Friendship across species borders: factors that facilitate and constrain heterospecific sociality

Abstract

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.This article is part of the theme issue 'Collective movement ecology'.

Spatial ecology of long‐tailed ducks and white‐winged scoters wintering on Nantucket Shoals

“…Heterospecific interactions likely serve a more critical role in stabilizing at-sea communities and population dynamics than previously thought (Berdahl et al 2016, Gil et al 2018, Sridhar and Guttal 2018. Nantucket Shoals produces some of the highest concentrations of gammarid amphipods and clams on the eastern seaboard, which could have populationlevel implications for sea ducks, forage fish, migratory seabirds, and marine mammals.…”

“…Perspective plots of long-tailed duck density in association with white-winged scoter, clam abundance, amphipod abundance, the western tidal front, and depth as predicted by generalized additive models (Table 1). Since clams and amphipods are spatially correlated on Nantucket Shoals, it makes sense for both long-tailed ducks and white-winged scoters to use each other as clues to the presence of prey, even though these birds commonly differ in specific dietary preference (Goodale et al 2010, Gil et al 2018, Sridhar and Guttal 2018. Since clams and amphipods are spatially correlated on Nantucket Shoals, it makes sense for both long-tailed ducks and white-winged scoters to use each other as clues to the presence of prey, even though these birds commonly differ in specific dietary preference (Goodale et al 2010, Gil et al 2018, Sridhar and Guttal 2018.…”

“…We propose that these pelagic ducks are using local enhancement to their mutual advantage, in a manner described Fig. theoretically by Holland (2015), Hein et al (2015), and Sridhar and Guttal (2018) in which two predators use each other as cues to multiple prey species that are themselves spatially associated. Perspective plots of white-winged scoter density in association with long-tailed ducks, clam abundance, amphipod abundance, the western tidal front, and depth as predicted by generalized additive models (Table 2).…”

“…We also test the novel idea that these two species of marine ducks, long-tailed ducks and whitewinged scoters, associate with one another on Nantucket Shoals, because, even though the ducks each eat different prey, these prey are themselves spatially correlated. We are not aware of any previous demonstration of such interaction in the marine environment, though their theoretical existence has been described (Hein et al 2015, Holland 2015, Sridhar and Guttal 2018. 1.…”

“…Around Nantucket, sea ducks commonly form extensive foraging rafts composed of thousands to tens of thousands of birds. Paying attention to publically available information (e.g., feeding flocks) to find food, and forming certain heterospecific partnerships may function to maximize profitability by minimizing search time for spatially aggregated cryptic prey (Morse 1970, Buckley 1996, Gr€ unbaum and Veit 2003, Sridhar and Guttal 2018. Sociality may function to drive flock formation and to enhance resource location (Hein et al 2015, Veit and Harrison 2017, Berdahl et al 2018, Zou et al 2018).…”