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150Chapter 8In the past century, similar evidence has been shown for non-human primates. In general, coordination is more apparent in affiliated individuals(Palagi et al., 2020; Pika et al., 2018). Many primate species with biparental care are characterized by their duetting behavior, a clear exampleof IIC. Importantly, such duets seem to be restricted to pair-bonding primatespecies (Haimoff, 1986). A famous example concerns siamangs (Symphalangus syndactylus), where duration and intensity of duetting correlated withrelationship quality (Geissmann & Orgeldinger, 2000); suggesting that thestronger the relationship, the smoother the song. Recently, these findingswere extended to facial mimicry in gibbons: pairs with strong facial mimicryhad a greater relationship quality (Florkiewicz, Skollar, & Reichard, 2018).Importantly, the direction of causality is not clear yet. It is likely that IICand pair-bond strength are embedded in a feedback loop; however, furtherresearch is needed to examine this notion.Physiological levelFew studies have investigated physiological synchrony in non-human animals, and data in pair-bonding contexts are especially rare. This also appliesto studies investigating physiological synchrony on a moment-to-moment basis (Kret, Tomonaga, & Matsuzawa, 2014), mainly due to methodologicalchallenges. The few studies that investigated pair-bonding species and physiological synchrony have established that pairs synchronize on a hormonallevel. For example, dyadic bond strength is associated with oxytocin synchrony in common marmosets (Callithrix jacchus) (Finkenwirth, van Schaik,Ziegler, & Burkart, 2015), and concentrations of hormones correlate in pairsof multiple bird species (Griffith, 2019). Hormonal synchrony is crucial during mating periods because the hormonal state of one partner might inducecourtship behavior, consequently changing the hormonal state and behavior of the other (M. C. Moore, 1982; Hirschenhauser, 2012). Comparablepatterns have been found in humans, where men whose testosterone levelscorrelate with their partner’s during pregnancy are more involved in raisingtheir child and maintaining their relationship (Saxbe et al., 2017). Thus,this preliminary evidence suggests that hormonal synchronization is relevantto establish a successful pair-bond and successfully care for offspring acrossspecies.The pair-bonding hypothesisHere, we have reviewed the literature on pair-bonding and IIC in humansand non-human animals. Our brief review suggests that IIC between partners might be a fundamental prerequisite for pair-bonding initiation, maintenance, and most likely, bi-parental care. Crucially, this prerequisite seemsIliana Samara 17x24.indd 150 08-04-2024 16:36