To the authors’ present knowledge, our work is the first one presenting similarities and differences in juvenile family dogs’ and miniature family pigs’ behaviour during interspecific interactions with humans. As found in our first study, pigs show some similar patterns of spontaneous human-oriented behaviours as family dogs in an unrestricted neutral context without the presence of food (Control condition), as reflected by the time spent with orienting towards the human’s body in close proximity and also the frequency of establishing physical contact with the human. The experience of being fed by the human and the presence of food (Food condition) intensify these behaviours, as well as orienting to the human’s face in both species. However, the presence of food seems to affect pigs’ orientation to the human’s body to a greater extent than that of dogs. Furthermore, our results also point out that the experience of being fed by the human plays a major role in triggering the display of face-orientation only in pigs but not in dogs. Most dogs tended to orient voluntarily at the unfamiliar human partner’s face even when they had not been provided any food right before, while the majority of the pig subjects did not perform any face-orientation during the Control condition and they exhibited this behaviour to an overall lesser extent. In our second study, only dogs but not pigs showed evidence of spontaneously relying on a human distal dynamic-sustained pointing as directional signal for locating a hidden food reward in a two-way object choice test. As a group, neither of the two species was successful in following the momentary version of the pointing gesture, whereas all pigs (while only some of the dogs) showed a clear lateralized behaviour (i.e., overall side bias).
The found similarities in the two species’ behaviour towards the unfamiliar human partner in Study 1, such as approaching her voluntarily within close proximity, touching her body, etc. let us infer that the socialization background of our subjects in terms of making the presence of humans in general safe and comfortable can be regarded as comparable. This allows for ruling out that any emerging differences should be due to pigs being generally more fearful of the situations and/or the experimenter than dogs. The observation that both species were performing a considerable amount of explorative behaviour in the Control condition—besides the explicitly coded ones—gives further support to the above argument. All this is important, since multiple works with farm pigs pointed out the appearance of fearful or aggressive behaviours at the onset of the experimental procedures that made it necessary to include several occasions of familiarization procedures before the start of the data collection (e.g., Bensoussan et al. 2016; Nawroth et al. 2013, 2014). Therefore, working with family pigs instead of farm animals supports the comparability of the two species’ behaviour in the present experimental procedures.
Besides the similarities in young dogs’ and pigs’ spontaneously exhibited human-oriented behaviours, our findings imply an important difference between the two species in their readiness to orient at the human partner’s face. This behaviour (and more specifically the establishment of eye contact) is widely reported in dogs; it typically appears spontaneously without special training and is regarded as an interspecific communication skill that domestication seems to have uniquely strengthened in them (e.g., Hare et al. 2002; Miklósi et al. 2003). An earlier study comparing dogs’ human-oriented gazing with that of another domestic species found that dogs gazed earlier and for longer duration at the human (not specifying body parts) than cats did in a problem solving situation in a feeding context (Miklósi et al. 2005). The authors explained the found differences in terms of the two species’ different levels of independence from humans, which might as well be rooted in the differential selection pressures during the course of domestication. Such evolutionary differences hold true for dogs and pigs as well. Pigs were not selected for working in close cooperation with humans that would have made it important for them to seek information about human attentional states, which might explain—on an evolutionary level—to some extent the species’ overall lower tendency in displaying specifically the face-oriented behaviour. Along with these, it is also important to note here that all experimenter-oriented behaviours were scored from within a max. distance of 30 cm from the experimenter (because of practical reasons). Due to anatomical differences between the two species, however, it might be more difficult for a pig than for a dog to raise the head at the necessary angle to perform face-orientation for a longer period (while body-orientation can be performed even with a more neutral head position). Consequently, we cannot rule out the possible influence of this anatomical factor on the above outlined findings.
Interestingly, the above does not apply if we consider orienting towards the human’s body. Pigs and dogs exhibited Body-orientation for a similar duration in the Control condition, while pigs tended to orient more towards the human’s body than dogs in the Food condition. One possible explanation to the fact that the presence of food intensified pigs’ body-oriented behaviour to a greater extent than that of dogs might lie in the difference between the two species’ motivational states to receive the expected food from the human. The two species might also differ in their persistence to anticipate food that they had just experienced to get, which might as well relate to general motivational differences.
Looking at the functional perspective, the clear appearance of pigs’ face-orientation in the Food condition only, along with the increase in that of dogs and the increased body-orientation and body-touching of both species in the Food condition might indicate the communicative, attention-getting nature of these behaviours not only in dogs, but also in pigs. Furthermore, considering the apparent intensification of all the measured experimenter-oriented behaviours in the presence of food, we can assume an underlying role of simple associative learning mechanisms, i.e., learning through former daily routines about food coming from the human, which all subjects must have experienced. Nawroth et al. (2013) reported on the tendency of juvenile farm pigs for discriminating two humans based on their attentive states (i.e., head orientation) after some training (Nawroth et al. 2013), while adult family dogs were reported to have the ability of recognizing human attention without any training introduced specifically for such purpose (Gácsi et al. 2004). These can be related to our finding that young pigs display face orientation almost exclusively in the feeding context, which also suggests the necessity of previous learning processes for this behaviour to appear apparently in this species, whereas it seems to be displayed more unconditionally in young dogs.
In general, all pigs vocalized during Study 1, while only a few dogs did. The vast majority of pigs’ vocalizations during both short sessions can be best characterized by—without aiming here for precise classification based on acoustic parameters—the general “grunt” label (Tallet et al. 2013). While pigs are highly vocal species and have been reported to produce diverse call types across different situations (e.g., Tallet et al. 2013; Linhart et al. 2015; for review, see Marino and Colvin 2015), a “grunt” is not a situation-specific call type and it might be produced across a wide range of social and non-social contexts (Linhart et al. 2015). Although recent studies have identified qualitative differences in grunts across different arousal states (Linhart et al. 2015) or emotional valence (Briefer et al. 2019), qualitative analysis based on acoustic parameters was beyond the scope of the present study, and quantitative results do not provide evidence neither for the influence of the context, nor for the interspecific communicative nature of pigs’ calls in the present experimental setup.
Along with all the above, we found important differences in the two species’ readiness in spontaneously adjusting their behaviour according to the visual communicative signals of a human in a two-way choice task. Although pigs’ visual acuity is known to be poorer than that of humans and dogs (Zonderland et al. 2008), we know that pigs can be trained to follow even the distal human pointing gesture in a comparable setting to ours (Nawroth et al. 2014), which shows that they possess the sensory abilities as well as the cognitive capacities that are necessary to fulfil the task. This indicates that the main difference between dogs and pigs in this sense is probably not in their cognitive capacity of learning human communicative cues. Since humans have proven to act as naturally salient social stimuli for dogs (e.g., Gácsi et al. 2005), this might serve as a facilitating mechanism in learning about human behaviour even without specific training. In contrast, there are no data indicating that the same would also hold true for pigs, and the different selection pressures during the two species’ domestication do not imply that either. Along with this, we cannot rule out that further social experience with humans would enable pigs to perform successfully in the same task at an older age, although former comparisons of dogs’ and wolves’ performances revealed that in some cases, even extensive socialization with humans does not necessarily help overcome natural species differences (Miklósi and Soproni 2006).
Interestingly, the side bias that emerged in 100% of the pig subjects during the test trials disappeared in three individuals during the following control session, while for the rest of the subjects, it was consistent in time (i.e., they retained the bias to the same side as previously). There is growing literature on lateralization including both population-level responses to certain stimuli (e.g., Siniscalchi et al. 2013; Andics et al. 2016) and individual-level motor lateralization (Tomkins, Thomson, and McGreevy 2010). Lateralized behaviour is commonly reported for choice paradigms as well, not only in farm animals (e.g., Kaminski et al. 2005; Nawroth et al. 2014), but a proportion of dog subjects, in two-way choice experiments, are also often reported to develop a preference to one side (e.g., Gácsi et al. 2009b; Prato-Previde et al. 2008). This decision-making rule—other than following the human cue—can also be considered a cost-efficient strategy yielding 50% success rate, and we might suppose that it is followed when the task itself is too difficult for the subject. There is evidence from the literature that pigs are able to use their spatial memory flexibly; they can be trained to either return to a location, where they previously found food (“win-stay” task) or to use the memory of a previously discovered food site to forage elsewhere (“win-shift” task), and they seemed to be more successful in the latter task (Marino and Colvin 2015). However, our findings—in accordance with other reports (Nawroth et al. 2014)—point out that in a two-way choice task, young pigs’ spontaneous overall performance pattern is rather similar to a “win-stay” strategy. Moreover, their tendency to develop preference for one side is quite robust; seems to be mostly consistent in time and context. For pigs, foraging is a natural species-specific activity, during which they heavily rely on spatial cues (Marino and Colvin 2015). Thus, they might have a stronger general inclination to use spatial cues than dogs, which might as well account for the found differences between the two species with regard to their predispositions to develop lateralized behaviour.
Apart from showing bias to one side, most of the pigs (6/8) made successful choices to both sides throughout the course of Study 2 (Fig. 6). This means that 75% of the pig subjects found food reward on both sides, so the lack of experience about either of the two sides being rewarded could possibly explain the choice pattern of those two individuals that only chose the same side. Also, we found no evidence for the location of the first successful choice influencing the side to which the subjects eventually developed a preference. During the initial training trials, we deliberately avoided baiting the actual test locations to exclude the possibility of biasing the subjects’ choice in the first test trial to the last rewarded side in the training trial. Along with these we cannot rule out that introducing the baited locations already in the training trials might have led to a potential better overall test performance for pigs.
Dogs’ performance in the distal dynamic-sustained pointing trials is in line with other data available in the literature (Kaminski and Nitzschner 2013). As opposed to several previous findings, however, dogs as a group were not successful in DM trials, although individual-level analysis revealed that two subjects’ performance was above chance level in this condition, and other studies with young dogs also reported that, in spite of the success on group level, less than half of the individuals performed above chance (e.g., Gácsi, Kara, et al. 2009a). The exact experimental setup and procedure details were also found to affect performance in the pointing task (Pongrácz et al. 2013). Consequently, slight differences in our procedure compared to others’ (e.g., lack of pre-training to both sides, as, e.g., in Gácsi et al. 2009b or Virányi et al. 2008) or the setup, such as somewhat bigger distance between the two objects and/or the tip of the pointing finger and the container (as in, e.g., Virányi et al. 2008) could also cause the task to be more difficult for the dogs participating in our study.
Limitations
One particular advantage of this study, in line with some previous ones comparing, e.g., the interspecific social skills of dogs and cats (Miklósi et al. 2005) or wolves (e.g., Virányi et al. 2008) is that we aimed to ensure that subjects are raised in similar environments providing comparable social stimulation by humans. This reduces the chance that any differences that emerged are due to a general determinative difference in the two species’ experience with humans. Nevertheless, we have to keep in mind that we do not have exact information for either species about the extent to which the owners reinforced any of those behaviours during socialization that we specifically tested for in our two studies (e.g., establishment of eye-contact/face-to-face orientation or following any version of the pointing gesture). Almost all pig owners kept a dog as well as a pig, and a general request for them upon the pigs’ arrival was to treat the pig in a similar manner as they would treat a dog, as much as possible. In spite of this, we cannot rule out the possibility that the owners still behaved differently in general with pigs and dogs (i.e., having different overall attitudes, expectations, the quality of social bonding may be different, etc.), which in turn could affect daily learning and, therefore, have potential influence on subjects’ test performance.
The supervised socialization of the piglets, as well as the a priori selection of the owners to fit the strict enrolment criteria proved to be demanding and time consuming tasks. Due to this we had the opportunity to work with only a finite amount of subjects, which we need to take into account when evaluating our results. Furthermore, the peculiarity of the pig population, as well as the fact that most of the pig subjects belonged to the same Minnesota miniature breed (see Online resource 2 for details) also makes the generalization of the results in this sense limited. Considering the dog subjects, we tried to enrol a diversity of breeds in both studies (see Online resource 2 for details), since there is evidence that breed group could potentially affect performance in interspecific communicative tasks (e.g., Gácsi et al. 2009b; Passalacqua et al. 2011).