Hassett et al. (2008) – “Sex differences in rhesus monkey toy preferences parallel those of children”

Aim #1 (sex differences)

To investigate sex differences in toy preferences of rhesus monkeys

Aim #2 (socialization/biological)

To see if socialization processes, or biological mechanisms affect sex stereotypical toy choice in rhesus monkeys.

Aim #3 (monkeys/children)

To see if there are sex differences in toy choice of rhesus monkeys compared to human children.

Sampling technique

Opportunity.

Features/Characteristics of the Sample

• Rhesus monkeys

• Members of a multi-male, multi-female social group

• Total of 135 that lived together for more than 25 years. (before cut-down for experimentation → 82 males/females)

• Full age-range of group members from infants to adults.

• Housed located at Yerkes National Primate Research Center Field Station (Atlanta, GA)

Exceptions (Cut-downs) from the Original Sample

• 14 animals not included in experimentation due to varying hormonal treatments prenatally.

• 39 newborn (0-3 months) infants were not included due to difficultly in determining their gender.

• Therefore, researchers were left with 61 females and 21 males as potential subjects.

Details of the finalized experimental group.

• 61 females and 21 males

• Housed in their natal group

• 25mx25m outdoor compounds with attached temperature-controlled indoor quarters.

• Water continuously available

• Animals fed monkey chow twice daily; supplemented once per day with fruits and vegetables.

Experimental Type

Laboratory Experiment

Research Methods

• Observations

  • Controlled

  • Non-participant

  • Covert

  • Structured; Time/Event Sampling

Type of data collected

• Quantitative Data

  • Average frequencies and durations for each behavior each monkey participated in throughout the trials.

  • Calculations on total number and total duration of interactions with each toy type.

Behavioral Checklist

This was used throughout the study to code the interactions with plush and wheeled objects the monkeys had throughout the study.

Independent Variables

• Monkey’s genders

  • Male

  • Female

Dependent Variables

• Whether the monkeys interacted with the toys

• The social rank of the monkey

• The age of the monkey

Experimental Design

• Independent measures

  • Two conditions (groups) of monkeys were compared: male and females.

Apparatus (Toys; Wheeled)

• Considered to be “Masculine”

• Ranged from 16cm to 46cm in size

• All had six wheels.

Apparatus (Toys; Plush)

• Considered to be “Feminine”

• Ranged from 14cm to 73

• All soft to touch

Examples of Plush Toys

• Winnie-the-Pooh

• Koala bear hand puppet

• Teddy Bear

• Scooby-Doo

• A Turtle

Examples of Wheeled Toys

• Wagon

• Truck

• Car

• Dump Truck

Apparatus (other)

• Video cameras to record the monkeys’ interactions.

• An app called “Hondobs” on palm pilots

  • Used to watch the procedure videos

  • Two independent observers recorded behaviors from the checklist on this.

Controls

• All monkeys went through the seven, 25-minute trials at the research center.

• One wheeled and one plush toy were positioned ten meters apart, and their positions were counterbalanced

• All trials were recorded

• All videos were watched by two independent observers.

Trial Setup (procedure)

• Seven 25-minute trials were conducted.

• Trials took place in a large indoor/outdoor enclosure housing the social group.

Pre-Trial Preparations (procedure)

• Subjects and other social group members were sequestered indoors.

• One wheeled and one plush toy were placed outdoors, 10m apart.

• The left or right placement of toys was counterbalanced across trials.

Trial Execution (procedure)

• Monkeys were released into the outdoor area.

• Each toy and any interacting animal was videotaped.

• Separate cameras were used for each toy.

Post-Trial Procedures (procedure)

• Toys were removed after each trial.

• Two observers coded interactions and behaviors from videotapes.

• Observers worked together to achieve consensus on identity and behaviors.

Unexpected Event (procedure)

• In one trial, a plush toy was torn into multiple pieces.

• This led to the trial ending 7 minutes early.

Behavior Data Collection (Data Analysis)

• All instances of specific behaviors were counted to determine frequency.

• Continuous behaviors were recorded with onsets and offsets to measure duration.

Data Normalization (Data Analysis)

• Subjects participated in different numbers of trials.

• Raw frequencies and durations were divided by the number of trials per subject to calculate average behavior frequency or duration.

Exclusion Criteria (Data Analysis)

• Subjects with fewer than five total behaviors were excluded from analysis.

• This resulted in the exclusion of 3 males and 14 females.

• The final sample included 23 females and 11 males.

• This sample of 34 monkeys was used for data analysis

Plush Toy Interaction (Males vs. Females - Frequency)

• Males: Mean = 2.06, S.D. = 9.21

• Females: Mean = 7.97, S.D. = 10.48

Plush Toy Interaction (Males vs. Females - Duration)

• Males: Mean = 0.53 min, S.D. = 1.41

• Females: Mean = 1.49 min, S.D. = 3.81

Wheeled Toy Interaction (Males vs. Females - Frequency)

• Males: Mean = 9.77, S.D. = 8.86

• Females: Mean = 6.96, S.D. = 4.92

Wheeled Toy Interaction (Males vs. Females - Duration)

• Males: Mean = 4.76 min, S.D. = 7.59

• Females: Mean = 1.27 min, S.D. = 2.2

Results (Preference of Toy)

• Males significantly preferred wheeled toys compared to the plush toys.

• Females showed no significant preference for plush toys over the wheeled toys.

Results (Frequency of Interactions)

• Males and females showed no significant difference in the frequency of interactions of wheeled toys compared to plush toys.

• Males showed significantly lower frequency of play with the plush toys compared to the females.

Results (Duration of Interaction)

• Males’ duration of interaction was significantly longer with wheeled toys compared to the plush toys.

• Females’ duration of interaction was not significantly different between wheeled toys and plush toys.

Magnitude of preference for sex-typical toys

A preference score for sex-typical toys was calculated by subtracting interaction frequencies and durations:
males (wheeled - plush), females (plush - wheeled).

Magnitude of preference for sex-typical toys

• Males’ had a significantly higher frequency “magnitude of preference” score compared to the females.

• Males’ had a significantly higher duration “magnitude of preference” score compared to the females.

Results (Significant Preferences)

• Males significantly preferred wheeled toys, while the females did not significantly prefer the wheeled or plush toys.

Results (rank/age differences in relation to preferences)

• Rank differences were observed only among females.

• Higher rank positively correlated with duration of interaction with plush toys but not wheeled toys.

• Higher rank positively correlated with frequency of interaction with both toys.

• Females with no toy preference ranked lower than those who preferred plush toys.

• There were no age differences according to preferences in the females.

Conclusions of the Study

• Sex-typed preferences in humans may be due to biological differences because even without differences in socialisation, monkeys showed preferences similar to human infants.
  

• Male monkeys similar to boys (children), have a strong preference for masculine-type toys. Whereas, female monkeys are more variable in their toy preferences.

• Toy preferences reflect behavioural and cognitive biases which have been influenced by hormones.