Worldwide, domestic cats (Felis silvestris catus) outnumber domestic dogs (Canis familiaris). Despite cats’ success in human environments, dog social cognition has received considerably more scientific attention over the last several decades []. A key aspect of what has been said to make dogs unique is their proclivity for forming attachment bonds, including secure attachments to humans [], which could provide scaffolding for the development of human-like socio-cognitive abilities and contribute to success in human environments []. Cats, like dogs, can be found living in social groups or solitarily, depending on early developmental factors, resource distribution, and lifetime experiences such as human interaction []. Despite fewer studies, research suggests we may be underestimating cats’ socio-cognitive abilities []. Here we report evidence, using behavioral criteria established in the human infant literature [], that cats display distinct attachment styles toward human caregivers. Evidence that cats share social traits once attributed to dogs and humans alone would suggest that broader non-canine-specific mechanisms may be needed to explain cross-species attachment and socio-cognitive abilities.

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7 Harlow H.F. The nature of love. 8 Wanser S.H.

Udell M.A.R. Does attachment security to a human handler influence the behavior of dogs who engage in animal assisted activities?. 5 Bowlby J. Attachment and Loss: Attachment, Second Edition. 6 Ainsworth M.D.S.

Bell S.M. Attachment, exploration, and separation: illustrated by the behavior of one-year-olds in a strange situation. 8 Wanser S.H.

Udell M.A.R. Does attachment security to a human handler influence the behavior of dogs who engage in animal assisted activities?. 9 Schöberl I.

Beetz A.

Solomon J.

Wedl M.

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Kotrschal K. Social factors influencing cortisol modulation in dogs during a strange situation procedure. 6 Ainsworth M.D.S.

Bell S.M. Attachment, exploration, and separation: illustrated by the behavior of one-year-olds in a strange situation. In our study, cats and owners participated in a Secure Base Test (SBT), an abbreviated strange situation test used to evaluate attachment security in primates [] and dogs []. During this test, the subject spends 2 minutes in a novel room with their caregiver, followed by a 2-minute alone phase, and then a 2-minute reunion phase (see Supplemental Information for details). Cats were classified into attachment styles by expert attachment coders using the same criteria used in the human infant [] and dog literature []. Upon the caregiver’s return from a brief absence, individuals with secure attachment display a reduced stress response and contact-exploration balance with the caretaker (the Secure Base Effect), whereas individuals with an insecure attachment remain stressed and engage in behaviors such as excessive proximity-seeking (ambivalent attachment), avoidance behavior (avoidant attachment), or approach/avoidance conflict (disorganized attachment) [].

5 Bowlby J. Attachment and Loss: Attachment, Second Edition. Figure 1 The main attachment styles observed in the cat–human bond. Show full caption (A) Proportion of three main attachment styles observed in kittens and adult cats. (B) Cat with secure attachment. (C) Cat with insecure-ambivalent attachment. (D) Cat with insecure-avoidant attachment. The SBT was conducted with kittens aged 3–8 months. Seventy kittens were classified into an attachment style (see Supplemental Information) and 9 kittens were unclassifiable. Of the classifiable kittens, 64.3% were categorized as securely attached and 35.7% were categorized as insecurely attached ( Figure 1 ). Of the insecure kittens, 84% were ambivalent, 12% avoidant, and 4% disorganized. To determine if attachment style could be predicted by differential socialization and reinforcement opportunities alone, a portion of the kittens were enrolled in a 6-week training and socialization intervention with their caretaker following baseline. When comparing 39 class and 31 control kittens, there were no significant differences in the number of kittens classified as secure or insecure either at baseline (Fishers, p = 0.14) or approximately 2 months later at follow-up (Fisher’s, p = 1.0). These results indicate that although social reinforcement is likely a factor that contributes to the development of an attachment style, once an attachment style has been established between the members of a dyad, it appears to remain relatively stable over time, even after a training and socialization intervention []. Indeed, we found the proportion of secure and insecure kittens at follow-up mirrored that of baseline, with 68.6% displaying secure attachment and 31.4% displaying insecure attachment. At the individual level, 81% of kittens retained the same secure base designation (secure/insecure) at retest (Binomial, p < 0.0001). This may suggest that heritable factors, such as temperament, also influence attachment style and could contribute to its stability.

5 Bowlby J. Attachment and Loss: Attachment, Second Edition. 5 Bowlby J. Attachment and Loss: Attachment, Second Edition. Because cats, like most domesticated animals, retain several juvenile traits into maturity and remain dependent on humans for care, we predicted that attachment behavior toward a primary caretaker would be present in adulthood. To evaluate this, 38 cats over one year of age participated in the SBT. Distinct attachment styles were evident in adult cats ( Figure 1 ), with a distribution similar to the kitten population (65.8% secure, 34.2% insecure). To evaluate the possibility that differences between cats classified as secure or insecure could be attributed to different responses to stressful situations rather than distinct responses to the return of the owner, we evaluated the frequency of meow vocalizations produced in the alone phase of the SBT as a measure of separation distress. Attachment theory predicts that both secure and insecure-ambivalent individuals should show signs of distress including vocalizations during this phase with little difference between secure and insecure-ambivalent individuals []. Insecure-avoidant and disorganized individuals should show less separation distress []. As predicted, cats categorized as secure or insecure-ambivalent showed equivalent levels of separation distress in the alone phase as measured by vocalization frequency (U(32) = 68.5, Z = –0.843, p = 0.40; see Video S1 Timestamp 8:08), and both groups showed significantly more attachment distress than insecure-avoidant and disorganized cats (H(2) = 7.39, p = 0.025).

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