Size- and sex-specific predation on dung flies by amphibian and arthropod predators – size match matters

Because predator-prey interactions in nature are multifarious, linking phenomenological predation rates to the underlying behavioural or ecological mechanisms is challenging. Size- and sex-specific predation has been implicated as a major selective force keeping animals small, affecting the evolution of body size and sexual size dimorphism. We experimentally assessed predation by various amphibian (frogs and toads) and arthropod predators (bugs, flies, spiders) on three species of dung flies with similar ecology but contrasting body sizes, sexual size dimorphism and coloration. Predators were offered a size range of flies in single- or mixed-sex groups. As expected based on optimal foraging theory, some anurans (e.g. Bufo bufo) selected larger prey, thus selecting against large body size of the flies, while others (Bombina variagata and Rana esculenta) showed no such pattern. Small juvenile Rana temporaria metamorphs, in contrast, preferred small flies, as did all arthropod predators, a pattern that can be explained by larger prey being better at escaping. The more mobile males were not eaten more frequently or faster than the cryptic females, even when conspicuously colored. Predation rates on flies in mixed groups permitting mating activity were not higher, contrary to expectation, nor was predation generally sex-specific. We conclude that the size-selectivity of predators, and hence the viability selection pattern exerted on their prey, depends foremost on the relative body sizes of the two in a continuous fashion. Sex-specific predation by single predators appears to contribute little to sexual dimorphism. Therefore, the mechanistic study of predation requires integration of both the predator’s and the prey’s perspectives, and phenomenological field studies of predation remain indispensable

Spatial distribution and risk factors associated with Salmonella enterica in pigs

The importance of pork in the transmission of Salmonella spp. to humans has led to the development of control programmes worldwide. For this, knowledge on the epidemiology of the infection in the production system is fundamental to the efficacy of the regulations. Our objective was to determine the prevalence and spatial distribution of Salmonella-infected farms in the central region of Argentina, and to identify the predominant serotypes and epidemiological factors associated with an increased risk of infection. Salmonella was isolated from 22 of 52 sampled farms, for a farm prevalence of 42·3% (95% confidence interval 28·4–56·1). The most frequent serotypes isolated were S. Typhimurium and S. Derby, which have often been considered of public health concern in the region. Limited evidences of global and local clustering in the region under study were found, and the type of feed and presence of diarrhoeic pigs were significantly associated with having Salmonella shedders in the farm. This highlights the need to evaluate microbiological controls at the farm level, and demonstrates the usefulness of the spatial tools to identify areas of greatest risk when processing pork at slaughterhouse, which could contribute to increasing the food safety of pork products.Fil: Parada, Julian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Río Cuarto. Facultad de Agronomía y Veterinaria. Departamento de Patología Animal; ArgentinaFil: Carranza, Alicia Isabel. Universidad Nacional de Río Cuarto. Facultad de Agronomía y Veterinaria. Departamento de Patología Animal; ArgentinaFil: Alvarez, Julio. University of Minnesota; Estados UnidosFil: Pichel, M.. Dirección Nacional de Instituto de Investigación. Administración Nacional de Laboratorio e Instituto de Salud "Dr. C. G. Malbrán"; ArgentinaFil: Tamiozzo, Pablo Jesus. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Río Cuarto. Facultad de Agronomía y Veterinaria. Departamento de Patología Animal; ArgentinaFil: Busso, Juan Jose. Universidad Nacional de Río Cuarto. Facultad de Agronomía y Veterinaria. Departamento de Patología Animal; ArgentinaFil: Ambrogi, Arnaldo. Universidad Nacional de Río Cuarto. Facultad de Agronomía y Veterinaria. Departamento de Patología Animal; Argentin

Viability selection by invertebrate predators in the polyphenic black scavenger fly Sepsis thoracica

Predation is a major factor influencing the fitness and life history of animals. Two key traits affecting prey survival are body size and coloration. Sepsis thoracica males display a sigmoid relationship between these 2 traits, defining a size threshold above which investment in melanin drastically drops, producing small melanic (black) or large amber morphs. In trying to understand the evolution of this rare dimorphism, we performed laboratory predation experiments to estimate the intensity of adult viability selection exerted by various arthropod predators (bugs, flies, and spiders) on male body size and coloration. Selection was performed against 2 different backgrounds mimicking the natural habitat (dung and grass) in which the camouflage and/or warning effect of the morphs should vary. Body size was mainly under positive selection (larger survived better), which overpowered selection on coloration and varied somewhat among predator species but not backgrounds. No disruptive selection was found, nor did selection change the sigmoid relationship between the 2 traits. We conclude that, for this fly, predator evasion and escaping skills determined by body size are more effective against invertebrate predators than its conspicuousness determined by coloration, contrasting what has been found for vertebrate predators, where prey coloration is important and negative selection on size dominates. Because arthropod predators have strong effects on insect populations, the positive directional selection imposed by invertebrate predators is likely an important force driving the evolution of body size in S. thoracica and insects in general

Disruptive sexual selection on male body size in the polyphenic black scavenger fly Sepsis thoracica

Sexual selection has 2 main components, female preference and male–male competition, which can lead males to adopt alternative reproductive tactics to optimize their reproductive success. Two traits that significantly influence reproductive success are body size and coloration, as they can facilitate access to females through male contests or as female attractors. We investigated whether, and if so which mechanism of sexual selection contributes to the maintenance, and possibly even the establishment, of 2 almost discrete male morphs in the polyphenic black scavenger fly Sepsis thoracica (Diptera: Sepsidae): small and black, or large and amber. We performed 2 complementary laboratory experiments to evaluate the mating success of the different male morphs and the behaviors (of both males and females) presumably mediating their mating success. We found evidence for intraspecific disruptive sexual selection on male body size that is mediated by male–male interactions, and significant positive directional selection on body size that interacted with (directional) selection on coloration, likely contributing to the origin and/or maintenance of the threshold relationship between the 2 traits in this species. The simultaneous occurrence of disruptive selection and polyphenism in S. thoracica supports the role of sexual selection in the intraspecific diversification of coupled traits (here body size and coloration), which could be a speciation starting point

VARIATION OF FEMALE PREFERENCE INFLUENCES THE MAINTENANCE OF PHENOTYPIC DIVERSITY

Female preference is one of the main components of sexual selection that can affect intraspecific phenotypic diversity. In some animal species, the phenotypic landscape of one of the sexes presents a bimodal or multimodal distribution and these phenotypes also differ in their reproductive tactics. Here, I evaluated how female preference influences the coexistence of two male alternative reproductive tactics in the jumping spider Maevia inclemens. I employed no-choice tests to analyze variation of female preference between different locations and between the two morphs.Females present a negative-frequency dependent sexual preference choosing the rare morph. Additionally, the female preference functions differ between morphs and these differences can contribute to the maintenance of the discreetness of the traits of the morphs. These results highlight the importance of variation of female preference in the maintenance of alternative reproductive tactics. They also show that negative-frequency dependent selection and diversifying selection contribute to the coexistence of both morphs in the species.Master of ScienceEcology and Evolutionary BiologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/101964/3/Busso-MS_Thesis.pd

Disruptive sexual selection on male body size in the polyphenic black scavenger fly Sepsis thoracica

Males of the fly Sepsis thoracica, which breeds in livestock dung, are either small and black or large and amber. In trying to understand the evolution of this rare phenomenon, we found that large amber males were more successful in obtaining mates, while small black males had greater mating success than males of intermediate size and color. This was mediated by different alternative aggressive mating behaviors that mainly differed in extent

Data from: Disruptive sexual selection on body size in the polyphenic black scavenger fly Sepsis thoracica

Sexual selection has two main components, female preference and male-male competition, which can lead males to adopt alternative reproductive tactics to optimize their reproductive success. Two traits that significantly influence reproductive success are body size and coloration, as they can facilitate access to females through male contests or as female attractors. We investigated whether, and if so which mechanism of sexual selection contributes to the maintenance, and possibly even the establishment, of two almost discrete male morphs in the polyphenic black scavenger fly Sepsis thoracica (Diptera: Sepsidae): small and black, or large and amber. We performed two complementary laboratory experiments to evaluate the mating success of the different male morphs and the behaviors (of both males and females) presumably mediating their mating success. We found evidence for intraspecific disruptive sexual selection on male body size that is mediated by male-male interactions, and significant positive directional selection on body size that interacted with (directional) selection on coloration, likely contributing to the origin and/or maintenance of the threshold relationship between the two traits in this species. The simultaneous occurrence of disruptive selection and polyphenism in S. thoracica supports the role of sexual selection in the intraspecific diversification of coupled traits (here body size and coloration), which could be a speciation starting point

Viability selection by invertebrate predators in the polyphenic black scavenger fly Sepsis thoracica

Males of the black scavenger fly Sepsis thoracica, which breeds in livestock dung, are either small and black or large and amber. In trying to understand the evolution of this rare phenomenon, we confronted flies of different sizes and colors with invertebrate predators in different backgrounds. We found that generally larger individuals survived better and that their coloration played a secondary role. Predation cannot be the primary cause of the male dimorphism

Data from: Viability selection by invertebrate predators in the polyphenic scavenger fly Sepsis thoracica

Predation is a major factor influencing the fitness and life history of animals. Two key traits affecting prey survival are body size and coloration. Sepsis thoracica males display a sigmoid relationship between these two traits, defining a size threshold above which investment in melanin drastically drops, producing small melanic (black) or large amber morphs. In trying to understand the evolution of this rare dimorphism, we performed laboratory predation experiments to estimate the intensity of adult viability selection exerted by various arthropod predators (bugs, flies, spiders) on male body size and coloration. Selection was performed against two different backgrounds mimicking the natural habitat (dung and grass) in which the camouflage and/or warning effect of the morphs should vary. Body size was mainly under positive selection (larger survived better), which overpowered selection on coloration and varied somewhat among predator species but not backgrounds. No disruptive selection was found, nor did selection change the sigmoid relationship between the two traits. We conclude that, for this fly, predator evasion and escaping skills determined by body size are more effective against invertebrate predators than its conspicuousness determined by coloration, contrasting what has been found for vertebrate predators, where prey coloration is important and negative selection on size dominates. Because arthropod predators have strong effects on insect populations, the positive directional selection imposed by invertebrate predators is likely an important force driving the evolution of body size in S. thoracica and insects in general