Energetic underpinnings of yellow dung fly mating success in the field

Foraging provides the basis for animal reproduction, but requires energy and time to be sustained, entailing a trade-off. Whereas females should maximize their time foraging for resources, males should minimize their foraging time by optimizing time budgets to maximize their access to mating partners. Mark-resight field studies are difficult and hence uncommon for small insects. Yellow dung flies (Scathophaga stercoraria L.) abound on pastures in cold-temperate regions across the northern hemisphere. Adult flies lick nectar from flowers for energy, but require small insect prey to produce eggs and sperm. Males wait for females around fresh cow dung, but at one point also need to replenish their energy and/or sperm reserves in the surrounding vegetation. Their foraging time budgets should depend on their body size, nutritional energy reserves, availability of sperm, competitor and female density. Marked male dung flies whose nutritional status was experimentally manipulated – water only (null control); water + sugar (energy replenishment); or water, sugar + Drosophila prey (energy and sperm replenishment) – were repeatedly observed on an experimental pasture for an entire day. Both nutrient types were expected to increase the mating success of especially large males. The total number of resighted males seen copulating was lowest for water-treated flies. Mating success was positively related to body size. The distance travelled between dung pats was greater for males fed sugar or prey and also increased with body size, while pat residence times decreased with size. No differences were found between the sugar- and prey-fed groups. Crucially however, there was no evidence in the field for a time budget or mating advantage of small males when nutrients were limited.