The past 2 decades have seen fruit flies being widely adopted for research on social behavior and aggression. This fruitful research, however, has not been well tied to fruit flies’ natural history. To address this knowledge gap, I conducted a field study. My goal was to inform future research conducted in artificial surroundings, and to inspire new investigations that can rely more heavily on fruit flies’ actual natural behavior. My two main novel findings were first, that flies in the field showed significant sociability, as they formed social groups rather than dispersed randomly among fruits of similar quality. Second, males showed fair levels of aggression towards each other as indicated by a lunging rate of 17 per hour, and lower rates of wing threat and boxing. Courtship was the most prominent activity on fruits, with females rejecting almost all males’ advances. This resulted in an estimated mating rate of 0.6 per female per day. Flies showed a striking peak of activity early in the mornings, even at cold temperatures, followed by inactivity for much of the day and night. Flies, however, handled well high temperatures approaching 40 °C by hiding away from fruit and concentrating activity in the cooler, early mornings. My field work highlights a few promising lines of future research informed by fruit flies’ natural history. Most importantly, we do not understand the intriguing dynamics that generate significant sociability despite frequent aggressive interactions on fruits. Males’ responses to female rejection signals varied widely, perhaps because the signals differed in information content perceived by flies but not humans. Finally, flies tolerated cold early mornings perhaps owing to fitness benefits associated with increased mating and feeding opportunities at this time. Flies were adept at handling very high temperatures under the natural daily temperature fluctuations and availability of shelters, and this can inform more realistic research on the effects of global warming on animals in their natural settings.

Fruit flies (Drosophila melanogaster) have served as a leading model system in many branches of biology, and the twenty-first century has seen them being successfully adopted for research on social behavior as well. Given the fundamental similarity among animals, research on social behavior in fruit flies helps us understand social behavior in all animals including humans. Furthermore, such research provides us with essential insights on the mechanisms that underlie social disorders

Research on social behavior may be categorized into aggression, defined as the use of physical force against conspecifics, and sociability, characterized as the tendency to engage in friendly activities with conspecifics. Such activities may include feeding together, traveling in a group, and communal resting or sleeping. While there had been early mentions of fruit fly aggression, research on the topic has intensified relatively recently owing to their adoption for neurobiological and genetic analyses of aggression. Similar to aggression, hints of fruit fly sociability existed for a long time, but focus on the topic is recent. The characterization of cis vaccenyl acetate (cVA) as an aggregation pheromone of fruit flies implied social attraction, which led to research on its adaptive significance  as well as its mechanisms of action. Recent research on sociability in fruit flies has indicated social synchronization of the circadian clock, reliance on social information gleaned from conspecifics, the formation of social groups and collective response to threat

To fully understand the evolutionary biology of social behavior, we must link it to its fitness consequences through its effects on access to food, shelter and mates. Laboratory-based analyses of fruit fly social behavior indeed include some of these factors in their protocols. While such protocols seem reasonable, they are, unfortunately, not based on established knowledge about fruit fly social behavior in natural settings. The fundamental reason for this discord is the striking rarity of field work quantifying the natural history of fruit flies. Very few researchers have systematically documented fruit fly behavior in the field, and the majority of those who did focused on sexual behavior. In his review of courtship behavior in Drosophila, Spieth devoted two pages to describing behavior in the field, which were probably based on his own unpublished observations. Markow and colleagues conducted perhaps the most elaborate field work on D. melanogaster as well as a few other fruit fly species. Other useful field studies include Partridge et al.and Soto-Yéber et al.. Remarkably, however, 33 years after Partridge et al. noted that they did not observe fly aggression in their study and hence “the role of fighting in the field remains mysterious”, and in spite of the upsurge in research on fruit fly aggression, the natural history of combat in fruit flies is still an enigma. Similarly, we know nothing about fruit flies’ sociability in nature.

To help alleviate the striking lack of information about the natural history of social behavior in fruit flies, I conducted a field study that included live observations as well as 60 h of video recording to address the following questions. First, what is the social organization of flies at fruit and away from fruit? Specifically, do flies prefer to feed and rest together? Second, how frequent is aggression between males, between females, and between males and females? Do males rely on aggression to monopolize fruit? Are males aggressive towards other males while pursuing females? How do receivers of aggression respond? While my focus was sociability and aggression, I also wished to gather information about other important fruit fly natural history features that we know too little about. Specifically, I asked what is the daily pattern of fly activity and distribution? What is the frequency of males pursuing females? How do females respond to males? What is the frequency of mating? Finally, how often do females and males approach copulating flies to potentially glean information relevant to mate choice copying?

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