Peer review of: Wild primates copy higher-ranked individuals in a social transmission experiment (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
Canteloup and coworkers present a study on social learning dynamics in a group of wild vervet 
monkeys. Their study is based on an open diffusion experiment in which individuals learn to open 
artificial foraging boxes. To analyze their data the authors use a variant of network-based diffusion 
analysis (NBDA). Besides testing for the occurrence of social learning the method allowed testing for 
various details of the learning processes, including tests for social learning strategies. Thus, the 
authors found for example that social learning dynamics in this experiment were consistent with a 
social learning strategy to preferentially copy higher ranking individuals. The presented method and 
results are particularly valuable for understanding social learning dynamics in wild animals and how 
individual-level processes translate into group-level cultural dynamics. 
 
In general I liked this manuscript very much. However, for the framework of the study it could be 
useful to explicitly consider additional individual-level processes that could influence group-level 
dynamics. In their current conceptual framework the authors included (1) social network structures 
(L49-54), (2) social learning mechanisms, i.e. the cognitive mechanisms underlying the process of 
social learning (L55-58), and social learning strategies, i.e. learning biases based on the content of 
behavior or its context. (L58-67). I think there are at least two additional processes that could 
influence group-level cultural dynamics: (1) attention biases that are not captured by social networks 
of shared space use. E.g. an individual could spend equal time with low and high ranking individuals 
but pay more attention towards what higher ranking individuals are doing, which could create a 
learning bias that is not captured by social network structure or social learning strategies (at least not 
the kind of strategies that are inferred in this study). (2) performance biases, where some individuals 
would consistently perform a certain task more often and thus could serve more often as a model for 
others. E.g. after learning a behavior, higher ranking individuals might perform behaviors more often 
compared to lower ranking individuals (which could occur even if monopolization is not possible). 
Again this could generate a bias towards learning from higher ranking individuals that is not 
determined by social network structure or social learning strategies. 
 
Considering these additional biases might provide a more solid conceptual foundation for the current 
study. In addition, a discussion of these additional biases would help a lot to better understand the 
applied version of NBDA. The way the input networks were constructed, the method automatically 
controls for any biases generated by social structure and potential attention and performance biases. 
This is because edges in the networks were only added if a naïve individual was in a position of closely 
observing a model performing a new behavior. This is very different from a lot previous applications of 
NBDA in which static networks were used. In these versions, it was not possible to control for potential 
attention and performance biases. I think it could be particularly important that readers understand 
such differences to correctly interpret the results and correctly apply this methods in future studies. 
 
Regarding the interpretation of the results it might be worth mentioning that not investigated 
attention or performance biases could enhance or weaken the inferred the learning bias of 
preferentially copying higher ranking individuals. Perhaps such additional biases could also explain the 
discussed differences to previous studies? Ideally the conducted study would be complemented by 
analyzing different kinds of biases, or the combined effect of all biases. However, such an extension 
might be beyond the scope of the current study. 
 
Minor comments: 
L182 and L188: Non-significant results should not be taken as evidence for the absence of an effect. I 
suggest rewording to something like “no statistically significant preferences could be detected”. In 

addition, I suggest to show the actual p-value instead of writing “p>0.05”. 
L373: It would be very useful to clarify what a “manipulation event” is. Is it just the manipulation of 
the box or is it a successful manipulation, where the individual succeeded in opening the box? I guess 
the latter (see also L413), but it is not entirely clear. 
L577: the journal should be PLoS Biology 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This paper presents a study on social learning in wild vervet monkeys that combines an experimental 
with a modelling approach based on network based diffusion analysis (NBDA). Two groups of wild 
vervet monkeys were first tested in a foraging task in which they were confronted with two sets of 8 
artificial fruits each. Each fruit could be opened in two ways, either by lifting a lid or pulling a drawer, 
to access a food reward. The empirical question was whether a given subject would be affected by the 
technique that other group members had employed when the subject was still naive , i.e. it had not 
succeeded to open one of the artificial fruits before. The NBDA was used to contrast different modes of 
transmission in the group, specifically option specific (OS) learning vs. cross-option (CO) social 
learning. The best fit was found for the OS model. Furthermore, an effect of rank on social learning 
was detected, with subjects being more likely to learn the opening of the box when they had observed 
higher-ranking subjects first. This is an interesting approach, but I have a number of concerns. 
 
• I am not convinced that copying indeed takes place. To demonstrate this, the authors should report 
very clearly in the text how many subjects who had only seen ONE technique then actually used this 
technique. Indeed, an inspection of the data from Noha shows the following sequence of events: Lif 
used the Drawer (apu=pull[?; legend is missing]), but Avo, who was attending, used lift. The next 
animal also used lift and had observed Avo using lift, but the next animal (Ness) used "srtpu (pull?)" 
and had just observed both options. Given that the group has only 12 subjects, out of which only 10 
learned to open the box, and discounting the first subjects that obviously did not learn to open the 
box socially, this leaves N=9 subjects for which I have doubts for 2 already. I did not go through the 
rest of the data. On a side note, remember that all data need to be annotated, i.e. all states of a 
variable need to be explained. 
• Alternatively, if the authors define social learning as using the technique that the vervets have 
observed more frequently than the other one before, then the problem arises that the outcome 
becomes hard to distinguish from a simple model that combines priority-of-access, i.e. dominants 
have access first, with a general bias for lift. In other words, the strong bias for the lift option may 
create the artefact that the animals are doing what the others are doing, while they may simply all 
largely prefer one option. 
• If the authors can rule out the problems mentioned above about social learning, they should 
consider the possibility that the rank effect may simply be due to the fact that subjects are more likely 
to “learn” from dominants because these have access the boxes first and therefore it is more likely 
that they have been observed. 
• What is the theoretical background that gives rise to Cross-Option learning? This appears to bit of a 
straw-man … Why would a subject be more likely to do the OPPOSITE of what they have observed? 
• Much more descriptive information on the actual behavior needs to be given: what is the latency to 
approach once the first subject has opened the box; what is the distance at which it is reasonable to 
assume that subjects are attending. How many subjects typically observed another one? Were there 
any cases where two subjects were available as models, and if so, how was this coded? What if the 
animals used different techniques? 
• The sample size is small (only 2 groups and few subjects in one group). I therefore strongly suggest 
to simulate different data sets that reflect different transmission processes for groups of different size 

and composition and check the outcome. For instance, what if the all subjects learn to open the box 
individually, but have a bias of using the lift option. Would the outcome of the diffusion analysis yield 
a similar pattern or could the two processes be distinguished? 
• It is worrying that one of the outcomes of the model is that “After having learnt one option, 
individuals were 31x faster to subsequently asocially learn the other option […]” (abstracts and a 
number of places in the ms). Firstly, faster THAN WHAT? And secondly, what are the time scales? Are 
the authors implying that animals first learn socially to use one SPECIFIC option, in e.g. 2 minutes, 
and then it takes them only e.g. 4 seconds to discover the other option individually? More information 
on the actual data is needed to convince the reader that the network diffusion model actually fits the 
data. 
• I don’t think it is necessary to invoke “culture” in this transmission experiment. Too much hyperbole 
here. “Social diffusion” is just fine. But as I said above, it is not really clear what the animals are 
learning from each other, and which parts (if any) of the observed behavior are facilitated by the 
presence of others. 
• All terms need to be clearly defined. It is well known that people have different ideas what ‘copying’ 
means, etc. E.g. Tomasello distinguishes between imitation and copying; others don’t (line 55 ff). 
 
Minor comments 
 
Line 25: twice as fast 
 
Line 85: this study does not address social innovation, cut these parts, they are distracting. 
 
97: what is the difference between ii) and iii)? 
 
102: specify directed social learning 
 
104 ff: too vague – use the section to develop predictions 
 
156: explain what the number means in the text 
 
163: compared, not compare 
 
168: “capable of social transmission that could support …” odd wording, rewrite 
 
341: “not enough”  “not sufficiently” 
 
371: “thanks to”  “using a” 
 
373: distance? 
 
378: “have then been”  “were” 
 
379: “who are participated the most”: please specify 
 
406: why individual analyses? Why not at the group level? 
 
536: “The thicker an edge is, the bigger …” 
 
I have two version of Figure 2, one has orange names, the other does not. Orange on green is bad for 
people who are color blind.