Table 2. Summary of changes observed in each chimpanzee with progressive aging.
Summaries describe the differences between the first and last field season each individual was sampled (although models underlying each result used data from all field seasons). The term ‘Possible Mild Increase/Decrease’ is used to note where we identified a change for a particular metric, but this change was considerably smaller than for other individuals, and therefore could be due to chance. We address these instances on a case-by-case basis within the Results. Dashed lines (-) indicate where we found no strong evidence for behavioural change. Names of males are listed in all capitals; females’ names are in capitals and lower case.
| Individual | Sex | Attendance | Engagement | Tool Selection Time | Oil-Palm Nut Cracking | Summary | ||
|---|---|---|---|---|---|---|---|---|
| Total Time | # Actions | # Strikes | ||||||
| Fana | F | Decrease | Decrease | Increase | Possible mild increase | - | - | Rate of attendance at the outdoor laboratory decreased, and Fana engaged with nuts and stones less often when present. Fana took longer to select tools. However, her general tool-using efficiency was mostly constant. |
| Jire | F | Decrease | Possible Mild Decrease | Increase | Mild increase | - | - | Rate of attendance at the outdoor laboratory decreased. Jire took longer to select stone tools. General tool-using efficiency was mostly constant, but Jire took slightly longer to crack oil-palm nuts in later years. |
| TUA | M | - | Possible Mild Increase | Decrease | - | - | - | Tua became slightly faster at selecting tools, and his tool-using efficiency was constant across years. |
| Velu | F | Decrease | Decrease (although sample size was small in 2011) | - | Increase | Increase | Increase | Rate of attendance at the outdoor laboratory decreased. Velu engaged with nuts and stones less often when present. Velu took longer to crack oil-palm nuts and used more striking actions. |
| Yo | F | Decrease | Possible Mild Increase | Increase | Increase | Increase | Increase | Rate of attendance at the outdoor laboratory decreased. When present, Yo spent a slightly greater proportion of time engaging with nuts and stones. Yo took much longer to select tools and demonstrated the largest reduction in tool-using efficiency. |