Mixed-species groups describe active associations among individuals of two 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 ignoring 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. Further, 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-species and mixed-species groups form through similar mechanisms of active attraction. However, we found that mixed-species groups showed elevated inter-species male-male interactions, relative to inter-population or inter-genotype 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.

Please see detailed methods section in the manuscript

Please see the relevant manuscript for details of how the data were collected

In the attached dataset, each row is data from a single patch in a single video of a single trial. The columns have the following meanings:

Column A: index autogenerated by R

combo: letters corresponding to the identities of the genotypes present in the trial

month: the month the video was recorded

day: the day the video was recorded

f1gen: the genotype of 1 set of 5 females

            b = blue; g = green; p = pink; r = red; y = yellow

m1gen: the genotype of 1 set of 5 males

f2gen: the genotype of the other set of 5 females

m2gen: the genotype of the other set of 5 males

f1col: the paint color identifying the “f1” females

m1col: the paint color identifying the “m1” females

f2col: the paint color identifying the “f2” females

m2col: the paint color identifying the “m2” males

per: the time of day (“period”) during which the video was recorded

            1 = morning; 2 = evening

hour: the hour during the video in which the randomly chosen 10-second sample occurred

minute: the minute during the video in which the randomly chosen 10-second sample occurred

view: the patch identity ranging from 0 to 3 for the 4 total patches

f1n: number of “f1” females recorded on the patch at t=0

m1n: number of “m1” males recorded on the patch at t=0

f2n: number of “f2” females recorded on the patch at t=0

m2n: number of “m2” males recorded on the patch at t=0

m1_m1: number of fly-seconds of male-male interactions between m1 males during the 10-second sample

m2_m2: number of fly-seconds of male-male interactions between m2 males during the 10-second sample

m1_m2: number of fly-seconds of male-male interactions directed from m1 males to m2 males during the 10-second sample

m2_m1: number of fly-seconds of male-male interactions directed from m2 males to m1 males during the 10-second sample

m1_f1: number of fly-seconds of male-female interactions directed from m1 males to f1 females during the 10-second sample

m1_f2: number of fly-seconds of male-female interactions directed from m1 males to f2 females during the 10-second sample

m2_f1: number of fly-seconds of male-female interactions directed from m2 males to f1 females during the 10-second sample

m2_f2: number of fly-seconds of male-female interactions directed from m2 males to f2 females during the 10-second sample

groupID: identifies each trial (concatenation of sorted combo and date)

f1spp: population and species information for “f1” females

            cos.mel = US melanogaster; car.mel = Caribbean melanogaster; sim = simulans

f2spp: population and species information for “f2” females

m1spp: population and species information for “m1” males

m2spp: population and species information for “m2” males

spp.comb1: indicates combination of populations and species present 

male.ints: total male-male interactions

m.within: “within” male-male interactions (see manuscript)

mf.within: “within” male-female interactions (see manuscript)

mf.among: “among” male-female interactions (see manuscript)

sorted.combo: same as combo but with duplicates removed for more accurate comparison