The aim of this study was to investigate how intrinsic and extrinsic factors affect curiosity, i.e., neotic responses and exploratory tendencies in wild orangutans. Our results showed that age, the presence of association partners that approached the apparatus, and food availability significantly affected levels of neophobia, neophilia, and exploration shown towards a novel experimental apparatus.
We found that immatures spent significantly more time visually exploring the experimental apparatus than adults (Fig. 1A, Table 1) and showed higher levels of neophilia (i.e., significantly shorter approach latencies and a trend for approaches to closer distances) than adults. This is in line with findings on exploratory tendency and neophilia from a variety of species tested in captivity including primates, bats, dogs, hyaenas, and several bird species [9, 16, 40, 67–74]; But see [22, 34, 75]). Furthermore, in wild African great apes, immatures show stronger looking reactions to novel camera trap devices [42] and are more likely to explore novel food items than other age classes [43]. On the proximate immediate level, higher exploratory tendencies and neophilia in immatures can be explained by the Spare-time Hypothesis, which states that youngsters have more free time available because they experience reduced environmental stress (such as the need to find food or to be vigilant, which are usually taken care of by their caregivers and/or other group members) and social distractions (such as the ones resulting from mating and reproduction) [22, 25]. However, in our study, 5 of the 8 immature focal individuals were independently ranging juveniles. These independent juveniles need to sustain their growing bodies, while they range largely on their own or in small playful peer groups, and thus likely experience increased environmental stress and social distractions. From an evolutionary perspective, it stands to reason that immatures have an innate disposition to be exploratory and neophilic because they need to learn about their environment to develop their skill and knowledge repertoires [Needing to Learn Hypothesis: 76]. For young individuals, the whole world is novel, and thus having temporary mechanisms in place which ensure that they can learn about the world is certainly adaptive [77]. Higher exploratory and neophilic tendencies in immatures enable learning about resources and thus to sustain themselves are also in line with the Necessity Hypothesis, which sees ecological needs as the biggest drivers of innovation (see below) [1, 4, 73, 78].
Interestingly and against our initial prediction, our findings showed that wild immature orangutans have a stronger neophobic reaction to the experimental apparatus than adults, in that they were more likely to show signs of agitation during the experimental trials. If high levels of exploration and neophilia in immatures ensure that learning opportunities are realized, high levels of neophobia may serve to protect them from potential dangers while doing so [4, 26, 34]. A certain level of agitation/excitement and alertness when exploring novel stimuli may ensure that risks of predation and injury are minimized, while the heightened/excitement-induced awareness may also improve memory retention and ultimately increase connection formation and therefore learning [reviewed by 79]. According to the Dangerous Niche Hypothesis, species and individuals which are exposed to higher risks should show higher levels of neophobia [4]. The combination of a small body size and semi-solitary lifestyle indeed likely puts immature orangutans at increased ecological risk. However, from our results, it remains unclear if immature orangutans have an intrinsic tendency to be more neophobic than adults or if their neophobic reactions were triggered by them confronting the stimulus more closely (see above). High levels of neophobia in combination with high levels of neophilia and exploration have been suggested to be a great ape characteristic [26] and have also been found in several bird species, including corvids and psittacines which are among the most innovative bird taxa [4, 19, 80].
In terms of the effects of social factors, we found that visual exploration, and to some extent neophilia (measured in approach distances), increased when at least one association partner was present that approached the experimental apparatus (Figs. 2A and C, Table 1). Consistent with these results, wild orangutans’ natural exploration behavior is positively affected by associations on the developmental and immediate proximate level [47, 48]. Increased levels of neophilia through social effects are in line with a large number of findings across a variety of taxa [reviewed by 26]. However, somewhat in contrast to our findings, African great apes show shorter visual exploration of novel stimuli with increasing number of current association partners [42]. Interestingly, this study of African apes did not take the behavior of the association partners into account. Therefore, the seemingly contrasting results may imply that response-specific facilitation leads to increased exploration of a novel stimuli but not general social facilitation. The Social Information Hypothesis states that individuals confronted with novel stimuli should rely on social cues to assess if the stimuli is worth it and safe to explore [26, 41]. Forss et al. [26] explain the innovation paradox, i.e., that large innovation repertoires are often found in slowly developing species that show a combination of high levels of neophobia and high exploratory tendency: high levels of intrinsic neophobia can be overcome by social information obtained from experts. Our results suggest that rather than affecting neophobia per se, response facilitation leads to an increased likelihood that individuals investigate novel stimuli and increases the intensity with which they explore the stimuli.
As to environmental effects, we found that high food availability (and thus, likely, high energy levels) correlates with increased visual exploration of the experimental apparatus. However, we also found a trend for lower levels of neophilia (shown in longer latencies to approach the experimental apparatus) when food availability was high. In the innovation literature, there is an ongoing debate about whether necessity or opportunity is the mother of invention, i.e., whether individuals are more prone to innovate when they experience the ecological pressure to do so (e.g., during food shortages or periods of increased energetic stress), or when they encounter suitable ecological conditions and stimuli (e.g., the resources and materials needed for innovations) and/or have increased amounts of energy and time available [44, 78, 81, 82]. In line with the Opportunity Hypothesis, our results suggest that high energy levels lead to an increased investment in gaining information about a novel stimulus. However, the trend towards a negative effect of food availability on neophilia may mean that it is during low energy periods when the stimulus is indeed actively investigated. If an individual’s energetic state affects exploration and neophilia in different directions, novel stimuli are most likely turned into innovation at a certain optimal level of prevailing ecological pressure, rather than at its extremes. Notably, however, for the Suaq Balimbing population, habitat food availability is generally higher than for most other orangutan populations. The experienced food availability over the course of this study ranged from 5.8 to 14.0. For most orangutan populations, the lower part of this range is around or above the yearly maximum. Therefore, with our experiment, we are likely unable to properly assess the effects of low food availability on wild orangutans’ curiosity.
Our findings may have important implications for experimental behavioral testing of animals, including cognitive tests where it can be difficult to differentiate between low level performance and a lack of motivation to interact with an experimental apparatus. This is particularly true in the wild, where long habituation periods are often not possible. When testing individuals, the first step is to get them to participate in the experiment. This includes overcoming their fear of the testing procedure and ensuring their motivation to interact with it [26]. Our results imply that individuals’ readiness to participate in behavioral experiments is likely affected by their age, as well as social and environmental factors. These factors should thus all be taken into account when conducting behavioral experiments in the wild.