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152Chapter 8output. Importantly, for both humans and non-human animals it remainsto be established at what level the coordination will be present: behavioral,physiological, or both.The link between IIC and pair-bonding is a natural extension of previous work that links specific behavioral phenomena to pair-bonding. Forexample, Julian Huxley already reported on the function of courtship rituals in 1914. Huxley extensively studied courtship displays in Great CrestedGrebes (Podiceps cristatus) and argued that coordinated actions and the resulting emotional synchrony functioned to strengthen their pair-bond [(Huxley, 1914), p. 516]: “I believe that the courtship ceremonies serve to keepthe two birds of a pair together, and to keep them constant to each other”.Thus, Huxley explicitly proposed IIC as a mechanism for pair-bonding. Asimilar approach was taken to explain vocal duetting in birds [60]. Thebottom line of these models is that performing coordinated displays helpsthe initiation of a new pair-bond, strengthens an existing pair-bond and inturn, improving the quality of bi-parental care. Thus, we have integratedboth the notion that pair-bond strength is essential for reproductive fitness,and the notion that IIC is crucial for establishing and maintaining such apair bond. Furthermore, we illustrate that IIC itself might play a role inreproductive fitness.Conclusion and future directionsTo delineate whether and when IIC is adaptive in pair bonding, it is crucialto compare humans to other animals. Therefore, interdisciplinary studiesby biologists and psychologists are essential. Recent theoretical models integrating findings from non-human animals and humans (Prochazkova &Kret, 2017; de Waal & Preston, 2017) have highlighted the importance ofIIC for affiliation. However, the challenges inherent to such research (e.g.,subtlety of emotional cues, Kret, 2015) illustrate the need to focus on othermeasures, such as responses of the autonomic nervous system. Autonomicresponses (e.g., pupil size change, blushing, sweating) are linked to emotionsand are not under voluntary control (Prochazkova & Kret, 2017). Previousresearch has shown that pupil size (Kret et al., 2014) and facial temperature (Kano, Hirata, Deschner, Behringer, & Call, 2016) can be effectivelyused in research with non-human primates. These methods provide excitingopportunities to study physiological synchrony in non-human animals.In the present review, we provided a comparative overview of the relationship between IIC and pair-bonding. We have outlined the relationship ofIIC and pair-bonding, as well as bi-parental care. However, we did not findsufficient evidence to delineate the direction of causality. In other words,does IIC actually cause a stronger pair-bond, or do individuals that arecompatible just show better coordinated behavior? An explicitly comparTable 1. Overview of research designs suitable for each pair-bonding stagefor humans and non-human primatesStage of pair-bond Humans Non-human animalsInitiation Speed-date paradigms Mate-choice arenasDuring speed dates, the In a mate-choice arena, onedaters’ behavioral and individual is confronted with multiplephysiological linkage can be potential mates. Behavioral andmeasured and used to predict physiological linkage can bedate outcomes measured and used to predict mate preference.Maintenance Correlational studies Correlational studiesIn a longitudinal setup, the In a correlational setup, variation inbehavioral and physiological relationship quality can be linked tolinkage of couples can be variation in IIC, such as vocalmeasured (e.g., from the duetting or mutual courtshipstart of the relationship) and displays.correlated with indicators ofrelationship satisfaction.Bi-parental care Correlational studies Cross-foster studies*In a longitudinal or cross- In cross-foster studies, some eggssectional setup, the behavioral are removed from the nest of theirand physiological linkage of biological parents and raised bycouples can be measured (e.g., surrogates. This allows one to studyfrom the start of the relationship) the effect of IIC while controlling forand correlated with investment in genetic quality of the offspring. Thus,bi-parental care and relevant the effect of parental IIC on parentalmeasures of reproductive fitness care can be examined in isolation.(e.g., health or developmental For an example, see (Ihle et al., 2015; Riesche et al., 2018).measures).Cross-over/serial breeding studies**In cross-over designs, individuals canbe sequentially paired with partnerswith whom they vary in IIC. Thiswithin-subject design allows thestudy of the effect of parental IIC onparental care while controlling forindividual quality of theparents. For an example, see (Rutstein et al., 2005).Correlational studiesIn a longitudinal or cross-sectionalsetup, the behavioral andphysiological linkage of pairs can bemeasured (e.g., from the start of thepair-bond) and correlated withinvestment in bi-parental care andrelevant reproductive fitnessmeasures (e.g., offspring quantityand/or offspring survival).* in birds and some primate species (e.g., marmosets)** in serially monogamous birds or primatesTable 1. Overview of research designs suitable for each pair-bonding stagefor humans and non-human primatesStage of pair-bond Humans Non-human animalsInitiation Speed-date paradigms Mate-choice arenasDuring speed dates, the In a mate-choice arena, onedaters’ behavioral and individual is confronted with multiplephysiological linkage can be potential mates. Behavioral andmeasured and used to predict physiological linkage can bedate outcomes measured and used to predict mate preference.Maintenance Correlational studies Correlational studiesIn a longitudinal setup, the In a correlational setup, variation inbehavioral and physiological relationship quality can be linked tolinkage of couples can be variation in IIC, such as vocalmeasured (e.g., from the duetting or mutual courtshipstart of the relationship) and displays.correlated with indicators ofrelationship satisfaction.Bi-parental care Correlational studies Cross-foster studies*In a longitudinal or cross- In cross-foster studies, some eggssectional setup, the behavioral are removed from the nest of theirand physiological linkage of biological parents and raised bycouples can be measured (e.g., surrogates. This allows one to studyfrom the start of the relationship) the effect of IIC while controlling forand correlated with investment in genetic quality of the offspring. Thus,bi-parental care and relevant the effect of parental IIC on parentalmeasures of reproductive fitness care can be examined in isolation.(e.g., health or developmental For an example, see (Ihle et al., 2015; Riesche et al., 2018).measures).Cross-over/serial breeding studies**In cross-over designs, individuals canbe sequentially paired with partnerswith whom they vary in IIC. Thiswithin-subject design allows thestudy of the effect of parental IIC onparental care while controlling forindividual quality of theparents. For an example, see (Rutstein et al., 2005).Correlational studiesIn a longitudinal or cross-sectionalsetup, the behavioral andphysiological linkage of pairs can bemeasured (e.g., from the start of thepair-bond) and correlated withinvestment in bi-parental care andrelevant reproductive fitnessmeasures (e.g., offspring quantityand/or offspring survival).* in birds and some primate species (e.g., marmosets)** in serially monogamous birds or primatesative approach [e.g., as in voles: (L. J. Young, Winslow, Nilsen, & Insel,1997)] can be fruitful in answering this and many other questions such as,do closely related species that differ with regards to bi-parental care andpair bonding also differ in the amount and contexts in which IIC is occurs?Further development of comparative theoretical will allow us to explain IICfindings in humans and other animals and advance the understanding of thismulti-faceted relationship.dictions. Importantly, for humans and non-human animals it remains tobe established at what level the coordination will be present: behavioral,physiological, or both.The link between IIC and pair-bonding is a natural extension of previous work that links specific behavioral phenomena to pair-bonding. Forexample, Julian Huxley already reported on the function of courtship rituals in 1914. Huxley extensively studied courtship displays in Great CrestedGrebes (Podiceps cristatus) and argued that coordinated actions and the resulting emotional synchrony functioned to strengthen their pair-bond (Huxley, 1914, p. 516): “I believe that the courtship ceremonies serve to keep thetwo birds of a pair together, and to keep them constant to each other”. Thus,Huxley explicitly proposed IIC as a mechanism for pair-bonding. A similarapproach was taken to explain vocal duetting in birds (Wickler, 1980). Thebottom line of these models is that performing coordinated displays helpsthe initiation of a new pair-bond, strengthens an existing pair-bond and inturn, improving the quality of bi-parental care. Thus, we have integratedboth the notion that pair-bond strength is essential for reproductive fitness,and the notion that IIC is crucial for establishing and maintaining such apair bond. Furthermore, we illustrate that IIC itself might play a role inreproductive fitness.Conclusion and future directionsTo delineate whether and when IIC is adaptive in pair bonding, it is crucialto compare humans to other animals. Therefore, interdisciplinary studiesby biologists and psychologists are essential. Recent theoretical models integrating findings from non-human animals and humans (Prochazkova &Kret, 2017; de Waal & Preston, 2017) have highlighted the importance ofIIC for affiliation. However, the challenges inherent to such research (e.g.,subtlety of emotional cues, Kret, 2015) illustrate the need to focus on othermeasures, such as responses of the autonomic nervous system. Autonomicresponses (e.g., pupil size change, blushing, sweating) are linked to emotionsand are not under voluntary control (Prochazkova & Kret, 2017). Previousresearch has shown that pupil size (Kret et al., 2014) and facial temperature (Kano, Hirata, Deschner, Behringer, & Call, 2016) can be effectivelyused in research with non-human primates. These methods provide excitingopportunities to study physiological synchrony in non-human animals.In the present review, we provided a comparative overview of the relationship between IIC and pair-bonding. We have outlined the relationship ofIIC and pair-bonding, as well as bi-parental care. However, we did not findsufficient evidence to delineate the direction of causality. In other words,does IIC actually cause a stronger pair-bond, or do individuals that arecompatible just show better coordinated behavior? An explicitly comparIliana Samara 17x24.indd 152 08-04-2024 16:36