Can Population-Level Laterality Stem from Social Pressures? Evidence from Cheek Kissing in Humans

Lateralised courtship behaviour and its impact on mating success in Ostrinia furnacalis (Lepidoptera: Crambidae)

Lateralisation is a well-established phenomenon observed in an increasing number of insect species. This study aims to obtain basic details on lateralisation in courtship and mating behaviour in Ostrinia furnacalis, the Asian corn borer. We conducted laboratory investigations to observe lateralisation in courtship and mating behaviours in adult O. furnacalis. Our goal was also to detect lateralised mating behaviour variations during sexual interactions and to elucidate how these variances might influence the mating success of males. Our findings reveal two distinct lateralised traits: male approaches from the right or left side of the female and the direction of male turning displays. Specifically, males approaching females from their right side predominantly exhibited left-biased 180° turning displays, while males approaching females from the left-side primarily displayed right-biased 180° turning displays. Notably, left-biased males, executing a 180° turn for end-to-end genital contact, initiated copulation with fewer attempts and began copulation earlier than their right-biased approaches with left-biased 180° turning displays. Furthermore, mating success was higher when males subsequently approached the right side of females during sexual encounters. Left-biased 180° turning males exhibited a higher number of successful mating interactions. These observations provide the first report on lateralisation in the reproductive behaviour of O. furnacalis under controlled laboratory conditions and hold promise for establishing reliable benchmarks for assessing and monitoring the quality of mass-produced individuals in pest control efforts.

Modulation of behavioural laterality in wild New Caledonian crows (Corvus moneduloides): Vocalization, age and function

The New Caledonian crow (Corvus moneduloides) is known for displaying a unique set of tool-related behaviours, with the bird's bill acting as an individually consistently lateralized effector. However, we still fail to understand how such laterality develops, is modulated or even if its expression is consistent across other behavioural categories. Creating the first ethogram for this species allowed us to examine laterality and vocalisations in a population of wild, free-flying New Caledonian crows using detailed analyses of close-up video footage. We revealed the existence of an overall strong left-sided bias during object manipulation only and which was driven by the adult crows of our focal population, the stabilization of individual preferences occurring during the birds' juvenile years. Individually, at least one crow showed consistent side biases to the right and left within different behavioural categories. Our findings highlight previously unknown variability in behavioural laterality in this species, thus advocating for further investigation. Specifically, we argue that a better understanding of the New Caledonian crow's biology and ecology is required if one wishes to pursue the promising comparative road that laterality could be connected to the evolution of tool-making.

The effects of sex and handedness on masturbation laterality and other lateralized motor behaviours

Masturbation is a common human behaviour. Compared to other unimanual behaviours it has unique properties, including increased sexual and emotional arousal, and privacy. Self-reported hand preference for masturbation was examined in 104 left-handed and 103 right-handed women, and 100 left-handed and 99 right-handed men. Handedness (modified Edinburgh Handedness Inventory, EHI), footedness, eyedness, and cheek kissing preferences were also measured. Seventy nine percent used their dominant hand (always/usually) for masturbation, but left-handers (71.5%) were less consistently lateralized to use their dominant hand than right-handers (86.5%). Hand preference for masturbation correlated more strongly with handedness (EHI), than with footedness, eyedness, or cheek preference. There was no difference in masturbation frequency between left- and right-handers, but men masturbated more frequently than women, and more women (75%) than men (33%) masturbated with sex aids. For kissing the preferred cheek of an emotionally close person from the viewer's perspective, left-handers showed a left-cheek preference, and right-handers a weaker right-cheek preference. The results suggest that hemispheric asymmetries in emotion do not influence hand preference for masturbation but may promote a leftward shift in cheek kissing. In all, masturbation is lateralized in a similar way to other manual motor behaviours in left-handed and right-handed men and women.

The "right" side of sleeping: laterality in resting behaviour of Aldabra giant tortoises (Aldabrachelys gigantea)

Although some studies investigated lateralization in reptiles, little research has been done on chelonians, focusing only on few behaviours such as righting response and escape preference. The aim of this study was to investigate lateralization in Aldabra giant tortoises (Aldabrachelys gigantea), focusing on asymmetrical positioning of the limbs and the head during resting behaviour, called sleep-like behaviour, involving both wild tortoises and individuals under human care. Subjects of the study were 67 adult Aldabra tortoises (54 free ranging on Curieuse, 13 under human care in Mahè Botanical Garden). For each tortoise observed during sleep-like behaviour, we recorded the position of the head (on the left, on the right or in line with the body midline) and we collected which forelimb and hindlimb were kept forward. Moreover, the number of subjects in which limbs were in a symmetrical position during the sleep-like behaviour was recorded. Based on our results, the number of tortoises with asymmetrical position of head and limb was higher (head: 63%; forelimbs: 88%; hindlimbs: 70%) than the number of tortoises with symmetrical position of the head and the limb. Regarding the head, throughout the subjects found with the asymmetrical position of the head during sleep-like behaviour, tortoises positioning the head on the right (42%) were more than those sleeping with the head on the left (21%). We found a relationship between the position of the forelimbs and hindlimbs during sleep-like behaviour. We reported no differences between Mahè (under human care) and Curieuse (wild) tortoises. Findings of this preliminary study underlined traces of group-level lateralization in head positioning during the sleep-like behaviour, possibly due to a left-eye/right-hemisphere involvement in anti-predatory responses and threatening stimuli as reported in reptiles and other vertebrates. This study aims at adding data on brain lateralization, often linked to lateralized behaviours, in reptiles, especially in chelonians.

Handedness Development: A Model for Investigating the Development of Hemispheric Specialization and Interhemispheric Coordination

The author presents his perspective on the character of science, development, and handedness and relates these to his investigations of the early development of handedness. After presenting some ideas on what hemispheric specialization of function might mean for neural processing and how handedness should be assessed, the neuroscience of control of the arms/hands and interhemispheric communication and coordination are examined for how developmental processes can affect these mechanisms. The author’s work on the development of early handedness is reviewed and placed within a context of cascading events in which different forms of handedness emerge from earlier forms but not in a deterministic manner. This approach supports a continuous rather than categorical distribution of handedness and accounts for the predominance of right-handedness while maintaining a minority of left-handedness. Finally, the relation of the development of handedness to the development of several language and cognitive skills is examined.

Asymmetries in social touch-motor and emotional biases on lateral preferences in embracing, cradling and kissing

In human social interaction, affective touch plays an integral role to communicate intentions and emotions. Three of the most important forms of social touch are embracing, cradling and kissing. These behaviours have been demonstrated to be lateralized, but the underlying mechanisms are still not well understood. Both motor and emotive biases have been suggested to affect laterality of social touch. We aimed to systematically investigate how motor preferences and emotive biases influence the lateralization of embracing, cradling and kissing within the same sample. Participants performed all three forms of social touch in neutral, positive and negative emotional conditions. Like a previous study, we found a rightward bias for embracing that was modulated by both motor preferences and the emotional content of the situation. Kissing and cradling were not influenced by motor preferences. In general, a negative emotional connotation of the situation led to a reduction of lateral biases in social touch, independent of the individual direction.

Differential roles of the right and left brain hemispheres in the social interactions of a free-ranging ungulate

Despite the abundant empirical evidence on lateralized social behaviours, a clear understanding of the relative roles of two brain hemispheres in social processing is still lacking. This study investigated visual lateralization in social interactions of free-ranging European bison (Bison bonasus). The bison were more likely to display aggressive responses (such as fight and side hit), when they viewed the conspecific with the right visual field, implicating the left brain hemisphere. In contrast, the responses associated with positive social interactions (female-to-calf bonding, calf-to-female approach, suckling) or aggression inhibition (fight termination) occurred more likely when the left visual field was in use, indicating the right hemisphere advantage. The results do not support either assumptions of right-hemisphere dominance for control of various social functions or hypotheses about simple positive (approach) versus negative (withdrawal) distinction between the hemispheric roles. The discrepancy between the studies suggests that in animals, the relative roles of the hemispheres in social processing may be determined by a fine balance of emotions and motivations associated with the particular social reaction difficult to categorize for a human investigator. Our findings highlight the involvement of both brain hemispheres in the control of social behaviour.

Opposite asymmetries of face and trunk and of kissing and hugging, as predicted by the axial twist hypothesis

The contralateral organization of the forebrain and the crossing of the optic nerves in the optic chiasm represent a long-standing conundrum. According to the Axial Twist Hypothesis (ATH) the rostral head and the rest of the body are twisted with respect to each other to form a left-handed half turn. This twist is the result, mainly, of asymmetric, twisted growth in the early embryo. Evolutionary selection tends to restore bilateral symmetry. Since selective pressure will decrease as the organism approaches symmetry, we expected a small control error in the form of a small, residual right-handed twist. We found that the mouth-eyes-nose (rostral head) region shows a left-offset with respect to the ears (posterior head) by up to 0.8° (P < 0.01, Bonferroni-corrected). Moreover, this systematic aurofacial asymmetry was larger in young children (on average up to 3°) and reduced with age. Finally, we predicted and found a right-sided bias for hugging (78%) and a left-sided bias for kissing (69%). Thus, all predictions were confirmed by the data. These results are all in support of the ATH, whereas the pattern of results is not (or only partly) explained by existing alternative theories. As of the present results, the ATH is the first theory for the contralateral forebrain and the optic chiasm whose predictions have been tested empirically. We conclude that humans (and all other vertebrates) are fundamentally asymmetric, both in their anatomy and their behavior. This supports the thesis that the approximate bilateral symmetry of vertebrates is a secondary feature, despite their being bilaterians.

Hugs and kisses - The role of motor preferences and emotional lateralization for hemispheric asymmetries in human social touch

Social touch is an important aspect of human social interaction - across all cultures, humans engage in kissing, cradling and embracing. These behaviors are necessarily asymmetric, but the factors that determine their lateralization are not well-understood. Because the hands are often involved in social touch, motor preferences may give rise to asymmetric behavior. However, social touch often occurs in emotional contexts, suggesting that biases might be modulated by asymmetries in emotional processing. Social touch may therefore provide unique insights into lateralized brain networks that link emotion and action. Here, we review the literature on lateralization of cradling, kissing and embracing with respect to motor and emotive bias theories. Lateral biases in all three forms of social touch are influenced, but not fully determined by handedness. Thus, motor bias theory partly explains side biases in social touch. However, emotional context also affects side biases, most strongly for embracing. Taken together, literature analysis reveals that side biases in social touch are most likely determined by a combination of motor and emotive biases.

Social Risk Dissociates Social Network Structure across Lateralized Behaviors in Spider Monkeys

Reports of lateralized behavior are widespread, although the majority of findings have focused on the visual or motor domains. Less is known about laterality with regards to the social domain. We previously observed a left-side bias in two social affiliative behaviors—embrace and face-embrace—in captive Colombian spider monkeys (Ateles fusciceps rufiventris). Here we applied social network analysis to laterality for the first time. Our findings suggest that laterality influences social structure in spider monkeys with structural differences between networks based on direction of behavioral bias and social interaction type. We attribute these network differences to a graded spectrum of social risk comprised of three dimensions.

Mother and offspring lateralized social behavior across mammalian species

Findings on nonprimate mammals place the issue of mother-infant lateralized relations in a broader context, demonstrating that humans are one of many species showing this feature. The remarkable interspecies consistency in the direction of lateralization points to a continuity between lateralized mother-infant interactions in primates and nonprimate mammals and suggests ancient evolutionary roots of human cradling bias. The results from species which, in contrast to primates, have no direct involvement of forelimbs in mother-infant spatial interactions clearly support the perceptual origin of this type of lateralization. A right hemisphere advantage for social functions relevant to mother-infant interactions is the most probable background for the left-sided biases in the behavior of mothers and infants. Recent findings suggest the contribution of lateralized mother-infant interactions to biological fitness. Mother and infant both can gain advantage from keeping the other on the left side.

Embracing your emotions: affective state impacts lateralisation of human embraces

Humans are highly social animals that show a wide variety of verbal and non-verbal behaviours to communicate social intent. One of the most frequently used non-verbal social behaviours is embracing, commonly used as an expression of love and affection. However, it can also occur in a large variety of social situations entailing negative (fear or sadness) or neutral emotionality (formal greetings). Embracing is also experienced from birth onwards in mother-infant interactions and is thus accompanying human social interaction across the whole lifespan. Despite the importance of embraces for human social interactions, their underlying neurophysiology is unknown. Here, we demonstrated in a well-powered sample of more than 2500 adults that humans show a significant rightward bias during embracing. Additionally, we showed that this general motor preference is strongly modulated by emotional contexts: the induction of positive or negative affect shifted the rightward bias significantly to the left, indicating a stronger involvement of right-hemispheric neural networks during emotional embraces. In a second laboratory study, we were able to replicate both of these findings and furthermore demonstrated that the motor preferences during embracing correlate with handedness. Our studies therefore not only show that embracing is controlled by an interaction of motor and affective networks, they also demonstrate that emotional factors seem to activate right-hemispheric systems in valence-invariant ways.

Captive gorillas' manual laterality: The impact of gestures, manipulators and interaction specificity

Relationships between humans' manual laterality in non-communicative and communicative functions are still poorly understood. Recently, studies showed that chimpanzees' manual laterality is influenced by functional, interactional and individual factors and their mutual intertwinement. However, what about manual laterality in species living in stable social groups? We tackled this question by studying three groups of captive gorillas (N=35) and analysed their most frequent manual signals: three manipulators and 16 gesture types. Our multifactorial investigation showed that conspecific-directed gestures were overall more right-lateralized than conspecific-directed manipulators. Furthermore, it revealed a difference between conspecific- and human-directed gestural laterality for signallers living in one of the study groups. Our results support the hypothesis that gestural laterality is a relevant marker of language left-brain specialisation. We suggest that components of communication and of manipulation (not only of an object but also of a conspecific) do not share the same lateralised cerebral system in some primate species.

Assessment and analysis of human laterality for manipulation and communication using the Rennes Laterality Questionnaire

Despite significant scientific advances, the nature of the left-hemispheric systems involved in language (speech and gesture) and manual actions is still unclear. To date, investigations of human laterality focused mainly on non-communication functions. Although gestural laterality data have been published for infants and children, relatively little is known about laterality of human gestural communication. This study investigated human laterality in depth considering non-communication manipulation actions and various gesture types involving hands, feet, face and ears. We constructed an online laterality questionnaire including 60 items related to daily activities. We collected 317 594 item responses by 5904 randomly selected participants. The highest percentages of strong left-lateralized (6.76%) and strong right-lateralized participants (75.19%) were for manipulation actions. The highest percentages of mixed left-lateralized (12.30%) and ambidextrous (50.23%) participants were found for head-related gestures. The highest percentage of mixed right-lateralized participants (55.33%) was found for auditory gestures. Every behavioural category showed a significant population-level right-side bias. More precisely, participants were predominantly right-lateralized for non-communication manual actions, for visual iconic, visual symbolic, visual deictic (with and without speech), tactile and auditory manual gestures as well as for podial and head-related gestures. Our findings support previous studies reporting that humans have left-brain predominance for gestures and complex motor activities such as tool-use. Our study shows that the Rennes Laterality Questionnaire is a useful research instrument to assess and analyse human laterality for both manipulation and communication functions.

The right way to kiss: directionality bias in head-turning during kissing

Humans have a bias for turning to the right in a number of settings. Here we document a bias in head-turning to the right in adult humans, as tested in the act of kissing. We investigated head-turning bias in both kiss initiators and kiss recipients for lip kissing, and took into consideration differences due to sex and handedness, in 48 Bangladeshi heterosexual married couples. We report a significant male bias in the initiation of kissing and a significant bias in head-turning to the right in both kiss initiators and kiss recipients, with a tendency among kiss recipients to match their partners’ head-turning direction. These interesting outcomes are explained by the influences of societal learning or cultural norms and the potential neurophysiological underpinnings which together offer novel insights about the mechanisms underlying behavioral laterality in humans.

Correction: Can Population-Level Laterality Stem from Social Pressures? Evidence from Cheek Kissing in Humans

[This corrects the article DOI: 10.1371/journal.pone.0124477.].