and in none of the tasks did fish caught from a high population density site outperform fish from a low-density site. g is thus unlikely a default result of how brains are designed, and not an automatic consequence of variation in social complexity. Rather, the results may reflect that g requires a minimal brain size, and thus explain the conundrum why the average mammal or bird has a roughly 10 times larger brain relative to body size than ectotherms. Ectotherm brains and cognition may therefore be organized in fundamentally different ways compared to endotherms.

animal behavior and cognition↗