The flies, whose scientific name is Scathophaga stercoraria, were randomly assigned to three nutritional regimes and kept in a laboratory climate chamber for at least 24 and up to 48 hours. One group received water only, a second received water and sugar as a substitute for nectar, and a third received water, sugar and small Drosophila prey. The supplemented flies were held at approximately 20 degrees Celsius before being returned to the pasture. From early morning until late evening, as many as five observers searched the pasture and freshly deposited dung pats for individually marked males, recording each fly’s identity, pairing status and time of observation.

Of the 540 males released, 231, or approximately 43%, were resighted at least once. Forty-six of the resighted flies were seen only once at a single dung pat, while 114 occupied one pat for longer and 117 switched pats at least once. The researchers used those observations to estimate mating success, the distance from one pat to the next, the number of sightings per day, the total number of pats visited, and the time between a male’s first and last appearance. These measures allowed laboratory feeding conditions to be linked with individual behavior in the field, an approach that is unusually difficult for small insects.

The experiment recorded 89 copulations, with 59 males copulating once, eight twice, two three times and two four times. The proportion of resighted males seen copulating was 0.22 in the water-only treatment, 0.35 in the sugar treatment and 0.40 in the sugar-plus-prey treatment. These values pointed toward greater mating success among supplemented males, but the treatment effect was not statistically significant, partly because relatively few copulations were observed. The average number of flies resighted also tended to be lower in the water group, with 10 per 30 released, compared with 15.5 in the sugar group and 13 in the sugar-plus-prey group, although the treatment differences did not reach the stated significance threshold.

Initial body mass was positively related to mating success regardless of feeding treatment. Larger males also traveled greater estimated distances when moving to their next pat and showed a trend toward visiting more pats. Their minimum residence time at a pat generally declined with body size, apart from males in the two supplemented groups, a pattern interpreted as evidence of greater energetic reserves and activity. No body-mass-by-treatment interaction was significant for any measured variable, meaning the data did not support the prediction that extra nutrients would help large males disproportionately or that smaller males would gain a mating advantage when energy was limited.

Travel distance was the one behavioral variable significantly affected by nutrition. Males receiving sugar alone moved a mean estimated 28.19 meters to the next pat, and those receiving sugar plus prey moved 23.90 meters, compared with 16.41 meters for males given only water. The number of pats visited, number of times seen during the day and total active time did not differ significantly among the feeding treatments. Mean active time was 129.1 minutes for water-fed males, 182.3 minutes for sugar-fed males and 146.9 minutes for males fed sugar and prey. Initial body mass, as expected from random assignment, also did not differ significantly among treatments.

The field setting addressed a distinctive feature of yellow dung fly reproduction. Adult flies use nectar for energy and insect prey to support egg and sperm production. Females spend much of their time foraging away from dung and visit a pat when ready to lay eggs. Males are more often found waiting on or around fresh dung, attempting to mate with arriving females and guard them during oviposition, although intense competition can make defense of a pat impractical. A male therefore has to choose repeatedly between remaining where females may arrive and leaving to replenish energy or sperm. Those decisions may depend on body size, nutritional reserves, sperm availability, and the local densities of rivals and potential mates.

Sugar and prey were expected to have different functions. Sugars fuel flight and fighting, while proteins obtained from insect prey can support sperm production; yellow dung flies cannot convert sugar into protein. The researchers also reasoned that larger males might benefit more from supplementation because they have greater absolute requirements for energy and sperm. The observed differences were limited: adding Drosophila prey did not yield a clear advantage over sugar alone in copulation rate, and no significant body-mass-by-treatment interaction was detected. The nutritional treatments may have overlapped in practice because the author reported that yellow dung flies can obtain sugar from prey and can be maintained in the laboratory on prey without a separate sugar source.

Sperm reserves offer another possible explanation for the limited difference between diets. Male sperm is initially produced after adult emergence when prey is available, a process that takes about three to seven days, and production may continue during adult life. Stored sperm can support approximately four to five copulations. Consequently, many sugar-fed males may still have retained enough sperm for mating after capture even though they received no insect prey during the 24-to-48-hour treatment. The experiment therefore tested the effects of short-term supplementation on a population whose age and initial nutritional and reproductive condition were not known, rather than creating males with precisely controlled reserves from emergence onward.

Several limitations restrict the conclusions. The adult ages of the captured males were not estimated because the proposed wing-based age-grading method required wing removal and digital analysis, leaving age as an uncontrolled influence on flight performance, searching and reproductive condition. The observations covered only six days: three in spring and three in autumn across two years. Fly density and body size can change substantially within a season, and at least four generations overlap annually in Switzerland. Daily weather also varied strongly among dates, and the day effect was prominent in the statistical analysis. Marked males also disappeared quickly from the focal pasture, and emigration could not be distinguished from death. These factors lowered effective sample sizes and probably contributed to the lack of statistical significance for the key mating comparison.

Despite those constraints, the mark-resight design showed that individual behavior and reproductive outcomes can be measured in a small field insect. The clearest measured nutritional effect was longer movement between dung pats, not a demonstrated rise in copulation success. Body size remained positively associated with mating success, and the expected interaction between size and nutrition did not emerge. The study therefore offers a bounded lesson about the energetic economics of mating: readily available sugar is associated with greater travel capacity, but in this experiment it did not reliably translate into more observed copulations, and adding insect prey produced no clear additional mating benefit over sugar alone.

Subject of Research: Field experiment on how nutrition and body size affect male yellow dung fly movement and mating success.

Article Title: Energetic underpinnings of yellow dung fly mating success in the field

Article References: Blanckenhorn, W. U. (2021). Energetic underpinnings of yellow dung fly mating success in the field. Alpine Entomology, 5, 61-67. https://doi.org/10.3897/alpento.5.68153

Image Credits: AI Generated

DOI: 10.3897/alpento.5.68153

Keywords: yellow dung fly, mating success, nutritional status, body size, field experiment, mark-resight method, foraging trade-offs, insect behavior, dung pats, sexual selection, Energetic, underpinnings