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151IIC in pair-bonding8to be deeply rooted and extends beyond humans. Similar to humans, someanimal species are faced with ultimate challenges relating to bi-parental careand relationship maintenance, such as producing, defending, and providingfor their offspring together. All these challenges are easier to address whenbehavior is well-coordinated. Thus, when investigating the origin of partnerbond-related behavior in humans, we should not overlook data from speciesfacing similar challenges, namely raising offspring while relying on anotherindividual. Therefore, we posit that a comparative framework integratingIIC and pair-bonding provides exciting opportunities to study the adaptivevalue of IIC in romantic relationships.Here, we re-introduce and build upon the pair-bonding hypothesis. Thishypothesis suggests that in species with bi-parental care, pair-bond strengthis crucial for successful breeding (Rasmussen, 1981). While Rasmussen(1981) refers to only the relationship between pair-bond strength and reproductive output, we specifically suggest that IIC could be the underlying mechanism. First, IIC and pair-bond strength might form a positivefeedback loop, so that coordination between individuals increases and thepair-bond can stand the test of time. Increased IIC might in turn improvereproductive output because of improved offspring care; however, sustainingthe existing pair-bond in itself might also be beneficial. Indeed, divorcing may bear reproductive consequences, such as the need to search for anew partner. Second, IIC might mainly function to set a high baseline pairbond strength during initial stages of bonding, so that only well-coordinatedcouples will be established. Although not mutually exclusive, the first explanation is well-supported by literature showing that reproductive successand IIC increase over time (Griggio & Hoi, 2011; Sánchez-Macouzet et al.,2014).The idea that IIC plays a pivotal role in pair-bonding and reproductivesuccess of a pair results in three main predictions. First, in species withbi-parental care, IIC should be apparent during courtship, because highamounts of IIC are necessary to successfully raise offspring. Second, wellcoordinated pairs that perform coordinated displays should have a strongerand more durable pair-bond than other pairs. Third, well-coordinated pairsshould have higher reproductive fitness (reflected in either more offspring orhigher survival rate) than less coordinated pairs. These predictions can forexample be studied using cross-over designs (Rutstein, Gilbert, & Tomkins,2005). In Table 1, we outline a few options to investigate these questionsin humans and non-human animals. For example, pair-bond strength canbe quantified by incorporating measures of proximity and grooming or allopreening (Kenny, Birkhead, & Green, 2017; Silk, Cheney, & Seyfarth, 2013).Consequently, their relationship with coordination (e.g., synchrony, Nedelec& Beaver, 2014) can be investigated to understand whether between-pairvariation in pair-bond strength is associated with between-pair variation inIIC. A comparative framework provides clear advantages to test these preIliana Samara 17x24.indd 151 08-04-2024 16:36