Social Partners Across Species
Part of the challenge in thinking broadly about sociality lies in the dominance of evolutionary models predicated on competing individuals—from genes on up to conspecifics. This frame obscures attention to forms of cooperation, certainly, but it also impoverishes attempts at developing and deploying a “sociological imagination” across species boundaries. But breaking from this frame and cultivating that imagination are facilitated by attending to and thinking about interspecies friendship. Hari Sridhar and Vishwesha Guttal provide a means of doing so in their article, “Friendship across Species Borders,” in Philosophical Transactions of the Royal Society B.
Their work article documents the diversity of heterospecific sociality and provides a conceptual framework “to understand how individuals choose a conspecific or a heterospecific as their social partner.” With the latter, Sridhar and Guttal counter the assumption that fundamentally different mechanisms must be at work in the former. To the extent that interspecies friendships have drawn notice, they have limited by an assumption that some radically different form of sociality must be invoked. If individuals are picking social partners across species boundaries, the thinking went, then the ways they are doing so must deviate from how they develop and maintain relations with conspecifics. With some reflection, though, this assumption must appear odd; surely the capacity to establish social partner relationships must tap into or derive from parallels in interpretive cues and signaling regarding threat potential or channels of interest an even affection. Certainly, some perception and acknowledgement of homologous forms of sociality are at work here. And that’s a key finding of the article: “our literature survey shows that heterospecific sociality based on mechanisms similar to single-species sociality is reported from many taxa, contexts and for various benefits.” They’re working through a range of models that each afford more or less attention to such underlying mechanism: social information benefits, social partner choice, and direct grouping benefits. The one they formulate emphasizes qualitative differences between species while asserting a shared capacity for sociality: “Our conceptual framework modifies existing social information use models in ways that make it generally applicable to any situation involving social partner choice from among both conspecifics and heterospecifics.” That this capacity is rendered via the metaphor of “currency” risks reifying such interactions in a market-based rationality, but there’s more here than just their model.
First, Sridhar and Guttal generate an impressive tally of heterospecific relationships. “Our survey revealed 203 unique combinations of taxa (at order level or above) and ecological contexts of heterospecific sociality that are likely to be based on mechanisms similar to single-species sociality.” These relations (cooperative, mutualistic, commensalistic, etc) range from bacteria to fungi, marine and terrestrial invertebrates, and to samples from all major vertebrate groups; they list amphibians, lizards, snakes, birds, mammals, both freshwater and marine habitats, and across different life cycle stages “including eggs, young ones, juveniles and adults.” These are also drawn from a diversity of ecological contexts: foraging, parental care, courtship, basking, resting, traveling, etc. The benefits of interspecies sociality are also broad (e.g. food, protection, thermoregulation, reducing energetic costs) but the outsized gain is protection from predators: “when the driver of sociality is a predator or an abiotic variable, traits important for social benefit and competition cost need not be the same.” That is, the risk dilution entailed by an ensemble of species who share a similar predator will likely not entail heighted degrees of completion, since their foraging tendencies probably differ. “Although body-size and foraging behaviour are likely to be strongly correlated among conspecifics, the same is unlikely to be the case among a set of heterospecifics.”
From this view, Sridhar and Guttal “suggest that animal sociality is best-viewed along a continuum from conspecifics to heterospecifics.” One end of this spectrum is quite familiar: social benefits only obtainable from conspecifics. The other end is unfamiliar and warrants more attention. They’ve provided a model for generating observations and analysis of heterospecific interactions, which are admittedly less frequent than between conspecifics. But frequency is likely not the core impediment. First and foremost, as noted above, is the dominant attention to competing individuals. But alongside this is one that the authors raise: these kinds of interactions are hard to see and follow because they entail nuanced and fleeting alignments of spatial/temporal positions and shifting ecological activity. So for groups in motion, such heterospecific relations may be largely absent, though not impossible. Whichever taxa, though, there remains the challenge of widening and adapting an analytical focus that is species-myopic. That is, we need better field techniques to start registering and thinking through this wider frame of sociality. Developing these will depend a great deal on generating a social imagination that can recognize underlying commonalities in such relation.
Hari Sridhar and Vishwesha Guttal, 2018, “Friendship across Species Borders: Factors that Facilitate and Constrain Heterospecific Sociality,” Philosophical Transactions of the Royal Society B 373, no. 1746 (May 2018), https://doi.org/10.1098/rstb.2017.0014.
Our understanding of animal sociality is based almost entirely on single species sociality. Heterospecific sociality, although documented in numerous taxa and contexts, remains at the margins of sociality research and is rarely investigated in conjunction with single-species sociality. This could be because heterospecific and single-species sociality are thought to be based on fundamentally different mechanisms. However, our literature survey shows that heterospecific sociality based on mechanisms similar to single-species sociality is reported from many taxa, contexts and for various benefits. Therefore, we propose a conceptual framework to understand conspecific versus heterospecific social partner choice. Previous attempts, which are all in the context of social information, model partner choice as a trade-off between information benefit and competition cost, along a single phenotypic distance axis. Our framework of partner choice considers both direct grouping benefits and information benefits, allows heterospecific and conspecific partners to differ in degree and qualitatively, and uses a multi-dimensional trait space analysis of costs (competition and activity matching) and benefits (relevance of partner and quality of partner). We conclude that social partner choice is best-viewed as a continuum: some social benefits are obtainable only from conspecifics, some only from dissimilar heterospecifics, while many are potentially obtainable from conspecifics and heterospecifics.