How we ultimately compare other primates with one another has implications for how we characterize biodiversity, and for how we understand our own place, as humans, in nature.

Models (referential or strategic) are methodological tools, as are regression analysis or RIA for fecal hormones. As with all methods, people can make mistakes using models (look at the single species hypothesis, a strategic model based on the competitive exclusion principle which held that robust and gracile australopithecines were male and female of the same species). Nevertheless, sometimes people get things right.

The most productive form of chimpanzee referential modeling lies precisely in understanding the reasons why populations differ; the focus is on the dynamics of change, not on static comparison. Savanna chimpanzees are important for understanding human evolution not because they are australopithecines (a stunningly obvious point made by some of the Ardipithecus team recently), but because comparing them with forest chimpanzees (multiple populations of each) allows us to identify patterns in how a primate of comparable brain size, body size, etc. adapts to open environments. It is ABOUT process and variation. I discuss this at greater length in my 1996 paper cited by Strier.

This is not to say that static and/or normative models CAN’T sometimes be informative — but as Fuentes rightly cautions, they must be used with great care or they constrain our thinking, to our cost.

As Strier and many of the participants in this discussion have noted, holding a concept that variation at multiple levels is the norm can be a powerful tool in sincere attempts to utilize the comparative method. It is also worth including the possibility that phenotypic plasticity and niche construction play core roles as processes in shaping behavior and the evolution of behavioral systems. Strier’s relatively rare experience of long-term research on a single population of primates enables her move this call along based on actual data and set of relevant experiences, an important contribution indeed.

We now have good reason to believe that macaque youngest ascendancy and baboon dominance are not inherent properties of the species, but reflect predictable results of simple behaviors played out in certain demographic contexts. The puzzling dichotomous variation in langur social organization appears driven by the intersection of individual behavioral tactics with population density. There are species norms, but at least some elements of them lie in the interaction of individuals with their environment, not exclusively “inside” the individuals. At the same time, species-typical “natures” do exist (c.f. Rosenblum’s macaque comparisons in the 1960s, and more recently Clarke and Boinski 1995 — Am. J. Primatol. 37:103). Strier challenges us to work out which is which, and to understand the mechanisms underlying variation and adaptation at both intraspecific and interspecific levels.

Strier (and King) point to an important sea-change in how we approach the study of socioecology. They (and I) advocate shifting from an emphasis on discrete categorization of taxa and investigation of differences between categories to a focus on continuous variation and the mechanisms responsible (both outside animals, like demography, and inside them, like the emotion/cognition which makes up temperament [aka behavioral syndromes]). The field could not have started this way; the categories we’ve constructed give us a way to frame variation that otherwise would have appeared vast and confusing and purely historical (c.f. Nimmo). But time to move on.

I am persuaded by Koenig and Borries’ response to Thierry that it is too soon to abandon “the socioecological model”; it has not yet received the “true testing [that] would require comparing ecological conditions and all the predicted aspects for MULTIPLE populations and MULTIPLE species.” (Evol. Anthropol. 18: 166, 2009; original emphasis). They appear to be recognizing the same issue as Strier: that understanding variation below the species level is critical to interpreting both the patterns we see across species, and more importantly the mechanisms by which those patterns arise. However, while that testing goes on we can – and should – work towards the ideal world of Strier’s last paragraph. One promising tool for this is social networks analysis, which may offer useful ways to quantify relationships and thus take comparative studies beyond categorical approaches (see e.g. Wey et al., Anim. Behav. 75: 333, 2008).

One quibble: in a response, Strier extends her ideas to “all long-lived, social beings”; this may be too restrictive. Interest in intraspecific variation has been fueled by e.g. behavioral syndromes in guppies, and even squid may show complex variation responding to the intersection of temperament, demography and habitat (Sinn et al., Beh. Ecol. Sociobiol. 64: 693, 2010).

This would also be consistent with Richie Nimmo’s proposal that behavioral variation is the normative pattern for many, if not all, primates. But, I would not be prepared to dismiss the power of evolution in shaping the patterns in this variation across different species and populations and depending on the particular behaviors in question. Pursuing new analytical methods for characterizing this variation would require greater integration across units of study, from individuals to groups and populations, without abandoning an evolutionary approach to understanding behavior.

In this context, the recent formal division of Brachyteles arachnoides, and the consequent transformation of Strier’s monkeys into B. hypoxanthus, affords a salutary lesson. This taxonomic judgment is instrumental, to enlist science in the cause of conserving the animals, and as a result of such interests, the number of official primate species has more than doubled over the last two decades. Such taxonomic inflation, strategically over-tabulating taxonomic diversity in the interests of the animals themselves, is well-known and is by no means restricted to the primates, as Strier notes. Nevertheless, the principal physical feature now distinguishing the two zoological species of Brachyteles from one another is simply their complexion.

This is consequently a very instructive situation, in which the scholarly study of humans should most definitely not be guided by primate biology.

It seems to me that the fundamental problems being raised by the extent of sub-species variation and adaptation for the underlying assumption of species normativity that underpins the logic of inter-species comparison is an essentially philosophical one. It is also one which has played a key role in the development of the social sciences, perhaps unsuprisingly given their focus on humans, those most variable and adaptive of social animals. It seems ultimately to raise the fundamental question as to what is a norm, and what sort of ontological architecture do we unwittingly invoke when we employ a comparative logic based on the assumption of such units as might render norms meaningful.

Perhaps the normative itself needs to be understood differently, not as bound to species units assumed to be pinned to some evolutionarily rooted biological essence, away from which they may diverge only so far, but instead as something always relatively contingent, transient, and determined by historical, demographic and ecological events and processes.

Taking this a step further, describing the variation within a species necessarily assumes a norm from which there is variation, but what if variation is the norm? If the norm is relativised through and through, then the logic of comparison would have to become less nomothetic and more ideographic. In other words, it would have to move towards treating all groups and even individuals as in principle unique, rather than viewing them as merely a particular individual expression of a biological collectivity which is given explanatory primacy.

I thoroughly applaud the working through of some of the implications of acknowledging that ‘species’ is itself a contested category, not something objectively and indisputably given. I agree that beginning foundationally from homogenised species-units rather than observed behaviour for the purposes of comparison therefore seems deeply problematic. Again, the necessary reponse seems to me to be a move away from a deductive rationalism and towards a more inductive empiricism. I am therefore very enthusiastic about the suggestion that primatological comparisons should begin from the assumption of intra-species variation, begin from difference, to use the sociological parlance, and then look for recurrent patterns or structures, rather than beginning from a buried assumption of intra-species normativity or homogeneity and then looking for variation.

This seems to follow logically from the recognition that these creatures are ‘cultural’, which surely suggests that what needs to be explained is not variation – this is to be expected, but the persistence of certain behavioural patterns – how do these get reproduced over time?

I also note with particular interest that the impact of human incursion upon many primate species, in dividing geographic groups from each other for example, has impacted profoundly on their patterns of behavioural adaptation, as well as (through the influence of conservation and ‘taxonomic inflation’) our classification of these groups, even to the extent of influencing what is deemed a distinct ‘species’. From my own perspective this underlines the extent to which both the human (primate) ‘subjects’ of taxonomic classification and the nonhuman primate ‘objects’ of classification are all bound up in the same historical-political-ecological nexus. All knowledge production inevitably occurs ‘inside’ this matrix, rendering the search for an objective species norm somewhat illusory. Species are fluid, not only phylogenetically, but because their identification is always the product of political-social-historical contingencies.

In light of this, the identification of species-normative behaviours for the purposes of setting up evolutionary models seems thoroughly misconceived, especially given the plasticity of both classificatory practices and actual behavioural variation. It seems plausible that there is no evolutionary norm, no point at which there were no contingent impacts upon behavioural patterns, no externalities. Even prior to the impact of humans, there would have been ecological, climatic, demographic and inter-species relational contingencies. In other words, it seems that if we are to follow through the implications of acknowledging that these species are ‘social’ beings, then we must also acknowledge that they are historical beings, with no behavioural ‘essence’ outside of their historical development. If they have an essence, the factors which ‘distort’ it have always already been there.

In my work on gestural communication of African apes, and in reading my colleagues’ work on field and captive study of ape behavior, I’m struck by how this behavioral plasticity embraces emotional responses as well as cognitive ones (though I realize this dichotomy is itself suspect). The “anecdotes are not data” school of primatology may still be kicking around, mistakenly assuming that study of individuals need be anecdotal, but much work (by e.g., M. Nakamura and B. Smuts as well as A. Fuentes) now shows that an individualistic focus on emotion and cognition may be cutting-edge and exciting as well as scientifically rigorous.

To stick with the great apes for a moment, just because I know them best, here’s how I see it: Every great ape is a self-aware, thinking and feeling self. There is no generalized “ape nature” or “chimpanzee nature”, or even “Gombe chimpanzee” or “Tai chimpanzee nature”. The intersection of developmental dynamics, cultural tradition and individual life history produces the variable responses to local conditions of which Strier writes.

Might this behavioral plasticity be phylogenetically conserved, so that it was present in the common ancestor of great apes and humans? Might anthropologists use this line of thinking to challenge some of the more reductive models of human behavior and its evolution coming from certain avenues within evolutionary psychology? The debate Strier mentions about chimpanzees, Ardipithecus, and referential models is surely engaging, but we can look more broadly. Can the primatological focus on behavioral plasticity at the individual as well as the populational level tell us something that matters conceptually about the evolution of human behavior?

I think the answer is yes. If we have learned anything from our colleagues who have interrogated an earlier wave of reductive claims, e.g., from Keller’s Century of the Gene or Marks’s What It Means to be 98% Chimpanzee, we must look hard at models that give pride of place to simplistic biology in understanding changes in hominid and human populations. When genes, or oxytocin, or fixed brain modules are championed “”over and above the power of contingent developmental and social processes**, might the study of apes offer a cautionary tale? Apes may focus our gaze in ways that help us think in comparative contexts more clearly.