Response to novel objects and foraging tasks by common marmoset {Callithrix jacchus) female pairs

Abstract Many studies have shown that environmental enrichment can significantly improve the psychological well-being of captive primates, increasing the occurrence of explorative behavior and thus reducing boredom. The response of primates to enrichment devices may be affected by many factors such as species, sex, age, personality and social context. Environmental enrichment is particularly important for social primates living in unnatural social groupings (i.e. same-sex pairs or singly housed animals), who have very few, or no, benefits from the presence of social companions in addition to all the problems related to captivity (e.g. increased inactivity). This study analyses the effects of enrichment devices (i.e. novel objects and foraging tasks) on the behavior of common marmoset (Callithrix jacchus) female pairs, a species that usually lives in family groups. It aims to determine which aspects of an enrichment device are more likely to elicit explorative behaviors, and how aggressive and stress-related behaviors are affected by its presence. Overall, the marmosets explored foraging tasks significantly longer than novel objects. The type of object, which varied in size, shape and aural responsiveness (i.e. they made a noise when the monkey touched them), did not affect the response of the monkeys, but they explored objects that were placed higher in the enclosure more than those placed lower down.Younger monkeys were more attracted to the enrichment devices than the older ones. Finally, stress-related behavior (i.e. scratching) significantly decreased when the monkeys were presented with the objects; aggressive behavior as unaffected. This study supports the importance of environmental enrichment for captive primates and shows that in marmosets its effectiveness strongly depends upon the height of the device in the enclosure and the presence of hidden food. The findings can be explained ifone considers the foraging 3 behavior of wild common marmosets. Broader applications for the research findings are suggested in relation to enrichment. Key words: common marmoset, female pairs, environmental enrichment, animal welfare. Boredom, due to the absence of new and/or adequate stimuli, is a major problem for laboratory and zoo primates 2,3 . Enrichment devices, such as novel objects and foraging tasks, may improve the psychological well-being of animals as these devices provide new and/or additional stimuli to the monkeys that may elicit exploration, manipulation, and attempts to gain food items from them Many studies on environmental enrichment have focused their attention on common marmosets (Callithrix jacchus 7 ), which are commonly kept in laboratories and zoos. Laboratories often require housing common marmosets in same-sex pairs for research purposes or practical reasons, but female pairs, as well as male pairs, are an unnatural social grouping for this species. However, for enrichment devices to be effective in improving the well-being of captive animals (by increasing explorative behavior and reducing abnormal behaviours), their presence should decrease, or at least not adversely affect, the rates of aggression and stress-related behavior. 5 Despite the number of studies on environmental enrichment, little is understood about which object properties are most effective in improving common marmoset wellbeing. In addition, object properties may interplay with other factors known to affect responses such as sex, social context, personality, and age Methods Study animals and housing conditions The study animals were 32 common marmoset female pairs, housed at the were fed once a day (approximately at 13.00 hrs) and water was available ad libitum. 7 Apparatus Three different novel objects were used in this study, chosen for their different characteristics (e.g. responsiveness) and not for their "naturalness". The first enrichment device (novel object A) consisted of a cup (approximately 12 x 9 x 12 cm) filled with 10 small plastic test-tubes. The second one (B) consisted of a plastic bottle (approximately 17 x 9 x 9 cm) with a large hole on its side, filled with ten pieces of cloth. The third one (C) consisted of four film cases (approximately 9 x 6 x 9 cm) joined together by means of plastic strings, each containing a marble. Objects A and C were aurally responsive (i.e. they made a noise when the monkeys touched them). Object B was silent. The same novel objects, this time also containing ten raisins hidden in them, were used as foraging tasks in Experiment 1 (see below). The monkeys usually ate raisins twice a week as part of their diet. Procedure There were two experiments. Experiment 1 was designed to test whether the kind of enrichment device (i.e. novel object or foraging task) and relative age of the monkeys influenced the response of the monkeys towards the objects and, whether rates of stress-related and aggressive behaviors were affected by environmental enrichment. Experiment 2 examined the effect of object location on marmoset behavior. For Experiment 1, data were collected by BM on 20 female pairs, using a between subjects design. Novel objects were presented to ten female pairs, whilst foraging tasks were presented to the remaining ten (see Each object was presented to each pair of monkeys once, on three observation days with one-day intervalsPresentation order was counterbalanced among the pairs. The mean age of the pairs presented with novel objects did not differ to those presented with foraging tasks (25.8 ± 2.2 and 23.1 ± 2.1 months ± SE respectively, F (1,18) = 0.79, NS). The mean age of the older monkeys in each pair was 31.3 ± 2.2 months ± SE, and 17.6 ± 1.0 months ± SE for the younger monkeys. All the data were collected from a hide with a one-way mirror. Observation sessions were scheduled after cleaning and before feeding. Data were collected using focal pair sampling 30 on check-sheets for 20 minutes. The objects were removed at the end of the observation sessions. An additional 10-minutes observation session immediately preceded the presentation of each object in order to have baseline rates of allo-grooming, stress-related and aggressive behaviors. Latency to explore (defined as touching, manipulating, or playing with the novel object/foraging task), time spent exploring, and latency to eat the first food item contained in the foraging tasks were used as measures of the responsiveness of the monkeys towards the enrichment devices. Aggressive behavior (such as frown, tufts/ears flick or forward, arch bristle locomotion, tail raised present, cackle, cuff, and bite) and scratching were also recorded. Recent studies have shown that scratching is a reliable measure of stress in common marmosets 31,32 as it is in Old World monkeys 9 Experiment 2 was designed to assess whether responsiveness to the enrichment devices was due to the object properties (i.e. size, shape or aural responsiveness) or to its location in the enclosure. The same three novel objects used in the first part of the study were presented to 12 different female pairs (see NS). Data analysis The mean scores per hour of each pair were used in the data analysis, except when a comparison was made between the behavior of older and younger monkeys within each pair and when scratching was analyzed at the individual level. Data were analyzed using a series of mixed-design ANOVAs and repeated-measures ANOVAs. The between subjects factor was enrichment type (novel object or foraging task), and the within subjects factors were object (A, B or C), height (high, middle, low), time (before and during object presentation) and relative age (older or younger in pair). 10 Tukey post-hoc tests were run when a significant result was found 37 . Only significant interactions are presented. Presentation order was considered in the data analysis to assess whether habituation towards the objects was a factor that affected the response of the monkeys, but as no significant result was found these data are not presented for the sake of brevity. A Pearson correlation was run to relate the baseline level of scratching (i.e. frequency before the presentation of the objects) and its variation (i.e. baseline level minus frequency of scratching during the presentation of enrichment devices). All the tests were two-tailed and significance was set at p < 0.05. Results A 2 x 3 way ANOVA with enrichment type and object as factors showed that the latency to explore the foraging tasks was significantly shorter (F(1,18) = 16.73, P < 0.01) and time spent exploring them significantly longer than the novel objects F(1,18) = 5.16, p < 0.05). The response of the monkeys to the three enrichment devices varied significantly in relation to kind of object (latency to explore: F(2,36) = 19.96, P < 0.01, time spent exploring: F(2,36) = 21.60, P < 0.01; see These results show that the monkeys interacted significantly more with the foraging tasks than with the novel objects. They also suggest that other factors such as object properties or height in the enclosure affect the responses towards the enrichment devices, as the monkeys responded to them differentially. However, the experimental design did not allow assessment of the relative importance of these factors as each 11 object was presented in the same position. Experiment 2 was thus designed to determine which factor (i.e. object kind or its height in the enclosure) was primarily responsible for differences in marmoset responsiveness. To this end, two series of repeated-measures ANOVAs were run for Experiment 2. In the first one, data were divided by object Moreover, the monkeys spent more time exploring the objects when they were placed in the middle or high in the enclosure than when placed low (P < 0.01). There was no significant difference in the latency to explore and time spent exploring between objects placed in the middle and high in the cage. The effect of relative age To test whether the response towards the enrichment devices varied between older and younger members of the pairs a series of repeated-measures ANOVAs were run. Data from Experiment 1 for the three novel objects/foraging tasks were pooled together. Latency to explore did not significantly differ between older and younger monkeys (older monkeys: 371.4 ± 65.2 mean seconds ± SE, younger monkeys: 368.2 ± 68.9 mean seconds ± SE; F(1,19) = 0.00, NS). However, younger monkeys explored the 12 objects for longer than the older ones (older monkeys: 0.12 ± 0.02 mean ± SE, younger monkeys: 0.19 ± 0.02 mean ± SE; F (1,19) = 14.44, P < 0.01). Finally, no significant difference was found for latency to eat between older and younger monkeys within each pair (mean older monkeys: 474.11 ± 94.03 seconds ± SE; mean younger monkeys: 418.87 ± 103.01 seconds ± SE; F (1,9) = 0. 48, NS). The effect of environmental enrichment on allo-grooming, aggressive and stress-related behaviors Allo-grooming was never performed by female pairs whilst they were presented with the enrichment devices. However, this behavior was also rarely observed when the objects were absent (0.47 ± 0.19 mean percentage of sample bouts ± SE). As the marmosets spent a considerable proportion of their time exploring the enrichment devices, one might expect them to fight in order to monopolize the objects, this effect being stronger in female pairs presented with foraging tasks. To this end, the frequency of aggression and scratching before and during the presentation of the enrichment devices was analyzed. Scratching is a reliable measure of stress in common marmosets (see Methods section. Two 2 x 2 mixed-design ANOVAs were run using enrichment type and time as factors. Data were collapsed across the three novel objects/foraging tasks. No significant difference was found in rates of aggressive behaviors before and during the presentation of enrichment devices (F(1,18) = 4.53, NS, see No significant difference was observed between female pairs presented with foraging 13 tasks and those presented with novel objects (F(1,18) = 1.26, NS), but a significant interaction was found (F(1,18) = 14.64, P < 0.01). The decrease in scratching was greater for foraging tasks, than for novel objects. Finally, a significant positive correlation was found between the baseline level of scratching and the reduction in the occurrence of this behavior during the presentation of the enrichment devices (r s = 0.87, N = 40, P < 0.001). Discussion All the monkeys explored the enrichment devices but the foraging tasks were explored earlier and for longer than the novel objects were. The shorter latency to explore the foraging tasks suggests that the monkeys could smell the food hidden in the objects as soon as they approached them. Clearly, the presence of desired food in the foraging tasks accounts for the significant difference observed between foraging tasks and novel objects in time spent exploring, as this was the only factor that differed between them. Other studies have also shown that foraging tasks elicit the strongest responses in marmosets and tamarins This situation is more similar to the natural foraging experiences of wild marmosets and can be easily achieved even with artificial devices. Although the three objects used in this study differed from one another in size, shapeand aural responsiveness, the monkeys did not show any significant preference for any one of them. However, they explored the objects placed higher in the enclosure more than those on the ground indicating that this factor greatly affects its attractiveness in common marmosets, as already noted by Millar 16 . This result is not surprising as, in the wild, common marmosets occupy the lower strata of the canopy and rarely goto the Environmental enrichment is only effective when it not only decreases boredom but it also does not adversely affect rates of aggressive and stress-related behaviors. In this study, rates of aggressive behaviors were unaffected by the presence of the enrichment devices and scratching significantly decreased in their presence. Scratching and exploration are not mutually exclusive categories.The reduction in the occurrence of scratching was significantly greater in female pairs presented with foraging tasks than with novel objects, but this may be due to the higher baseline level of scratching in females pairs presented with foraging tasks ( Overall, these results support the importance of environmental enrichment for the psychological well-being of captive primates 3 . In particular, they are important as housing common marmoset females in same-sex pairs is an unnatural social grouping for this species, and the positive benefits of a social companion appear to be limited. Aggression among captive female marmosets is frequent (Introduction) and moreover female pairsexchange affiliative behaviors (i.e. allo-grooming) at extremely low rates. Therefore, enrichment devices may be extremely important for common marmoset female pairs (and probably for other captive primates living in similar unnatural social groupings), even if they are presented for relatively brief periods of time, as they reduce 16 boredom through increases in exploratory behaviour, decrease the occurrence of stressrelated behavior, and do not affect aggression within the pair 1