Male sperm reserves and copulation frequency in birds

Sperm competition intensity shapes divergence in both sperm morphology and reproductive genes across murine rodents

It remains unclear how variation in the intensity of sperm competition shapes phenotypic and molecular evolution across clades. Mice and rats in the subfamily Murinae are a rapid radiation exhibiting incredible diversity in sperm morphology and production. We combined phenotypic and genomic data to perform phylogenetic comparisons of male reproductive traits and genes across 78 murine species. We identified several shifts towards smaller relative testes mass, presumably reflecting reduced sperm competition. Several sperm traits were associated with relative testes mass, suggesting that mating system evolution selects for convergent suites of traits related to sperm competitive ability. We predicted that sperm competition would also drive more rapid molecular divergence in species with large testes. Contrary to this, we found that many spermatogenesis genes evolved more rapidly in species with smaller relative testes mass due to relaxed purifying selection. While some reproductive genes evolved rapidly under recurrent positive selection, relaxed selection played a greater role in underlying rapid evolution in small testes species. Our work demonstrates that postcopulatory sexual selection can impose strong purifying selection shaping the evolution of male reproduction, and that broad patterns of molecular evolution may help identify genes that contribute to male fertility.

Evolution of the germline mutation rate across vertebrates

The germline mutation rate determines the pace of genome evolution and is an evolving parameter itself¹. However, little is known about what determines its evolution, as most studies of mutation rates have focused on single species with different methodologies². Here we quantify germline mutation rates across vertebrates by sequencing and comparing the high-coverage genomes of 151 parent-offspring trios from 68 species of mammals, fishes, birds and reptiles. We show that the per-generation mutation rate varies among species by a factor of 40, with mutation rates being higher for males than for females in mammals and birds, but not in reptiles and fishes. The generation time, age at maturity and species-level fecundity are the key life-history traits affecting this variation among species. Furthermore, species with higher long-term effective population sizes tend to have lower mutation rates per generation, providing support for the drift barrier hypothesis³. The exceptionally high yearly mutation rates of domesticated animals, which have been continually selected on fecundity traits including shorter generation times, further support the importance of generation time in the evolution of mutation rates. Overall, our comparative analysis of pedigree-based mutation rates provides ecological insights on the mutation rate evolution in vertebrates.

Age-related changes in sperm traits and evidence for aging costs of sperm production in a sexually promiscuous passerine

In many animal species, organismal performance declines with age in a process known as aging or senescence. Senescence typically leads to a deterioration of physiological functionality and can impact the development of primary sexual phenotypes. Sperm production is a complex and costly process that is sensitive to changes in individual physiological state, yet remarkably little is known about age-related changes in sperm performance and aging costs of sperm production. Here we use a non-linear generalized additive mixed models (GAMM) modelling to evaluate age-related changes in postcopulatory sexual traits in the European barn swallow (Hirundo rustica rustica), a relatively short lived sexually promiscuous passerine species, where male extra-pair fertilization success has been shown to increase with age. We confirmed a positive relationship between sperm midpiece length and sperm velocity in this species. Within-male changes in sperm morphology and sperm velocity were in general absent, with only sperm length decreasing linearly with increasing age, although this change was negligible compared to the overall variation in sperm size among males. In contrast, the cloacal protuberance (CP) size changed nonlinearly with age, with an initial increase between the first and third year of life followed by a plateau. The results further indicate the existence of a trade-off between investments in sperm production and survival as males with large CP tended to have a reduced lifespan. This seems consistent with the idea of expensive sperm production and survival aging costs associated with investments in post-copulatory traits in this sexually promiscuous species.

Sperm Numbers as a Paternity Guard in a Wild Bird

Sperm competition is thought to impose strong selection on males to produce competitive ejaculates to outcompete rival males under competitive mating conditions. Our understanding of how different sperm traits influence fertilization success, however, remains limited, especially in wild populations. Recent literature highlights the importance of incorporating multiple ejaculate traits and pre-copulatory sexually selected traits in analyses aimed at understanding how selection acts on sperm traits. However, variation in a male’s ability to gain fertilization success may also depend upon a range of social and ecological factors that determine the opportunity for mating events both within and outside of the social pair-bond. Here, we test for an effect of sperm quantity and sperm size on male reproductive success in the red-back fairy-wren (Malurus melanocephalus) while simultaneously accounting for pre-copulatory sexual selection and potential socio-ecological correlates of male mating success. We found that sperm number (i.e., cloacal protuberance volume), but not sperm morphology, was associated with reproductive success in male red-backed fairy-wrens. Most notably, males with large numbers of sperm available for copulation achieved greater within-pair paternity success. Our results suggest that males use large sperm numbers as a defensive strategy to guard within-pair paternity success in a system where there is a high risk of sperm competition and female control of copulation. Finally, our work highlights the importance of accounting for socio-ecological factors that may influence male mating opportunities when examining the role of sperm traits in determining male reproductive success.

Zinc and Shell Flour as Innovative Natural Aromatase Blocker to Increase Testosterone Concentration

The hormone testosterone is essential in various biological processes, including reproductive function so its presence in the body must remain stable. Low concentrations of testosterone can affect secondary traits and libido in male animals. Testosterone concentrations are usually increased through hormonal induction, but this will harm long-term administration. The alternative offered in this problem is to give aromatase blockers both in the form of commercial drug injection (letrozole) and natural ingredients (shell powder). Aromatase blockers can inhibit the synthesis of estrogen from testosterone at certain doses that are usually different for each animal, administering aromatase blockers has been shown to increase testosterone concentration, in birds and chickens the administration of aromatase blockers did not have any significant side effects.

Early nighttime testosterone peaks are correlated with GnRH-induced testosterone in a diurnal songbird

Experimental manipulation has established testosterone as a potent, pleiotropic regulator coordinating morphology, physiology and behavior. However, the relationship of field-sampled, unmanipulated testosterone concentrations with traits of interest is often equivocal. Circulating testosterone varies over the course of the day, and recent reports indicate that testosterone is higher during the night in diurnal songbirds. Yet, most field studies sample testosterone during the morning. Sampling at times when levels and individual variation are low may be one reason relationships between testosterone and other traits are not always observed. Testosterone is regulated by the hypothalamic-pituitary-gonadal axis, with gonadotropin-releasing hormone (GnRH) initiating the endocrine cascade. Research has examined GnRH-induced testosterone levels with traits of interest, yet the relevance of these induced levels and their relationship with endogenously produced levels are not fully clear. Using photostimulated male great tits (Parus major) we tested the hypotheses that circulating testosterone levels peak during the night and that GnRH-induced testosterone concentrations are positively related to nightly testosterone peaks. Blood was sampled during first, middle or last third of night. One week later, baseline and GnRH-induced testosterone levels were sampled during mid-morning. Morning baseline testosterone levels were low compared with night-sampled levels that peaked during the first third of the night. Further, GnRH-induced testosterone was strongly positively correlated with levels observed during the first third of the night. These data suggest that morning testosterone samples likely do not reflect an individual's endogenous peak. Instead, GnRH-induced testosterone levels do approximate an individual's nightly peak and may be an alternative for birds that cannot easily be sampled at night in the field. These findings are likely to have implications for research aimed at relating traits of interest with natural variation in sex steroid hormone levels.

Life-long testosterone and antiandrogen treatments affect the survival and reproduction of captive male red-legged partridges (Alectoris rufa)

Abstract

Sexual steroids can play an important role as life-history organizers. In males, high circulating testosterone levels induce physiological/behavioral costs and benefits, leading to trade-offs. However, studies simultaneously testing the impact of these levels in both fitness components (survival and fecundity) during lifetime are scarce and limited to wild birds. To determine the mortality causes or hormonal manipulation impacts on male fertility is, nonetheless, a difficult task in free-ranging animals that could be easier in captivity. We longitudinally monitored captive red-legged partridges (Alectoris rufa) and exposed males to high exogenous testosterone levels, anti-androgens, or a control treatment during each breeding period throughout their lives. Theory predicts that individuals maintaining high androgen levels should obtain higher fitness returns via reproduction, but suffer reduced longevity. Testosterone-treated male partridges, accordingly, lived shorter compared to controls, since they were more prone to die from a natural bacterial infection. However, the same birds seemed to have a lower capacity to fertilize eggs, probably due to endocrine feedback reducing testicular mass. These results show that exogenous testosterone can exert unpredicted effects on fitness parameters. Therefore, caution must be taken when drawing conclusions from non-fully controlled experiments in the wild. Males treated with the androgen-receptor blocker flutamide did not outlive controls as predicted by the life-history trade-off theory, but their mates laid eggs with higher hatching success. The latter could be due to mechanisms improving sperm quality/quantity or influencing maternal investment in egg quality. Testosterone receptor activity/amount could thus be as relevant to fitness as testosterone levels.

Significance statement

It has repeatedly been hypothesized that high testosterone levels induce a cost in terms of reduced lifetime reproductive success. This can be due to reduced fecundity or via shorter lifespan. This is, however, only supported by a handful of studies, mostly in wild birds. We tested this in captive male red-legged partridges, which allowed us to determine reproductive success and mortality causes. We increased testosterone levels or blocked its action with antiandrogens throughout life. High testosterone levels reduced the survival by making birds more prone to die by infection. The eggs produced by their mates also showed lower hatching success, a probable manipulation artifact that should be considered in avian studies in the wild. Interestingly, the androgen-receptor blocker flutamide increased lifetime hatching success compared to controls, suggesting that androgen receptor amounts/activity are even more relevant to fitness than testosterone levels.

Male age and its association with reproductive traits in captive and wild house sparrows

Evolutionary theory predicts that females seek extra‐pair fertilizations from high‐quality males. In socially monogamous bird species, it is often old males that are most successful in extra‐pair fertilizations. Adaptive models of female extra‐pair mate choice suggest that old males may produce offspring of higher genetic quality than young males because they have proven their survivability. However, old males are also more likely to show signs of reproductive senescence, such as reduced sperm quality. To better understand why old males account for a disproportionally large number of extra‐pair offspring and what the consequences of mating with old males are, we compared several sperm traits of both captive and wild house sparrows, Passer domesticus. Sperm morphological traits and cloacal protuberance volume (a proxy for sperm load) of old and young males did not differ substantially. However, old males delivered almost three times more sperm to the female's egg than young males. We discuss the possibility of a post‐copulatory advantage for old over young males and the consequences for females mated with old males.

The effect of chronic and acute stressors, and their interaction, on testes function: an experimental test during testicular recrudescence

Organisms are expected to invest less in reproduction in response to a stressor, but theory predicts that this effect should depend on the frequency and duration of stressors in the environment. Here, we investigated how an acute stressor affected testes function in a songbird, and how chronic stressors influenced the acute stress response. We exposed male dark-eyed juncos (Junco hyemalis) either to chronic or minimal (control) disturbance during testicular recrudescence, after which we measured baseline testosterone, testosterone after an acute handling stressor, and capacity to produce testosterone after hormonal stimulation. In a 2×2 design, we then killed males from the two chronic treatment groups either immediately or after an acute stressor to investigate the effect of long- and short-term stressors on the testicular transcriptome. We found that chronically disturbed birds had marginally lower baseline testosterone. The acute stressor suppressed testosterone in control birds, but not in the chronic disturbance group. The ability to elevate testosterone did not differ between the chronic treatments. Surprisingly, chronic disturbance had a weak effect on the testicular transcriptome, and did not affect the transcriptomic response to the acute stressor. The acute stressor, on the other hand, upregulated the cellular stress response and affected expression of genes associated with hormonal stress response. Overall, we show that testicular function is sensitive to acute stressors but surprisingly robust to long-term stressors, and that chronic disturbance attenuates the decrease in testosterone in response to an acute stressor.

Fluff-thieving birds sabotage seed dispersal

Characterizing many species interactions as mutualisms can be misleading because some members of the interaction derive greater fitness benefits at the expense of other members. We provide detailed natural history data on a suspected bird-plant mutualism in South Africa where many species of birds use fluffy Eriocephalus seed material to construct their nests, potentially dispersing seeds for the plant. We focus on a common bird, Prinia maculosa, which invests heavily in gathering Eriocephalus material. Prinias spent 5 of their median 6-day nest construction period adding seed material to their nests and frequently travelled outside their territory boundary to gather Eriocephalus material. Yet, prinias gathered primarily Eriocephalus fluff and actively avoided gathering seeds. The average prinia nest contained only 6.6 seeds, but contained fluff from 579 seeds. These data suggest that prinias provide limited dispersal benefits to Eriocephalus plants. By contrast, the large amounts of Eriocephalus fluff in prinia nests, and the effort that prinias invest in gathering it, suggest that prinias benefit from constructing their nests with Eriocephalus material. We end by outlining hypotheses for possible fitness benefits that Eriocephalus material could provide prinias and other birds.

Índice gonadossomático e correlações entre dimensões e peso testiculares na codorna japonesa (Coturnix coturnix japonica) aos 60 dias de idade

As dimensões e o peso testicular foram avaliados em 25 machos adultos de codornas japonesas (Coturnix coturnix japonica) e correlacionados entre si, a fim de se buscar obter informações que possibilitem a criação de um parâmetro preditivo da capacidade de produção espermática. As aves foram abatidas e os testículos removidos para mensuração e pesagem. As dimensões e o peso entre os testículos esquerdo e direito foram comparados. Os parâmetros testiculares foram correlacionados entre si por meio da determinação do coeficiente de correlação linear. O índice gonadossomático também foi determinado, indicando alocação de 3,68% do peso corporal em testículos nos machos avaliados, resultado maior do que os descritos na literatura. Apesar da diferença de formato entre os testículos esquerdo e direito, o peso foi semelhante, sugerindo que não há diferença na capacidade de produção espermática entre eles. Diferença estatística entre os valores biométricos indica que o testículo esquerdo é mais curto e mais arredondado, enquanto o direito é mais longo e estreito. O peso corporal apresentou baixa correlação com o peso testicular, 0,14 e 0,12 para os testículos esquerdo e direito, respectivamente. Os parâmetros que melhor se correlacionaram com o peso testicular foram a largura e a espessura. Considerando-se a identificação de um parâmetro único para comparação entre machos, a espessura do testículo esquerdo apresentou coeficiente de correlação linear de 0,89 com o somatório do peso dos dois testículos. Com base nessa informação, métodos não invasivos, como a ultrassonografia, poderiam ser utilizados para estimar o potencial reprodutivo e auxiliar na comparação de machos de codornas japonesas em núcleos de seleção de aves elite, contribuindo para melhoramento genético da espécie.

Spermiophagy in the male reproductive tract of some passerine birds

In order to elucidate the locus and means of spermiophagy in passerine birds, we examined histologically the entire male reproductive tract of sexually mature birds of three passerine species with different forms of sperm competition, namely, the alpine accentor (Prunella collaris), the redflanked bush robin (Tarsiger cyanurus), and the Bengalese finch (Lonchura striata var. domestica). Spermiophagy occurred consistently and frequently in the epithelial layer of the seminal glomera and ejaculatory duct in each species, which were regularly identified by non-ciliated epithelial cells. The epithelial spermiophagy was occasional or infrequent in other portions of the seminal tract, and spermiophagy by macrophages was uncommon throughout the tract. Quantitative data in the seminal glomera and ejaculatory duct gave no clear answer concerning a possible relationship between the epithelial spermiophagy and different levels of sperm competition among these passerine species. In conclusion, the epithelial lining of the terminal region of the seminal tract is the main site for spermiophagy in the male reproductive tract of these passerine species, which activity serves to maintain the quality of semen by eliminating infertile spermatozoa as well as sperm remaining at the end of the breeding season.

Resolving variation in the reproductive tradeoff between sperm size and number

Spermatozoa are amongst the most variable cells, and three factors are thought to account for this variation in design: fertilization mode, phylogeny, and postcopulatory sexual selection. In addition, it has long been assumed that a tradeoff exists between sperm size and number, and although postcopulatory sexual selection affects both traits, empirical evidence for a tradeoff has so far been elusive. Our recent theoretical model predicts that the nature of a direct tradeoff between sperm size and number varies with sperm competition mechanism and sperm competition risk. We test these predictions using a comparative approach in two very different taxa with different sperm competition mechanisms: passerine birds (mechanism: simple raffle) and Drosophila fruit flies (sperm displacement). We show that in both groups, males increase their total ejaculate investment with increasing sperm competition risk, but whereas passerine birds allocate disproportionately to sperm number, drosophilids allocate disproportionately to sperm size. This striking difference between the two groups can be at least partly explained by sperm competition mechanisms depending on sperm size relative to the size of the female reproductive tract: in large animals (passerines), sperm numbers are advantageous in sperm competition owing to dilution inside the female tract, whereas in small animals (drosophilids), large sperm are advantageous for physical competition (sperm displacement). Our study provides two important results. First, we provide convincing evidence for the existence of a sperm size-number tradeoff. Second, we show that by considering both sperm competition mechanism and dilution, can we account for variation in sperm size between different taxa.

Sperm competition selects for sperm quantity and quality in the Australian Maluridae

When ejaculates from rival males compete for fertilization, there is strong selection for sperm traits that enhance fertilization success. Sperm quantity is one such trait, and numerous studies have demonstrated a positive association between sperm competition and both testes size and the number of sperm available for copulations. Sperm competition is also thought to favor increases in sperm quality and changes in testicular morphology that lead to increased sperm production. However, in contrast to sperm quantity, these hypotheses have received considerably less empirical support and remain somewhat controversial. In a comparative study using the Australian Maluridae (fairy-wrens, emu-wrens, grasswrens), we tested whether increasing levels of sperm competition were associated with increases in both sperm quantity and quality, as well as an increase in the relative amount of seminiferous tubule tissue contained within the testes. After controlling for phylogeny, we found positive associations between sperm competition and sperm numbers, both in sperm reserves and in ejaculate samples. Additionally, as sperm competition level increased, the proportion of testicular spermatogenic tissue also increased, suggesting that sperm competition selects for greater sperm production per unit of testicular tissue. Finally, we also found that sperm competition level was positively associated with multiple sperm quality traits, including the proportion of motile sperm in ejaculates and the proportion of both viable and morphologically normal sperm in sperm reserves. These results suggest multiple ejaculate traits, as well as aspects of testicular morphology, have evolved in response to sperm competition in the Australian Maluridae. Furthermore, our findings emphasize the importance of post-copulatory sexual selection as an evolutionary force shaping macroevolutionary differences in sperm phenotype.

Seasonal and social modulation of testosterone in Costa Rican rufous-collared sparrows (Zonotrichia capensis costaricensis)

Previous work shows that most birds breeding in northern temperate regions adjust production of testosterone in response to stage of the breeding cycle and in some cases following social interactions. In contrast, prior research suggests that tropical breeding birds are less likely to modulate testosterone in response to social interactions (the propensity to increase testosterone in response to social instability is known as the challenge hypothesis). To further test the challenge hypothesis in tropical birds, we investigated whether variation in season affects reproductive condition, aggressive behavior, and social modulation of testosterone in two populations of Costa Rican rufous-collared sparrow, Zonotrichia capensis costaricensis. We conducted our study at three distinct times of year: the dry season (March and May); the veranillo, a hiatus in the rainy season (July); and the late rainy season (November). Significantly more birds were in breeding condition in the dry season than in the rainy season or veranillo. In each time period, we collected baseline testosterone samples and conducted simulated territorial intrusions (STIs). Our study shows that testosterone is modulated with season independent of breeding condition, as testosterone levels were affected by season, breeding condition, and an interaction of the two factors. Males breeding in the dry season had higher plasma testosterone levels than non-breeding males in the dry season and both breeding and non-breeding males in the veranillo and rainy season. Males did not socially modulate testosterone in any season. Aggressive behaviors expressed during STIs did not differ among seasons with the exception that males sang fewer songs during the rainy season.

Endocrine correlates of alternative phenotypes in the white-throated sparrow (Zonotrichia albicollis)

Many vertebrate species exhibit alternative phenotypes (or morphs), in which one sex displays phenotypic variation equal to or greater than the variation between the sexes. Males in such species typically display differences in reproductive strategies and morphology. Steroid hormones such as testosterone are known modulators of reproductive behavior and morphology and therefore are obvious candidates for the mediation of phenotypic differences between morphs. We conducted a year-round study in the white-throated sparrow (Zonotrichia albicollis) that exhibits alternative phenotypes in plumage coloration and behavior in both sexes: during the breeding season, white-striped males and females are more aggressive and have higher song rates than tan-striped individuals. At the beginning of the breeding season, free-living white-striped males had higher plasma testosterone concentrations than tan-striped males. However, this finding might have been due to different social experiences because captive male morphs sampled at similar times of year did not differ in testosterone concentrations. Captive white-striped males had larger testis and cloacal protuberance sizes than tan-striped males, which might be related to the divergent mating strategies of the morphs. Male morphs showed similar increases in luteinizing hormone following injections of gonadotropin-releasing hormone, but white-striped males showed larger increases in testosterone, indicating differences between morphs in gonadal testosterone production. Females had low concentrations of testosterone, and morphs did not differ. Plasma dehydroepiandrosterone (DHEA) concentrations were elevated in both sexes and morphs during the breeding and non-breeding seasons. These data do not support the hypothesis that testosterone activates behavioral differences between alternative phenotypes in the white-throated sparrow. Alternative testable hypotheses include hormonal effects during early development and direct genetic effects.

Unusual Sperm Morphology in the Eurasian Bullfinch (Pyrrhula Pyrrhula)

The sperm of the Eurasian Bullfinch (Pyrrhula pyrrhula) differs markedly in gross morphology from that of all other passerines examined to date. In other passerines, the sperm head is pointed and helical, and the midpiece comprises a mitochondrial helix extending along the flagellum; whereas in the Eurasian Bullfinch, the sperm acrosome is rounded, not helical, and the midpiece is extremely short. In a pairwise study, using principal component analysis (PCA), we combined quantitative and qualitative sperm morphology traits and conducted a phylogenetic correlation to compare the sperm morphology of Eurasian Bullfinch and Beavan's Bullfinch (P. erythaca) with nine other pairs of congeneric passerines. The analysis revealed that Eurasian Bullfinch was a dramatic outlier in sperm morphology and that Eurasian and Beavan's bullfinches are more different than any other pair of species. Excluding Eurasian Bullfinch from the analysis showed that most variation in sperm morphology in the other species was attributable to phylogeny. The Eurasian Bullfinch also has extremely small testes for its body size, which indicates that sperm competition is infrequent in this species; we discuss the possibility that relaxed selection, via lack of sperm competition, may have contributed to the species' unusual sperm morphology.

Morfología Espermática Inusual en Pyrrhula pyrrhula

Sperm competition and diversity in rodent copulatory behaviour

The copulatory behaviour of male mammals is characterized by striking diversity in patterns of copulatory stimulation and ejaculation frequency. We conducted comparative analyses of rodents to investigate the potential influence of sperm competition in the evolution of copulatory behaviour. We found that increasing sperm competition is associated with faster rates of stimulation and earlier ejaculation among species in which males perform multiple intromissions prior to ejaculation, but with no overall change in the number of intromissions per ejaculation. Increasing sperm competition is also associated with a higher frequency of repeated ejaculations with the same female, and with shorter refractory periods between repeated copulations. Increasing sperm competition level thus appears to select for male ability to ejaculate more rapidly and more frequently with each female mated. As prolonged copulations are known to reduce female remating rates, these findings indicate that males may experience opposing selection pressures on copulatory behaviour with respect to offensive and defensive aspects of sperm competition. We conclude that sperm competition is likely to be an important selection pressure explaining diversity in mammalian copulatory behaviour.

Bill colour and correlates of male quality in blackbirds: an analysis using canonical ordination

Carotenoid-dependent plumage displays are widely assumed to be honest indicators of individual health or quality, which are used as cues during mate choice and/or agonistic signalling. Despite the fact that red, yellow and orange pigmentation of bills is common, and also variable between individuals, comparatively little is known about bill colouration as a condition-dependent trait. Furthermore, many studies of avian colouration are confounded by the lack of objective colour quantification and the use of overly simplistic univariate techniques for analysis of the relationship between the condition-dependent trait and individual quality variables. In this study, we correlated male blackbird bill colour (a likely carotenoid-dependent sexually selected trait) with body/condition variables that reflect male quality. We measured bill colour using photometric techniques, thus ensuring objectivity. The data were analysed using the multivariate statistical techniques of canonical ordination. Analyses based on reflectance spectra of male blackbird bill samples and colour components (i.e. hue, chroma and brightness) derived from the reflectance spectra were very similar. Analysing the entire reflectance spectra of blackbird bill samples with Redundancy Analysis (RDA) allowed examination of individual wavelengths and their specific associations with the body/condition variables. However, hue, chroma and brightness values also provided useful information to explain colour variation, and the two approaches may be complimentary. We did not find any significant associations between male blackbird bill colour and percent incidence of ectoparasites or cloaca size. However, both the colour component and full spectral analyses showed that culmen length explained a significant amount of variation in male blackbird bill colour. Culmen length was positively associated with greater reflectance from the bill samples at longer wavelengths and a higher hue value (i.e. more orange-pigmented bills). Larger males may have larger territories or be better at defending territories during male-male interactions, ensuring access to carotenoid food sources. Future studies should elucidate the relationship between bill colour and behavioural measures such as aggressiveness, territory size, song rate and nest attendance.

Repeated inseminations required for natural fertility in a wild bird population

In most bird species, pairs copulate many times before egg laying. The exact function of repeated inseminations (i.e. successful copulations) is unknown, but several suggestions have been made. We tested the hypothesis that repeated inseminations are required to ensure fertilization of eggs, by using an experimental method where free-ranging male collared flycatchers (Ficedula albicollis) were prevented from inseminating their mates. We show that egg fertility was lower when females had not copulated during the studied part of their fertile period. By counting sperm on the inner perivitelline layer of eggs, we estimated that a minimum of 86 sperm must reach the site of fertilization to ensure average fertility. Using the timing of inseminations and the numbers of sperm on successive eggs, we show that repeated copulations are necessary to achieve an average rate of fertilization of a single clutch. Our results thus provide evidence that repeated inseminations function to ensure fertilization success. We discuss possible constraints on sperm production and utilization that may have contributed to this pattern.

Latitudinal variation in testis size in six species of North American songbirds

Six songbird species were studied to determine how testis size varies in relation to latitude, using data from breeding areas at latitudes 7°N to 69°N. Three of the species (Carpodacus mexicanus, Vireo olivaceus, Geothylpis trichas) exhibited a significant positive relationship, one (Passerculus sandwichensis) a significant negative relationship, and two (Agelaius phoeniceus, Spizella passerina) no significant association between relative testis mass and latitude. These results suggest that sperm competition may also vary latitudinally, likely in response to geographic variation in breeding season length, extrapair mating intensity, and social mating system.

Male and female behaviour and extra-pair paternity in the wheatear

Behavioural observations and DNA fingerprinting were used to determine the relationship between male and female behaviours and levels of extra-pair paternity in the wheatear, Oenanthe oenanthe. Behavioural observations were consistent with the hypothesis that males attempted to ensure paternity by mate guarding, while pursuing extra-pair copulations (EPCs) primarily outside the fertile period of their pair female. The intensity of guarding varied with time of season and was greater at late nests. However, although males on territories with late nests also experienced high intrusion rates, the intensity of guarding was influenced more by the operational sex ratio (which was female skewed at early nests) than by intrusion rates per se. We suggest that early breeding males adopted a strategy of territorial defence to ensure paternity, as opposed to guarding their female directly (which late breeding males did), to capitalize on the increased opportunities to pursue EPCs in neighbouring territories. Females were less conspicuous than males in their pursuit of EPCs, were never seen off territory or observed to solicit extra-pair males directly, and rejected the majority of EPCs. The frequency of extra-pair paternity was 11% of 73 offspring, in 29% of 17 broods, and was not correlated with the intensity of guarding. Female cooperation appeared to be important for successful copulation, and extra-pair paternity is therefore likely to be a consequence of solicited, or at least accepted, EPCs. We discuss why females might have participated in EPCs. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Multiple mating and cooperative breeding in polygynandrous alpine accentors. II. Male mating tactics

I studied the timing and frequency of male copulatory behaviour in alpine accentors, Prunella collaris, in Japan, with respect to dominance status, age and development of the cloacal protuberance. Males used one of three mating tactics depending on their dominance status. Alpha males guarded several females in succession, but only during the females' fertile period, while gamma males copulated rarely and only with unguarded females. The tactics of beta males were flexible: with unguarded females they usually adopted a frequent-copulation tactic, but when two females had synchronous egg-laying periods, beta males associated closely with whichever female the alpha male left unguarded. When alpha males closely guarded a female, beta males guarded this same female for short periods before and after the guarding period of the alpha male. Subordinate rank restricted a male's opportunities for mating but males of all dominance ranks were sexually active and had fully developed cloacal protuberances. Frequent copulation plus part-time mate guarding (as found in beta males) and opportunistic copulation (as found in gamma males) appears to be a conditional strategy whereby young, subordinate males are 'making the best of a bad job'. A long-term study over 10 years showed that males moved up in social rank with increasing age but 78.9% of males remained subordinate because alpha males occupied the top rank for much of their lifetime. Copyright 1998 The Association for the Study of Animal Behaviour.