A fungal parasite selects against body size but not fluctuating asymmetry in Swiss subalpine yellow dung flies

Evidence for selective disadvantages of large body size remains scarce in general. Previous studies of the yellow dung fly Scathophaga stercoraria have demonstrated strong positive sexual and fecundity selection on male and female size. Nevertheless, the body size of flies from a Swiss study population has declined by ~10% 1993–2009. Given substantial heritability of body size, this negative evolutionary response of an evidently positively selected trait suggests important selective factors being missed. An episodic epidemic outbreak of the fungus Entomophthora scatophagae permitted assessment of natural selection exerted by this fatal parasite. Fungal infection varied over the season from ~50% in the cooler and more humid spring and autumn to almost 0% in summer. The probability of dying from fungal infection increased with adult fly body size. Females never laid any eggs after infection, so there was no fungus effect on female fecundity beyond its impact on mortality. Large males showed their typical mating advantage in the field, but this positive sexual selection was nullified by fungal infection. Mean fluctuating asymmetry of paired appendages (legs, wings) did not affect the viability, fecundity or mating success of yellow dung flies in the field. This study documents rare parasite-mediated disadvantages of large-sized flies in the field. Reduced ability to combat parasites such as Entomophthora may be an immunity cost of large body size in dung flies, although the hypothesized trade-off between fluctuating asymmetry, a presumed indicator of developmental instability and environmental stress, and immunocompetence was not found here

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. Key Words body size, energy reserves, field observations, food manipulation, foraging, mating success, Scathophaga stercoraria, reproductio

Are yellow dung flies domesticated cow dung specialists?

The theory of niche differentiation implies some extent of specialization of species with regard to key resources, notably food. Coprophagous (dung-eating) insect larvae play a critical role in the decomposition of livestock dung in modern and traditional agricultural grasslands. The yellow dung fly (Scathophaga stercoraria L.; Diptera: Scathophagidae) is one of the largest, most common and abundant dung decomposers on pastures in cold-temperate regions across the entire northern hemisphere. As this fly is often associated with domesticated cattle or dairy cows, which are commonly kept for human nutrition worldwide (beef, milk, cheese, etc.), it is sometimes suspected to be a cow dung specialist. However, yellow dung flies are regularly active on and around other dung types, and must have reproduced on dung of wild vertebrates before the domestication of cattle. We therefore experimentally studied the performance of yellow dung fly larvae on dung of various large domestic vs. wild mammals (cow, horse, wild boar, red deer) in the laboratory in Switzerland. Larval performance in terms of juvenile survival, egg-to-adult development time, growth rate, and final adult body size, the major life history indicators of individual reproductive success, did not vary greatly among the various dung types tested. Thus, yellow dung flies can successfully reproduce on multiple types of mammal (vertebrate) dung, wild and domestic, and are therefore dung generalists rather than specialists. We conclude that yellow dung flies are common in European low- and highlands because they could plastically shift to dung of common herbivorous livestock after their domestication without losing the ability to reproduce on dung of common wild mammals

Taxonomic resolution and treatment effects – alone and combined – can mask significant biodiversity reductions

Taxonomic resolution or uncertainty poses an important problem in biodiversity research. Assessment of biodiversity at the species level is most informative and preferred, but requires effort and expertise. Alternatively, researchers often bin species into higher taxa because they are unable to recognize them, or to save money and time. Here we analyse, by simulation and analytical modelling, the combined effects of dose-dependent mortality and taxonomic binning on biodiversity indices for a fictitious community of organisms. We asked (1) how binning species in a sample into higher taxa significantly affects biodiversity measures, and (2) whether dose-dependent mortality effects, alone or in combination with taxonomic uncertainty, are duly captured by classic biodiversity indices. Our study shows that haphazard binning into various taxonomic levels is legitimate and preferable to orderly binning (all taxa binned at the same level), because it provides the best resolution. We further show that binning will regularly obscure statistical detection of biodiversity differences, if only due to scaling of mean and variance. Also, treatment effects in combination with taxonomic uncertainty can introduce estimation biases of at times complex nonlinear and non-intuitive nature under any taxonomic resolution scenario, potentially including relative increases in the biodiversity index when intuitively decreases would be expected. We recommend being specific about the expected qualitative and quantitative effects of any treatment or natural comparison before formulating a hypothesis regarding biodiversity reductions. Our theoretical study should aid in this endeavour

Heritability of three condition surrogates in the yellow dung fly

Condition capture has been proposed as a general mechanism maintaining additive genetic variation, Va, in sexually selected traits under directional selection. It relies on two main assumptions: condition-dependent trait expression and Va in condition. Although there is evidence for the former, direct evidence that condition is heritable is scarce, although this is a requirement of most models of handicap sexual selection. We used a parent-offspring, full-sib, two-container laboratory breeding design in the yellow dung fly Scathophaga stercoraria to demonstrate the broad- and narrow-sense heritability of three surrogates of condition commonly used in sexual selection studies: lipid and glycogen reserves (i.e., physiological condition), body size, and fluctuating asymmetry. All three measures are nutrition dependent and have been linked to sexual selection in free-living yellow dung flies. While lipid reserves and body size were heritable, asymmetry and glycogen reserves were not. Moreover, the evolvability of physiological condition was higher than that of the other two traits. Of the three surrogates, physiological condition is most akin to the original definition, but all have their limitations. We conclude that condition is a useful heuristic concept in evolutionary ecology, but its practical value may be limited by the fact that it cannot be measured directl

No size-dependent reproductive costs in male black scavenger flies (Sepsis cynipsea)

Mating is generally assumed to be costly, but mating costs differ between the sexes. Although mating itself is considered cheaper for males, mate search and mate competition are cheaper for females. Nevertheless, studies increasingly reveal considerable mating costs for males, and these costs should depend on the body size of the individual. We investigated size-dependent predation (ecological) and energetic (physiological) mating costs in male black scavenger flies, Sepsis cynipsea (Diptera: Sepsidae), a model organism for studies of reproductive behavior. We addressed costs of mating by assessing predation risk for differently sized flies in male, female, and mixed-sex groups. Males were not more likely to be predated in mating or mate-search situations. Scathophaga stercoraria (Diptera: Scathophagidae) predators preferred smaller females and males as prey. Male movement in these different social situations does not proximately explain this size-selective predation, as small individuals were not more mobile. We addressed energetic costs of mating by measuring residual longevity (or starvation resistance) of starved males exposed to different mating situations. Copulation, courtship, interaction with reluctant females, or brief interactions with other males, all presumably increasing energy demand, did not significantly reduce longevity of males compared with males not interacting with other individuals. In general, small males died sooner when starved. Overall, we found no direct costs of mating for male S. cynipsea, but both predation and physiological costs were size dependen

The Effect of Fluctuating Temperatures During Development on Fitness-Related Traits of Scatophaga stercoraria (Diptera: Scathophagidae)

Development of ectotherms is highly temperature dependent. Studies using variable thermal environments can improve ecological relevance of data because organisms naturally face day-to-day stochastic temperature fluctuations as well as seasonal changes in the amplitude of such daily fluctuations. The objective of this study was to investigate if, and to what extent, the use of constant temperatures is justified in studies of the model species, yellow dung fly, Scatophaga stercoraria (L.). We examined the effect of temperature fluctuation on the expression of several life history traits and the effect on subsequent adult longevity. We used two fluctuating temperature treatments with the same mean but different amplitudes (15/21°C, 12/24°C; 12/12 h), and three constant temperature treatments spanning the wide temperature range faced in the wild (12, 18, and 24°C). Large temperature fluctuation was mostly detrimental (lower juvenile survival, slower growth, smaller body size, and longer development), whereas moderate temperature fluctuation usually gave responses similar to the constant regime. When developing in fluctuating temperatures, adult longevity (no effect), body size (lower), and wing shape (narrower wings) deviated from the expectations based on the constant temperature reaction norms, presumably because of acclimation responses. Contrary to some studies no obvious beneficial effects of moderate temperature fluctuation were observed. Instead, yellow dung flies seem to canalize development in the face of temperature fluctuation up to a point when detrimental effects become unavoidable. The relatively greater effects of extreme constant developmental temperatures question their biological relevance in experiment

Are yellow dung flies domesticated cow dung specialists?

The theory of niche differentiation implies some extent of specialization of species with regard to key resources, notably food. Coprophagous (dung-eating) insect larvae play a critical role in the decomposition of livestock dung in modern and traditional agricultural grasslands. The yellow dung fly (Scathophaga stercoraria L.; Diptera: Scathophagidae) is one of the largest, most common and abundant dung decomposers on pastures in cold-temperate regions across the entire northern hemisphere. As this fly is often associated with domesticated cattle or dairy cows, which are commonly kept for human nutrition worldwide (beef, milk, cheese, etc.), it is sometimes suspected to be a cow dung specialist. However, yellow dung flies are regularly active on and around other dung types, and must have reproduced on dung of wild vertebrates before the domestication of cattle. We therefore experimentally studied the performance of yellow dung fly larvae on dung of various large domestic vs. wild mammals (cow, horse, wild boar, red deer) in the laboratory in Switzerland. Larval performance in terms of juvenile survival, egg-to-adult development time, growth rate, and final adult body size, the major life history indicators of individual reproductive success, did not vary greatly among the various dung types tested. Thus, yellow dung flies can successfully reproduce on multiple types of mammal (vertebrate) dung, wild and domestic, and are therefore dung generalists rather than specialists. We conclude that yellow dung flies are common in European low- and highlands because they could plastically shift to dung of common herbivorous livestock after their domestication without losing the ability to reproduce on dung of common wild mammals

Weak sex-specific evolution of locomotor activity of Sepsis punctum (Diptera: Sepsidae) thermal experimental evolution lines

Elevated temperatures are expected to rise beyond what the physiology of many organisms can tolerate. Behavioural responses facilitating microhabitat shifts may mitigate some of this increased thermal selection on physiology, but behaviours are themselves mediated by physiology, and any behavioural response may trade-off against other fitness-related activities. We investigated whether experimental evolution in different thermal regimes (Cold: 15 °C; Hot: 31 °C; Intergenerational fluctuation 15/31 °C; Control: 23 °C) resulted in genetic differentiation of standard locomotor activity in the dung fly Sepsis punctum. We assessed individual locomotor performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24 °C to 45 °C using an automated device. We found no evidence for generalist-specialist trade-offs (i.e. changes in the breadth of the performance curve) for this trait. Instead, at the warmest assay temperatures hot-selected flies showed somewhat higher maximal performance than all other, especially cold-selected flies, overall more so in males than females. Yet, the flies' temperature optimum was not higher than that of the cold-selected flies, as expected under the 'hotter-is-better' hypothesis. Maximal locomotor performance merely weakly increased with body size. These results suggest that thermal performance curves are unlikely to evolve as an entity according to theory, and that locomotor activity is a trait of limited use in revealing thermal adaptation

Video analysis of the locomotory behaviour of Aedes aegypti and Ae. japonicus mosquitoes under different temperature regimes in a laboratory setting

Mosquito-borne diseases impose a high burden on human and animal health. Temperature strongly influences the physiology and life cycle of mosquitoes, but also the development and/or propagation of the pathogens they transmit. Thus, the vector capacity of mosquitoes depends strongly on temperature and their behavioural thermoregulation through microhabitat selection. Expanding on a previous study of static thermal preferences, the locomotory dynamics of temperate Aedes japonicus (reared from eggs collected in the field) and tropical Ae. aegypti (from a laboratory colony) was investigated at constant temperatures (10 °C, 25 °C, 40 °C) and in temperature gradients (10-20 °C, 20-30 °C, 30-40 °C). Blood-fed or non-blood-fed female mosquitoes were released in groups of 15 individuals into a Plexiglas box positioned on two thermoregulators connected by an aluminium plate to automatically monitor by video analysis mosquito flying, walking and resting duration, covered distances and velocity. Mosquitoes were predominantly resting, followed by walking and flying. At constant 10 °C, flights were rare and brief, and walking was slow. Most activity was observed at 25 °C for Ae. japonicus and 40 °C for Ae. aegypti. In the 30-40 °C gradient, activity of Ae. aegypti increased towards the cold end, suggesting active avoidance of very high temperatures. In the 20-30 °C gradient, edge effects were prominent, nevertheless revealing a greater proportion of mosquitoes gathered at the cooler end. Video analysis showed that this effect was not caused by a cold trap but represents true thermal preference. In the coolest gradient (10-20 °C), mosquitoes were active in all sectors without displaying a preference for either side. Overall, both the tropical and temperate mosquito species preferred cooler temperatures and actively avoided the hottest temperatures. Further studies with infected mosquitoes should provide important insights for developing models of vector-borne disease outbreaks