No Evidence Against the Greater Male Variability Hypothesis: A Commentary on Harrison et al.’s Meta-Analysis of Animal Personality

Harrison et al. (2021) set out to test the greater male variability hypothesis with respect to personality in non-human animals. Based on the non-significant results of their meta-analysis, they concluded that there is no evidence to support the hypothesis, and that biological explanations for greater male variability in human psychological traits should be called into question. Here, we show that these conclusions are unwarranted. Specifically: (a) in mammals, birds, and reptiles/amphibians, the magnitude of the sex differences in variability found in the meta-analysis is entirely in line with previous findings from both humans and non-human animals; (b) the generalized lack of statistical significance does not imply that effect sizes were too small to be considered meaningful, as the study was severely underpowered to detect effect sizes in the plausible range; (c) the results of the meta-analysis can be expected to underestimate the true magnitude of sex differences in the variability of personality, because the behavioral measures employed in most of the original studies contain large amounts of measurement error; and (d) variability effect sizes based on personality scores, latencies, and proportions suffer from lac of statistical validity, adding even more noise to the meta-analysis. In total, Harrison et al.’s study does nothing to disprove the greater male variability hypothesis in mammals, let alone in humans. To the extent that they are valid, the data remain compatible with a wide range of plausible scenarios.

Agonistic experience during development establishes inter-individual differences in approach-avoidance behaviour of crickets

Members of numerous animal species show consistent inter-individual differences in behaviours, but the forces generating animal “personality” or individuality remain unclear. We show that experiences gathered solely from social conflict can establish consistent differences in the decision of male crickets to approach or avoid a stimulus directed at one antenna. Adults isolated for 48 h from a colony already exhibit behavioural differences. Prior to staging a single dyadic contest, prospective winners approached the stimulus whereas prospective losers turned away, as they did also after fighting. In contrast, adults raised as nymphs with adult males present but isolated from them as last instar nymphs, all showed avoidance. Furthermore, adults raised without prior adult contact, showed no preferred directional response. However, following a single fight, winners from both these groups showed approach and losers avoidance, but this difference lasted only one day. In contrast, after 6 successive wins or defeats, the different directional responses of multiple winners and losers remained consistent for at least 6 days. Correlation analysis revealed examples of consistent inter-individual differences in the direction and magnitude of turning responses, which also correlated with individual aggressiveness and motility. Together our data reveal that social subjugation, or lack thereof, during post-embryonic and early adult development forges individuality and supports the notion of a proactive–reactive syndrome in crickets.

Species comparison of among- and within-individual variation and correlations

Individuals frequently differ consistently from one another in their average behaviors (i.e. “animal personality”) and in correlated suites of consistent behavioral responses (i.e. “behavioral syndromes”). However, understanding the evolutionary basis of this (co)variation has lagged behind demonstrations of its presence. This lag partially stems from comparative methods rarely being used in the field. Consequently, much of the research on animal personality has relied on “adaptive stories” focused on single species and populations. Here we used a comparative approach to examine the role of phylogeny in shaping patterns of average behaviors, behavioral variation, and behavioral correlations. In comparing the behaviors and behavioral variation for five species of Gryllid crickets we found that phylogeny shaped average behaviors and behavioral (co)variation. Variation in average exploratory behavior and response to cues of predator presence attributable to phylogeny was greater or comparable to the magnitude of “personality variation”. Likewise, magnitudes of variation were concordant with evolutionary relationships and behavioral correlations were consistent across species. These results suggest that phylogenetic constraints play an important role in the expression of animal personalities and behavioral syndromes and emphasize the importance of examining evolutionary explanations within a comparative framework.