The myology of the manus of microtine rodents

A study of the muscles of the manus of 13 genera and subgenera of microtine rodents revealed considerable variation which could be organized into 23 primitive-derived evolutionary hypotheses (transmutation series). The results, when compared with those for other mammals, produced functional/ecological hypotheses which were tested by comparing their predictions with the literature. The microtine manus is morphologically distinguished by nine characters and is adapted for a terrestrial/semi-fossorial niche. In particular, Dicrostonyx and Lemmus (distinguished by eight characters) possess the most highly fossorial manus, while the manus of Arborimus (distinguished by eight characters) contains a combination of primitive characters and derived characters which give the Tree vole the most highly developed 'clasping' ability among the microtines studied. The study has revealed characters which have potential systematic value but it is probable that parallelism has occurred in the microtine manus. When the manual characters are combined with characters from other functional complexes, their systematic value can be more clearly determined.     

Habitat selection responses of parents to offspring predation risk: an experimental test

The ability of nest predation to influence habitat settlement decisions in birds is widely debated, despite its importance in limiting fitness. Here, we experimentally manipulated nest predation risk across a landscape and asked the question, do migratory birds assess and respond to variation in nest predation risk when choosing breeding habitats? We examined habitat preference by quantifying the density and settlement date of eight species of migratory passerines breeding in areas with and without intact nest predator communities. We found consistently more individuals nesting in areas with reduced nest predation than in areas with intact predator assemblages, although predation risk had no influence on settlement or breeding phenology. Additionally, those individuals occupying safer nesting habitats exhibited increased singing activity. These findings support a causal relationship between habitat choice and nest predation risk and suggest the importance of nest predation risk in shaping avian community structure and breeding activity.     

Deceptive pollination of Dactylorhiza incarnata: an experimental test of the magnet species hypothesis

Floral deception, which mainly appears in highly evolved families such as Orchidaceae, was studied in Central Finland. In nectarless Dactylorhiza incarnata, the deceptive pollination system has been considered to function best in remote habitats such as marshes, where flowering plants attractive to pollinators are rare (remote habitats hypothesis). In contrast, the magnet-species theory predicts that a nectarless plant benefits from growing in the vicinity of nectarcontaining species. We tested these hypotheses by adding attractive, nectar-containg violets (Viola x wittrockiana) to orchid populations. The percentage of fruit set in D. incarnata was adversely affected by the violets, probably because interspecific exploitation competition for pollinators took place in favour of the violas at the expense of the orchids. This result gave no support for the magnet-species theory and supported the remote habitats hypothesis.     

Does avian malaria reduce fledging success: an experimental test of the selection hypothesis

Like many parasites, avian haematozoa are often found at lower infection intensities in older birds than young birds. One explanation, known as the “selection” hypothesis, is that infected young birds die before reaching adulthood, thus removing the highest infection intensities from the host population. We tested this hypothesis in the field by experimentally infecting nestling rock pigeons (Columba livia) with the malaria parasite Haemoproteus columbae. We compared the condition and fledging success of infected nestlings to that of uninfected controls. There was no significant difference in the body mass, fledging success, age at fledging, or post-fledging survival of experimental versus control birds. These results were unexpected, given that long-term studies of older pigeons have demonstrated chronic effects of H. columbae. We conclude that H. columbae has little impact on nestling pigeons, even when they are directly infected with the parasite. Our study provides no support for the selection hypothesis that older birds have lower parasite loads because parasites are removed from the population by infected nestlings dying. To our knowledge, this is the first study to test the impact of avian malaria using experimental inoculations under natural conditions.     

Evolution of viviparity in warm-climate lizards: an experimental test of the maternal manipulation hypothesis

The maternal manipulation hypothesis for the evolution of reptilian viviparity has been claimed to apply to any situation where gravid females are able to maintain body temperatures different from those available in external nests, but empirical data that support this hypothesis are very limited. Here, we tested this hypothesis using gravid females of a warm-climate lizard, Mabuya multifasciata, by subjecting them to five thermal regimes for the whole gestation period. We found gravid females selected lower body temperatures and thermoregulated more precisely than did nongravid females. Offspring produced in different treatments differed in head size, limb length and sprint speed, but not in overall body size or mass. Variation in morphological traits of offspring was induced primarily by extreme temperatures. Sprint speed of offspring was more likely affected by the mean but not by the variance of gestation temperatures. Gravid females maintained more stable body temperatures than did nongravid females not because these temperatures resulted in the optimization of offspring phenotypes but because the range of temperatures optimal for embryonic development was relatively narrow. Our data conform to the main predictions from the maternal manipulation hypothesis that females should adjust thermoregulation during pregnancy to provide optimal thermal conditions for developing embryos and that phenotypic traits forged by maternal thermoregulation should enhance offspring fitness.     

Helping behaviour in facultatively eusocial hover wasps: an experimental test of the subfertility hypothesis

A candidate explanation for the evolution of eusociality is that helpers are physiologically constrained such that helping is their only realistic option. We tested this subfertility hypothesis in a species of facultatively eusocial hover wasp (Hymenoptera, Stenogastrinae: Liostenogaster flavolineata) by seeing whether helpers that were forced to nest on their own were able to mature their own eggs. One focal helper was left alone on each of 22 nests, from which all other adult wasps (including the dominant) were permanently removed. After 18 days, all but one of the 19 focal helpers that remained on their nests had ovarian development and insemination status characteristic of dominants, and the majority had probably laid eggs. This was in striking contrast to the reproductive status of other helpers removed from the same nests at the start of the experiment. These results provide convincing experimental evidence that females do not become helpers because of some unconditional physiological constraint. There is currently no unequivocal support for the subfertility hypothesis in facultatively eusocial Hymenoptera lacking morphological castes. Copyright 1999 The Association for the Study of Animal Behaviour.     

The exchange hypothesis for the formation of chromatid aberrations: an experimental test using bleomycin

The association between bleomycin-induced chromatid aberrations and BUdR-label exchange between sister chromatids was investigated in order to evaluate Revell's exchange hypothesis for the formation of chromatid aberrations. The results of this study indicate that a larger than expected proportion of chromatid breaks can be accounted for by the exchange hypothesis though not all breaks are the result of incomplete exchange.     

Population regulation of Serengeti Wildebeeest: a test of the food hypothesis

Summary The food hypothesis proposes that density dependent mortality regulates populations through food shortage. For Serengeti wildebeest, we found an empirical relationship between dry season adult mortality rate, density and food supply. This relationship predicted that: (1) the population would stabilize between 1.0 and 1.5 million animals, (2) dry season mortality would be density dependent and sufficient to account for the levelling off of this large ungulate population. Recent observations have tested and confirmed these predictions.     

A test of the risk allocation hypothesis: tadpole responses to temporal change in predation risk

The risk allocation hypothesis predicts that temporal variationin predation risk can influence how animals allocate feedingbehavior among situations that differ in danger. We testedthe risk allocation model with tadpoles of the frog Rana lessonae,which satisfy the main assumptions of this model because theymust feed to reach metamorphosis within a single season, theirbehavioral defense against predators is costly, and they canrespond to changes in risk integrated over time. Our experiment switched tadpoles between artificial ponds with different numbersof caged dragonfly larvae and held them at high and low riskfor different portions of their lives. Tadpoles responded stronglyto predators, but they did not obey the risk allocation hypothesis:as the high-risk environment became more dangerous, there wasno tendency for tadpoles to allocate more feeding to the low-riskenvironment, and as tadpoles spent more time at risk, they didnot increase feeding in both environments. Our results suggestthat the model might be more applicable when the time spentunder high predation risk is large relative to the time requiredto collect resources.     

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty-one hydraulic regimes.
2. Enclosures containing the stonefly, Dinocras cephalotes , and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.
3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani , and the Chironomidae, were reduced in the presence of Dinocras , but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.
4. Densities of two other prey species rare in the diets of Dinocras ( Hydropsyche instabilis and Gammarus fossarum ) were generally unaffected by predators regardless of the hydraulic regime.
5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.     

Song sparrows, top carnivores and nest predation: a test of the mesopredator release hypothesis

Ground-nesting North American landbirds have declined in the longterm, including species with a variety of migratory strategies. The mesopredator release hypothesis explains declines by suggesting that the virtual elimination of top carnivores (large-bodied canids and felids) from much of North America has “released” populations of nest-destroying mesopredators (i.e., medium-sized terrestrial omnivores such as the raccoon Procyon lotor). The hypothesis predicts (1) higher nest success in the presence than in the absence of top carnivores, and (2) a positive relationship between mesopredator abundance and nest predation. Results from a 4-year "natural experiment" in the agricultural landscape of southern Michigan tended to support prediction 1. When coyotes (Canis latrans) were absent in 1993 and present in 1994, 1995 and 1996, Mayfield nest survival in song sparrows (Melospiza melodia), a ground-nesting landbird, increased significantly during the same 4-year interval. Relative frequency of nest predation, the most common cause of nest failure, declined significantly over the four years. Coyotes may have decreased nest predation in 1994–1996 by reducing the abundance of raccoons, apparently the main nest predator in the study area. In an experiment testing prediction 2, mesopredator abundance and predation rate on artificial nests were positively related, as predicted by the hypothesis. Although the present study is not wholly conclusive by itself, we cautiously suggest it contributes to a growing body of evidence derived from a number of studies supporting the mesopredator release hypothesis. Received: 23 May 1997 / Accepted: 12 January 1998     

Social competition and plasma testosterone profile in domesticated canaries: an experimental test of the challenge hypothesis

The challenge hypothesis predicts that plasma testosterone (T) concentration is high when male-male competitions are high and decreases when males are engaged in paternal care. In monogamous species, T concentration increases at the beginning of the breeding period and decreases after egg laying. According to the challenge hypothesis, increasing competition should also lead to T increase. The aim of our study was to test this hypothesis. In a first experiment, we measured the T profile of domesticated canaries housed with their mate in separated cages without competition. In a second one, we created a competition by housing male and female domestic canaries together (in an aviary) and emphasized this competition by limiting food access. We also studied social status effect. Our results showed no effect of social status in both sexes and no differences in female's T concentration. Concerning males, we obtained a clear monogamous T profile from the ones housed in a low competition situation and a polygamous profile from the others housed in high competition situation. Thus, our results support the hypothesis of the plasticity of the mechanisms controlling T concentration according to environmental conditions.     

Phenotypic variation in male guppies from natural inland populations: an additional test of Haskins' sexual selection/predation hypothesis

Synopsis In Venezuela, guppies,Poecilia reticulata, are found in a diversity of aquatic ecosystems fringing the western slope of the eastern Andes. These inland drainages are associated with large between-site variation in both physical and biotic environments. Numbers of potential guppy predators vary greatly between low diversity piedmont streams and high diversity llanos swamps. We analyze geographic variation in several phenotypic traits associated with conspicuousness of male guppies as an independent test of Haskins' (Haskins et al. 1961) hypothesis of natural balance between sexual selection (driving male conspicuousness) and predation (driving crypsis). Four sites were compared: a diverse llanos swamp (high predation), two Andean piedmont streams (intermediate and low predation), and a spring pool located south of Venezuela's Paría Peninsula (no aquatic predators). Because tropical wet and dry seasons are associated with varying degrees of environmental change at these sites, within-site seasonal samples provided an additional test of the role of selective predation on Male phenotypes. Phenotypic traits associated with both size and coloration supported the sexual selection/predation hypothesis. male guppies from the spring were generally more colorful than male guppies from inland drainages. Males from the diverse swamp site were smaller and less colorful than guppies from other sites. Males from the swamp population were smallest and least colorful during the early dry season, when diurnal aquatic piscivores were more diverse and abundant. Dominant spot pigments at each site were generally those that provided greatest contrast with the predominant environmental background color, lending further evidence that coloration patterns in male guppies enhance conspicuousness.