Decision letter after peer review:
Thank you for submitting your article “Bumblebees retrieve only the ordinal ranking of foraging options when comparing memories obtained in distinct settings” for consideration by eLife.
Your article has been reviewed by three peer reviewers, one of whom is a member of our Board of Reviewing Editors, and the evaluation has been overseen by Christian Rutz as the Senior Editor. The following individual involved in the review of your submission has agreed to reveal their identity: Alex Kacelnik (Reviewer #3).
The reviewers have discussed their reviews with one another, and the Guest Reviewing Editor has drafted this decision letter to help you prepare a revised submission.
While all three reviewers found merit in your paper, they raised several important concerns, indicating a need for conducting additional experiments. Here is a summary of the main issues:
Reviewer #1 suggests that the results of the first two experiments could be accounted for by a recency effect, where the higher item of the last comparison is better retrieved, and that in the last experiment, bumblebees’ choice for the higher-ranking item could be based on its higher absolute quantitative value in terms of sucrose solution.
Reviewer #2 asks for an experimental assessment of whether bees do respond differently to the different colors and concentrations used. Moreover, Reviewer #2, as well as Reviewer #1, noted that the huge literature on transitive inference in non-human animals could be relevant to the topic addressed by the authors and deserves to be considered.
Reviewer #3, similarly to Reviewer #1, is not convinced that the authors can exclude sensitivity to absolute properties, and suggests some important control experiments, the most important one being the following: training of bees with only A and only B, each in a context, and then giving a choice: if bees show a preference when the only available information is concentration, then the argument that they just cannot remember concentrations could be rejected.
Several other, more specific, comments were provided by the reviewers and should be taken into account by the authors in a revision. In conclusion, the reviewers feel that the paper is potentially publishable but more experimental work is needed to address several issues raised by these fascinating experiments.
Note: Please note that eLife has adopted the STRANGE framework, to help improve reporting standards and reproducibility in animal behaviour research. In your revision, please consider scope for sampling biases and potential limitations to the generalisability of your findings:
Reviewer #1 (Recommendations for the authors):
It seems to me that the study is interesting and highlights some fascinating ability of bees to adapt and implement different strategic behaviour to better succeed in different tasks.
Reviewer #2 (Recommendations for the authors):
Line 61-63: “Only recently” refers to the work of Pompilio and Kacelnik (1), which is already 12 years old. This does not seem “recent”.
Line 63: Please avoid superlative qualifications (“impressive” studies).
Line 70: The name REAL in capitals should be changed to the appropriate citation format.
Lines 118-124: The explanations provided for the lexicographic and the non-lexicographic combinations strategies are not intuitive and difficult to follow both for advised and unadvised readers. Please, be more didactic in your explanations.
Line 200-201: In fact, the work of Greggers and Menzel (1) shows that honey bees can memorize simultaneously four different feeding options providing different reward qualities (sucrose flow rate) and labeled with different colors (and they match their choice to their feeding properties). Mentioning/discussing this would be useful.
Line 205: I could not understand why you are citing here the work of Miriyala et al., (2), which is a work on electrophysiological properties of gustatory receptors with no relation to brain encoding of utility. Miriyala et al., measured peripheral gustatory activity, not central encoding of gustatory information.
1. U. Greggers, R. Menzel, Memory dynamics and foraging strategies of honeybees. Behavioral Ecology and Sociobiology 32, 17-29 (1993).
2. A. Miriyala, S. Kessler, F. C. Rind, G. A. Wright, Burst firing in bee gustatory neurons prevents adaptation. Curr Biol 28, 1585-1594 (2018).
Reviewer #3 (Recommendations for the authors):
L87: “if bumblebees remembered absolute values for options, their preference between B and C in the test should be identical”
The word ‘remembered’ should be changed to ‘were driven exclusively by their memory for’. An animal can ‘remember’ something but not use only this memory when other cues are available, as it appears elsewhere in the paper.
L97, Figure 1: It would have been nice for completeness to have a control where bees were trained with A(45) and C(30) in the absence of any competitor flower types, and then offered a choice of A vs. C. I assume that they would prefer A, but if the authors are correct in claiming that bees don’t retain absolute memories, even in that case the bees would be indifferent. If the bees in such a control preferred A, this would show that they DO remember quality when other cues are not competing. As it is, the data are compatible with attention to absolute concentration being overshadowed by information about ranking, rather than bees being unable to remember absolute values.
L130: “If bumblebees have memories for both ranking and absolute information, they should either prefer B over C because it is much higher in quality, or this value difference would cancel out the ranking difference (B < A while C > D) and their preference for B and C should be equal. On the other hand, a preference for the much lower reward quality flower C would indicate they have access to only ranking information.”
This logic is not completely tight. If, say, the difference in preference induced by absolute memories were X, and that induced by ranking were Y, where X <
L142: “remembered ranking would predict a similar preference. There was no difference in preference for C across experiments 1 and 2 (GLM: 95% CI = [-0.24 0.13], N = 80, P = 0.55; Figure 1B and D).”
From visual inspection of Figure 1B and D it appears that there is in fact a small (and noisy) greater preference in 1B than in 1C. The analysis presented shows that this is not statistically significant. The authors interpret that lack of significance as meaning lack of a difference. This should be tempered, for the usual reasons: lack of significance is not enough to confirm that the null hypothesis (lack of effect) is actually true.
L162 “Further, bumblebees’ equal preference for the equally ranked A and C flowers, despite their difference in sugar concentration, demonstrates absolute memories were not used (Table 1) and, in combination with experiments 1 and 2, confirms that bumblebees only use remembered ranking.”
I am not convinced. Say that an animal encounters flowers A and B in a random sequence. It then assigns a value to each according to their ranking in that context (A > B), regardless of their absolute concentration. But, each time the animal encounters B, how does it know that it is worse than A? It must be because it DOES remember A, otherwise it could not establish a ranking. In other words, bees may not use, or may not remember, absolute values when moving to new contexts, but they must be sensitive to absolute values, at least within a context. So, maybe there is only short-term memory for absolute value, or maybe that is erased (reset) whenever a new context is entered, but the task of ranking is impossible without some memory for concentration existing and being used.
Again, the simple control of training with only A and only B, each in a context, and then giving a choice would be highly informative: if they show a preference when the only available information is concentration, then the argument that bumblebees just cannot remember concentrations could be rejected.
L182 “In the unrewarding test with all four options available, bees preferred option A (GLM: 95% CI = [0.94,1.68], N = 20, P = 1.21e-06; Figure 2B), indicating that despite being susceptible to suboptimal outcomes in lab-created situations, using only memories of options’ ordinal ranking may be evolutionarily rational, i.e. can still lead to optimal choices within an ecologically relevant scenario.”
I don’t think the language here is used tightly. What Exp 4 shows is that while in 2-way choices bees showed strong context effects (Figure 1B), in 4-way choices animals’ preferences match the use of absolute memory as well as ranking. Exp 4 is presented as if the only difference with the earlier design were the training condition, but it also differs in the choice procedure (4 vs. 2 options). It is known that other animals are sensitive to the number of options at choice time. The claim that the situation in Exp 4 is more ecologically relevant than that in the previous experiments is not fully explained, and it would be good to make it more convincing. As it is, it appears somewhat post-hoc.
As elsewhere in the paper, the use of the words optimal and sub-optimal is a bit loose. Any partial preference is suboptimal, because an ‘optimal’ animal (which is a theoretical construct) allocates all its behaviour to the best option in the context. In this article, following a practice in many experimental psychology writings, the words are used to mean any bias in the favourable direction, regardless of its magnitude. How would one call a theoretical bee that showed exclusive preference for the best option, ‘super-optimal’, perhaps? By definition, it should not be possible to better optimal behaviour. One could simply say that the psychology of the animals may be adaptive under natural conditions, not ‘optimal’. In truth, I even don’t like this, because we can’t test whether animals are well designed or not. Optimality studies test models, not the notion that animals are well adapted.
L279: “The training was deemed complete when either a bee performed 200 landings or when a bee only landed on the mostly rewarded option for three consecutive bouts.”
This method could produce an artefactual bias. If a bee did 3 bouts in a flower other than the ‘most rewarded’, then the test continues, but if it does it in the richest one, the test stops. This traps good results that could occur at random in any sequence. Under the null hypothesis that bees have no preference, this would show a bias towards the most rewarded flower because the data are censored when a good response is shown. It is equivalent to keep adding individuals to a sample until a significant result is observed, and then stop. I assume that bees may never have shown 3 bouts on a lesser flower, and if this is the case, please state so.