Old age variably impacts chimpanzee engagement and efficiency in stone tool use
- Department of Biology, University of Oxford, United Kingdom
- Development and Evolution of Cognition Group, Max Planck Institute of Animal Behavior, Germany
- School of Biological and Behavioural Sciences, Queen Mary University of London, United Kingdom
- Department of Pedagogy, Chubu Gakuin University, Japan
- College of Life Science, Northwest University, China
- Department of Science, Gorongosa National Park, Mozambique
- Interdisciplinary Center for Archaeology and Evolution of Human Behaviour (ICArEHB), Universidade do Algarve, Portugal
- CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Portugal
- Wild Minds Lab, School of Psychology and Neuroscience, University of St Andrews, United Kingdom
- School of Anthropology and Museum Ethnography, University of Oxford, United Kingdom
- Faculty of Psychology and Educational Sciences, and Swiss Center for Affective Sciences, University of Geneva, Switzerland
- Department of Brain and Cognitive Sciences, University of Rochester, United States
Figures
Figure 1
The experimental set-up and behaviors at the outdoor laboratory.
(a) Yo, an elderly female (approximately 51 years old), has placed a coula nut (Coula edulis) on an anvil stone and is preparing to strike with the hammer stone. (b) An adult female (14 years old) using a leaf tool to drink water from the water point. (c) Yo and Velu (left to right, both approximately 56–58 years old), eating oil-palm fruits from an available raceme. (d) The central stone-tool matrix with numbered stones. (e) Yo (right; approximately 49 years old) selecting tools from the stone matrix.
Figure 2
Attendance and behavior of old-aged chimpanzees at the outdoor laboratory.
(a) Attendance rate for individuals at the outdoor laboratory between the 1999 and 2016 field seasons. Red points and lines indicate individuals are over 30 years old; black points and lines indicate individuals younger than 30 years. Lines are drawn for all individuals who attended the outdoor laboratory in two or more field seasons (individuals who were only sampled in one field season have blank spaces in the legend). The names of males are provided in all capitals, and females are provided with capital and lower-case letters. Focal old-aged individuals are indicated in the legend in bold. (b) Attendance data for the five focal individuals as they age from 1999 to 2016. (c) The proportion of total time individuals spent engaging in four different categories of behavior at the outdoor laboratory between 1999 and 2011 (data collected at the first outdoor laboratory location only).
Figure 3
Duration of stone-tool selection events.
(a) Tool-selection duration times for each old-aged individual. Color correlates with the number of stone tools removed from the matrix prior to that particular tool-selection event. Shape indicates the number of tools selected by an individual in a given tool-selection event. The lines and shaded areas represent a smoothed linear relationship describing the data for each individual. (b) A mixed effect model describing the duration of stone tool-selection events across a scaled parameter of the year for each field season. Individuals are included in the model as both a random intercept and slope. The plot shows the model’s prediction of the relationship between the duration of stone-tool selection and year for each individual, compared with the baseline fixed effect of year.
Figure 4 with 1 supplement
Metrics of efficiency for the cracking and processing of oil-palm nuts.
This plot only includes metrics in which random slope models outperformed corresponding null models (see Figure 4—figure supplement 1 for all metrics). Data is summarized using boxplots (central lines in the boxes indicate median values, and upper and lower boundaries of boxes indicate the 25th and 75th percentiles; lines express the range of data, and dots indicate outliers for a given individual in a given year). Sample size available in Table 1.All data relate to the cracking and processing of individual oil-palm nuts. These metrics include, for each nut cracked, (a) the total time taken, (b) the number of discrete actions, and (c) the number of hammer strikes.
Figure 4—figure supplement 1
Summary data on all efficiency metrics for all focal aging chimpanzees.
Appendix 3—figure 1
Metrics for the cracking and processing of both oil-palm and coula nuts.
Data is confined to individuals who cracked nuts from both species. Data for coula nuts is in red, and data for oil-palm nuts is in black. Data describes the cracking and processing of individual nuts, including (a) the total time taken; (b) the total number of actions used; (c) the number of unique types of actions used; (d) the number of hammer strikes; (e) the number of times the nut had to be placed and replaced on the anvil; (f) the number of reorientations of stone tools, and (g) the number of times stone tools were switched over, or switched out for new tools.
Videos
Video 1
Oil-palm nut cracking by Yo in 1999 and 2016.
Video 2
Coula nut cracking by Jire, Foaf and Yo in 2011.
Tables
Table 1
Summary of sampled observations for each focal old-age individual.
Total time observed includes all time individuals were present in the first 10 encounters of each field season (Observed Encounters). Dashed lines (-) represent where no data was collected for an individual in a given field season. Males have names in all capitals, whereas females have names in capitals and lower case.
| ID | DOB* | Date of Death (Age) | Age (years) | Observed Encounters (% including interaction with nuts or stone tools) | Total Time Observed (minutes) | Action Sequences Coded (for individual nuts) | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1999 | 2004 | 2008 | 2011 | 2016 | 1999 | 2004 | 2008 | 2011 | 2016 | 1999 | 2004 | 2008 | 2011 | 2016 | 1999 | 2004 | 2008 | 2011 | 2016 | |||
| Fana | 1956 | 2022 (67) | 44 | 49 | 53 | 56 | 61 | 10 (70%) | 10 (70%) | 10 (80%) | 7 (28.6%) | 10 (40%) | 280.6 | 275.2 | 184.2 | 105.7 | 283.5 | 80 | 77 | 80 | - | 70 |
| Jire | 1958 | Alive as of 2025 (67) | 42 | 47 | 51 | 54 | 59 | 10 (80%) | 10 (80%) | 10 (90%) | 10 (60%) | 10 (80%) | 339.4 | 349.9 | 275.0 | 282.1 | 308.6 | 100 | 91 | 104 | 42 | 82 |
| TUA | 1957 | 2013 (56) | 43 | 48 | 52 | 55 | - | 10 (70%) | 10 (80%) | 10 (80%) | 5 (60%) | - | 174.4 | 212.1 | 204.3 | 116.0 | - | 103 | 96 | 81 | 26 | - |
| Velu | 1959 | 2017 (58) | 41 | 46 | 50 | 53 | 58 | 10 (60%) | 10 (80%) | 7 (85.7%) | 1 (100%) | 7 (42.9%) | 189.4 | 281.8 | 158 | 23.1 | 202.4 | 85 | 99 | 50 | - | 5 |
| Yo | 1961 | 2021 (60) | 39 | 44 | 48 | 51 | 56 | 10 (80%) | 10 (70%) | 10 (90%) | 3 (33%) | 5 (40%) | 200.7 | 304.5 | 363.7 | 211.3 | 282.7 | 66 | 70 | 141 | 20 | 33 |
-
*
Date of birth (DOB) was estimated for these individuals at the start of longitudinal data collection at Bossou (1976). Thus, ages for individuals are estimates. Ages are approximated from January of the estimated year of birth, and the end of the final month of each field season (e.g. for the 1999-2000 field season – abbreviated to 1999 - ages are estimated using 29th February 2000 as the end of the field season).
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. |
Appendix 1—table 1
Ethogram of codable manipulations for observations of nut-cracking behaviors*.
Manipulations in bold are coded alongside a corresponding object. There are 6 possible corresponding objects: 1. Nut, 2. Hammer, 3. Anvil, 4. Kernel, 5. Shell, 6. Bare Hand. Codes in italics are used to denote the start and end of observable sequences. Coding commenced when individuals began interacting with stones, nuts, or nut fragments. Coding ceased when individuals began engaging in a new behavior, e.g. play, grooming, eating oil-palm fruits. On the occasions where an individual moved out of clear sight of the video recording, or behavior became obscured by an individual’s body position, sequences were terminated with a ‘Not Visible’ codon and marked as incomplete. This ethogram has been used in previous studies to collect data from the Bossou archive, see Howard-Spink et al., 2024.
| Action | Description |
|---|---|
| Bite | Place object in mouth and bite with teeth. Differs from ‘Eat’ as there is no consumption. Differs from ‘Store’ as the object moves in and out of the mouth while the chimp is stationary. Differs from ‘Peel Teeth’ as bite applies general force, whereas peel with mouth is to remove shell fragments from kernel dexterously. Differs from ‘Kiss’ as the object enters inside the mouth. |
| Brush | Brush objects off the anvil. |
| Drop | Place object(s) on ground. |
| Eat | Consume object. Differs from bite and store as it requires successful ingestion. |
| Flip | Flip object over. |
| Grasp | Grasp an object and move off the ground. |
| Kick | Apply rapid, hard force on the object with the foot, so that the object is displaced. |
| Kiss | Place object to lips or nose, but not inside of the mouth. |
| Pass | Pass object between hands. |
| Peel Hand | Peel shell off of kernel with hands. |
| Peel Teeth | Peel shell off of kernel with teeth. |
| Place | Place an object on an anvil. |
| Provide | Directly hand an object to another individual. |
| Pull Foot | Pull an object across the ground with foot. |
| Pull Hand | Pull an object across the ground with hand. |
| Push Foot | Push an object across the ground with foot. |
| Push Hand | Push an object across the ground with hand. |
| Rake Foot | Pull many objects towards oneself using foot/leg. |
| Rake Hand | Pull many objects towards oneself using hand/arm. |
| Reorient | Rotate object horizontally. |
| Roll Foot | Roll object along the floor with foot. |
| Roll Hand | Roll object along the floor with hand. |
| Spit | Let object fall from mouth or lips. |
| Stomp | Whilst standing, apply strong force to object with foot. |
| Store | Place object(s) in mouth for transportation. Separated from bite, eat, peel teeth as it is followed by transportation across the outdoor laboratory, before then being removed from the mouth intact. |
| Strike One Hand | Strike with one hand, (and associated object). |
| Strike Two Hand | Strike with two hands, (and associated object). |
| Support Foot | Support object with foot. |
| Support Hand | Support object with hand. |
| Take | Receive an object directly from another individual. |
| Throw | Throw object away from self, horizontally or vertically. |
| Touch Foot | Touch or grasp object without moving it using foot. |
| Touch Hand | Touch or grasp object without moving it using hand. |
| Relocate | Stand up and move to a new area; is immediately followed by another coding action. |
| Start | Start of a sequence. |
| End Bout | End of a sequence. |
| Not Visible | Individual moves out of view, and coding cannot continue. Differs from ‘End Bout’, as there is no evidence the individual has stopped engaging with stones, nuts or nut fragments. |
Appendix 2—table 1
Model output for attendance rates over each year.
| glmer(Encounters ~Year-Scaled*Age_Cohort + (1| ID)+offset(log(Field_Season_Duration)), family = Poisson) | ||||
|---|---|---|---|---|
| Random Effect | Variance | SD | ||
| ID (Intercept) | 0.055 | 0.235 | ||
| Fixed Effect | Estimate | SE | z value | p |
| (Intercept) | –0.570 | 0.107 | –5.35 | <0.001 |
| Year-Scaled | –0.123 | 0.078 | –1.57 | 0.116 |
| Old_Cohort | –0.198 | 0.141 | –1.40 | 0.160 |
| Year-Scaled:Old_Cohort | –0.220 | 0.096 | –2.29 | 0.022 |
Appendix 2—table 2
The number of stone-tool selection events sampled for each individual in each field season.
| Individual | Year | Total | ||||
|---|---|---|---|---|---|---|
| 1999 | 2004 | 2008 | 2011 | 2016 | ||
| Fana | 2 | 2 | 8 | 0 | 4 | 16 |
| Jire | 4 | 4 | 5 | 3 | 3 | 19 |
| TUA | 6 | 9 | 7 | 3 | 0 | 25 |
| Velu | 3 | 5 | 10 | 0 | 3 | 21 |
| Yo | 4 | 5 | 8 | 9 | 1 | 27 |
Appendix 2—table 3
Models of stone-tool selection duration across years, with AIC.
AIC values reported for models fitted by Maximum Likelihood. PAB is the encounter in which stone tool selection events occurred.
| Model | Formula | df | AIC |
|---|---|---|---|
| REML = F | |||
| Individual Random Slope and Intercept for Year-Scaled. | Log(Time)~Year-Scaled+Stones Selected + Stones Previously Taken + (1+Year-Scaled||ID) + (1|PAB) | 8 | 231 |
| Dropped Year-Scaled | Log(Time)~Stones Selected + Stones Previously Taken + (1|ID) + (1|PAB) | 6 | 234 |
| Null for All Fixed Effects | Log(Time)~1 + (1|ID)+ (1|PAB) | 4 | 234 |
| REML = T | |||
| Random Slope & Intercept | Log(Time)~Year-Scaled+Stones Selected + Stones Previously Taken + (1+Year-Scaled||ID) + (1|PAB) | 8 | 244 |
| Fixed Slope & Random Slope | Log(Time)~Year-Scaled+Stones Selected + Stones Previously Taken + (1|ID) + (1|PAB) | 7 | 246 |
Appendix 2—table 4
Summary output for mixed-effect model for tool selection duration, with individual as a random slope, and encounter and individual as random intercepts.
108 observations, 49 encounters, 5 individuals.
| Log(Time)~Year-Scaled+Stones Selected + Stones Previously Taken + (1+Year-Scaled||ID) + (1|PAB), REML = F | |||
|---|---|---|---|
| Random Effect | Variance | SD | |
| Encounter | 0.02 | 0.15 | |
| ID (Intercept) | 0.13 | 0.35 | |
| ID (Slope – Year-Scaled) | 0.05 | 0.22 | |
| (Residual) | 0.35 | 0.59 | |
| Fixed Effect | Estimate | SE | t value |
| (Intercept) | 1.86 | 0.22 | 8.43 |
| Year-Scaled | 0.14 | 0.12 | 1.18 |
| Two Stones Selected | 0.31 | 0.15 | 2.14 |
| Stones Previously Taken | –0.01 | 0.02 | –0.65 |
Appendix 2—table 5
Individual coefficients for the mixed-effect model for stone-tool selection duration, with individual as a random slope, and encounter and individual as random intercepts.
| Individual | Intercept | Scaled Year (Slope) |
|---|---|---|
| Fana | 1.93 | 0.27 |
| Jire | 1.80 | 0.17 |
| TUA | 1.39 | –0.17 |
| Velu | 1.76 | 0.07 |
| Yo | 2.41 | 0.35 |
Appendix 2—table 6
Summary output for the random slope mixed-effect model for the total time taken to crack and process oil-palm nuts over sampled field seasons.
1538 observations, 20 encounters, 5 individuals. Values rounded to 3.d.p.
| lmer(Log(Time)~Year-Scaled + (Year-Scaled | ID) + (1| Encounter), REML = F) | |||
|---|---|---|---|
| Random Effect | Variance | SD | |
| Encounter | 0.029 | 0.169 | |
| ID (Intercept) | 0.089 | 0.298 | |
| ID (Slope – Year-Scaled) | 0.002 | 0.039 | |
| (Residual) | 0.418 | 0.647 | |
| Fixed Effect | Estimate | SE | t value |
| (Intercept) | 2.657 | 0.142 | 18.767 |
| Year-Scaled | 0.077 | 0.030 | 2.605 |
Appendix 2—table 7
Summary output for the random slope mixed-effect model for the number of discrete actions used to crack and process oil-palm nuts over sampled field seasons.
1538 observations, 20 encounters, 5 individuals. Values rounded to 3.d.p.
| glmer(Action Events ~Scaled Year + (Year-Scaled |ID) + (1|Encounter), family = poisson) | |||
|---|---|---|---|
| Random Effect | Variance | SD | |
| Encounter | 0.084 | 0.289 | |
| ID (Intercept) | 0.029 | 0.170 | |
| ID (Slope – Year-Scaled) | 0.003 | 0.059 | |
| Fixed Effect | Estimate | SE | z value |
| (Intercept) | 2.563 | 0.102 | 25.241 |
| Year-Scaled | 0.022 | 0.029 | 0.769 |
Appendix 2—table 8
Summary output for the random slope mixed-effect model for the number of strikes of the hammer stone used to crack and process oil-palm nuts over sampled field seasons.
1538 observations, 20 encounters, 5 individuals. Values rounded to 3.d.p.
| glmer(Strikes ~Year-Scaled + (Year-Scaled |ID) + (1|Encounter), family = poisson) | |||
|---|---|---|---|
| Random Effect | Variance | SD | |
| Encounter | 0.157 | 0.397 | |
| ID (Intercept) | 0.042 | 0.204 | |
| ID (Slope – Year-Scaled) | 0.011 | 0.104 | |
| Fixed Effect | Estimate | SE | z value |
| (Intercept) | 1.462 | 0.131 | 11.149 |
| Scaled Year | 0.079 | 0.051 | 1.545 |
Appendix 2—table 9
Intercept and slope for individual random effects for nut metrics of total time, total number of actions, and total number of strikes.
Values rounded to 3.d.p.
| Metric | Individual | Intercept | Year-Scaled (Slope) |
|---|---|---|---|
| Total Time | Fana | 2.614 | 0.064 |
| Jire | 2.508 | 0.089 | |
| TUA | 2.328 | 0.036 | |
| Velu | 2.635 | 0.068 | |
| Yo | 3.200 | 0.128 | |
| Actions | Fana | 2.685 | –0.028 |
| Jire | 2.507 | 0.049 | |
| TUA | 2.416 | 0.043 | |
| Velu | 2.399 | –0.050 | |
| Yo | 2.814 | 0.098 | |
| Strikes | Fana | 1.415 | 0.008 |
| Jire | 1.321 | 0.051 | |
| TUA | 1.290 | 0.195 | |
| Velu | 1.476 | –0.043 | |
| Yo | 1.820 | 0.186 |
Appendix 2—table 10
Each unique action type employed by Yo in 1999 and 2016, described as a proportion of the first 1000 actions observed for Yo in each year.
The difference between years is calculated, and whether this difference is greater than 0.05 (5%) is determined. A direction of change between years is also indicated for each action.
| Action | 1999 | 2016 | 2016–1999 | Difference >0.05? | direction |
|---|---|---|---|---|---|
| brush SHELL | 0.01632302 | 0.00981354 | 0.00650948 | FALSE | decrease |
| drop ANVIL | 0.00429553 | 0.00392542 | 0.00037012 | FALSE | decrease |
| drop HAMMER | 0.03006873 | 0.00785083 | 0.02221789 | FALSE | decrease |
| drop KERNEL | 0.00085911 | 0.00294406 | 0.00208496 | FALSE | increase |
| drop NUT | 0.00429553 | 0.00392542 | 0.00037012 | FALSE | decrease |
| drop SHELL | 0.01546392 | 0.02551521 | 0.01005129 | FALSE | increase |
| eat KERNEL | 0.08676976 | 0.13837095 | 0.05160119 | TRUE | increase |
| flip ANVIL | 0.00257732 | 0 | 0.00257732 | FALSE | decrease |
| flip HAMMER | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| flip KERNEL | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| grasp ANVIL | 0.00429553 | 0.00392542 | 0.00037012 | FALSE | decrease |
| grasp HAMMER | 0.04553265 | 0.00588813 | 0.03964452 | FALSE | decrease |
| grasp KERNEL | 0.07044674 | 0.05888126 | 0.01156548 | FALSE | decrease |
| grasp NUT | 0.11340206 | 0.05691855 | 0.05648351 | TRUE | decrease |
| kiss KERNEL | 0.00257732 | 0 | 0.00257732 | FALSE | decrease |
| kiss NUT | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| pass HAMMER | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| pass KERNEL | 0.00171821 | 0 | 0.00171821 | FALSE | decrease |
| pass NUT | 0.01718213 | 0.00196271 | 0.01521942 | FALSE | decrease |
| peelhand SHELL | 0.00257732 | 0.00392542 | 0.0013481 | FALSE | increase |
| peelteeth SHELL | 0.05584192 | 0.12365064 | 0.06780871 | TRUE | increase |
| place HAMMER | 0.01202749 | 0.00196271 | 0.01006478 | FALSE | decrease |
| place KERNEL | 0.00601375 | 0.00098135 | 0.00503239 | FALSE | decrease |
| place NUT | 0.09621993 | 0.0451423 | 0.05107763 | TRUE | decrease |
| relocate | 0.00687285 | 0.00294406 | 0.00392879 | FALSE | decrease |
| reorient ANVIL | 0.00945017 | 0.00392542 | 0.00552475 | FALSE | decrease |
| rollhand HAMMER | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| spit SHELL | 0.00085911 | 0.00098135 | 0.00012225 | FALSE | increase |
| strikeonehand HAMMER | 0.35395189 | 0.44455348 | 0.09060159 | TRUE | increase |
| strikeonehand NUT | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| supportfoot ANVIL | 0.00343643 | 0.00294406 | 0.00049236 | FALSE | decrease |
| touchfoot ANVIL | 0.00085911 | 0 | 0.00085911 | FALSE | decrease |
| touchhand ANVIL | 0.00429553 | 0.00392542 | 0.00037012 | FALSE | decrease |
| touchhand HAMMER | 0.00515464 | 0 | 0.00515464 | FALSE | decrease |
| touchhand KERNEL | 0.00601375 | 0.00294406 | 0.00306968 | FALSE | decrease |
| touchhand NUT | 0.01460481 | 0.03140334 | 0.01679853 | FALSE | increase |
| touchhand SHELL | 0.00085911 | 0.00392542 | 0.00306631 | FALSE | increase |
| bite SHELL | 0 | 0.00098135 | 0.00098135 | FALSE | increase |
| brush HAMMER | 0 | 0.00098135 | 0.00098135 | FALSE | increase |
| grasp SHELL | 0 | 0.00294406 | 0.00294406 | FALSE | increase |
| reorient HAMMER | 0 | 0.00098135 | 0.00098135 | FALSE | increase |
| supporthand ANVIL | 0 | 0.00098135 | 0.00098135 | FALSE | increase |
Appendix 3—table 1
Summary of nut cracking and processing metrics for Tua, Jire, and Yo between 2008 and 2016.
Total time duration is summarized by mean and standard deviation. All other metrics are summarized by medians and interquartile ranges. Horizontal lines indicate where no data is available for a given year. Information on how each metric is defined and estimated can be found in the Methods.
| Individual | Metric | Year and Nut Type | |||
|---|---|---|---|---|---|
| 2008 | 2011 | 2016 | |||
| TUA | oil-palm (n=81) | coula (n=26) | - | ||
| Means (sd) | |||||
| Total Time (s) | 10.6 (6.34) | 54.7 (92.6) | - | ||
| Medians (IQR) | |||||
| Actions | 8 (4) | 25.5 (16) | - | ||
| Action Types | 6 (2) | 8 (3) | - | ||
| Strikes | 2 (2) | 7 (6.75) | - | ||
| Nut Positioning | 1 (0) | 2 (2.75) | - | ||
| Tool Reorientations | 0 (0) | 0 (0.75) | - | ||
| Tool Changes | 0 (0) | 0 (0) | - | ||
| Jire | oil-palm (n=104) | oil-palm (n=25) | coula (n=17) | oil-palm (n=82) | |
| Means (sd) | |||||
| Total Time (s) | 12.8 (8.37) | 21.8 (14.9) | 44.6 (21.6) | 21.9 (22.3) | |
| Medians (IQR) | |||||
| Actions | 8 (5) | 11 (6) | 26 (17) | 11 (7.75) | |
| Action Types | 6 (2) | 7 (3) | 11 (2) | 7 (2) | |
| Strikes | 2 (2) | 3 (3) | 7 (4) | 4 (2) | |
| Nut Positioning | 1 (1) | 1 (1) | 3 (1) | 1 (1) | |
| Tool Reorientations | 0 (0) | 0 (0) | 0 (1) | 0 (0) | |
| Tool Changes | 0 (0) | 0 (0) | 0 (0) | 0 (0) | |
| Yo | oil-palm (n=141) | coula (n=20) | oil-palm (n=33) | ||
| Means (sd) | |||||
| Total Time (s) | 28.1 (24.6) | 186 (183.4) | 55.9 (42.3) | ||
| Medians (IQR) | |||||
| Actions | 11 (6) | 88.5 (60.5) | 27 (14) | ||
| Action Types | 7 (1) | 11.5 (8.5) | 8 (1) | ||
| Strikes | 4 (2) | 27 (23) | 12 (8) | ||
| Nut Positioning | 1 (1) | 11 (22.25) | 2 (1) | ||
| Tool Reorientations | 0 (0) | 0.5 (4.25) | 0 (0) | ||
| Tool Changes | 0 (0) | 0 (7.25) | 0 (0) | ||
Appendix 4—table 1
Total time taken for Yo to crack oil-palm nuts in 2018.
Encounter numbers indicate where data was collected for the same encounter. Nut number indicates which nut (in chronological order) Yo was cracking for a specific encounter. Numbers missing from this chronology were removed as we could not reliably estimate start and end times.
| Encounter | Nut Number | Duration (s) |
|---|---|---|
| 9 | 6 | 145 |
| 9 | 7 | 73 |
| 9 | 8 | 24 |
| 9 | 9 | 29 |
| 11 | 1 | 220 |
| 11 | 3 | 173 |
| 11 | 4 | 165 |
| 11 | 5 | 54 |
| 11 | 6 | 42 |
| 11 | 7 | 17 |
| 11 | 8 | 36 |
| 11 | 9 | 75 |
| 11 | 10 | 37 |
| 11 | 17 | 35 |
| 11 | 18 | 59 |
| 11 | 19 | 54 |
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Old age variably impacts chimpanzee engagement and efficiency in stone tool use
eLife 14:RP105411.
https://doi.org/10.7554/eLife.105411.3
