Women, men, risk and energy: a reply to Koster's paradox of Aché foraging

Commenting on our recent paper [1], Koster [2] questions the way we represent Aché foraging. Unfortunately, Koster's critique misses the main point of our paper and instead proposes a paradox where Aché hunting is at once high-risk and reliable. We reluctantly respond to these criticisms here, focusing on two specific issues: first is the way in which resource acquisition risk was measured among Aché foragers, and second is the nature of foraging in the relatively homogeneous Aché environment. We cover these issues in the context of a broader discussion on Aché foraging.

To measure acquisition risk for different Aché resources, we relied on Kaplan & Hill's [3] measure of harvest asynchrony across families per trip for each resource. Koster is concerned that this way of measuring variance does not include the risk of daily acquisition failure. The main issue with these data is that they conflate failures to capture with failures to encounter or even search for an item. Rather than underestimating the effect of zeros, as Koster suggests, this should overestimate the impact of failed acquisition on variance. If most of the variance for some resources is introduced by failures to even search for an item, then this bias is the most likely cause of Aché gathered resources appearing to have higher variance than hunted resources [1], which is at odds with nearly all other analysis on the variance of hunting versus collecting. Despite this problem, this measure of variance still provides an estimate of acquisition risk. If, on any given trip, every family acquires similar amounts of the same resources, then this measure of standard deviation will be low and acquisition risk is likely to be low. If only a few individual families are successful, or only a few acquire much more than everyone else, the standard deviation across families would be high and so would the acquisition risk, with some families failing more than others. The former is the case for Aché: most hunted prey types were acquired each trip ([3], table 5) resulting in relatively low standard deviations and indicating that acquisition risk for hunted prey is relatively low. With such low risk, Aché men's large-game hunting is frequently successful and, as a result, men contribute more to subsistence than women. As we mention in our paper, this corresponds with recently published data on Aché hunting success and the division of labour by Hill & Kintigh [4] and Hill & Hurtado [5], which show that Aché hunters are successful about every other time they go hunting, unlike Martu kangaroo hunting (which is successful only once every seven hunts [6]) or Hadza hunting (which is successful about once every 29 days [7]).

Unlike Meriam and Martu, Aché live in an environment where (i) resources are rather homogenously distributed and (ii) animal resources occur at relatively high densities. When combined, these environmental parameters result in hunting bouts that rarely fail overall, because failure to capture one prey animal can easily be mitigated by the high probability that a hunter will subsequently encounter another animal. Thus, as described by standard models of prey choice, the encounter rate is the main predictor of acquisition risk, not pursuit time [2]. Given the homogeneity of the Aché environment, Koster is right that different types of animal prey are substitutable (which, as mentioned above, should actually reduce acquisition risk), and as such it may be more comparable to represent Aché foraging data as a single value. Drawing on another publication [8], Koster [2] notes that, on average, Aché acquire 12 000 calories per day with an s.d. of 13 243 [8], leading to a coefficient of variation (CV) of 1.1 [2]. This is a very low coefficient of variation, placing Aché foraging at about the same level of energy and variance as Meriam turtle-collecting [1]. This is the very comparison we made in our paper to describe a convergent division of labour—one in which energy and risk do not trade off with one another, and thus social and offspring provisioning goals can be met with similar resources. Even if, as Koster [2] suggests, high-energy resources pursued by Aché men have a collective CV around 1.6 [8], then this is still much lower variance than Martu hill kangaroo hunting (3.3) or Meriam diving (2.6) [1]. Unfortunately, Koster misses this point, instead making the paradoxical argument that these Aché values and the accompanied division of labour, in which men's hunting provides the bulk of acquired calories (87%), are on par with Meriam and Martu. This is equivalent to saying that Aché hunting is simultaneously high-risk and reliable; so reliable that it provides the bulk of acquired food, but so risky that it fails as frequently as Meriam and Martu hunting. This is simply incorrect. For many Meriam and nearly all Martu foraging activities, risk and energy trade off with one another: the greater the potential energetic reward, the greater the risk of acquisition failure. This is not the case for Aché foraging activities; as a result, men outproduce women because there are many larger resources that do not have excessive levels of risk. In agreement with our original analysis, Koster is correct that Aché should target high-energy resources for provisioning; however, Koster misses the reason why this is so: because, in the Aché environment, those resources are associated with a low risk of failure. Our point here is simple: if high-energy resources have a high risk of failure, then targeting such high-energy resources cannot result in reliable provisioning. As a result, production will be dominated by the reliable acquisitions of low-energy resources. Without saying anything about foragers' motivations, we highlight that the simple relationships between these variables are ecological and mechanistic. Framing the issue in this way also has implications for the gender division of labour: if men generally target high-energy resources and women generally aim to minimize risk, then variability in the presence or absence of a risk–energy trade-off will predict variability in the proportion of total production that men and women produce.

Understanding Aché foraging has been a contentious issue since many of the first studies were published. Koster [2] questions the usefulness of data collected while Aché were out on forest treks in what may be unrepresentative family groups, and is suspect over the way in which Aché data were aggregated. These are issues with the Aché foraging data overall; as such, Koster is more appropriately levelling a critique of the Aché data itself, not our use of it. While we agree with Koster that there may be reasons to be cautious about some of the ways in which data are collected and aggregated, we disagree that this should paralyse comparative behavioural ecological research. Some of the more fruitful findings this line of research has to offer come from comparative analyses of human behaviour in different ecological settings. In this case, we find that men and women seem to evaluate trade-offs between risk and energy differently. Based on the presence or absence of such trade-offs, populations in different environments are likely to have very different gender divisions of labour. We suggest that following this thesis should clarify recent debates over foraging decisions of men and women across diverse environments.