Development of foraging skills in two orangutan populations: needing to learn or needing to grow?

Abstract

Background: Orangutans have one of the slowest-paced life histories of all mammals. Whereas life-history theory suggests that the time to reach adulthood is constrained by the time needed to reach adult body size, the needing-to-learn hypothesis instead suggests that it is limited by the time needed to acquire adult-level skills. To test between these two hypotheses, we compared the development of foraging skills and growth trajectories of immature wild orangutans in two populations: at Tuanan (Pongo pygmaeus wurmbii), Borneo, and Suaq Balimbing (Pongo abelii), Sumatra. We collected behavioral data on diet repertoire, feeding rates and ranging competence during focal follows, and estimated growth through non-invasive laser photogrammetry.

Results: We found that adult-like diet repertoires are attained around the age of weaning and that female immatures increase their repertoire size faster than their male peers. Adult-level feeding rates of easy techniques are reached just after weaning, but several years later for more difficult techniques, albeit always before adulthood (i.e. age at first reproduction). Independent immatures had faster feeding rates for easy to process items than their mothers, with male immatures achieving faster feeding rates earlier in development relative to females. Sumatran immatures reach adult-level feeding rates 2-3 years later than their Bornean peers, in line with their higher dietary complexity and later weaning. The range-use competence of independently ranging and weaned immatures is similar to that of adult females. Body size measurements showed, immatures grow until female age of first reproduction.

Conclusions: In conclusion, unlike in humans, orangutan foraging skills are in place prior to reproduction. Growth trajectories suggest that energetic constraints, rather than skills, best explain the length of immaturity. However, skill competence for dietary independence is reached later where the adult niche is more complex, which is consistent with the relatively later weaning age with increasing brain size found generally in primates, and apes in particular.

Keywords: Body growth; Development; Diet repertoire; Feeding rates; Foraging skills; Life history; Needing-to-learn hypothesis; Orangutans; Ranging; Skill learning.

Fig. 1

Development of diet repertoire at Tuanan: Diet repertoire size in percentage of the mother’s diet repertoire size in relation to age (in years) for immatures that are still in permanent associations with their mothers. The dotted vertical line shows mean weaning age at the Tuanan population. The dashed horizontal line marks the mothers’ diet repertoire sizes (100 %)

Fig. 2

Development of feeding rates: Immatures’ feeding rates, expressed as percentage of their mother’s feeding rates, as a function of age and processing complexity of the food items, at Tuanan (a) and Suaq (b). The vertical dotted line shows mean weaning age at each population. The horizontal dashed line marks adult-level feeding rate (100 %)

Fig. 3

Development of Ramble ratios: Average daily ramble ratios for independently ranging immatues (juveniles) and mothers at Tuanan (a) and Suaq (b). The numbers next to the juvenile data points indicate ages (in years). The juvenile points are always paired with a data point of their mother collected in the same 5 month period (in 3 cases this data was not available)

Fig. 4

Body size development: Measures of whole arm, fore arm and upper arm versus age of the Suaq immatures in absolute values (a) and in percentage of the average adult females (b). Forearm length versus age of the immatures at Tuanan and Suaq in absolute values (c) and in percentage of the average adult females (d). The Tuanan data were retrieved from [54]

Fig. 5

Dietary complexity: Distribution of the different processing steps in percent of the total diet (for the food items that form the top 90 % of the total diet) of 4 adult females at Tuanan and Suaq each. The red line indicates the site frequency ratio of each given processing step (log (frequeny at Tuanan/frequency at Suaq +1))