Simplified song in an island population of the bush warbler Cettia diphone

Song complexity and the repertoire of the bush warbler Cettia diphone were studied in an island and a mainland population in Japan. The song complexity (number of modulations in a song) was lower in the island population than in the mainland one. On the other hand, the repertoire size (number of song types per male) was larger on the island. Founder effect probably does not influence the island song because colonization occurred a very long time ago. Sound transmission properties of the habitat and weak selection pressure for interspecific identification might have influenced the island song, but these cannot elucidate the simpler song on the island. There is a possibility that social conditions affect song complexity in the populations; the highly polygynous mating system of the mainland population yields strong selection pressure toward complex song through acquisition of mates and/or establishment of quality territories. On the other hand, variable songs on the island seem to be affected by cultural mutation. Received: September 3, 1999 / Accepted: December 9, 1999     

Characterization of 12 polymorphic microsatellite loci in the Japanese bush warbler Cettia diphone

Japanese bush warblers, Cettia diphone, are a common species in Japan and have a polygynous breeding system. In the breeding ground some males have their own territories with more than one female. Male floaters are also not uncommon in the breeding ground. To understand breeding strategy in this species, exact parentage should be elucidated. In order to obtain a tool for this purpose, we isolated 34 microsatellite loci from a genomic library in this species and developed primers for 12 loci. These primers were tested in the Japanese bush warbler and successfully amplified. In analyses of 49 unrelated individuals, allelic numbers ranged from two to 22, and observed heterozygosity (H_O) ranged from 0.27 to 0.854 except for two loci (Cdi29 and Cdi35a) with H_O < 0.1.     

Responses of the Bush Warbler Cettia diphone to artificial eggs of Cuculus cuckoos in Japan

Artificial eggs of six different colours and control eggs of Bush Warblers Cettia diphone were introduced into nests of Bush Warblers, a host of both Little and Himalayan Cuckoos Cuculus poliocephalus and C. saturatw in Japan. All control (chocolate-brown) and artificial red eggs were accepted; all grey and all white eggs were rejected. The rejection rates of orange, pink and orange spots on grey eggs were 8%, 369; and 55%, respectively. Bush arblers are more likely to reject eggs the more dissimilar they are from their own. The results strengthen the possibility that the chocolate-brown eggs of Little and Himalayan Cuckoos may have evolved through the discriminative ability of Bush Warblers and their intolerance towards dissimilar eggs.     

Spatial distribution of nests constrains the strength of sexual selection in a warbler

In socially monogamous species, extra‐pair paternity may increase the strength of intersexual selection by allowing males with preferred phenotypes to monopolize matings. Several studies have found relationships between male signals and extra‐pair mating, but many others fail to explain variation in extra‐pair mating success. A greater appreciation for the role that ecological contingencies play in structuring behavioural processes may help to reconcile contradictory results. We studied extra‐pair mating in a spatial context in the common yellowthroat (Geothlypis trichas), a territorial wood warbler. Over the course of 6 years, we observed 158 breeding attempts by 99 males, resulting in a total of 369 nests and 520 sampled nestlings. The spatial distribution of territories varied greatly, with males having between 0 and 10 close neighbours and between three and 39 neighbouring nestlings close enough to represent extra‐pair siring opportunities. Both within‐pair and extra‐pair reproductive success increased with breeding density, but the opportunity for sexual selection and strength of selection varied with density. Total variance in reproductive success was highest at low density and was mostly explained by variation in within‐pair success. In contrast, at high density, both within‐pair and extra‐pair successes contributed substantially to variance in reproductive success. The relationships between plumage and extra‐pair mating also varied by density; plumage was under strong sexual selection via extra‐pair mating success at high density, but no selection was detected at low density. Thus, ecological factors that structure social interactions can drive patterns of sexual selection by facilitating or constraining the expression of mating preferences.     

Genetic variation and island biogreography: Microsatellite and mitochondrial DNA variation in island populations of the Australian bush rat, Rattus fuscipes greyii

To understand the impact of various factors on the maintenance of genetic variation in natural populations, we need to focus on situations where at least some of these factors are removed or controlled. In this study, we used highly variable, presumably neutral, microsatellite and mtDNA markers to assess the nature of genetic variation in 14 island and two mainland populations of the Australian bush rat, where there is no migration between islands. Thus we are controlling for selection and gene flow. Both marker sets revealed low levels of diversity within the small island populations and extreme differentiation between populations. For six microsatellite loci, all of the small island populations had less genetic variation than the mainland populations; reduction in allelic diversity was more pronounced than loss of heterozygosity. Kangaroo Island, the large island population, had similar levels of diversity to the mainland populations. A 442 base pair (bp) section of the mtDNA control region was screened for variation by outgroup heteroduplex analysis/temperature gradient gel electrophoresis (OHA/TGGE). Only three of the 13 small island populations showed haplotypic diversity: Gambier (2), Waldegrave (2), and Eyere (3). The level of haplotypic diversity in the small island populations was similar to that on the mainland, most likely reflecting a recent population bottleneck on the mainland. In contrast, Kangaroo Island had 9 mtDNA haplotypes. The dominant factor influencing genetic diversity on the islands was island size. No correlation was detected between genetic diversity and the time since isolation or distance form the mainland. The combination of genetic drift within and complete isolation among the small island populations has resulted in rapid and extreme population divergence. Population pair-wise comparisons of allele frequency distributions showed significant differences for all populations for all loci (F _st = 0.11–0.84, R _st = 0.07–0.99). For the mtDNA control region, 92.6% of variation was apportioned between populations; only the Pearson islands shared a haplotype. Mantel tests of pair-wise genetic distance with pair-wise geographic distance showed no significant geographical clustering of haplotypes. However, population substructuring was detected within populations where sampling was conducted over a broader geographical range, as indicated by departures from Hardy-Weinberg equilibrium. Thus substructuring in the ancestral population cannot be ruled out. The dominant evolutionary forces on the islands, after the initial founder event, are stochastic population processes such as genetic drift and mutation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The intensity of sexual selection in relation to male sexual behaviour,female choice,and sperm precedence

In this paper we define sexual selection on males as the variance in numbers of mates per male and show how the intensity of this selection is affected by male sexual behaviour, female choice, sex ratio, and modes of sperm precedence. This definition coincides with Darwin's conception of sexual selection but differs from some post-Darwinian views. For systems of single-male paternity, we show that the intensity of total selection on male reproductive success equals the intensity of natural selection on female fertility, times the sex ratio, plus the intensity of sexual selection on males. The absolute intensity of sexual selection is unaffected by the system of sperm precedence. The application of the results to field studies is discussed.     

Experimental analyses of sexual and natural selection on short tails in a polygynous warbler

We believe that no experimental study has yet tested Darwin's idea that, as well as generating trait elaboration, intersexual selection might sometimes drive sex-biased trait reduction. Here we present the results of two experiments exploring the negative relationship between tail length and reproductive success in male golden-headed cisticolas (Cisticola exilis). In the first experiment, artificially shortening a male's tail produced a dramatic increase in his reproductive success, measured as either the number of females nesting or number of chicks Hedged on his territory. A second experiment, in which manipulated birds were flown through a maze, revealed that short tails also impose costs by reducing aerodynamic performance during slow-speed foraging flight. Because tail shortening yields reproductive benefits and viability costs, we conclude it has evolved via sexual selection. Disentangling exactly how short tails enhance male reproductive success is more difficult. Male-male competition appears partly responsible: aerodynamic theory predicts that tail reduction enhances high-speed flight and, in line with this, shortened-tail males spent more time engaged in high-speed aerial chases of rivals and defended higher-quality territories. However, shortened-tail males had higher reproductive success independent of territory quality and spent more time in aerial displays which may be directed at females. This suggests that tail shortening is also favoured via female choice based on male phenotype.     

Sexual networks: measuring sexual selection in structured,polyandrous populations

Sexual selection is traditionally measured at the population level, assuming that populations lack structure. However, increasing evidence undermines this approach, indicating that intrasexual competition in natural populations often displays complex patterns of spatial and temporal structure. This complexity is due in part to the degree and mechanisms of polyandry within a population, which can influence the intensity and scale of both pre- and post-copulatory sexual competition. Attempts to measure selection at the local and global scale have been made through multi-level selection approaches. However, definitions of local scale are often based on physical proximity, providing a rather coarse measure of local competition, particularly in polyandrous populations where the local scale of pre- and post-copulatory competition may differ drastically from each other. These limitations can be solved by social network analysis, which allows us to define a unique sexual environment for each member of a population: ‘local scale’ competition, therefore, becomes an emergent property of a sexual network. Here, we first propose a novel quantitative approach to measure pre- and post-copulatory sexual selection, which integrates multi-level selection with information on local scale competition derived as an emergent property of networks of sexual interactions. We then use simple simulations to illustrate the ways in which polyandry can impact estimates of sexual selection. We show that for intermediate levels of polyandry, the proposed network-based approach provides substantially more accurate measures of sexual selection than the more traditional population-level approach. We argue that the increasing availability of fine-grained behavioural datasets provides exciting new opportunities to develop network approaches to study sexual selection in complex societies.     

Repertoire size, sexual selection, and offspring viability in the great reed warbler: changing patterns in space and time

Only a few studies have focussed on the consistency of sexualselection patterns in space and time. One such case is the greatreed warbler (Acrocephalus arundinaceus), for which studiesin Germany in 1981–1982 and Sweden in 1987–1991strongly suggested that the size of a male's song repertoirewas the target of mate choice and sexual selection. Studyingthe same German population once again in 1994–2000, weinvestigated the consistency of these patterns over time aswell as between populations. Our reanalysis of the data from1981–1982 shows that male repertoire size was positivelycorrelated with male pairing success (harem size) and with clutchsize (adjusted for seasonal effects), whereas no such correlationswere found during 1994–2000 in the same population. Wesuggest that the earlier correlations were probably caused indirectlyby covariation with territory quality, and that a decline inpopulation size has changed the role of territory quality. Inthe Swedish population, an earlier study found a striking correlationbetween the size of a male's repertoire and the viability ofits offspring, suggesting good-gene effects. In contrast, wefound no such correlation in the German population, neitherin 1981–1982 nor in 1994–2000. We conclude thatrepertoire size does not seem to be a very reliable indicatorof variation in male quality. Interestingly, the analysis ofdata from 1994–2000 showed that male pairing success wasstrongly correlated with measures of strophe length and immediateversatility, traits that have been found to reflect male longevity.Future studies will have to show whether these performance-relatedtraits are more powerful indicators of male quality than isrepertoire size.     

On the moulding of senescence by natural selection in sexual and partly sexual populations

Under a wide variety of dynamic environmental conditions, natural selection appears to favor reproductive investment in a sexually produced offspring, carrying only half of the mother’s genes, over the investment in an asexually produced offspring, genetically identical to her. It is maintained that the same environmental conditions must affect the evolutionary cost and benefit of an investment in the prolongation of one’s own life versus an investment in sexual reproduction, in favor of the latter. The effects of different environmental conditions on the division of resources among sexual reproduction, asexual reproduction and prolongation of life are studied.     

Escapes from herbivory in relation to the structure of mangrove island macroalgal communities

Summary Some shallow habitats that surround mangrove islands exhibit abruptly discontinuous macrophyte boundaries; in other regions, plant distributional patterns are less defined. Where distinct boundaries do occur, fleshy algae predominate on the roots of the red mangrove, Rhizophora mangle, which do not contact the bottom sediments (hanging roots), while calcifying algae dominate on the substratum-penetrating roots and banks (=embedded-root habitat) surrounding the mangrove thickets. Considerable natural-history and floristic information reveals that the fleshy hanging-root species are not specialists, for that type of habitat. Experimental transplants showed that on banks and embedded roots where there typically are abundant macroherbivores (particularly sea urchins), most fleshy algae are eliminated.The dominants of the hanging-root habitat (e.g, Acanthophora spicifera, Spyridia filamentosa, Caulerpa racemosa var. Occidentalis) are 6–20 times more susceptible to herbivores than the dominants of the embedded-root habitat (e.g., Halimeda opuntia f. triloba, H. monile). Consequently, we suggest the former are relegated to the spatial refugia from herbivores (=non-coexistence escapes) provided by the hanging roots. Factors associated with these palatability differences include higher average calorific values (6.5 times) of the fleshy hanging-root dominants, greater proportions of organic content (2.6 times) and the general absence of calcification. The dominants of the embedded-root habitat show reduced edibility as a probable consequence of low calorific values, heavy calcification and potential herbivore-detering secondary metabolites. Correlative evidence and preliminary experimental results tentatively indicate that, in the absence of macroherbivores, the hanging-root dominants, which exhibit production rates 4.7 times greater than the dominants of the embedded-root habitat, are better competitors for space.We suggest that variations in herbivory are responsible, in part, for maintaining greater algal diversity in mangrove systems. At a study site with abundant sea urchins, five algal species were found only in the embedded-root habitat three species were confined to the hanging roots, while three others occurred in both. At an urchin-free site, no macrophytes were found only on embedded-root substrata, while one (in trace amounts) was found only on hanging-root habitat and eight occurred in both. We predict that in the absence of herbivores, the species assemblage characteristic of the hanging-roots would exclude many of the dominants from the embedded-root habitat.     

Age-specific patterns of infection with haemosporidians and trypanosomes in a warbler: implications for sexual selection

Although the selective loss of individuals susceptible to disease can favor the evolution of female preference for older males, the interrelationship between age, infection, longevity, and mating success remains poorly characterized in natural populations. In a longitudinal study of 61 male common yellowthroats (Geothlypis trichas), we found that the probability of infection with hematozoa increased as males aged from 1 to 5 years. Despite a significant, negative association between infection and longevity that partially masked age-effects, the odds that a male was infected with Trypanosoma, Plasmodium, or Leucocytozoon increased 71–212% per year. Nearly 75% of males in their first breeding season were either uninfected or infected with only a single parasite, while 50% of older males were infected with at least two parasites and 16% were infected with all three. No males escaped infection after their second breeding season. Older males were also more likely to sire extra-pair young (EPY) and, as a consequence, infection with multiple parasites was associated with a fourfold increase in the odds of producing EPY. Unlike younger males, 80% of the oldest males had a history of either surviving chronic infection or recovering. Combined with previous work showing higher diversity at the major histocompatibility complex among older males, our results suggest that the song and plumage traits that signal male age in common yellowthroats also, perforce, signal resistance to parasites. By preferring older males, females may obtain good genes for disease resistance even in the absence of any effect of infection on male ornamentation.     

The evolution of male infanticide in relation to sexual selection in mammalian carnivores

The evolution of infanticide by males has often been explained by the sexual selection hypothesis, which posits that infanticide improves male reproductive success by shortening the interbirth intervals of the mothers of the killed offspring. In Carnivora, however, the fitness advantages assumed in this hypothesis have been shown in only a few species, and it has been argued that male infanticide may be nonadaptive in pinniped carnivores. According to the sexual selection hypothesis, male infanticide is expected to be more prevalent in species in which males are subjected to stronger sexual selection through intrasexual competition over mates. We examined a phylogenetically corrected relationship between male infanticide and sexual size dimorphism (SSD) as a measure of the intensity of sexual selection in carnivores. Our analyses failed to detect a significant association between the occurrence of male infanticide and SSD across carnivores, although they showed that, among fissipeds (typically terrestrial carnivores), males in species with stronger male-biased SSD are significantly more likely to commit infanticide. This suggests that the evolution of male infanticide is correlated with intense sexual selection in fissipeds. In pinnipeds (Odobenidae, Otariidae, and Phocidae), there was no significant association between male infanticide and SSD. Assuming that SSD represents the intensity of sexual selection on males, this result is consistent with the argument that infanticide by male pinnipeds is not a sexually selected behaviour.     

Balancing selection,sexual selection and geographic structure in MHC genes of Great Snipe

Signatures of balancing selection are often found when investigating the extremely polymorphic regions of major histocompatibility complex (MHC) genes, and it is generally accepted that selective forces maintain this polymorphism. However, the exact nature of the selection is controversial. Theoretical studies have mainly focused on overdominance and/or frequency dependent selection while laboratory studies have emphasised the role of mate choice. Empirical field data, on the other hand, have been relatively scarce. Previously we have found that geographic structure in MHC class II genes of the Great Snipe (Gallinago media) is too pronounced to be explained by neutral forces alone. Here we test the hypothesis that sexual selection on MHC alleles may be influencing this geographic structure between mountain and lowland populations. We found evidence of balancing selection acting on MHC genes in the form of a higher rate of amino-acid changing substitutions compared to silent substitutions in the peptide binding regions. Not only natural selection but also sexual selection may influence MHC polymorphism in this bird because certain MHC alleles have been found to be associated with higher male mating success. Contrary to predictions from negative frequency dependent selection, males carrying locally rare alleles did not have a mating advantage. Instead, the mating success of alleles in a mountain population was positively correlated to their relative frequency in the mountains compared to the lowlands, implying that locally adapted MHC alleles may also be favoured by sexual selection.     

Acoustic communication between the sexes of the bush cricket, Leptophyes punctatissima

ABSTRACT. Both sexes of the speckled bush cricket, Leptophyes punctatissima (Bosc) (Tettigoniidae) produce sound by stridulation. The sound is used in courtship. The male sings for periods throughout the day and night, and there is a peak of activity in the early afternoon. The female sings only in response to a male chirp. The male increases his rate of singing if he receives replies from a female. After 3 days isolation from male song, the female replies most readily to male song mimics of short duration (10 ms) whose carrier frequency is 30 or 45 kHz. Females that have been isolated from male song for 10 days respond less readily than those isolated for 3 days. The short duration of the songs of both sexes reduces their value as locating signals. This disadvantage may be outweighed by the fact that a short signal reduces the chance of a predator detecting the singer by acoustic or visual cues.     

Tail length in birds in relation to tail shape,general flight ecology and sexual selection

Relative tail length (longtailedness) of Palearctic birds was assessed by the standardized residuals of log–log regressions of tail length on wing length and tarsus length. The mean degree of tail shortening was greater than mean degree of tail lengthening, but there was a greater frequency of extreme long-tailed than short-tailed species. Longtailedness was greater in ornamental pin, lyre, deep forked and graduated shaped tails. These shapes (except graduated, for which data were lacking) were also relatively long-tailed according to shortest-rectrix lengths, this extra length potentially contributing compensatory lift. In forked tails, tail ratio increased linearly with longtailedness to above the aerodynamic optimum, and thus the most elongated forked tails were also more deeply forked. Tail shortening was marked for rounded tails, a surprising result in view of their slightly ornamental shape. Phylogenetically independent contrasts showed significantly greater longtailedness in graduated than square-tailed species, confirming the species-wide analysis. In phylogenetically independent contrasts of longtailedness and ecological factors, short-tailed species had significantly greater flight distances than medium-tailed species, but long- and medium-tailed species did not differ in migratory distance, foraging distance, overall flight distance or importance of aerial foraging. The data suggest that ecological factors, i.e. natural selection, are more important in the evolution of short-tailedness than longtailedness in birds, and that an additional influence of sexual selection on tail length and shape is also widespread.     

Cultural ecology and the genetical structure of Nicobar island populations

The population of Nicobar is not a single random mating population but divided into a number of subpopulations within each of which essentially random mating takes place. Heterogeneity tests indicate that there is a significant difference among subpopulations for the ABO blood group system but not for the MN system. The overall gene frequencies of the ABO system were: r = 0·914; p = 0·033; q = 0·053. The gene frequencies of the MN system are much more consistent in the area as a whole than in the ABO system. The gene frequencies of the MN system were: M = 0·92 and N = 0·08. The Wahlund's principle yields the value of Ø = 0·0358 for the MN system and the unweighted mean value of Øs equals 0·0301 for the ABO system. The founder effect may have a far greater effect than the effects of chance in the genetical structure of Nicobar subpopulations.     

Phylogeography and genetic structure of northern populations of the yellow warbler (Dendroica petechia)

Phylogeographic patterns of intraspecific variation can provide insights into the population-level processes responsible for speciation and yield information useful for conservation purposes. To examine phylogeography and population structure in a migratory passerine bird at both continental and regional geographical scales, we analysed 344 bp of mitochondrial DNA (mtDNA) control region sequence from 155 yellow warblers (Dendroica petechia) collected from seven locations across Canada and from Alaska. There is a major subdivision between eastern (Manitoba to Newfoundland) and western (Alaska and British Columbia) populations which appears to have developed during the recent Pleistocene. Some localities within these two regions also differ significantly in their genetic composition, suggesting further subdivision on a regional geographical scale. Eastern and western birds form distinct phylogeographic entities and the clustering of all western haplotypes with two eastern haplotypes suggests that the western haplotypes may be derived from an eastern lineage. Analyses based on coalescent models support this explanation for the origin of western haplotypes. These results are consistent with important features of Mengel's model of warbler diversification. From a conservation perspective they also suggest that individual populations of migrant birds may form demographically isolated management units on a smaller scale than previously appreciated.