Introduction

Exploring the social relationships between humans and their companion animals is key to understanding the development and nature of interspecific socio-cognitive skills. Dogs (Canis familiaris) exhibit human-analogue attachment towards their owner1 with similar function2 and certain mechanisms3 to that of infant-mother relationships. There is growing research interest on how such a particular relationship can evolve. Even though the human social niche—in which human-animal relationships can be realised—is clearly needed for the development of attachment bonds towards the owner, previous studies on differently socialised adult4 and puppy5 dogs showed negligible environmental effects on the dog-owner bond. In line with this, differences in the human-oriented bond between dogs and wolves raised in a similar human social environment support the role of genetic changes (that domestic dogs have undergone since the dog-wolf lineage split) in dogs' preparedness to form attachment with humans (e.g.2,5). Although there is some debate about the extent to which the relationship between intensely socialised wolves and dogs with their owners is similar6,7, it is important to remark that no studies demonstrated attachment in wolves that meet with all of its operational criteria8. To find out if the genetic effects supporting dog–human attachment are caused by domestication in general or more specifically by the artificial selection that increased dogs' dependence on humans9, one should also investigate whether similarly kept individuals of other domesticated species are attached to their owners. Beyond dogs, however, cats are the only companions whose social bond to the owner has been investigated to date, but cat results are contradictory (see10,11, but see12,13) and the only direct comparison of dogs and cats using the same standard procedure did not provide evidence for cats’ attachment towards their owner13.

Recently, miniature variants of domestic pigs (Sus scrofa domesticus) (later referred to as pigs) turned out to be popular companion animals despite originally being selected for other purposes (i.e. for meat stock14, and later for medical research15). Similarly to dogs, pigs are group-living animals16 that, unlike cats, have a similar role in human families to that of dogs when kept as companions. While their species predispositions differ, these similarities still make companion pigs a more feasible comparison to dogs than other species living in human families. But whether companion pigs exhibit attachment behaviours towards their owner is unknown.

While 'attachment' is a broad term frequently used as a synonym for various special bonds in general psychology and in animal behaviour literature, here, under ‘attachment’, we refer to a specifically defined phenomenon that enables us to draw strict parallels between companion animal-owner and infant–mother attachment bonds. According to this definition, ‘attachment’ is an organisational construct belonging to a behavioural system17 that regulates long-lasting asymmetrical social relationships18 in which the attached individual depends on a security-providing attachment figure19. The function of attachment is to enhance the attached individual’s chance for survival and learning through keeping it in the proximity of the attachment figure, therefore it plays an important role in infant-mother20 as well as in dog-owner relationships (e.g.,2,21,22). Attachment is associated with reward-related neural mechanisms3,23, can develop early in life and is relatively stable in time in case of both humans (e.g.,24,25) and dogs26. Attachment can be assessed along objectively measurable behavioural patterns20,27. Specifically, these behaviours are (1) contact- and proximity-seeking with the attachment figure during exploration (secure base effect) and in case of danger (safe haven effect), (2) separation distress-related behaviours in the absence of the attachment figure, and (3) specific behavioural changes upon reunion. Importantly, all these above operational criteria must be present in an attachment relationship. While various special bonds are reported between individuals of different species (e.g.,28) and even towards inanimate objects (e.g., human children29, adults30, non-human primates31)—some characterised by behaviours also crucial in the attachment system—, the dog-owner relationship stands out as the only non-intraspecific bond that is proven to simultaneously fulfil all criteria of attachment in accordance with the definition described above2,4,32,33,34. Companion dog-owner attachment is also unique8 in terms of the fact that dogs can form attachment towards their owners both in their natural environment (within the human social niche) and during adulthood2. Studies investigating the social bond in non-human primates towards humans typically test hand-raised or captive animals, and mostly focus on non-adult subjects (e.g.,35).

The Strange Situation Test (SST)—a validated laboratory test (e.g.,36) with various precursors in the examination of social bonds across species37—is widely used to investigate attachment towards human individuals both intraspecifically (i.e. infant-mother relationships, e.g.,38) and interspecifically (e.g., dog-owner relationships39,40,41). The SST has not yet been used, however, to test pigs.

Domestic pigs, compared to dogs, exhibit various different, but also many similar human-oriented behaviours. For instance, research on farm pigs revealed pigs’ willingness to make social contact with humans (e.g.,42,43,44,45) already from a young age (e.g.,44), as has also been reported in dogs9. Furthermore, recent comparative studies on young companion pigs and dogs kept in a similar human social niche revealed that the two species orient towards and seek contact with humans to a similar degree in various situations46,47. But, as compared to dogs, the available knowledge on pigs’ interspecific social competence and affinity is still scarce.

In this comparative study, using the SST context, we investigated whether young companion pigs show attachment towards their owner, and whether pigs’ and dogs’ attachment-specific behaviours differ towards the owner and the stranger to a similar extent. We hypothesised that (H1) if domestication in general, along with socialisation in the human environment contributes to the emergence of attachment towards a human owner in social species, then intensely socialised companion pigs and dogs will behave differently with the owner and the stranger to a similar degree. Conversely, (H2) if the specific selection that has led to increased dependence on humans in dogs is a main genetic factor for interspecific attachment to arise, then only dogs and not pigs will show the specific behaviour patterns that fulfil the criteria of attachment. To investigate the temporal stability of pigs’ behaviour in the SST, we tested pigs twice, at the age of ~ 9 and ~ 18 months.

Methods

Participants

In this comparative study, we tested companion pigs (Sus scrofa domesticus) and dogs (Canis familiaris) living in human families in a comparably similar social environment. Pigs, as well as dogs, were considered family members and were exposed to similarly close human contact from the age of ~ 8 weeks.

Twelve juvenile companion pigs participated in the study (5 neutered females, and 7 neutered males, Minnesota and mixed miniature variants). To make pigs’ rearing environment highly similar to that of well-socialised family dogs, pigs’ adoption process was supervised and guidelines were provided by the Neuroethology of Communication Lab (ELTE, Budapest, Hungary). During this, owners were taught how to handle pigs at home, socialise them to humans, expose and habituate them to different environments and transportation, etc. To investigate the temporal stability of pigs’ behaviour, pigs were tested twice, at the age of 8.75 ± 3.64 months and 17.69 ± 2.57 months. At the time of the first test, pigs lived together with their owners for approximately 5–6 months. One pig entered the research program a few months older (at the age of 14 months) than the others. Due to a possible order effect, this pig’s test was considered as a first, and it was tested only once. One pig was used to pilot the protocol, thus its first (pilot) SST was left out of the analyses. The second test of a third pig is also missing as before the second test this subject died. As a result of these, 11 pigs participated in the first SST and 10 pigs participated in the second SST. We had a total of 12 pig subjects, as individuals participating in the two tests were not exactly the same. For more details about pigs’ rearing conditions see the Supplementary Material of the current study, the doctoral dissertation of Pérez Fraga48 and the ‘Online Resource 2’ belonging to the study of Gerencsér et al.46.

To compare pigs’ and dogs’ behaviour, 17 companion dogs were also tested once (9 intact and 2 neutered females, and 5 intact and 1 neutered males; mean ± SD age = 8.78 ± 0.77 months). Most of the dog owners were volunteers of the Family Dog Project (https://familydogproject.elte.hu/), and some of them applied for the tests through different social media platforms or questionnaires of the Family Dog Project. The socialisation background of the dogs was similar to that of the pigs. Our dog participants were of many different breeds (16 pure breeds: 1 Hungarian vizsla, 2 golden retriever, 1 Jack Russell terrier, 1 shih tzu, 2 American staffordshire terrier, 1 Tervueren, 2 beagle, 1 papillon, 1 mudi, 1 puli, 1 English cocker spaniel, 1 border collie, 1 labradoodle; and 1 mongrel). This increased the representativeness of our results to dogs in general. Importantly, prior to the experiment, pigs and dogs have never been tested in the SST and had no prior experience with the test location.

Experimental setup

Here, we used a Strange Situation Test (SST) that consists of several episodes during which the subject is either alone, or with the potential attachment figure and/or a stranger in an unfamiliar environment. Based on Bowlby’s evolutionary approach20, this situation triggers specific patterns of attachment-related behaviours. In the SST, the stranger, together with the strange situation in the unfamiliar environment, and with the separation from the owner evoke moderate stress in the subjects38. The use of SST protocols involving the encounter with a stranger is common and well-established in dog SST experiments, yielding similar, robust, and replicable results across studies (for reviews see,1,2,21). The stress provoking effect of these protocols is confirmed by physiological studies33,49,50,51. Despite these, it is conceivable that in adult, well-socialised and friendly dogs/pigs the stranger may not remarkably increase the stress level even in the absence of the owner. But even in this case, basic differences in the subjects’ behaviours must be observable in the presence of the two persons, enabling the assessment of attachment.

Our SST protocol was based on the one used to investigate dog-owner attachment in the studies of Gábor et al.3 and Lenkei et al.52. The SST took place in the behavioural lab (6.27 m × 5.4 m) (Fig. 1) unfamiliar to the pigs and the dogs at the Department of Ethology, Eötvös Loránd University, Budapest, Hungary. Two chairs were placed in the middle of the room, facing each other, 1.5 m apart. The lab had two doors which were used by the stranger and the owner during the test (they always used the one closer to their assigned chair). The owner’s and the stranger’s chairs were randomised and balanced across tests. There were two additional chairs (for pigs) or tables (for dogs) with 6 wooden building blocks on them. These tables with the building blocks on them3,52 functioned as random objects for owners to perform actions neutral for their animals while ignoring them. This allowed us to observe if the owner's activity in themselves would affect the subjects’ proximity seeking behaviour, similarly to the case of human infants, who monitor the actions of the mother even if that is not directly related to them.

Figure 1
figure 1

Experimental setting. Schematic drawing of the basic experimental setup of the Strange Situation Test.

Full size image

Various pet toys (e.g., balls) were placed in a playing area between the chairs. Subjects’ behaviour was recorded using four ceiling-mounted cameras strategically positioned to provide visibility of the entire lab. The same owner participated in both tests of the pigs, with the exception of one pig, who moved to another human family between the two SSTs and consequently changed its primary caregiver.

Importantly, excessively high levels of stress can result in extreme behaviours (i.e. panic or freezing) in which case attachment is not assessable. Pigs are prey animals whose stress level easily raises, so to increase their comparability with dogs (make their Anxiety scores similar), a “calming” box or blanket in which they usually sleep was placed at the corner of the room opposite the playing area. According to our previous experiences with pig testing and with the SST piloting, the lack of a calming object could have caused too high levels of stress in pigs as indicated by their extreme vocalisations or escaping behaviours. We assumed that the familiar scent of the calming box/blanket decreased the stress level of pigs.

Protocol of the Strange Situation Test

The SST consisted of 6, 2-min-long episodes presenting the participants with differently stressful situations influenced by the presence and the absence of the owner and a stranger (Table 1). The owner and the stranger performed different activities the description of which can be found in Table 2. At the beginning of the test, owners got a blue-tooth headset playing a timed audio file with pre-recorded instructions, ensuring each activity to be performed in the given time by the owner. Strangers knew the protocol by heart and used a stopwatch to keep the timing. Before the test, an Experimenter welcomed the owners, provided them a detailed explanation of the process, and ensured that everything went well during the test. The experimenter was never the same person as the stranger, so both pigs and dogs met the stranger for the first time during the test.

Table 1 Detailed protocol of the Strange Situation Test.
Full size table
Table 2 Description of the activities performed by the owner (O) and the stranger (S) during the SST.
Full size table

Coding of behaviour

Based on the observed behaviour patterns, three major factors were formed and analysed; Attachment to the owner, Anxiety in the strange situation, and Acceptance of the stranger (willingness to interact with an unfamiliar person) (see39). This three-factor coding is often used in recent dog attachment papers (e.g.,3,52). For dogs, the same behaviours were coded as in the previous studies, but, for pigs, we made some slight modifications to better fit to their behaviour and emotional displays (Table 3). For example, normal tail wagging most times indicates positive emotions in both species53,54 while intense tail wagging or an increased tail wagging frequency is associated with stress in pigs54,55. In addition, in the SST context, dogs typically vocalise when stressed (except during playing and greeting). In contrast, pigs are a very vocal species16, thus their vocalisations were more carefully specified to ensure that we only code those that indicate stress56,57,58,59. Based on our observations from the past years with pigs, pigs’ reaction time is slower than that of dogs, which was also considered during the behaviour coding.

Table 3 Detailed description of the scoring of pigs’ and dogs’ behaviour in the SST.
Full size table

Analysis

One–one independent second coder coded > 10% of the pig and dog videos. From these, inter-rater reliability was calculated based on Cohen’s Kappa. This achieved 0.8–1 value indicating an excellent agreement in case of pigs, and 0.7–0.8 value indicating substantial agreement in case of dogs60.

To investigate whether pigs and dogs show specific patterns of attachment behaviours (i.e. they behave differently with the owner and the stranger in the SST context), the three symmetrically coded relevant variables between the owner and the stranger were compared similarly to the cat-dog comparative study of Gácsi et al. (submitted)13. These three variables were: (i) standing by the door in the presence of the owner (DoorO-1) vs. the stranger (DoorS-1) when the other person is absent, (ii) approaching the entering owner (GreetO-1) vs. stranger (GreetS-1) while the other person is in the room, and (iii) following the leaving owner (FollowO-1) vs. stranger (FollowS) while the other person stays in the room. In the case of both pigs and dogs, the number of individuals that showed/ did not show the relevant behaviour in the presence of the owner/stranger were compared via Related-Samples McNemar Change Test.

To compare the factor scores (Attachment, Anxiety, Acceptance) of the two species Mann–Whitney U tests or Independent-Samples t-Tests were used depending on the results of the Shapiro–Wilk normality test. To test the possible association between the factors in case of both pigs and dogs, Pearson correlation was used. All of the previous pig analyses were based on the data of the first pig SST to avoid the influence of a possible order effect. To test temporal stability of pigs’ behaviour across the first and the second SST, Pearson correlation and Wilcoxon signed-rank tests or Paired-Samples t-Tests were used depending on the results of the Shapiro–Wilk normality test. The pig who changed its owner between the two tests was also involved in these analyses. This decision was influenced by (1) the consideration that if this pig exhibited different attachment patterns, it would weaken our results, and (2) indirect evidence from the relevant dog literature, which indicates that dogs' attachment does not decrease following a change in ownership, as demonstrated by group-level results26,51,61.

IBS SPSS Statistics 23.0 was used for the statistical analyses.

Ethics statement

The experiment was conducted at the behavioural lab of the Department of Ethology, Eötvös Loránd University, Budapest, Hungary. All applicable institutional, national and international guidelines for the use and care of animals were followed. The study was non-invasive and did not cause any harm to the animals. We state that the study is reported in accordance with ARRIVE guidelines. The local ethical committee (Állatkísérleti Tudományos Etikai Tanács, Pest Megyei Kormányhivatal Élelmiszerlánc-Biztonsági és Állategészségügyi Igazgatósága in Budapest, Hungary) approved the experiment (# PE/EA/430-6/2018). Pig and dog owners were volunteers, did not get any monetary compensation and gave written informed consent.

Results

The comparison of the three symmetrically coded variables between the owner and the stranger revealed significant differences in dogs’ behaviour towards the two persons (Fig. 2A). More specifically, dogs were more likely to (1) stand by the door when the owner was absent and the stranger was present compared to the reverse situation (DoorS-1 vs. DoorO-1: df = 1, P = 0.003), (2) follow the leaving owner to the door while the stranger was present compared to the reverse situation (FollowO-1 vs. FollowS: df = 1, P < 0.000) and (3) approach the entering owner than the entering stranger while the other person was present (GreetO-1 vs. GreetS-1: df = 1, P = 0.004). None of these owner vs stranger comparisons revealed significant differences in pigs (DoorO-1 vs. DoorS-1: df = 1, P = 1.00; FollowO-1 vs. FollowS: df = 1, P = 0.50; GreetO-1 vs. GreetS-1: df = 1, P = 0.625).

Figure 2
figure 2

Results of the Strange Situation Tests. (A) Pigs’ and dogs’ behaviour along the three symmetrically coded variables between the owner and the stranger. For detailed description of the behaviours, see Table 3. (B) Pairwise comparison of pigs’ and dogs’ Attachment, Acceptance and Anxiety scores. (C) Attachment and Anxiety score correlation in pigs and dogs. (D) Similar tendency of pigs’ Attachment and Acceptance scores between their first and second SST. O: owner, S: stranger, SST: Strange Situation Test, ns: not significant, *P < 0.01, **P < 0.005, ***P ≤ 0.001. Dogs: N = 17 dogs (A–D); Pigs: N = 11 (A–C), D: N = 9 (D).

Full size image

The factor score comparison between the two species revealed significant differences in case of the Attachment (U = 163.0, P = 0.001) and Acceptance (t = 2.518, df = 26, P = 0.009) scores. Specifically, dogs had higher Attachment, while pigs had higher Acceptance scores than that of the other species. Such between species difference was not present in case of the Anxiety score (t = 0.585, df = 26, P = 0.282). For factor score details, see Fig. 2B and Table S1.

In both species, we found positive Attachment and Anxiety score correlations (Pig: r = 0.659, P = 0.028; Dog: r = 0.665, P = 0.004), but there was no association regarding the other two factors in either species (Attachment and Acceptance (Pig: r = − 0.332, P = 0.319, Dog: r = − 0.283, P = 0.271); Acceptance and Anxiety scores (Pig: r = 0.089, P = 0.795, Dog: r = 0.037, P = 0.888)) (Fig. 2C).

When testing the temporal stability of pigs’ behaviour in the SST, we found no differences between their first and second Attachment (Z = 7.500, P = 0.141), Anxiety (t = − 0.405, df = 8, P = 0.348) and Acceptance (t = − 0.236, df = 8, P = 0.410) scores indicating that their scores at the group level did not indicate a significant change between their first and second tests. Pearson correlation revealed that pigs’ Attachment (r = 0.612, P = 0.080) and Acceptance (r = 0.600, P = 0.087) scores, but not Anxiety scores (r = − 0.015, P = 0.970), showed a similar tendency in time (Fig. 2D).

Discussion

In this study, by testing young individuals of two domesticated social species, we demonstrated differences between the nature of companion pig–owner and companion dog–owner relationships. Specifically, we found that in the Strange Situation Test (SST) (1) dogs but not pigs showed significantly different behaviours towards the owner and a stranger along three major attachment-specific variables, (2) dogs’ Attachment (towards the owner) score was higher than that of pigs, (3) pigs’ Acceptance (of the stranger) score was higher than that of dogs. Furthermore, (4) Attachment scores positively correlated with Anxiety scores in both species and (5) pigs’ Attachment and Acceptance scores showed a similar tendency in time. Our results demonstrate that, contrary to dogs, pigs typically do not show those behaviour patterns that characterize human infant-mother attachment in the SST.

This research contributes to the understanding of how interspecific attachment towards humans may evolve. The fact that dogs but not pigs showed a specific differentiation between the owner and the stranger indicate that only dogs’ relevant behaviours fulfilled the criteria of attachment. Considering that pigs and dogs are both domestic and social species, and that the tested individuals were kept similarly as companions, we can reject the hypothesis that domestication alone, or in interaction with the human family environment is enough to trigger companion animal–human attachment. The fact that pigs’ attachment scores did not increase between their first and second tests shows that even a longer period of time was not enough for the development of an attachment bond in pigs. Even though companion pigs and dogs occupy a very similar human social niche nowadays, only dogs have been under selective pressure to fit into this niche. Indeed, while for farm pigs the main selection criteria were excessive breeding and optimising meat stock14 and miniature pigs were later selected for medical research15, dogs were specifically bred for direct social interactions with humans9. These results coincide with the result of those cat studies revealing no owner-oriented attachment in cats12,13. That is, unlike dogs' artificial selection, the natural selection process62 and the different living conditions of domestic cats, might have not led to the degree of dependence on their owner necessary to develop a bond that fulfils all operational criteria of attachment13. In sum, our results suggest that dogs’ artificial selection that resulted in increased dependence on humans, facilitated the development of dogs’ human-analogue attachment towards their owner.

Differences between dogs’ and pigs’ owner-related behaviour in the SST may also stem from the distinct characteristics of their pre-domesticated ancestors. That is, the ways dogs and pigs form relationships with humans may partly be rooted in the intraspecific social world of their ancestors and can thus be linked to that of their closest living relatives, wolves and wild boars. Wolves possess a complex social network in which their fitness and survival are greatly reliant on cooperation with their mates for activities such as breeding and hunting. They reside in groups consisting of closely related family members of both sexes in which individuals depend on each other and are attached to the pack63. Regarding their relationship with humans, it is important to note that even intensely socialised wolves do not form attachment to their caregiver8,64. They, however, do exhibit signs of stress when separated from their handler and seek contact upon reunion when they are on a leash65. Wolves’ special social world and ability to express behaviours that are also important in the attachment system might serve as a baseline that, in interaction with domestication, allows for the development of human–dog attachment bonds.

Wild boars, similarly to wolves, are renowned for their gregarious nature and intricate social structures66. They even exhibit a division of labour among individuals, particularly in the context of cooperative breeding. But, in contrast to wolf packs, wild boars’ group organisation is the classic form of fission–fusion societies in which females form matriarchal units with their offspring and the boars only adhere to the group during the mating season67. In addition, pigs’ foraging strategy is centred around locating food patches68 during which individuals rely more on themselves. Due to these between-species differences, wild boars may not exhibit the same high degree of dependence on their mates as wolves. Thus, the ancestors of wild boars may have possessed less potential than the ancestors of wolves to, following their domestication, become socially attached partners of humans, which could also contribute to the different owner-directed behaviours of pigs and dogs in the SST context.

As early experiences with humans influence adult domestic animals’ (including dogs’ and pigs’) human-oriented behaviours69,70, one may argue that the divergence between pigs’ and dogs’ behaviour in the SST could be caused by the different lengths of their socialisation-sensitive periods. Dogs’ socialisation-sensitive period to humans seems to be somewhat longer (between 3 and 13 weeks of age)71,72 than that of pigs (between 2 and 10 weeks of age)73. Our subjects were adopted by their human family no earlier than 8–10 weeks of age, which may mark the end of the socialisation-sensitive period for pigs and the middle of this period for dogs. So, one alternative explanation for pigs not forming an attachment bond could be the insufficient amount of interaction with their owner during the socialisation-sensitive period. However, before adoption all subjects had extended experiences with humans, and early experience with the owner specifically is not required for the formation of attachment in dogs. Indeed, dogs acquired later than 3 months of age can also develop an attachment bond2. Even pet dogs living with a second caregiver26, adult shelter dogs74, and assistant dogs raised by a puppy walker prior to being placed with a disabled person51,61 can form attachment with their new owners. Therefore, differences in pigs’ and dogs’ human-oriented attachment are unlikely to be caused by different experiences with their owners in the sensitive period.

Even though our group-level results suggest that forming attachment to humans is not a general feature of companion pigs, we do not claim that its development is impossible on the individual level. In fact, there were a few pigs that got high attachment scores (i.e. similar to those of most dogs), at least in their first test. These pigs exhibited most of the behaviours specific to attachment measurable in the SST context. Individual variability in the attachment behaviour of both dogs75 and humans27,76 is most commonly attributed to differences in the individuals’ previous experiences with the attachment figure. While the environment of pig participants was controlled, and their owners handled them similarly, there is a possibility that some owners may have provided more security to the animals in ambiguous situations, potentially contributing to higher attachment scores. However, if this was the case, it should have resulted in a further increase in attachment scores for the second SST, which was not observed for either pig who got outstandingly high attachment scores in the first SST. Although some genetic variability could have played a role in the attachment behaviour of the pigs, the effect of differences in the selection forces that shaped the development of miniature pig variants involved in this study is unlikely. Each line of miniature pigs was selected for medical purposes15. This is further supported by indirect evidence showing that attachment behaviour of dogs is similar across breed types52. As, based on our sample size, we cannot draw firm conclusions on the causes of individual differences in pigs’ behaviour in the SST, thus future studies are required to reveal how developmental and inherited factors might form the bond between companion pigs and their owners. Our study raises the possibility that certain pigs may have the potential to show similar attachment behaviours towards their owners in the SST context as most dogs do.

The relative stability of individual pigs’ owner- and stranger-related behaviours, and that none of the group level factor scores increased or decreased across the two tests support the stability and repeatability of the SST in pigs. These findings also suggest that pigs’ willingness to interact with unfamiliar people and social relationship with the owner are relatively stable over time. While it seems that companion pigs typically do not show attachment to humans, some kind of social bond to their owners is indicated by the positive correlation between Anxiety and Attachment scores: higher levels of situational stress in pigs, just as in dogs, led to increased proximity-seeking behaviours. This may suggest that the owner can provide a certain level of security to the animals, as it was also observed in horses77. Proximity-seeking behaviour in stressful situations is one feature of but does not imply attachment; it has also been observed in other social bond types, especially in young individuals78. Pigs’ willingness to interact with unfamiliar humans as well is shown by their higher acceptance scores as compared to those of dogs. The relative temporal stability of pigs’ acceptance score suggests that pigs’ willingness to interact with unfamiliar humans was unlikely to be related to individual experiences; instead, it was probably due to some species-specific traits, such as neophilia79. That is, the high amount of time pigs spent in the vicinity of the stranger was likely part of their exploratory behaviour42.

Notwithstanding that companion pig-dog comparative experiments hold great potential for breakthroughs in understanding the origins and development of interspecific social relationships and communicative skills, their limitations must also be taken into account. (1) One may argue that the same behaviours may reflect different inner states in pigs and dogs which makes their direct comparison questionable. We, however, took special care to fit behaviours to emotional displays accurately and provided functionally matching behaviour descriptions for pigs and dogs (for details, see the “Methods” section). Importantly, as attachment is characterised by highly similar behavioural components across species (e.g.,80), this functional matching of behaviours was the most straightforward when scoring Attachment. (2) As the number of companion pigs is much lower than that of companion dogs, comparative studies present a trade-off between control over environmental similarity and sample size. In the current study, optimising for comparability, we opted for using companion pigs participating in our long-term research project, verifiably kept under conditions highly similar to that of dogs—and this resulted in a relatively low sample size. (3) Differentiating between passive anxious and passive relaxed behaviours is challenging in both pigs and dogs, potentially leading to slight inaccuracies in the Anxiety scores. To mitigate this, no Anxiety scores were given to animals in passive but comfortable positions (e.g., lying on the floor). (4) To equalise the Anxiety scores of pigs and dogs to an appropriate, moderate level, we introduced a calming box/blanket for pigs, but not for dogs, the presence or scent of which might have impacted their Attachment scores and Acceptance-related behaviours differently, even without direct interaction with this object. However, the facts that pigs’ and dogs’ Anxiety scores did not differ significantly, that all five pigs who received scores in the HideO measure consistently stayed (hid) under/behind O's chair and none of them did so at the calming box/blanket for more than half of the time of the sit phases support that this design difference had no significant direct effect on between-species differences of Attachment or Acceptance scores.

In sum, our results show that despite the many similarities of companion pigs and dogs—such as living conditions, similar role in human families, intense exposure to human social stimuli, and belonging to domesticated, vocal, social and group-living species—the nature of companion pig-owner and dog-owner relationships differ significantly. The special human analogue attachment relationship of dogs with their owners does not seem to appear in companion pigs as a general feature. We conclude that domestication and intense exposure to human social stimuli during development are not sufficient to trigger attachment towards humans in companion animals. This suggests that, along with certain species predispositions, the specific selection that had led to increased dependence on humans was an essential component for the evolutionary emergence of companion animal-human attachment.