Sex differences in children's toy play are robust and similar across cultures []. They include girls tending to play more with dolls and boys more with wheeled toys and pretend weaponry. This pattern is explained by socialization by elders and peers, male rejection of opposite-sex behavior and innate sex differences in activity preferences that are facilitated by specific toys []. Evidence for biological factors is controversial but mounting. For instance, girls who have been exposed to high fetal androgen levels are known to make relatively masculine toy choices []. Also, when presented with sex-stereotyped human toys, captive female monkeys play more with typically feminine toys, whereas male monkeys play more with masculine toys []. In human and nonhuman primates, juvenile females demonstrate a greater interest in infants, and males in rough-and-tumble play. This sex difference in activity preferences parallels adult behavior and may contribute to differences in toy play []. Here, we present the first evidence of sex differences in use of play objects in a wild primate, in chimpanzees (Pan troglodytes). We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males.

Main Text

4 Whiten A.

Goodall J.

McGrew W.C.

Toshisada N.

Reynolds V.

Sugiyama Y.

Tutin C.E.G.

Wrangham R.W.

Boesch C. Chimpanzee cultures. 5 Humle, T. (in press). The tool use repertoire of Bossou chimpanzees. In The Chimpanzees of Bossou and Nimba, T. Matsuzawa, T. Humle and Y. Sugiyama, eds. (Tokyo: Springer-Verlag). During 14 years of observation of the Kanyawara chimpanzee community in Kibale National Park, Uganda, we found that chimpanzees used sticks in four main ways: as probes to investigate holes potentially containing water or honey; during aggression, either as props in displays or as weapons (throwing or hitting) in aggression towards conspecifics or other species; during solitary or social play; and in a behavior we term ‘stick-carrying’. Stick-carrying consisted of holding or cradling detached sticks (median length, 36 cm; median weight, 112 g; n = 6 recovered sticks). The juveniles carried pieces of bark, small logs or woody vine, with their hand or mouth, underarm or, most commonly, tucked between the abdomen and thigh (Supplemental information). Individuals carried sticks for periods of one minute to more than four hours during which they rested, walked, climbed, slept and fed as usual. From all-events recording in chimpanzee parties from 1993 to 2006 we found that stick-carrying represented 38.9% of total stick use observations (n = 301), and 10.0% of object use observations (n = 1170). Regular stick-carrying has no discernible function and has not been reported from other chimpanzee studies [], even where stick use has been carefully documented for many years [].

6 Boesch C.

Boesch H. Tool use and tool making in wild chimpanzees. Figure 1 Age and sex differences in the rate of stick-carrying in chimpanzees. Show full caption Females: circles, solid line. Males: triangles, dashed line. Age categories referred to in the main text are infants (0–4.9 years (yrs)), juveniles (5–7.9 yrs), adolescents (8–14.9 yrs), and adults (>15 yrs, male; after full sexual swelling, females). To control for age and satisfy small samples of individuals per age category, each individual was assigned to one of 11 age-classes. Assignation to age-class was determined by the individual's mean age between their start and end of observations, 1993-2006. Age-classes, together with sample sizes of females and males, respectively (total 37 females, 31 males), were: 0–1 yr (1,2), 1–2 yrs (5,4), 2–4 yrs (1,5), 4–6 yrs (3,2), 6–8 yrs (4,1), 8–10 yrs (1,2), 10–12 yrs (1,1), 12–14 yrs (1,1), 14–16 yrs (4,1), 16–20 yrs (2,1) and >20 yrs (14,11). Mean stick-carrying rates across individuals were higher for females than for males (Wilcoxon signed-rank T = 3; n = 10 age-classes, P = 0.017 (2-tailed)). No stick-carrying was observed for individuals in the 8–10 year age-class. Note that although the figure shows stick-carrying by individuals whose mid-point was in the adult range, no female carried a stick after becoming a mother. The frequency of stick-carrying peaked among juveniles and was higher in females than males ( Figure 1 ). This sex difference could not be explained by a general propensity for females to use objects more than males [], because several types of object use were in fact male-biased, including weapon use by adolescents and adults (medians per 1000 hours: male 0.09, n = 19; female 0.00, n = 30; Mann-Whitney U = 597, P = 0.002, 2-tailed) and use of leaves by adults for wiping their bodies, particularly for cleaning genitals after copulation (male 0.87, n = 14; female 0.00, n = 24; U = 344, P = 0.0002).

Probing, by contrast, showed a similar age-sex distribution to stick-carrying, with juvenile and adolescent females using probes more often than same-aged males (male 0.00, n = 13; female 0.48, n = 10; U = 108.5, P = 0.004; adults: male 0.00, n = 14; female 0.00, n = 24; U = 182.0, P = not significant (n.s.)). However individual differences in the frequency of probing and stick-carrying were not correlated (Spearman R = 0.20, n = 10 juvenile and adolescent females, P = n.s.) and there was no overlap in the diameters of carried sticks and probes: carried sticks (range 2–7 cm diameter, n = 6) were at least twice as thick as probes (0.3–1 cm, n = 14; Supplemental Information). There was thus no evidence of stick-carrying being motivationally or functionally related to probing.

2 Pellegrini A.D.

Smith P.K. The Nature of Play: Great Apes and Humans. 7 Wrangham R.W.

Peterson D. Demonic Males: Apes and the Origins of Human Violence. 8 Matsuzawa T. The death of an infant chimpanzee at Bossou, Guinea. We suggest instead that sex differences in stick-carrying are related to a greater female interest in infant care, with stick-carrying being a form of play-mothering (i.e. carrying sticks like mother chimpanzees carrying infants). Several lines of evidence support this hypothesis. First, in the few instances when we observed adult females carrying sticks (five females, six events), the behavior always occurred prior to the female's first birth. Thus, unlike probing and other object use, stick-carrying ceased with motherhood. Second, unlike other types of stick use, carried sticks were regularly taken into day-nests (on at least 25 occasions; six females, two males) where individuals rested and were sometimes seen to play casually with the stick in a manner that evoked maternal play. Third, the capacity for young chimpanzees to direct care towards objects has been reported in apes raised by humans [] and is indicated by two detailed reports from the wild of chimpanzees treating sticks like dolls. At Kanyawara, an 8-year-old male carried and played with a small log for four hours and also made a separate nest for it []; and an 8-year-old female at Bossou (Guinea) carried a log, including patting it like “slapping the back of an infant”, while her mother was carrying her sick infant sibling [].