Relationships between food resources, foraging patterns, and reproductive success in the water pipit, Anthus sp. spinoletta

A basic but rarely tested assumption in optimal foraging theoryis that positive relationships exist between the foraging patternof an animal, its short-term benefits in feeding, and its long-termfitness. We present evidence for these relationships for a centralplace foraging situation. We studied the foraging behavior ofadult water pipits (Anthus sp. spinoletta) feeding nestlingsin an Alpine habitat near Davos, Switzerland, with the followingresults: (1) searching effort decreases with increasing distancefrom the nest, (2) the amount of prey and the proportion oflarge items brought to the nest increases with increasing foragingdistance, (3) water pipits do not forage according to habitatavailability, but prefer vegetation types with the highest fooddensity (mainly grass and herbs) and avoid those with the lowest,and (4) this selectivity is only expressed when the birds foragemore than 50 m from the nest, i.e., usually outside the territory.Among the several potential interpretations of these results,the most parsimonious is that foraging decisions are based onprofitability, i.e., on the net energy gain per time unit. Additionally,we found that food conditions translate into fitness: the numberof fledglings per nest is related positively to the averageprey biomass at the foraging place and negatively to the averagedistance between the foraging place and the nest. Maximum economicdistances, which were predicted from this food-fitness relationship,agreed well with the actual foraging distances observed. Thissuggests a dose connection between foraging decisions and fitness.In addition to the theoretical issues, some conservation issuesare also briefly discussed.     

Patterns of natural selection on size at metamorphosis in water frogs

Strategies for optimal metamorphosis are key adaptations in organisms with complex life cycles, and the components of the larval growth environment causing variation in this trait are well studied empirically and theoretically. However, when relating these findings to a broader evolutionary or ecological context, usually the following assumptions are made: (1) size at metamorphosis positively relates to future fitness, and (2) the larval growth environment affects fitness mainly through its effect on timing of and size at metamorphosis. These assumptions remain poorly tested, because data on postmetamorphic fitness components are still rare. We created variation in timing of and size at metamorphosis by manipulating larval competition, nonlethal presence of predators, pond drying, and onset of larval development, and measured the consequences for subsequent terrestrial survival and growth in 1564 individually marked water frogs (Rana lessonae and R. esculenta), raised in enclosures in their natural environment. Individuals metamorphosing at a large size had an increased chance of survival during the following terrestrial stage (mean linear selection gradient: 0.09), grew faster and were larger at maturity than individuals metamorphosing at smaller sizes. Late metamorphosing individuals had a lower survival rate (mean linear selection gradient: -0.03) and grew more slowly than early metamorphosing ones. We found these patterns to be consistent over the three years of the study and the two species, and the results did not depend on the nature of the larval growth manipulation. Furthermore, individuals did not compensate for a small size at metamorphosis by enhancing their postmetamorphic growth. Thus, we found simple relationships between larval growth and postmetamorphic fitness components, and support for this frequently made assumption. Our results suggest postmetamorphic selection for fast larval growth and provide a quantitative estimate for the water frog example.     

Choosy females and indiscriminate males: mate choice in mixed populations of sexual and hybridogenetic water frogs (Rana lessonae, Rana esculenta)

For several decades, behavioral ecologists have studied theeffects of the environment on the behavior of individuals;but only fairly recently they have started to ask the reversequestion: how do the behavioral strategies of individuals affectthe composition and dynamics of populations and communities?Although intuitively obvious, this feedback from individualto higher levels is difficult to demonstrate, except in systemswith exceptionally fast and marked responses of the populationsto the behavior of its members. Such a system exists in sperm-dependentspecies. In European water frogs, for instance, successfulreproduction of a hybrid species (R. esculenta, genotype LR)requires mating with one of its parental species (R. lessonae,genotype LL), except in the rare cases where hybrids are triploid.The sexual host LL, however, should avoid matings with the sexual parasite LR, because the resulting LR offspring willeliminate the L genome from their germ line. In this studywe investigate how this conflict is solved. Since water froghybrids come in both sexes, rather than as females only likein other sperm-dependent systems, we performed the tests withboth females and males. One individual was given a choice betweentwo individuals of the opposite sex, one an LL and the otheran LR. In both species, females showed the predicted preferencefor LL males, whereas males did not discriminate between LLand LR females. On the individual level, we interpret the sexdifference in choosiness by the lower costs from mating withthe wrong species (LR) and the higher benefits from matingwith large individuals in males than in females. In "normal"species, male preference for large (i.e. more fecund) femalesis advantageous, but in this system such a choice can resultin mating with the larger LR females. With respect to the structureand dynamics of mixed populations, we discuss that the observed female preference is consistent with the higher mating successof LL males found in nature. Hence, mate female choice is astrong candidate for a mechanism promoting coexistence of thesperm-dependent hybrid and its sexual host. This confirms predictionsfrom previous theoretical models.     

Chromosome-specific DNA repeat probes.

In research as well as in clinical applications, fluorescence in situ hybridization (FISH) has gained increasing popularity as a highly sensitive technique to study cytogenetic changes. Today, hundreds of commercially available DNA probes serve the basic needs of the biomedical research community. Widespread applications, however, are often limited by the lack of appropriately labeled, specific nucleic acid probes. We describe two approaches for an expeditious preparation of chromosome-specific DNAs and the subsequent probe labeling with reporter molecules of choice. The described techniques allow the preparation of highly specific DNA repeat probes suitable for enumeration of chromosomes in interphase cell nuclei or tissue sections. In addition, there is no need for chromosome enrichment by flow cytometry and sorting or molecular cloning. Our PCR-based method uses either bacterial artificial chromosomes or human genomic DNA as templates with alpha-satellite-specific primers. Here we demonstrate the production of fluorochrome-labeled DNA repeat probes specific for human chromosomes 17 and 18 in just a few days without the need for highly specialized equipment and without the limitation to only a few fluorochrome labels.     

Mutation accumulation and fitness effects in hybridogenetic populations: a comparison to sexual and asexual systems

Background  

Female only unisexual vertebrates that reproduce by hybridogenesis show an unusual genetic composition. They are of hybrid origin but show no recombination between the genomes of their parental species. Instead, the paternal genome is discarded from the germline prior to meiosis, and gametes (eggs only) contain solely unrecombined maternal genomes. Hence hybridogens only transmit maternally inherited mutations. Hybridity is restored each generation by backcrossing with males of the sexual parental species whose genome was eliminated. In contrast, recombining sexual species propagate an intermixed pool of mutations derived from the maternal and paternal parts of the genome. If mutation rates are lower in female gametes than males, it raises the possibility for lower mutation accumulation in a hybridogenetic population, and consequently, higher population fitness than its sexual counterpart.     

The Relative Importance of Habitat Structure and of Prey Characteristics for the Foraging Success of Water Pipits (Anthus spinoletta)*

While many studies on foraging have related energy gain to the density and the size of prey, only few have investigated whether and how habitat structure modifies the gain through affecting foraging success. In this study, the influences of habitat structure and prey characteristics on the foraging success of water pipits, Anthus spinoletta, were investigated experimentally. The birds take longer to find prey in tall than in short vegetation. The effects of vegetation on searching times differ between prey types. These differences are probably caused by variation in prey behaviour and in cryptic colouration, but not by prey size. Searching times increase with decreasing density for mealworms and tipulids, but not for caterpillars. Handling large prey items requires more time than handling smaller prey. Tipulids and caterpillars, which were offered alive, are handled for a longer time than dead mealworms of corresponding size. The success of attacks on flying insects is probably influenced by the prey's flight speed: fast houseflies are missed more often than slow tipulids. Overall, the results show that the time costs of foraging water pipits are influenced to a comparable degree by vegetation structure, by prey density and by other specific prey characteristics such as camouflage, hiding behaviour or agility. The amount of food gathered per unit time is determined primarily by factors that affect searching times, and less by handling and travelling times. Insertion of our data into an optimal diet model leads to the prediction that water pipits should be generalist foragers, which agrees with the observed behaviour.     

Ecological determinants of extrapair fertilizations and egg dumping in Alpine water pipits (Anthus spinoletta)

Behavioral ecology has successfully explained the diversityin social mating systems through differences in environmentalconditions, but diversity in genetic mating systems is poorlyunderstood. The difference is important in situations whereparents care for extrapair young (EPY) originating from extrapairpaternity (EPP), extrapair maternity (EPM), and intraspecificbrood parasitism (IBP). In birds, IBP and EPM are rare, butEPP is widespread and highly variable among species and populations.Explanations for this variability are controversial, mainlybecause detailed ecological information is usually lacking inpaternity studies. Here we present results of the first studyto identify the ecological determinants of extrapair activitiesfor both sexes of the same species, the water pipit (Anthusspinoletta). DNA fingerprints of 1052 young from 258 nests revealedEPP in 5.2% of the young from 12.4% of the nests. EPM and IBP,both involving egg dumping (EDP), each occurred in 0.5% of theyoung from 1.9% of the nests. Nests with and without EPY couldnot be distinguished by traits of the breeders and by reproductivesucccess, but they differed with respect to ecology: nests withEPP young were characterized by asynchronous clutch initiation,nests with EPM and IBP young were characterized by higher overlapwith neighboring territories and closer proximity to communalfeeding sites. We suggest that chance events, resulting fromthe temporal and spatial distribution of broods, offer a betterexplanation for the occurence of extrapair activities than femalesearch for genetic or phenotypic benefits. This possibilityof "accidental" extrapair reproduction as an "ecological epiphenomenon"with low potential for selection should also be considered forspecies other than the water pipit.     

Nestling provisioning in water pipits (Anthus spinoletta): do parents go for specific nutrients or profitable prey?

In this study, we investigated whether free-living insectivorous water pipits (Anthus spinoletta) choose prey according to biochemical quality as measured by protein, lipid, carbohydrate, energy and water contents and/or according to profitability as measured by density, size and catchability. Food preference – expressed in relation to availability – is estimated for 22 arthropod taxa (families and orders). Uni- and multivariate statistics detected no relationships between food preference and nutrient contents, but revealed that more larger prey items are fed to nestlings than smaller ones, both for all prey taken together and within individual taxa. Furthermore, slow-flying arthropods, which are easier to catch, were usually preferred over walking and fast-flying ones. Combined with results from previous studies on the effects of vegetation, prey density and catchability on search times and energy intake, these findings suggest that water pipits select their prey primarily to maximize profitability, i.e. energy intake per unit time. Qualitative traits seem to be important only for specific taxa. For example, toxins or poor digestibility may be responsible for the avoidance of heteropterans, beetles and ants and for feeding the nestlings fewer tipulids than expected at high tipulid densities. Received: 18 January 1999 / Accepted: 14 June 1999     

Investment and relatedness: A cost/benefit analysis of breeding and helping in the pied kingfisher (Ceryle rudis)

Helping at the nest in birds is often termed altruism. However, so far, no study has ever demonstrated high costs to a helper's own lifetime reproductive success (=direct fitness), nor its compensation through benefits from relatives other than its own offspring (=indirect fitness). In this paper on pied kingfishers (Ceryle rudis) the relationship between investment, relatedness and inclusive fitness (expressed in terms of genetic equivalents) is investigated for breeding males, and males that help either relatives (=primary helpers) or strangers (=secondary helpers). With respect to guarding nests against predators and feeding young, primary helpers invest as much as breeders, but secondary helpers contribute significantly less. These differences in status and investment (measured in energy expenditure) affect the birds' future to such an extent that primary helpers have a lower chance of surviving and mating than secondary helpers. However, their costs in direct fitness are compensated by pronounced benefits to indirect fitness, resulting from improved survival of siblings and parents. An attempt is made to calculate the inclusive fitness of birds following different strategies over a 2-year period. It is concluded that (a) breeding is superior to helping and helping superior to doing nothing and (b) that kin-selection must be invoked to explain why surplus males choose the more costly primary helper strategy instead of the cheaper secondary helper strategy. Alternative explanations, including group selection, parental manipulation and reciprocity, are discussed.     

FROM CLONAL TO SEXUAL HYBRIDS: GENETIC RECOMBINATION VIA TRIPLOIDS IN ALL-HYBRID POPULATIONS OF WATER FROGS

Speciation via interspecific hybrids is very rare in animals, as compared to plants. Whereas most plants overcome the problem of meiosis between different chromosome sets by tetraploidization, animal hybrids often escape hybrid sterility by clonal reproduction. This comes at the expense of genetic diversity and the ability to purge deleterious mutations. However, here we show that all-hybrid populations of diploid (LR) and triploid (LLR and LRR) water frogs ( Pelophylax esculentus ) have secondarily acquired sexual reproduction. First, in a crossing experiment analyzed with microsatellite markers, triploid hybrids of both sexes and genotypes (LLR and LRR) recombined their homospecific genomes. Second, the great majority of natural populations investigated had low multilocus linkage disequilibrium, indicating a high recombination rate. As predicted from mating system models, the L genome had constant, low levels of linkage disequilibrium, whereas linkage disequilibrium in the R genome showed a significant reduction with increasing proportion of recombining triploids. This direct evidence of sexual reproduction in P. esculentus calls for a change of the conventional view of hybridogens as clonally reproducing diploids. Rather, hybridogens can be independent sexually reproducing units with an evolutionary potential.