Comparing single- and mixed-species groups in fruit flies: differences in group dynamics, but not group formation

Abstract

Mixed-species groups describe active associations among individuals of 2 or more species at the same trophic level. Mixed-species groups are important to key ecological and evolutionary processes such as competition and predation, and research that ignores the presence of other species risks ignoring a key aspect of the environment in which social behavior is expressed and selected. Despite the defining emphasis of active formation for mixed-species groups, surprisingly little is known about the mechanisms by which mixed-species groups form. Furthermore, insects have been almost completely ignored in the study of mixed-species groups, despite their taxonomic importance and relative prominence in the study of single-species groups. Here, we measured group formation processes in Drosophila melanogaster and its sister species, Drosophila simulans. Each species was studied alone, and together, and one population of D. melanogaster was also studied both alone and with another, phenotypically distinct D. melanogaster population, in a nested-factorial design. This approach differs from typical methods of studying mixed-species groups in that we could quantitatively compare group formation between single-population, mixed-population, and mixed-species treatments. Surprisingly, we found no differences between treatments in the number, size, or composition of groups that formed, suggesting that single- and mixed-species groups form through similar mechanisms of active attraction. However, we found that mixed-species groups showed elevated interspecies male–male interactions, relative to interpopulation or intergenotype interactions in single-species groups. Our findings expand the conceptual and taxonomic study of mixed-species groups while raising new questions about the mechanisms of group formation broadly.

Identifying the social context of single- and mixed-species group formation in large African herbivores

“…Most importantly for the current analysis, we were unable to identify or track specific individuals within any group. For example, detailed movement and social data for each individual would allow more accurate estimation of group formation, including whether the group was single- or mixed-species at the moment each individual joined [26,35].…”

“…How the spatio-temporal scales of group formation differ and overlap among species, and the influence of these processes on the formation and disintegration of mixed-species groups, is a fascinating area for future research. Ultimately, large-scale observational data, like those available via camera traps, can identify important patterns whose mechanisms may be further dissected with more focused observations of individuals and complemented by laboratory manipulation of more tractable organisms [26].…”

“…R. Soc. B 378: 20220105 with the common definition of a group as a set of individuals that have a higher likelihood of interacting with one another than with other individuals on the landscape owing to their spatial proximity [2,26]. The criteria used to determine adequate proximity for defining a group varies among studies, even for the same species [66,67].…”

“…Despite recent progress in understanding the costs and benefits of different group types, we still know relatively little about the social and behavioural processes that lead to the formation of single-versus mixed-species groups [26][27][28]. The current conceptual models of mixed-species group formation primarily focus on the roles of local ecological variables in determining the times and places in which mixed-species groups will be observed [13,21,[29][30][31].…”

“…Despite the importance of social partner availability in the formation of single-species groups, we know very little about the mechanisms that produce variation in heterospecific partner availability and the influence of partner availability on mixed-species group formation. Main challenges to understanding group formation mechanisms include the difficulty of observing group formation, particularly for larger or wild animals, and the difficulty of conducting experiments that directly measure preference [26]. The logistical challenges of measuring group formation are further compounded for studies of mixed-species groups, as multiple species must be observed or manipulated.…”