Written by: Kaleigh Rhoads


 Pan troglodytes, commonly known as chimpanzees, are often kept in captivity in zoos, research facilities, or private owners. As highly social and intelligent animals, chimpanzees in captivity require Enrichment for comfort and mental stimulation. My goal is to determine what types of Enrichment are most preferred by the chimpanzees at the Oakland Zoo and identify any patterns associated with preference.

I hypothesize that the preferred type of Enrichment will differ between males and females. Also, in line with the Yerkes study, I predict that the females will spend more time moving enrichment objects such as blankets and plush toys than the males but that females’ preference will be less precise. In this study, I will be performing ten minute focal follows of each chimpanzee and record the time spent in contact with enrichment objects.

I have grouped the enrichment objects into three categories: manipulatable items, moving objects, and natural objects (objects that mimic natural behaviors). After many observations, I’ve concluded a clear sexual division of preference for enrichment type. Manipulatable items are the preferred enrichment type of males, while moving objects are the select enrichment types. 


 Sex differences in toy preference have long been observed in human children from an early age and even appear to further sex differences in cognitive and social development (Alexander, 2003). More than just for a child’s amusement, toys can also provide opportunities for object manipulation and exploration that enhances sexually dimorphic spatial abilities (Alexander, 2003).

It is in this way that sexually dimorphic toy preferences are early foundations of adult gender roles. Alexander (2003) suggests that these preferences are based not only on social learning but also indicate a biological preparedness for a “masculine” or “feminine” gender role. Alexander’s study of sex-typed toy preferences and visual development submits that these preferences are based on an innate bias for processing object movement, color, and form and adds to this argument that androgenized girls show more masculine visual development and perceptions (Alexander, 2003).

 The percent of contact time with female-typical toys was more significant in female vervet monkeys than males (Alexander and Hines, 2002). 

 In the last several decades, the view of human behavior as a product of evolution has gained ground and respectability, thus making human behavior subject to the same explanatory framework as animal behavior (De Waal, 2002). As humans’ closest evolutionary relatives, sharing at least 98.5% of our DNA, chimpanzees have become a popular research subject and model for human behavior (Gibbons, 1998).

 This research aims to investigate the difference in preference for enrichment objects among the chimpanzee population at the Oakland Zoo. I hypothesize that male and female chimpanzees will differ in their preferred enrichment types. 

 Study Species

 Pan troglodytes or chimpanzees are a sexually dimorphic ape with males weighing 88.2 to 132 lbs and females, on average, weighing 70.5 to 104 lbs (Rowe, 1996). Males and females have an average length of 2.68 ft from nose to rump while on all fours (Rowe, 1996). Chimpanzees are terrestrial and arboreal, and terrestrial locomotion includes quadrupedal knuckle-walking and occasional bipedalism (Doran, 1996).

Chimpanzees are diurnal and in the wild, build, and sleep in nests at night (Rowe, 1996). Wild chimpanzees span across twenty-two countries in equatorial Africa but about 77% of the total population lives in Gabon and Congo (Cowlishaw and Dunbar, 2000). With such a broad distribution, chimpanzees have adapted to live in various habitats, including dry savannas, evergreen rainforests, montane forests, swamp forests, and dry woodland-savanna mosaics (Goodall, 1986).

Chimpanzees are specialized frugivores, but their diet also includes seeds, leaves, bark, resin, honey, and animal matter, including eggs and small to medium sized mammals (Goodall, 1986). Chimpanzees live in fission-fusion societies ranging from 20-150 individuals, though they spend most of their time traveling in smaller groups, which may consist of one sex, both sexes, or a female with offspring (Van Lawick-Goodall, 1968).

 The chimpanzee population at the Oakland Zoo includes four females and three males. Eddie is a twenty-two-year-old male who came from a private owner in 2010. Bernie is an eighteen-year-old male who came from a private owner with Eddie in 2010.

 The enclosure at the Oakland Zoo includes an indoor section that is not visible to the public and an outdoor enclosure with a hardwire mesh fence as walls and roof. Attached to the outer side of the hardwire mesh fencing are several plastic feeders designed for foraging. The enclosure substrate is dirt with some grass and includes large rocks and a fake termite mound. There are several large structures in the enclosure with multi-leveled platforms.

Some of these platforms have hammocks hanging above them. There are also several sizeable horizontal metal beams near the ceiling. Several long ropes are hanging from the ceiling that nearly reaches the ground. One of these ropes has a large hard plastic ball hanging at its end. There is also a play structure made from large wooden logs and beams. In one corner, there is a small platform near the ceiling of the enclosure.

There is a large observation window at either end of the section, but tall foliage is planted along one long side of the square, making it difficult for zoo visitors to see through.


 Based on the methods set forth by Hasset et al. (2003), I chose to categorize enrichment objects available to the chimpanzees by specific object properties that made the categories comparable to stereotypical gender assignments. The first category, which I call “manipulatable,” includes plastic cars, plastic wagons, plastic barrels, large plastic spools, and plastic shovels.

The second category, which I call “soothing,” includes blankets, towels, clothing items, and plush toys. A third, sex-neutral category, which was not welcomed by Hasset et al. (2003) but was included by Alexander and Hines (2002), and is available to the chimpanzees in a zoo setting, I call “naturalistic.” The naturalistic category includes enrichment objects which attempt to mimic wild chimpanzee foraging behaviors such as hanging food dispensers, a fake termite mound, a KongTM, and PVC pipes with food inside. 

 Over nine weeks, I conducted multiple ten-minute focal follows on each of the seven chimpanzees at the Oakland Zoo. During a focal follow, I recorded whenever the focal chimpanzee interacts with an object of the three categories above. I also record the onset and offset time that this interaction occurs. As this is an observational study, and I cannot manipulate the chimpanzees or enclosure, I follow several rules for these focal follows.

The first rule is that if the current focal chimpanzee enters the indoor section and is no longer visible, I pause the stopwatch for up to one minute. If the focal chimpanzee re-enters the outdoor enclosure within one minute, I resume the focal follow. The third rule for observation is regarding the use of larger enrichment objects. Some larger items such as plastic cars and barrels are used as either Enrichment (manipulating, carrying, moving, or throwing objects) or as a part of the landscape (no further interaction beyond sitting on top of the item).

When a focal has no other interaction with an object besides sitting on top of it, I do not record the business. 

 Analysis of this data compares male to female chimpanzees in both frequencies of interaction with each enrichment category and the average duration of these interactions. This analysis will determine if there is a sexual division in the preferred enrichment type of the chimpanzee population at the Oakland Zoo. 

 Data Analysis

 I began my data analysis by arranging the individual interaction data first by focal, then by date. Once I had determined the duration of each focal-enrichment interaction in seconds, and the total time in minutes that I had spent observing each focal, I began to interpret the data by an individual chimp. First, I was interested in the frequency of interactions with each enrichment type.

To do this, I divided the total number of interactions observed with one enrichment type by the total number of minutes spent watching that particular chimp. Then I multiplied that number by 60 to find the rate of occurrences per hour for each chimp. I did this for each enrichment type and each chimp. I also looked at the rate of events per hour for all Enrichment to determine if one sex spent significantly more time with Enrichment in general.

Once I had this data for each chimp, I found the mean rate of occurrence per hour for all females for each enrichment type. I then did the same with males. The column graph below shows this visually.

 Rate of Occurrences per Hour

 Focal Name Type 1 Type 2 Type 3 Total

 F1 (Abbey) 0.00 1.50 4.50 6.00

 F2 (Amy) 0.77 4.60 0.77 6.13

 F3 (Andi) 1.44 4.32 1.20 6.96

 F4 (Caramia) 2.29 3.43 1.14 6.86

 M1 (Bernie) 3.43 1.71 0.86 6.00

 M2 (Eddie) 5.00 0.00 3.00 8.00

 M3 (Moses) 3.43 2.57 0.00 6.00

 Female Average 1.12 3.46 1.90 6.49

 Male Average 3.95 1.43 1.29 6.67

 I organized the female averages for the rate of occurrence per hour into a pie graph to visualize the interactions with each enrichment type about the total observed enrichment interactions by females. I then followed the same procedure for male businesses.

 Next, I was interested in the duration of time spent with each enrichment type. Again, I began by looking at each chimp individually. For each chimp, I separated my data tables into enrichment types. I recorded the minimum, maximum, and mean amount of time in seconds observed spent with that enrichment type. I then found the average minimum, maximum, and mean for males and females for each enrichment type.

The “stock: high-low-close” graphs below show this visually.

 Type 1 Duration (sec)

 Min Max Mean

 F1 (Abbey) 0.00 0.00 0.00

 F2 (Amy) 10.00 313.00 114.00

 F3 (Andi) 8.00 94.00 35.33

 F4 (Caramia) 6.00 91.00 40.50

 M1 (Bernie) 10.00 53.00 35.00

 M2 (Eddie) 15.00 210.00 67.80

 M3 (Moses) 3.00 39.00 23.25

 Female Average 6.00 124.50 47.46

 Male Average 9.33 100.67 42.02

 Type 2 Duration (sec)

 Min Max Mean

 F1 (Abbey) 600.00 600.00 600.00

 F2 (Amy) 6.00 600.00 205.61

 F3 (Andi) 2.00 600.00 130.89

 F4 (Caramia) 5.00 600.00 234.33

 M1 (Bernie) 4.00 512.00 258.00

 M2 (Eddie) 0.00 0.00 0.00

 M3 (Moses) 9.00 399.00 167.33

 Female Average 153.25 600.00 292.71

 Male Average 4.33 303.67 141.78

 Type 3 Duration (sec)

 Min Max Mean

 F1 (Abbey) 38.00 70.00 54.00

 F2 (Amy) 8.00 12.00 10.33

 F3 (Andi) 3.00 15.00 7.20

 F4 (Caramia) 39.00 60.00 49.50

 M1 (Bernie) 70.00 70.00 70.00

 M2 (Eddie) 2.00 17.00 10.33

 M3 (Moses) 0.00 0.00 0.00

 Female Average 22.00 39.25 30.26

 Male Average 24.00 29.00 26.78


 Regarding the frequency of use, the data shows that observed interactions with enrichment type 1 were over three and a half times more frequent in males than in females. The data also shows that males were more than two and a half times more likely to interact with enrichment type 1 than enrichment type 2. Male interactions with enrichment type 1 account for 59% of the observed businesses with all males’ enrichment types; this suggests that my prediction was correct that this would be the preferred enrichment type by males.

Interactions with enrichment type 2 were almost two and a half times more frequent in females than males, and females were over three times as likely to interact with enrichment type 2 as enrichment type 1. Female interactions with enrichment type 2 account for 53% of females’ total observed enrichment interactions; this suggests that the preferred enrichment type for females is enrichment type 2.

Though I predicted that females would show more variability in their enrichment preferences, I did not find this figure to be significantly different from males in my results. Males and females exhibited similar preferences for enrichment type 3, accounting for 29% of all observed female-enrichment interactions, and 19% of all experimental male-enrichment businesses.

Males and females did not differ significantly in their overall rate of occurrence for all enrichment interactions. 

 Regarding the average duration of chimpanzee-enrichment interactions, males and females showed a similar average duration spent with enrichment type 1. Though females spent an average of five seconds more than males during enrichment type 1 interactions, I do not find this figure significantly. The data regarding duration of enrichment type 2 interactions shows that females, on average, spent more than twice the amount of time as males did during these interactions.

The data shows no significant difference between the sexes with the duration of enrichment type 3 businesses.

 Overall, I’ve determined from this data that although males interacted with enrichment type 1 more frequently, the duration of these interactions differed far more significantly between individuals than between sexes. I’ve also determined that females interact with enrichment type 2 more often than males, but for a more extended period. Males and females did not show significant differences in either frequency or duration of interactions for enrichment type 3, suggesting that this is a genuinely sex-neutral category. 


 This study proved my hypothesis correct that male and female chimpanzees differ in their preferred enrichment type; this is consistent with Alexander and Hines’s (2002) acknowledgment of a sex-neutral enrichment type. 

 Alexander and Hines (2002) categorized a pot as a female-typical object while I found this to apply anthropomorphic meaning to an item that does not exist among vervet monkeys. I ordered similar objects (plastic buckets, cups, and shovels) as male-typical based on their hard, plastic, and manipulatable physical characteristics. Alexander and Hines (2002) also differed in that they categorized a plush animal as a sex-neutral object.

I instead thought of this as more female-typical and classified all plush animals as enrichment type 2 based on its characteristics of being comforting and soothing. On several occasions, I observed one female, Amy, nurturing and kissing plush animals, adding confidence to this decision to categorize all plush animals as female-typical. In this study, the third category of natural enrichment objects was unique.

Due to these differences in enrichment categories, I did not observe the increased variability in female preference, followed by Hasset et al. (2008) and Alexander and Hines (2002).

 During this study, I face several challenges in my observations, which can bias my results. Because I did not have physical or visible access to the indoor chimpanzee enclosure, I was forced to cut several focal follows short and not account for the Enrichment used while out of my sight. Also, because the male chimpanzees spent significantly less time in the outdoor enclosure than the females, I could dramatically obtain less observation time among males than females.

Though I accounted for this during data analysis, more observation time for males would be preferable. Among all enrichment categories, I found a variation of preference within both sexes. Though the means support a sexual division in taste, a larger sample size would be preferable to support a theory of inherent sexually dimorphic enrichment preferences. 

 Preferences differed between sexes and between individuals. For example: while one female, Amy, almost without exception, consistently interacted with blankets, clothes, and plush toys, her mother, Abbey, had only one occurrence of interaction with type 2 enrichment observed. Due to Enrichment’s different preferences among a single chimpanzee population, I submit that it is essential to include a variety of enrichment types to occupy chimpanzees in captivity.

Though not used by all chimps, enrichment type 1 provides mental stimulation, enrichment type 2 provides comfort, and enrichment type 3 allows natural behaviors to occur that mimic wild chimpanzee behavior. All of these are beneficial to maintaining a captive chimpanzee population.

 References Cited

 Alexander, Gerianne M. “An Evolutionary Perspective of Sex-Typed Toy Preferences: Pink, Blue, and the Brain.” Archives of Sexual Behavior. 32.1 (2003): 7-14. 

 Alexander, Gerianne M., and Melissa Hines. “Sex differences in response to children’s toys in nonhuman primates ( Cercopithecus aethiops sabaeus ).” Evolution and Human Behavior. 23. (2002): 467-479. Web. 

 Cowlishaw G, Dunbar R. 2000. Primate conservation biology. Chicago: Univ Chicago Pr. p 498.

 De Waal, Frans. Tree of Origin: What Primate Behavior Can Tell Us about Human Social Evolution. Cambridge: First Harvard University Press, 2002.

 Doran DM. 1996. Comparative positional behavior of the African apes. In: McGrew WC, Marchant LF, Nishida T, editors. Great ape societies. Cambridge (England): Cambridge Unive Pr. p 213-24.

 Gibbons, Ann. “Comparative Genetics: Which of Our Genes Make Us Human?” Science. 281.5382 (1998): 1432-1434.

 Goodall, J., 1986. The chimpanzees of Gombe. Cambridge (MS): Belknap Pr. p 673.

 Hasset, Janice M., Erin R. Seibert, and Kim Wallen. 2008 Sex differences in rhesus monkey toy preferences parallel those of children. Hormones and Behavior. 54.3: p 359-364. 

 Macdonald D, editor. 2001. The encyclopedia of mammals. Volume 2, Primates, and large herbivores. New York: Facts on File. P 930.

 Rowe, N., 1996. The pictorial guide to the living primates. East Hampton (NY): Pogonias Pr. p 263

 Van Lawick-Goodall, Jane (1968). “The Behaviour of Free-Living Chimpanzees in the Gombe Stream Reserve”. Animal Behaviour Monographs (Rutgers University) 1 (3): p 167