Early maternal loss leads to short- but not long-term effects on diurnal cortisol slopes in wild chimpanzees

  1. Cédric Girard-Buttoz
  2. Patrick J Tkaczynski
  3. Liran Samuni
  4. Pawel Fedurek
  5. Cristina Gomes
  6. Therese Löhrich
  7. Virgile Manin
  8. Anna Preis
  9. Prince F Valé
  10. Tobias Deschner
  11. Roman M Wittig
  12. Catherine Crockford

  • Curated by eLife

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    Summary: This paper tests the biological embedding model by asking whether and how early maternal loss affects cortisol levels and diurnal cortisol slopes among wild chimpanzees at the Tai Forest, Côte d'Ivoire. The results suggest that maternal loss alters the HPA stress axis in wild chimpanzees, but these effects are not visible later in life. Authors suggest that the lack of a later life association between maternal loss and cortisol levels may be due to selective early mortality of individuals with high cortisol levels but did not provide any survival or behavioural data to show that orphans and non-orphans differ in any fitness-related traits other than cortisol. Furthermore, the association between cortisol and the HPA axis is in the opposite direction to that observed in humans and there seems to be no significant increase in cortisol in orphans compared to non-orphans. Overall, the study is the result of extensive fieldwork, the number of samples collected is impressive and the subject is very interesting.

    The analyses will benefit greatly if the authors use effect sizes and confidence intervals for inferences instead of p-values. This may solve the significance threshold issues. Moreover, the reliance on p-values seem to limit the value of the data. For example, authors suggest that results from model 1 should be treated with caution because the full model is not significantly different from the null model, but by relying on it as the key finding of the study without exploring effect sizes, it does not seem that they did exercise sufficient caution.

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Abstract

The biological embedding model (BEM) suggests that fitness costs of maternal loss arise when early-life experience embeds long-term alterations to hypothalamic-pituitary-adrenal (HPA) axis activity. Alternatively, the adaptive calibration model (ACM) regards physiological changes during ontogeny as short-term adaptations. Both models have been tested in humans but rarely in wild, long-lived animals. We assessed whether, as in humans, maternal loss had short- and long-term impacts on orphan wild chimpanzee urinary cortisol levels and diurnal urinary cortisol slopes, both indicative of HPA axis functioning. Immature chimpanzees recently orphaned and/or orphaned early in life had diurnal cortisol slopes reflecting heightened activation of the HPA axis. However, these effects appeared short-term, with no consistent differences between orphan and non-orphan cortisol profiles in mature males, suggesting stronger support for the ACM than the BEM in wild chimpanzees. Compensatory mechanisms, such as adoption, may buffer against certain physiological effects of maternal loss in this species.

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  1. Version published to 10.7554/elife.64134 on eLife
  2. eLife

    Reviewer #3:

    The results of this study suggest that maternal loss alters the HPA stress axis in wild chimpanzees, but these effects are transient and are not evident later in life.

    Overall the study is the result of much careful fieldwork. The number of cortisol samples is impressive and these are robustly analysed. The conclusions are carefully and thoroughly discussed.

    I have very few comments, in part because I am not a specialist in stress hormones and so cannot fully assess the laboratory analysis or interpretation, but in part because my view is that this is a high-quality thorough study and a well-written manuscript.

    My only major point is that I am aware that measurement of cortisol is difficult in the wild. It is possible to inadvertently measure metabolites other than cortisol, and the most robust way to measure cortisol is using a challenge and subsequent measurements. While I cannot adequately assess this aspect of the manuscript, I think it is important that the other reviewers/editor ensure the hormone measurements are appropriate.

  3. eLife

    Reviewer #2:

    The paper submitted by Girard-Buttoz and colleagues asks whether and how early maternal loss affects cortisol levels and diurnal slopes among wild chimpanzees at Tai Forest, Côte d'Ivoire. The major claim of the paper is that, like humans, chimpanzees experience altered HPA functioning after maternal loss, including alterations to both diurnal slope and overall cortisol levels. However, their chimpanzee orphans exhibited patterns in diurnal slope that were opposite to their predictions (predicted blunted slopes, observed steeper slopes). The authors should be commended for their efforts in collecting a large number of samples for this analysis. However, I am not convinced that it is sufficient for investigating the hypotheses put forth here and, therefore I am also not convinced that their results are solid. I also have concerns about the theoretical grounding for the paper.

    1. My principal concerns with this paper, as written, revolve around the methods/results. First and foremost, I am not convinced that the authors have the sufficient sample size to evaluate the predictions/hypotheses outlined in the introduction. While 849 urine samples is a large number, and again, their efforts here should be commended, the sample spread is actually quite thin once it is spliced up into appropriate categories, especially considering how many samples were collected per individual year, on average. As the authors indicate throughout and especially when describing their modeling approach, cortisol is inherently a very noisy hormone impacted by myriad factors- including age in at least one other densely-sampled chimpanzee community. I'm also surprised that time of day was modeled quadratically. It is my understanding that humans, other populations of chimpanzees, and other mammals follow a sigmoidal curve which should be modeled with a third-order term as well. For these reasons, it's difficult to tell whether model 1A is not significant because of insufficient sample or a true lack of predictive power. Additionally, I'm concerned that the paper seems to focus so much on the results from a single model term in a model that did not reach significance.

    2. Despite acknowledging that the "significance of these predictors should be interpreted with caution" because model 1a did not reach significance, the authors make very strong claims about the results in the discussion- and also feature the finding of that model in the title of the paper. That seems problematic to me- especially because the insignificant model results (more intense diurnal slopes among immature orphans) diverge from the expectations set forth by other works in humans and non-humans. The finding that this is to do with higher-than-expected morning cortisol is puzzling given that evening levels are generally considered more responsive or plastic. However, this could also be an artefact of fitting the models without the third-order term for time.

    3. The introduction needs refinement to help clarify and specify the authors' arguments.

    (a) Does the biological embedding model always lead to negative fitness outcomes? Or is it possible that phenotypic adjustments might be adaptive, or even just making the best of a bad job (e.g. earlier death, but not death today)?

    (b) Throughout the introduction it is unclear whether and where the authors refer to the human clinical literature as opposed to animal literature. It is also unclear how human patterns are similar versus different from those observed in animals. Further, I would recommend that the authors include a deeper review of the animal literature (e.g. early experimental work with macaques, cortisol at other chimpanzee field sites/captivity). It's also unclear whether and where the authors refer more broadly to early life adversity (and what this means for humans vs. animals) versus more specifically to maternal loss. Additionally, there should be further discussion specifically related early maternal loss (rather than "early life adversity" which can include a lot of different factors) focused on the nutritional and social obstacles associated with early maternal loss, how these related to HPA functioning, and how these effects are expected to change during development (Plasticity? Flexibility? The role of HPA in responding to changing environmental conditions?). What about the adaptive calibration model which posits that the HPA can readjust during particular periods of developmental reorganization?

    1. It is difficult to assess the discussion without first dealing with the problems in the introduction/methods. However, despite their claims in the results section, it does not seem that the authors interpreted the results of model 1a with caution.
  4. eLife

    Reviewer #1:

    A very interesting paper testing the biological embedding model in a wild long-lived mammal using an impressive dataset. However, the results for immature orphans are not entirely straight forward. The effect on the HPA axis is in the opposite direction to humans and there seems to be no significant increase in cortisol compared to non-orphans overall - it depends on time since maternal loss. The paper would be improved by communicating this more clearly and discussing exactly why this pattern may be different to that in humans. Some of the evolutionary ideas discussed in the paper also need to be more clearly conveyed or thought through.

    Substantive concerns:

    1. There are important sections in the introduction (L125-128 particularly) and discussion (L403-409) about the evolution of the HPA response and differences between humans and other mammals that are unclear. Greater detail on the evolutionary logic being used, the precise hypotheses being suggested and references to back the ideas up are required (further details in minor comments).

    2. Table2/Model 1a doesn't directly test whether orphans have higher cortisol than non-orphans (or no p-value reported in table 2) and CIs in table 1 suggest that there is not a significant difference. Therefore, categorical statements that orphans have higher cortisol levels don't seem to be entirely justified. However, model 1B demonstrates that cortisol declines with years since maternal loss and figure 3 supports the idea that orphans do have higher cortisol than non-orphans in the first 2 years following maternal loss but that this declines to levels similar to those of non-orphans after 2 years. Could a statistical test be run to back this up? Perhaps instead of using a binary variable for orphan status (yes/no) it could be analysed as categories (orphaned within 2 years, orphaned more than 2 years ago, not orphaned as an immature) which could be used to directly test this and back up statements e.g. recently orphaned immatures had higher cortisol levels than non-orphans. A broader concern is why likelihood ratio tests have been used to calculate p values (and for only some of the predictors) rather than reporting the output from the models themselves. Could you explain what the benefit of this is over reporting values from the actual models and/or also provide the model outputs?

    3. The effect on cortisol slopes found in this study is in the opposite direction to that in humans. This is discussed in some detail but is lacking clarity in places and I think it would help to make this difference more obvious - it is really a key finding of the paper not a secondary point. The expected pattern is very nicely set out in the introduction so it would be good to format the discussion so there is a paragraph that outlines exactly how the results differ from hypothesized:

    (a) that the effect on cortisol slopes is in the opposite direction

    (b) that only the cortisol levels of recently orphaned immatures are significantly different to non-orphan immatures and then brings in the ideas discussed about why these differences may be present. I think this would really help communicate the findings more clearly, bringing the discussion more inline with what is set out in the introduction.

  5. eLife

    Summary: This paper tests the biological embedding model by asking whether and how early maternal loss affects cortisol levels and diurnal cortisol slopes among wild chimpanzees at the Tai Forest, Côte d'Ivoire. The results suggest that maternal loss alters the HPA stress axis in wild chimpanzees, but these effects are not visible later in life. Authors suggest that the lack of a later life association between maternal loss and cortisol levels may be due to selective early mortality of individuals with high cortisol levels but did not provide any survival or behavioural data to show that orphans and non-orphans differ in any fitness-related traits other than cortisol. Furthermore, the association between cortisol and the HPA axis is in the opposite direction to that observed in humans and there seems to be no significant increase in cortisol in orphans compared to non-orphans. Overall, the study is the result of extensive fieldwork, the number of samples collected is impressive and the subject is very interesting.

    The analyses will benefit greatly if the authors use effect sizes and confidence intervals for inferences instead of p-values. This may solve the significance threshold issues. Moreover, the reliance on p-values seem to limit the value of the data. For example, authors suggest that results from model 1 should be treated with caution because the full model is not significantly different from the null model, but by relying on it as the key finding of the study without exploring effect sizes, it does not seem that they did exercise sufficient caution.

  6. Version published to 10.1101/2020.10.15.340893v1 on bioRxiv