Genes, Brain and Behavior (2013) 12: 446–452 doi: 10.1111/gbb.12044

Genetic and environmental contributions to theexpression of handedness in chimpanzees(Pan troglodytes)

W. D. Hopkins†,‡,∗, M. J. Adams§ andA. Weiss¶,∗∗

†Division of Developmental and Cognitive Neuroscience,Yerkes National Primate Research Center, Atlanta, GA, USA,‡Neuroscience Institute and Language Research Center,Georgia State University, Atlanta, GA, USA, §Department ofAnimal and Plant Sciences, University of Sheffield, Sheffield,UK, ¶Scottish Primate Research Group, and ∗∗Department ofPsychology, School of Philosophy, Psychology and LanguageSciences, The University of Edinburgh, Edinburgh, UK*Corresponding author: W. D. Hopkins, Division of Develop-mental and Cognitive Neuroscience, Yerkes National PrimateResearch Center, Atlanta, GA 30322, USA.E-mail: whopkins4@gsu.edu

Most humans are right-handed and, like many behav-

ioral traits, there is good evidence that genetic factors

play a role in handedness. Many researchers have argued

that non-human animal limb or hand preferences are not

under genetic control but instead are determined by ran-

dom, non-genetic factors. We used quantitative genetic

analyses to estimate the genetic and environmental con-

tributions to three measures of chimpanzee handedness.

Results revealed significant population-level handedness

for two of the three measures – the tube task and man-

ual gestures. Furthermore, significant additive genetic

effects for the direction and strength of handedness

were found for all three measures, with some mod-

ulation due to early social rearing experiences. These

findings challenge historical and contemporary views of

the mechanisms underlying handedness in non-human

animals.

Keywords: Asymmetry, chimpanzees, genetics, handed-ness, heritability, language evolution, laterality, motor skill,praxic, tool use

Received 18 February 2013, revised 4 April 2013 and 21 April2013, accepted for publication 22 April 2013

One feature of human behavior is right-handedness. Thoughthere is some cultural variation (Perelle & Ehrman 1994;Porac & Coren 1981; Raymond & Pontier 2004), around90% of humans are right-handed and the archeologicalrecord dates right-handedness to at least 2 mya (Uomini2009), suggesting it has a long evolutionary history.Studies of families and twins suggest that handedness

has a genetic component (Carter-Saltzman 1980; Hicks &Kinsbourne 1976; Klar 1999; Medland et al. 2009; Sicotteet al. 1999; Warren et al. 2012). Overall, these studiesindicate that about 25% of the variability in handedness isattributable to shared genetic factors and that 75% of thevariability is due to shared environmental factors. Severalgenetic models have been proposed to explain the nearuniversal expression of human right-handedness (Annett2002; Corballis et al. 2012; Klar 1999; Laland et al. 1995;McManus & Bryden 1992; Yeo et al. 2002). Though bothsingle and two allele genetic models of human handednesshave been hypothesized, each model proposes that right-handedness is genetically coded and that left-handedness is aconsequence of random non-genetic factors. Environmentalfactors, including prenatal position in the fetus (Previc 1991),early asymmetrical intrinsic and extrinsic stimulation (Michel1981) and social learning (Provins 1997) have also beenproposed explanations for the preponderance of humanright-handedness. Lastly, it has been hypothesized thatthe asymmetrical function of the brain and behavior offerselective advantages in some species and therefore lateralityhas some evolutionary benefit (Ghirlanda & Vallortigara2004). This model proposes that divided functions betweentwo hemispheres has a computational advantage and thatcoordination of social behavior among conspecifics may haveselected for conformity for consistent left-right differencesin behavior and the brain (Ghirlanda & Vallortigara 2004).

Historically population-level handedness has been con-sidered uniquely human, having evolved as a specificadaptation shortly after the Pan-Homo split (Bradshaw &Rogers 1993; Corballis 1992; Warren 1980). This historicalview was supported primarily by two bodies of research.First, there was little if any evidence of population-level limband hand preferences in non-human primates and moredistantly related vertebrates (Corballis 1992; Warren 1980).Second, early attempts to selectively breed for directionalpaw preferences in mice were unsuccessful (Collins 1975;Collins 1985), leading some to conclude that directionalbiases, unlike human handedness, were not under geneticcontrol but influenced by random environmental factors innon-human animals (Warren 1980).

The late evolutionary emergence of handedness hasrecently been challenged. For example, population-levelbehavioral asymmetries have been found in vertebrates(MacNeilage et al. 2009; Rogers & Andrew 2002; Strockenset al. 2012; Vallortigara & Rogers 2005) and even someinvertebrates (Fransnelli et al. 2012). There is also some evi-dence that behavioral asymmetries in fish and chimpanzeesmay be under genetic influence (Bisazza et al. 2000b) but

446 © 2013 John Wiley & Sons Ltd and International Behavioural and Neural Genetics Society

Genetic and environmental contributions to the expression of handedness

the exact contribution of genetic to non-genetic factors isunclear from these reports (but see Vallortigara & Bisazza2002). With specific reference to handedness, thoughthere is some inconsistency in findings between species,studies in non-human primates have revealed evidence ofpopulation-level biases, particularly among great apes (Fagotet al. 1991; Hook-Costigan & Rogers 1997; Hopkins 2006;MacNeilage et al. 1987; McGrew & Marchant 1997). Forinstance, to date more than 500 chimpanzees from fourdifferent populations have been tested for hand use on atask requiring coordinated bimanual actions called the tubetask (Hopkins et al. 2011; Llorente et al. 2010) and smallbut significant right-hand biases have been found in eachsample population. Further, based on the distribution ofhandedness for several tasks in chimpanzees, Annett (2006)suggested that a possible genetic basis might be presentin this species, though expressed less strongly than inhumans.

We sought to estimate the degree to which geneticand environmental effects contribute to chimpanzee hand-edness across several tasks (see Fig. 1a). To estimatecontributions of genetic and environmental factors tothe expression of handedness, we capitalized on twoattributes of captive chimpanzees residing in differentresearch facilities within USA. First, the existence ofwell-documented pedigrees dating back, in many cases,to the founder animals allowed us to determine precisemeasures of relatedness among all the apes in the sample.Second, for a variety of reasons unrelated to this study,some chimpanzees housed in these research facilities havebeen reared primarily by humans in nursery type settings.In contrast, others were born and raised in social groups ofconspecifics. As many of these differently reared individualsare related, we are able to compare the heritability ofhandedness among individuals raised in very different earlysocial environments.

Methods

SubjectsData were collected from captive chimpanzees living in threeresearch facilities including the Yerkes National Primate ResearchCenter, M. D. Anderson Cancer Center and Alamogordo PrimateFacility. Hand preference was measured with three tasks includingsimple reaching (n = 345; 189 females, 156 males), manual gestures(n = 347; 195 females, 152 males) and the tube task (n = 542;304 females, 238 males), a measure of hand use for coordinatedbimanual actions. Samples sizes varied across tasks dependingupon the availability of subjects for testing and subjects’ willingnessto perform the tasks. Within each cohort, there were mother-and human-reared chimpanzees. Human-reared chimpanzees wereseparated from their mothers within the first 30 days of life, dueto unresponsive care, injury or illness (Bard 1994; Bard et al. 1992).These chimpanzees were placed in incubators, fed standard humaninfant formula (non-supplemented) and cared for by humans untilthey were sufficiently able to care for themselves, at which timethey were placed with other infants of the same age until theywere 3 years old (Bard 1994; Bard et al. 1992). At 3 years of age,human-reared chimpanzees were integrated into larger social groupsof adult and sub-adult chimpanzees. Mother-reared chimpanzeeswere not separated from their mother for at least 2.5 years of life andwere raised in nuclear family groups ranging from 4 to 20 individuals.

Handedness measures

Simple reachingOn the first trial, a raisin was thrown into the subject’s homeenclosure. The raisin was thrown by the experimenter to a locationat least 3 m from the subject, such that they had to locomote tothe location of the raisin, pick up the raisin and bring it to theirmouth for consumption. When the chimpanzee picked up the raisin,the experimenter recorded the hand used as left or right. Oneand only one reaching response was recorded each trial to ensureindependence of data. The trial ended after subjects retrieved theraisin. For all subsequent trials, subjects were required to locomote atleast three strides between reaching responses to allow for posturalreadjustment between trials. A median of 51 reaching responseswere obtained from each subject (range = 11–145).

The tube taskEach chimpanzee received two tests and a minimum of 30 responseswere obtained from each chimpanzee. For the tube task, peanutbutter is smeared on the inside edges of polyvinyl chloride tubesapproximately 15 cm in length and 2.5 cm in diameter. Peanut butteris smeared on both ends of the tube and is placed far enough downthe tube such that the subjects cannot lick the contents completelyoff with their mouths, but rather must use one hand to hold the tubeand the other hand to remove the peanut butter. The tubes werehanded to subjects in their home enclosures and a focal samplingtechnique was used to collect individual data from each subject. Themedian number of responses obtained from each subject was 67(range = 1–759).

Manual gesturesAt the onset of each trial, an experimenter would approachthe chimpanzee’s home cage and center themselves in front ofthe chimpanzee at a distance of approximately 1.0–1.5 m. If thechimpanzee was not already positioned in front of the experimenterat the onset of the trial, the chimpanzee would immediately movetoward the front of the cage when the experimenter arrived withthe food. The experimenter then called the chimpanzee’s name andoffered a piece of food until the chimpanzee produced a manualgesture. Only responses in which the chimpanzee unimanuallyextended his or her digit(s) through the cage mesh to request the foodwere considered a response. Other possible manual responses suchas cage banging or clapping were not counted as a gesture. Two-handed gestures, although rare, were not scored, nor were gesturesthat were produced by the chimpanzee prior to the experimenterarriving in front of the chimpanzee’s home cage. The median numberof manual gesture responses obtained from each subject was 33(range 2–492).

Hand preference calculationFor each subject and measure, a handedness index was calculatedfollowing the formula [HI = (R − L)/(R + L)] where R and L indicatedthe frequencies in left and right-hand use. Handedness index valuesranged from −1.0 to 1.0 with positive values indicating right-handpreferences and negative values indicating left-hand preferences.We used the HI indices to visualize the distribution of handednessand to classify subjects into handedness categories for descriptivepurposes.

Heritability analysisWe estimated the heritability of the measures with a generalizedlinear mixed ‘animal model’ (Kruuk 2004; Lynch & Walsh 1998;Wilson et al. 2009) that decomposes the phenotypic variance intogenetic and environmental effects. We conducted the analyses usingBayesian estimators to more readily incorporate the non-normaldistribution of the data (proportions of hand use) and to be ableto test alternative hypotheses even if data for a given parameterestimate was sparse. The models included sex and rearing status asfixed effects. We conducted the same model building procedure onthe directional and strength measures of handedness data.

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Figure 1: Descriptive Handedness Data for Each Task. Left column: pictures depicting chimpanzees performing each handednesstasks (a) manual gesturing, (b) performing the tube task and (c) simple reaching. Right column: histograms depicting the handednessdistribution based on HI scores for (a) manual gestures, (b) tube task and (c) simple reaching. Dark bar indicates mean HI score foreach task and the dotted bars represent the 95% confidence intervals. Manual gestures and the tube task showed a population-levelright-hand skew.

Using the whole sample of both human- and mother-rearedchimpanzees, we first estimated the narrow-sense heritability,which captures the proportion of resemblance between relativesattributable to transmitted genes. The residual variance in this modelcaptured the unique environment variance, an estimate of effectsthat make related individuals differ from one another. Additivegenetic variance was fit using the additive genetic relationship matrixof pairwise relatedness among individuals constructed from the pedi-gree of the whole sample. We fit a multivariate model using all threehandedness measures for either direction or strength to estimatethe variance of each measure and the covariances among measures.

In a Bayesian analysis, known or assumed information about theparameters being estimated can be set up in their prior distribution.The prior distribution is then combined with the likelihood from thedata to yield a sample from the posterior probability distributionof model parameters. The variance components were modeled as

being drawn from an Inverse-Wishart distribution, which has twoparameters: a matrix describing where the distribution is centeredand a shape (or ‘degree of belief’) parameter that specifies how tightlyconcentrated the distribution is around its center. We used a priormatrix for the additive genetic variance that was equivalent to heri-tability of 0.25 and a shape parameter (ν = 3) that was non-informativefor the heritability of each measure. In other words, we specified aprior probability for the heritability that was uniform between 0 and1. Each proportional outcome, i.e. number of observed uses of rightand left hands, on a particular task was treated as binomially dis-tributed (where, arbitrarily, right hand = ‘success’ for directional biasand dominant hand = ‘success’ for strength) using a logit link func-tion. We calculated heritability on the latent scale as σ 2

A/(σ 2A + σ 2

E)where σ 2

A is the additive genetic variance and σ 2E is the resid-

ual variance. We did not include the distribution specific variance(π2/3) in the denominator to yield heritability as a repeatability on the

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original scale (Nakagawa & Schielzeth 2010), which would be moreappropriate for phenotypes expressed a limited number of times.The heritability estimates take out measurement error and adjustfor fixed effects. We also used this model to estimate the geneticand environmental correlations among the handedness measures. Agenetic correlation represents the extent to which hand use in thedifferent tasks is influenced by the same genetic effects while theenvironmental correlations are from effects unique to each individual.

We next tested whether the heritability of each measure differedby rearing status. To determine this we constructed bivariatemodels for each measure by considering the same handednessmeasure in the mother- or human-reared environments as twoseparate traits (Falconer 1952; Lynch & Walsh 1998). Althoughindividuals only had values for the environment they were raisedin, the genetic covariance could be estimated across environmentsbecause chimpanzees had relatives who were reared in the differentenvironment. We examined the proportion of samples for which thetrait’s heritability in the human-reared condition was less than for themother-reared condition. We used a prior with a shape parameterν = 3 that gave equal prior probability for a correlation over the range−1 to +1. Because individuals were reared in only one environment,there was no information to estimate a residual covariance so it wasfixed to zero in all of these models.

Then, using only the mother-reared chimpanzees, we estimatedthe effect of the maternal environment on handedness usingmaternal ID as the predictor. The maternal environment variancecaptures non-genetic sources of resemblance between siblings,such as from the intrauterine environment or early learning fromthe mother. In pedigree-based models it also captures non-additivegenetic variance that contributes to resemblance between siblingsbut not between parents and offspring (Wilson et al. 2009).

We fit models by Markov Chain Monte Carlo (MCMC) estimationusing MCMCglmm (Hadfield 2010) in R (R Development Core Team2012). We modeled the distribution of the outcomes using themultinomial2 family, first verifying that an additive over-dispersionmodel was appropriate by regressing each proportional outcomeon sex and rearing status using a generalized linear model with aquasi-binomial error structure and checking that the over-dispersionparameter was >1. We ran each model four times for 1 × 106 MCMCiterations, discarded the first half of each chain and thinned them to1000 samples to represent the joint posterior distribution. We alsoexperimented with different priors for the additive genetic varianceof 10%, 25% and 50% of the adjusted phenotypic variance. Wemonitored convergence of the chains by, for each parameter, check-ing that the difference in means of the first 10% and the last 50% ofthe sample (Geweke 1992) had z-scores <|2| and that the autocorre-lation between retained samples was <0.1 using the coda package(Plummer et al. 2006) and by a visual inspection of the trace plots.

We combined the posterior distributions from each run of amodel to make parameter inferences. To determine population-level preference, we calculated the average handedness index usingthe model-estimated means. This index represents the expectedproportion of hand use in a given task for a ‘genetically average’chimpanzee and averaging over sex and rearing condition effects.We determined whether there was a significant right-hand (or left-hand) bias using a two-tailed p-value as twice the proportion ofsamples that were above (or below) 0.

We summarized parameter estimates using means and 95%coverage intervals (CIs) and compared models using the DevianceInformation Criterion (DIC) averaged over four runs of each model.The lowest DIC is considered the best model in terms of using amodel to predict future data (Gelman et al. 2004). The DIC can beused to compare models in terms of the parameters they fit and thepriors used for those parameters.

Results

We used the model-estimated fixed effect intercepts (con-ditioning over effects of sex, rearing status and relatednessamong subjects) to calculate mean HI scores for eachmeasure to determine if they differed from a hypothetical

value of zero, which would be the predicted HI valueif there was no population-level hand preference or ifpreference was bimodally distributed. Population-level righthandedness was found for the tube task (mean HI = 0.20,95% CI = 0.19–0.29, P < 0.001) and for manual gestures(0.28, CI = 0.21–0.37, P < 0.001) but not simple reaching(0.02, CI = −0.04–0.09, P = 0.47) (Fig. 1). Neither sexnor rearing history was significantly associated with handpreference. Chimpanzees on average used their dominanthand 68% of the time for simple reaching (CI = 67–70%),74% (CI = 72–76%) of the time for gesturing and 80%(CI = 79–82%) of the time in the tube task. Direction ofsimple reaching was phenotypically correlated with boththe gesture (r = 0.22, P < 0.01) and tube tasks (r = 0.26,P < 0.01), though these latter two measures were not signif-icantly correlated (r = 0.05, P > 0.05). For the absolute valueof the handedness index, representing handedness strength,only the reach and tube tasks were significantly correlated.

Heritability analyses

The direction and strength of handedness showed moderateand significant heritability for all three tasks. It should also benoted that estimates of heritability were higher for strengthcompared to direction in hand use for all three tasks (seeTable 1). Using alternative priors had little effect on theheritability estimates. For mother-reared individuals, thematernal environment contributed moderately to variancein handedness for all three measures. The heritability ofdirection for mother-reared individuals was higher for thereach and tube tasks but lower for the gesture task. Ofthese only the heritability of direction for the tube task wassignificantly different between rearing conditions (P = 0.03).

Genetic and environmental correlations across tasks

None of the genetic correlations among the measures,indicating the extent to which handedness in each taskis influenced by the same set of genes, was significantlydifferent from 0 (Table 2). However, we did identify asignificant genetic correlation between hand preferencefor simple reaching and the tube task for mother-rearedindividuals. The unique environmental correlations betweenreach-gesture and reach-tube for handedness direction, butnot for strength, were also significantly positive.

Discussion

Consistent with human family and twin studies (Franckset al. 2002), we found that directional bias and strength wereheritable in chimpanzees. In addition, we found that strengthof laterality appeared to be under greater genetic controlthan direction. This finding is somewhat consistent withprevious results on selective breeding for paw preferencesin mice (Collins 1985) that show that the strength butnot direction of paw preferences can be selectively bred;however, unlike mice, directional biases in hand preferencewere also heritable in the chimpanzees. It is thus likely that,in chimpanzees, a similar genetic mechanism might code forstrength of hand preference and that directional biases are

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Table 1: Variance proportion coefficients for each handedness measure

Model Rearing VPC Reach Gesture Tube

Direction1. Genetic effects All h2 0.36 (0.19–0.61) 0.34 (0.18–0.56) 0.24 (0.10–0.43)2. Rearing status Human h2 0.36 (0.17–0.65) 0.43 (0.17–0.79) 0.22 (0.08–0.48)

Mother h2 0.49 (0.26–0.75) 0.37 (0.18–0.62) 0.48 (0.22–0.75)3. Maternal effects Mother h2 0.46 (0.25–0.70) 0.39 (0.18–0.68) 0.61 (29–0.83)

m2 0.38 (0.21–0.56) 0.24 (0.13–0.38) 0.23 (0.10–0.38)Strength1. Genetic effects All h2 0.67 (0.40–0.97) 0.46 (0.28–0.69) 0.44 (0.27–0.62)2. Rearing status Human h2 0.61 (0.40–0.81) 0.48 (0.24–0.76) 0.48 (0.25–0.73)

Mother h2 0.58 (0.38–0.76) 0.49 (0.30–0.70) 0.49 (0.27–0.72)3. Maternal effects Mother h2 0.47 (0.31–0.63) 0.51 (0.32–0.69) 0.45 (0.24–0.68)

m2 0.48 (0.32–0.64) 0.40 (0.25–0.55) 0.31 (0.18–0.48)

Parameter estimate means given with 95% CIs in brackets.VPC, variance proportion coefficient; h2, narrow-sense heritability; m2, maternal environment coefficient, calculated as repeatabilitieson the latent scale.

Table 2: Correlations among handedness measures

Direction Strength

Reach Gesture Reach Gesture

PhenotypicGesture 0.22 (0.11, 0.32) 0.05 (−0.06, 0.15)Tube 0.26 (0.16, 0.36) 0.05 (−0.06, 0.15) 0.14 (0.03, 0.24) −0.09 (−0.19, 0.02)Additive geneticGesture -0.01 (−0.40, 0.57) 0.14 (−0.28, 0.47)Tube 0.33 (−0.24, 0.62) 0.03 (−0.44, 0.49) 0.09 (−0.23, 0.46) -0.28 (−0.58, 0.04)Additive genetic (mother)Gesture −0.18 (−0.57, 0.45) 0.26 (−0.21, 0.49)Tube 0.45 (0.01, 0.81) −0.21 (−0.63, 0.38) −0.01 (−0.36, 0.33) −0.40 (−0.64, 0.03)Additive genetic (human)*Gesture 0.44 (−0.43, 0.78) −0.06 (−0.44, 0.38)Tube 0.56 (−0.31, 0.82) 0.67 (−0.40, 0.78) 0.25 (−0.82, 1.0) −0.34 (−0.67, 0.29)Unique environmentalGesture 0.26 (0.07, 0.57) −0.08 (−0.82, 0.42)Tube 0.36 (0.11, 0.51) 0.08 (−0.13, 0.29) 0.26 (−0.23, 0.46) 0.05 (−0.21, 0.37)Maternal environmentalGesture 0.00 (−0.42, 0.40) −0.03, (−0.36, 0.27)Tube 0.37 (−0.03, 0.71) −0.04 (−0.44, 0.44) 0.10 (−0.22, 0.42) −0.12 (−0.41, 0.28)

Phenotypic correlations point estimates from handedness indices with 95% confidence intervals in brackets. Additive genetic, uniqueenvironment and maternal environment correlations from animal model with posterior mode and 95% CIs in brackets. Correlationssignificantly different from 0 were highlighted in bold at P < 0.05.

subsequently induced by consistent postnatal environmentalfactors. These early social experiential factors might then,in turn, interact with genetic factors in the determination ofdirectional biases in hand use. In short, genetic factors playa significant role in determining handedness but postnatalenvironmental variables may have a significant additiveeffect on their expression. The role that environmentalmechanisms play on the development of behavioral asym-metries in vertebrates has been studied extensively (Rogerset al. 2013). What postnatal environmental mechanismsmight influence chimpanzee handedness is not clear.However, there are several candidates among lateralizedbehavioral traits in chimpanzee neonates and mothers (Fagot& Bard 1995; Hopkins 2004; Hopkins & Bard 1995; Hopkins& Bard 2000; Hopkins & De Lathouwers 2006; Manning

et al. 1994), including asymmetries in head orientation orlateral biases in maternal cradling or nipple preferences.We also cannot rule out that observations of maternalhand use by the offspring might have some impact onhandedness. Any of these explanations is consistent withthe moderate effect from the maternal environment foundin mother-reared individuals. We found some evidence forgenetic effects differing between rearing environments,where the heritability of the tube task was higher amongmother-reared individuals. One possibility is that factorsin the human-reared environment override the individual’sgenetic predisposition, at least with respect to the tube task.

The results further showed that there were significantunique environmental correlations between reaching andmanual gesture, and the tube task, though the associations

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were not particularly strong. In contrast, the geneticcorrelations were weak and the only correlation significantlydifferent from zero was found in mother-reared individuals.These findings differ from some reports of the heritability ofhuman hand preferences (Medland et al. 2009; Warren et al.2012) where genetic correlations among tasks are usuallysignificant. This suggests a possible species differences inthe genetic architecture underlying human and chimpanzeehandedness.

One caution in making comparisons between human andchimpanzee handedness is the tasks used to assess handpreference and the manner in which they are selected.Human handedness is typically quantified by use of question-naire and the items placed on the survey have been explicitlydeveloped to measure the construct handedness. In otherwords, the questionnaire items are selected, a priori, to mea-sure handedness. In contrast, the measures used in this andmany studies of non-human primate handedness, are typi-cally obtained by observing hand use. Moreover, the behav-iors measured are often selected based on conveniencerather than whether they elicit reliable hand use in eachindividual. These varying approaches may potentially explainthe differences in genetic correlations between tasks andwarrants further investigation, using approaches not unlikethose recently employed by Forrester et al. (2011a,b; 2012).

In summary, the findings reported here extend the hand-edness literature by reporting on the quantitative estimationof heritability in chimpanzee, and, indeed, non-humanprimate hand preferences (but see Fears et al. 2011). Theoverall results show that population-level task specific hand-edness was present in the common ancestor of humansand chimpanzees 5–6 mya and was under some geneticcontrol at that time. What advantages and disadvantagesare explicitly linked to handedness remains unclear but onepossibility is that asymmetrical functions conferred someadaptive advantage in either cognitive and/or motor functionsthat increased fitness for certain individuals or populations(Vallortigara & Rogers 2005). In chimpanzees, the mostobvious potential lateralized behavior under selection wouldlikely have been in the realm of tool use (McGrew &Marchant 1999) but this remains to be tested. Nonetheless,as others have argued, duality of function as an adaptivefunction can be accomplished without the emergence ofpopulation-level asymmetries (Ghirlanda & Vallortigara 2004).Thus, other evolutionary factors may have come into play inthe emergence of population-level asymmetries such as theneed for cooperation in complex social systems (for examplein fish, see Bisazza et al. 2000a). Further studies on the roleof genes and experiential factors on the development ofhandedness and other aspects of behavioral lateralizationwill provide invaluable data on the evolution of hemisphericspecialization in primates, including humans.

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Acknowledgments

This research was supported by NIH grants NS-42867, NS-73134, HD-56232 and HD-60563 and Cooperative AgreementRR-15090. We appreciate assistance of Jamie Russell, JenniferSchaeffer, Dr. Steve Schapiro and Molly Gardner for assistance indata collection. American Psychological Association and Instituteof Medicine guidelines for the treatment of chimpanzees inresearch and were followed during all aspects of this study.

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