Sperm competition in the zebra finch, Taeniopygia guttata

Transcriptomic changes in the posterior pallium of male zebra finches associated with social niche conformance

Animals plastically adjust their physiological and behavioural phenotypes to conform to their social environment-social niche conformance. The degree of sexual competition is a critical part of the social environment to which animals adjust their phenotypes, but the underlying genetic mechanisms are poorly understood. We conducted a study to investigate how differences in sperm competition risk affect the gene expression profiles of the testes and two brain areas (posterior pallium and optic tectum) in breeding male zebra finches (Taeniopygia castanotis). In this pre-registered study, we investigated a large sample of 59 individual transcriptomes. We compared two experimental groups: males held in single breeding pairs (low sexual competition) versus those held in two pairs (elevated sexual competition) per breeding cage. Using weighted gene co-expression network analysis (WGCNA), we observed significant effects of the social treatment in all three tissues. However, only the treatment effects found in the pallium were confirmed by an additional randomisation test for statistical robustness. Likewise, the differential gene expression analysis revealed treatment effects only in the posterior pallium (ten genes) and optic tectum (six genes). No treatment effects were found in the testis at the single gene level. Thus, our experiments do not provide strong evidence for transcriptomic adjustment specific to manipulated sperm competition risk. However, we did observe transcriptomic adjustments to the manipulated social environment in the posterior pallium. These effects were polygenic rather than based on few individual genes with strong effects. Our findings are discussed in relation to an accompanying paper using the same animals, which reports behavioural results consistent with the results presented here.

Differential effects of continuous and intermittent daytime food deprivation periods on metabolism and reproductive performance in diurnal zebra finches

We investigated whether food availability effects on metabolism and reproduction are the result of the sum effect of daily feeding (food availability) and starvation (food deprivation) periods. Adult zebra finches were paired and subjected to a time-restricted feeding (TRF) regimen consisting of continuous and intermittent daytime food deprivation periods. Birds were given food during the 12-h day for a total of 4-h in the evening (1 *4-h, hour 8-12), or in 2 splits of 2 h each (2 * 2-h) or 4 splits 1 h each (4 * 1-h), with controls on food ad libitum, until they had the first egg clutch. TRF caused significant changes in hepatic expression of metabolism-associated sirt1, egr1, pparα and foxo1 genes despite no difference in the food intake, body mass and blood glucose levels. Importantly, TRF resulted in a significant reduction in plasma testosterone and estradiol levels, delayed nest-building and egg laying, and reduced clutch size. Concurrently, under TRF regimes, we found a significantly lower expression of th and mtr genes linked with motivation and affiliation (but not of dio2, dio3, gnrh1 and gnih genes linked with gonadal maturation) in the hypothalamus, and of star and hook 1 genes in the testes and star, cyp19 and erα genes in the ovary. These results demonstrate the importance of daily food deprivation times on the metabolism and reproduction, and suggest a possible provisioning of energy available from daily feeding for the maintenance of body condition at the expense of reproduction performance in diurnal animals.

Male social niche conformance? Effects of manipulated opportunity for extra-pair mating on behavior and hormones of male zebra finches

Success in sperm competition is an important determinant of male fitness in mating systems with female multiple mating. Thus, sperm competition risk represents a key dimension of the male social environment to which individual males are expected to adaptively adjust their reproductive phenotype. Such adaptive phenotypic adjustment we here refer to as male social niche conformance. In this pre-registered study, we investigated how male zebra finches, Taeniopygia guttata, adjust their behavior to sperm competition risk. We experimentally manipulated the opportunity for extra-pair mating to create two levels of sperm competition risk: 1) Single-pair, no sperm competition risk; 2) Double-pair, sperm competition risk. We compared male courtship, mate guarding, copulation rates, and aggression between the treatment groups. To identify hormonal correlates of male behavioral adjustment, we measured plasma testosterone and corticosterone levels before and after the social treatment started. Contrary to our pre-registered predictions, males from the Double-pair treatment group decreased courtship rates compared to those from the Single-pair group, and Double-pair males responded less aggressively towards intruders than Single-pair males. Testosterone levels decreased over the breeding cycle, but social treatment had no effect on either testosterone or corticosterone levels. Our results indicate that male zebra finches do not intensify courtship or competitive reproductive behaviors, or upregulate key hormones when another breeding pair is present. Although we found no evidence for the predicted adaptive behavioral responses to sperm competition risk, we show that male zebra finches plastically adjust their behavior to their social environment.

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.

Developmental polychlorinated biphenyl exposure influences adult zebra finch reproductive behaviour

Polychlorinated biphenyls (PCBs) are worldwide chemical pollutants that have been linked to disrupted reproduction and altered sexual behaviour in many organisms. However, the effect of developmental PCB-exposure on adult passerine reproductive behaviour remains unknown. A commercial PCB mixture (Aroclor 1242) or an estrogenic congener (PCB 52) were administered in sublethal amounts to nestling zebra finches (Taeniopygia guttata) in the laboratory to identify effects of developmental PCB-exposure on adult zebra finch reproductive parameters. Results indicate that although traditional measures of reproductive success are not altered by this PCB dosage, PCBs do alter sexual behaviours such as male song and nesting behaviour. Males treated with PCB 52 in the nest sang significantly fewer syllables than control males, while females treated with Aroclor 1242 in the nest showed the strongest song preferences. PCB treatment also caused an increase in the number of nesting attempts and abandoned nests in the Aroclor 1242 treatment relative to the PCB 52 treatment, and offspring with control fathers fledged significantly earlier than those with fathers treated with Aroclor 1242. Behavioural differences between males seem to best explain these reproductive effects, most notably aggression. These findings suggest that sublethal PCB-exposure during development can significantly alter key reproductive characteristics of adult zebra finches, likely reducing fitness in the wild.

Differential sperm-egg interactions in experimental pairings between two subspecies and their hybrids in a passerine bird

Speciation research has largely overlooked reproductive barriers acting between copulation and the formation of the zygote (i.e., postmating, prezygotic [PMPZ] barriers), especially in internally fertilizing vertebrates. Nonetheless, it is becoming clear that PMPZ reproductive barriers can play a role in the formation and maintenance of species boundaries. We investigated sperm-egg interactions in the recently diverged subspecies pairs of the long-tailed finch, Poephila acuticauda acuticauda and P. a. hecki, to explore potential PMPZ barriers. Specifically, we compared the number of sperm reaching the perivitelline layer (PVL) of the ova, and hence the site of fertilization, in both intra- and inter-subspecies pairings and pairings of F1 hybrid adults with one parental subspecies. Although we found no difference in PVL sperm number among intra- and inter-subspecific pairs, a significantly lower number of sperm reached the site of fertilization in a backcross pairing. As low numbers of PVL sperm appear to be associated with low fertilization success in birds, our findings offer insight into the potential role of postcopulatory processes in limiting gene flow between the subspecies and may help explain the relatively narrow hybrid zone that exists in the wild in this species. Though further work is needed to gain a comprehensive understanding of the morphological, physiological, and molecular mechanisms underlying our results, our study supports the role of PMPZ reproductive barriers in avian speciation, even in recently diverged taxa, that may not yet be fully genetically incompatible.

Extra-pair paternity in the black-headed gull: is it exceptional among colonial waterbirds?

Although nesting at high densities promotes frequent interactions with individuals other than social mates, the rate of extra-pair paternity (EPP) in colonial waterbirds is generally low. So far, the highest EPP rate within this group has been reported in the black-headed gull Chroicocephalus ridibundus breeding in a colony in Czech Republic (33% broods with extra-pair offspring). In this study, we used a set of microsatellite loci to analyse parentage in 34 gull families (102 nestlings) from northern Poland. We recorded intra-specific brood parasitism in 10.8% of broods, but no evidence for EPP was found. The striking discrepancy between our results and previous estimates of EPP rate in the black-headed gull strongly indicate that promiscuity is a context-dependent strategy in this species and that its prevalence may greatly vary between populations. We recommend that a special care should be taken while using single-population data on EPP in the inter-specific comparative analyses.

Divorce in the socially monogamous zebra finch: Hormonal mechanisms and reproductive consequences

Up to 80% of all bird species are socially monogamous. Divorce (switching partners) or pair disruption (due to the death of a partner) has been associated with decreased reproductive success, suggesting social monogamy is a strategy that may maximize fitness via coordination between partners. Previous studies have demonstrated the effects of divorce and pair disruption on immediate reproductive success. Here, we used a paired experimental design in the zebra finch (Taeniopygia guttata) to examine the hormonal mechanisms that modulate parental behavior and reproductive success in response to a partnership change (hereafter divorce). Specifically, we examined the effects of divorce on the avian stress hormone corticosterone (CORT) in both parents and nestlings, parental behaviors (incubation and nestling provisioning), prolactin (PRL), and reproductive success. We found that divorce resulted in delayed clutch initiation, reduced clutch mass, and an increase in nestling CORT response to a standardized stressor. These effects on reproductive investment and chick CORT response were not clearly determined by parental endocrine responses. Divorce had no effect on the level of parental CORT. PRL levels were highly correlated within a pair regardless of treatment, were negatively related to the investment that males made in incubation, and increased in experimental males as a result of pair disruption. This study demonstrates the fundamental impact which divorce has not only on reproduction, but also the physiological stress responses of offspring and suggests that in socially monogamous animals the maintenance of a stable partnership over time could be advantageous for long term fitness.

Polyspermy in birds: sperm numbers and embryo survival

Polyspermy is a major puzzle in reproductive biology. In some taxa, multiple sperm enter the ovum as part of the normal fertilization process, whereas in others, penetration of the ovum by more than one sperm is lethal. In birds, several sperm typically enter the germinal disc, yet only one fuses with the female pronucleus. It is unclear whether supernumerary sperm play an essential role in the avian fertilization process and, if they do, how females regulate the progression of sperm through the oviduct to ensure an appropriate number reach the ovum. Here, we show that when very few sperm penetrate the avian ovum, embryos are unlikely to survive beyond the earliest stages of development. We also show that when the number of inseminated sperm is limited, a greater proportion than expected reach and penetrate the ovum, indicating that females compensate for low sperm numbers in the oviduct. Our results suggest a functional role for supernumerary sperm in the processes of fertilization and early embryogenesis, providing an exciting expansion of our understanding of sperm function in birds.

Embryonic and postnatal telomere length decrease with ovulation order within clutches

Telomere length (TL) in early life has been found to be predictive of subsequent lifespan. Factors such as parental TL, parental age and environmental conditions during development have been shown to contribute to the observed variation in TL among individuals. One factor that has not hitherto been considered is ovulation order, although it is well established that the last hatched/born offspring in a brood or litter often show relatively poor subsequent performance. We examined the within- and across-clutch effect of ovulation order on TL in embryos of zebra finches experiencing the same controlled incubation conditions (N = 151), and tested whether any such ovulation order effects remained detectable in adults (N = 122). Irrespective of clutch and egg size, TL in early-stage embryos (72 h incubation) markedly decreased with within-clutch ovulation order; the difference in TL of first and last-laid embryos was equivalent to the average within-individual telomere loss over the entire period of nestling and juvenile life. This ovulation-order effect occurred only within but not across clutches, and was still evident in adults. Given that TL in early life predicts lifespan, our results suggest that parental effects on telomere length could contribute to the known poor performance of later-ovulated family members.

Testosterone secretion in a socially monogamous but sexually promiscuous migratory passerine

The steroid hormone testosterone (T) influences a multitude of traits critical to reproduction in vertebrates. In birds, high male T supports territory establishment and mate attraction, but is thought to interfere with parental care. Interspecific comparisons indicate that migratory species with short, synchronous breeding seasons have the highest peak T, and that the seasonal profile of T exhibits a rapid decline with the onset of incubation by females. We describe the T profile of the migratory, socially monogamous, and biparental Eastern Kingbird (Tyrannus tyrannus) from the high desert of eastern Oregon, USA, where breeding occurs within a short 2-3 month period. Eastern Kingbirds are socially monogamous but exhibit high rates of extra-pair paternity as ∼60% of broods contain extra-pair young. We therefore evaluate whether Eastern Kingbirds exhibit the "typical" T profile expected for a synchronously breeding migratory species, or whether T is maintained at a more constant level as would be predicted for a species with opportunities for mating that extend over a majority of the breeding season. Our samples were divided into six periods of the reproductive cycle from territory establishment to the feeding of fledglings. T did not change across stages of the nest cycle. Instead, T declined with sampling date and nest density, and increased with the number of fertile females in the population. Male kingbirds advertise their presence through song for most of the breeding season, and we suggest that T is maintained throughout most of the breeding season because male fitness is equally dependent on within- and extra-pair reproductive success.

Sperm Competition Risk and Sexual Coercion Predict Copulatory Duration in Humans

A man whose romantic partner is sexually unfaithful is at risk of sperm competition and cuckoldry-unwitting investment in offspring to whom he is genetically unrelated. Men, therefore, may have evolved mechanisms to solve the adaptive problems of sperm competition and cuckoldry. The current research investigates another potential anti-cuckoldry tactic: reducing in-pair copulation (IPC) duration, thereby more quickly placing his sperm into competition. We hypothesize that IPC duration will be negatively correlated with female infidelity (Hypothesis 1). We further hypothesize that IPC duration will be negatively correlated with sexual coercion (Hypothesis 2). Results of Study 1 (men's reports, n = 410) indicate that both men's perceptions of female infidelity and men's sexual coercion predict shorter IPC duration. Results of Study 2 (women's reports, n = 455) did not provide statistical support for the study hypotheses. The current research provides an initial investigation of men's adjustment of copulatory duration and suggests that men reduce IPC duration and ejaculate more quickly at the couple's most recent copulation, in response to greater risk of sperm competition and in the context of sexual coercion.

Long sperm fertilize more eggs in a bird

Sperm competition, in which the ejaculates of multiple males compete to fertilize a female's ova, results in strong selection on sperm traits. Although sperm size and swimming velocity are known to independently affect fertilization success in certain species, exploring the relationship between sperm length, swimming velocity and fertilization success still remains a challenge. Here, we use the zebra finch (Taeniopygia guttata), where sperm size influences sperm swimming velocity, to determine the effect of sperm total length on fertilization success. Sperm competition experiments, in which pairs of males whose sperm differed only in length and swimming speed, revealed that males producing long sperm were more successful in terms of (i) the number of sperm reaching the ova and (ii) fertilizing those ova. Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important. The mechanisms underlying these interactions are poorly understood, but we suggest that differences in sperm storage and utilization by females may contribute to the outcome of sperm competition.

Of mice and men: what animal research can tell us about context effects on automatic responses in humans

Automatic responses play a central role in many areas of psychology. Counter to views that such responses are relatively rigid and inflexible, a large body of research has shown that they are highly context-sensitive. Research on animal learning and animal behavior has a strong potential to provide a deeper understanding of such context effects by revealing remarkable parallels between the functional properties of automatic responses in human and nonhuman animals. These parallels involve the contextual modulation of attitude formation and change (automatic evaluation), and the role of contextual contingencies in shaping the particular action tendencies in response to a stimulus (automatic behavior). Theoretical concepts of animal research not only provide novel insights into the processes and representations underlying context effects on automatic responses in humans; they also offer new perspectives on the interface between affect, cognition, and motivation.

Social facilitation of male song by male and female conspecifics in the zebra finch, Taeniopygia guttata

Zebra finches are a ubiquitous model system for the study of vocal learning in animal communication. Their song has been well described, but its possible function(s) in social communication are only partly understood. The so-called 'directed song' is a high-intensity, high-performance song given during courtship in close proximity to the female, which is known to mediate mate choice and mating. However, this singing mode constitutes only a fraction of zebra finch males' prolific song output. Potential communicative functions of their second, 'undirected' singing mode remain unresolved in the face of contradicting reports of both facilitating and inhibiting effects of social company on singing. We addressed this issue by experimentally manipulating social contexts in a within-subject design, comparing a solo versus male or female only company condition, each lasting for 24h. Males' total song output was significantly higher when a conspecific was in audible and visible distance than when they were alone. Male and female company had an equally facilitating effect on song output. Our findings thus indicate that singing motivation is facilitated rather than inhibited by social company, suggesting that singing in zebra finches might function both in inter- and intrasexual communication.

Learning in intimate connections: Conditioned fertility and its role in sexual competition

BACKGROUND

Studies of sexual conditioning typically focus on the development of conditioned responses to a stimulus that precedes and has become associated with a sexual unconditioned stimulus (US). Such a sexually conditioned stimulus (CS) provides the opportunity for feed-forward regulation of sexual behavior, which improves the efficiency and effectiveness of the sexual activity.

OBJECTIVE AND DESIGN

The present experiments were conducted to provide evidence of such feed-forward regulation of sexual behavior in laboratory studies with domesticated quail by measuring how many fertilized eggs were produced by the female after the sexual encounter. During the conditioning phase, male and female quail received a conditioned stimulus paired with the opportunity to copulate with each other.

RESULTS

Sexual conditioning increased the number of eggs that were fertilized as a consequence of copulation, especially if both the male and the female were exposed to the sexual CS. This conditioned fertility effect occurred with a range of CS durations and CS types. The conditioned fertility effect also occurred in situations involving sexual competition. When two males copulated with the same female, DNA fingerprinting showed that the male whose sexual encounter was signaled by a sexual CS was responsible for most of the resulting offspring. Sexual conditioning also reduced the first-male disadvantage in fertilization that occurs when two males copulate with the same female separated by several hours. Another significant finding was that sexual conditioning attenuated the usual drop in fertilization rate that occurs when the same male copulates with two females in succession.

CONCLUSION

These results show that sexual conditioning increases the number of offspring that are produced in both isolated male-female encounters and in situations that involve two males copulating with the same female or one male copulating with more than one female. By increasing fertilization rates, sexual conditioning can alter genetic transmission across generations and shape evolutionary change.

From Mate Retention to Murder: Evolutionary Psychological Perspectives on Men's Partner-Directed Violence

In response to the tragically high incidence and negative consequences of female-directed violence in intimate relationships, a large literature has been dedicated to the investigation of the proximate and the ultimate or evolutionary predictors of men's partner-directed violence. Evolutionary psychology offers a framework for investigating the design of evolved information-processing mechanisms that motivate costly behaviors such as men's partner-directed violence. We review several forms of men's partner-directed violence, including insults, sexual coercion, physical violence, and homicide, from an evolutionary psychological perspective and with a particular focus on the adaptive problem of paternity uncertainty. The problem of paternity uncertainty is hypothesized to have selected for the emotion of male sexual jealousy, which in turn motivates men's nonviolent and violent mate retention behaviors. We review empirical evidence for the relationships among paternity uncertainty, male sexual jealousy, and men's partner-directed violence. We propose that a comprehensive understanding of men's partner-directed violence will be achieved only by careful consideration of both proximate and ultimate causes.

Suspicions of female infidelity predict men's partner-directed violence

Archer's argument regarding sex differences in partner violence rests on a general account of between-sex differences in reproductive strategies and in social roles. However, men's partner-directed violence often is predicted by perceived risk of female infidelity. We hypothesize that men's partner-directed violence is produced by psychological mechanisms evolved to solve the adaptive problem of paternity uncertainty.

Sperm Supply and Egg fertilization in the Ostrich (Struthio camelus)

We used egg break-out and spermatozoa trapped in the perivitelline layer of eggs to test the hypothesis that sperm supply and egg fertilization rate are high in the ostrich. Egg fertilization status was determined at break-out by the appearance of the germinal disc (GD) and then the perivitelline layer overlying the GD region was collected to count sperm (SpermOPVL) under fluorescence following staining with 4',6'-diamidino-2-phenyindole (DAPI). The study was carried out on commercial ostrich farms over two laying seasons. In the first year, 229 eggs from nine randomly chosen ostrich pens comprising pairs, trios (two females, one male) and larger groups were collected for 1 week of every month of laying. Eggs contained 253 +/- 18 SpermOPVL/mm2 of the GD (mean +/- SEM; range 0-1330). Egg fertilization rate averaged 89.4 +/- 3.4% and varied from 78.6 to 98.2% between pens. Month had no effect on sperm supply or egg fertilization status. Eggs from paired birds (sex ratio 1 : 1) had less sperm in the GD than the eggs from pens with a higher sex ratio. In the second year, 150 eggs from seven pens, each containing only one male and either one, two or three females, were studied for 2 weeks at the beginning (winter), middle (spring) and end (summer) of laying. Eggs contained 364 +/- 45 SpermOPVL/mm2 of the GD (range 0-2880). Season had no effect on sperm supply or egg fertilization. The number of SpermOPVL varied between pens, assumed to be due to variation between individual males. The number of SpermOPVL increased as the sex ratio increased only when very high-ranking males were excluded from the analysis. Egg fertilization rate was 94.4 +/- 3.1% but varied from 64.0 to 100% between pens. Egg fertilization was not affected by season or sex ratio. Low fertilization rates were observed in two pens and appeared related to the lack of synchrony between timing of laying and sperm production in the first, and lack of mating in the second pen. We conclude that ostrich flocks generally have high rates of egg fertilization and any infertility is associated with lack of sperm supply.

Males' evolutionary responses to experimental removal of sexual selection

We evaluated the influence of pre- and post-copulatory sexual selection upon male reproductive traits in a naturally promiscuous species, Drosophila melanogaster. Sexual selection was removed in two replicate populations through enforced monogamous mating with random mate assignment or retained in polyandrous controls. Monogamous mating eliminates all opportunities for mate competition, mate discrimination, sperm competition, cryptic female choice and, hence, sexual conflict. Levels of divergence between lines in sperm production and male fitness traits were quantified after 38-81 generations of selection. Three a priori predictions were tested: (i) male investment in spermatogenesis will be lower in monogamy-line males due to the absence of sperm competition selection, (ii) due to the evolution of increased male benevolence, the fitness of females paired with monogamy-line males will be higher than that of females paired with control-line males, and (iii) monogamy-line males will exhibit decreased competitive reproductive success relative to control-line males. The first two predictions were supported, whereas the third prediction was not. Monogamy males evolved a smaller body size and the size of their testes and the number of sperm within the testes were disproportionately further reduced. In contrast, the fitness of monogamous males (and their mates) was greater when reproducing in a non-competitive context: females mated once with monogamous males produced offspring at a faster rate and produced a greater total number of surviving progeny than did females mated to control males. The results indicate that sexual selection favours the production of increased numbers of sperm in D. melanogaster and that sexual selection favours some male traits conferring a direct cost to the fecundity of females.

Effects of humidity on reproductive behavior in male and female zebra finches (Taeniopygia guttata)

Zebra finches (Taeniopygia guttata) are regarded as opportunistic breeders with reproductive behaviors mediated by short-term proximal environmental conditions. This article provides experimental evidence for the role of humidity in reproductive behaviors. Zebra finches were subjected to experimentally manipulated high levels of relative humidity. Males gathered more nest material and sang more. Females showed no relationship between humidity and gathering nest materials, egg laying, or changes in rates of vocalization.

No Evidence for Effects of Breeding Density and Male Removal on Extrapair Paternity in the Pied Flycatcher

Molecular methods have brought new insight to the study of mating patterns. Extrapair fertilizations (EPF) have proven to be widespread among bird species irrespective of social relationships. Ecological factors, such as breeding density and synchrony, have been suggested as contributors to variation in EPF rates. Absence of a male during the female fertile period may also increase the probability of EPFs. In this study, we examined experimentally whether breeding density and male absence before egg-laying influence the EPF rate in Pied Flycatchers (Ficedula hypoleuca). DNA fingerprinting revealed that 4.4% (10/225) of nestlings were the result of EPFs and 7.0% (3/43) of broods included at least one offspring sired by a male other than that attending the nest. We did not find any evidence of higher EPF rate with higher breeding density as stated by the density hypothesis. Contrary to expectation, EPFs were more frequent, though not quite statistically significantly, at low breeding density: all three EPF nests were found in low-breeding-density areas. There was no evidence that EPF rate was affected by a 4 h male removal. In all cases of EPF, an old female was mated with a yearling male, which suggests that older females mated with browner yearling males may have adjusted their initial mate choice through EPCs to acquire indirect genetic benefits. Pied Flycatcher females may be constrained to accept a mate of lower than preferred quality, especially at low breeding density, due to the cost of searching for alternative males.

Territoriality as a paternity guard in the European robin, Erithacus rubecula

To investigate the relative importance of paternity defences in the European robin we used behavioural observations, simulated intrusions and temporary male removal experiments. Given that paired males did not increase their mate attendance, copulation rate or territory size during the female's fertile period, the most frequently quoted paternity assurance strategies in birds were absent. However, males with fertile females sang and patrolled their territories more regularly, suggesting that territorial motivation and vigilance were elevated when the risk of cuckoldry was greatest. In addition, there was a significant effect of breeding period on response to simulated intrusions: residents approached and attacked freeze-dried mounts more readily in the fertile period. During 90-min removals of the pair male in the fertile period, neighbours trespassed more frequently relative to prefertile and fertile period controls and appeared to seek copulations with unattended females. When replaced on their territories, males immediately increased both song rate and patrolling rate in comparison with controls. We propose that male robins sing to signal their presence, and increase their territorial vigilance and aggression in the fertile period to protect paternity. Copyright 2000 The Association for the Study of Animal Behaviour.

Copulation rate and sperm use by female bearded tits, Panurus biarmicus

We examined functional and mechanistic aspects of sperm competition in the bearded tit by determining (1) the variation in copulation rate between birds breeding alone or with other males present, and (2) the number of sperm present in the female reproductive tract estimated from the number of sperm trapped on the perivitelline layers of eggs. Females copulated at a higher rate with their partner when other males were present, but this did not translate into more sperm on eggs, possibly because insemination rates exceeded the female's sperm storage capacity. The rate of sperm loss from the female's reproductive tract, which is an important variable of sperm competition models, was obtained for the first time for a wild bird and was relatively high compared with other birds. This suggests that bearded tits copulate frequently because a single insemination is insufficient to fertilize the whole clutch, and females thus have to copulate during the egg-laying period to avoid infertile eggs. This finding is the first empirical support for the fertility insurance hypothesis. In line with this we discuss the significance of the interspecific variation in rate of sperm loss in relation to mating strategies in general, and the evolution of multiple mating rather than improved sperm storage in bearded tits with particular regard to the pronounced extrapair copulation behaviour of females. Copyright 1998 The Association for the Study of Animal Behaviour.

Do female pigeons trade pair copulations for protection?

Male pigeons, Columba livia, employ intense mate guarding and frequent copulation apparently as strategies to ensure their paternity. The aim of this study was to investigate the benefits to females of mate guarding by males and frequent copulation. Field observations showed that females initiated the majority of copulations and females that solicited copulations more frequently were guarded more closely by their partner. Experimental removal of guarding male partners showed that: (1) unguarded, fertile females suffered increased harassment from extrapair males which reduced their foraging efficiency; and (2) unguarded, fertile females did not seek copulations with extrapair males. Various explanations for frequent pair copulation initiated by females are discussed and we conclude that female pigeons trade pair copulations for protection (mate guarding) against sexual harassment from other males. Copyright 1998 The Association for the Study of Animal Behaviour.