Nest size and hatchling sex ratio in chinstrap penguins

Variation in the sex ratio at hatching in the chinstrap penguin Pygoscelis antarctica was investigated, using molecular sexing to test predictions of sex allocation theory. The sex ratio was slightly male-biased (0.54) but did not differ significantly from parity. The proportion of males increased with nest size, an estimator of parental quality in chinstrap penguins. High-quality parents were able to produce and rear a higher proportion of male offspring, the more costly sex in this slightly sexually dimorphic species. Our results may be in agreement with Trivers and Willards (1973) argument on biases in the offspring sex ratio being contingent on parental condition or quality.     

Influence of age on reproductive performance in the Seychelles warbler

I studied age-related breeding performance of the cooperativelybreeding Seychelles warbler (Acrocephalus sechellensis) on CousinIsland, Seychelles, during 14 years. The annual number of youngthat fledged is significantly related to territory quality andnumber of helpers in the breeding group. Accounting for thesefactors and for the partner's breeding experience, annual productionof fledglings by breeding birds increases from 2 to 5 yearsand decreases beyond 5 years of age. Age-related changes inreproductive success within breeding individuals, paired withthe same experienced partner and occupying the same breedingterritory with similar amount of food from 2 to 8 years of age,show that Seychelles warblers have higher hatching success andproduce more hatchlings and fledglings as they become older.This is probably not a response to decreasing residual reproductivevalue caused by decreasing life expectancy as they become older.Primiparous warblers produce the same number of fledglings asmultiparous warblers of the same age, have a similar life expectancyas multiparous warblers, and occupy territories with similaramounts of resources available for reproduction. As all warblershave similar access to food before reproduction and similarforaging efficiency, the low reproductive success in youngerwarblers cannot be ascribed to differences in environment butto the bird's ability to breed successfully (e.g., a resultof previous helping and/or breeding experience). Effects ofsenescence on reproduction begin to occur from age 6 for bothsexes. From that age, eggs have lower hatching success, butfledging success remains the same. Birds that start breedingat a young age on a given quality territory produce more fledglingsin their lifetime than birds that delay breeding. Many old birdsnever fledge young of their own. They have not been able tobreed because of a shortage of breeding territory vacancies.[BehavEcol 7: 417–425 (1996)]     

Experimental evidence for innate predator recognition in the Seychelles warbler.

Nest predation is a major determinant of fitness in birds and costly nest defence behaviours have evolved in order to reduce nest predation. Some avian studies have suggested that predator recognition is innate whereas others have stressed the importance of learning. However, none of these studies controlled for the genetic origin of the populations investigated and the effect of unfamiliarity with the predator. Here we determined whether experience with a nest predator is a prerequisite for nest defence by comparing predator recognition responses between two isolated but genetically similar Seychelles warbler (Acrocephalus sechellensis) populations, only one of which had experience of the egg predating Seychelles fody (Foudia sechellarum). Individuals in the predator-free population significantly reduced nest guarding compared to individuals in the population with the predator, which indicates that this behaviour was adjusted to the presence of nest predators. However, recognition responses (measured as both alarm call and attack rates) towards a mounted model of the fody were equally strong in both populations and significantly higher than the responses towards either a mounted familiar non-predator and a mounted, novel, non-predator bird species. Responses did not differ with a warbler's age and experience with the egg predator, indicating that predator recognition is innate.     

MHC-based patterns of social and extra-pair mate choice in the Seychelles warbler

The existence and nature of indirect genetic benefits to mate choice remain contentious. Major histocompatibility complex (MHC) genes, which play a vital role in determining pathogen resistance in vertebrates, may be the link between mate choice and the genetic inheritance of vigour in offspring. Studies have shown that MHC-dependent mate choice can occur in mammal and fish species, but little work has focused on the role of the MHC in birds. We tested for MHC-dependent mating patterns in the Seychelles warbler (Acrocephalus sechellensis). There was no influence of MHC class I exon 3 variation on the choice of social mate. However, females were more likely to obtain extra-pair paternity (EPP) when their social mate had low MHC diversity, and the MHC diversity of the extra-pair male was significantly higher than that of the cuckolded male. There was no evidence that females were mating disassortatively, or that they preferred males with an intermediate number of MHC bands. Overall, the results are consistent with the 'good genes' rather than the 'genetic compatibility' hypothesis. As female choice will result in offspring of higher MHC diversity, MHC-dependent EPP may provide indirect benefits in the Seychelles warbler if survival is positively linked to MHC diversity.     

Sex-specific associative learning cues and inclusive fitness benefits in the Seychelles warbler

In cooperative breeding vertebrates, indirect fitness benefits would be maximized by subordinates that accurately assess their relatedness to group offspring and preferentially help more closely related kin. In the Seychelles warbler (Acrocephalus sechellensis), we found a positive relationship between subordinate-nestling kinship (determined using microsatellite marker genotypes) and provisioning rates, but only for female subordinates. Female subordinates that helped were significantly more related to the nestlings than were nonhelpers, and the decision to help appears to be based on associative learning cues. High levels of female infidelity means that subordinates cannot trust their legitimacy through the male line, consequently they appear to use the continued presence of the primary female, but not the primary male, as a reliable cue to determine when to feed nestlings. By using effective discrimination, female subordinates are able to maximize the indirect benefits gained within a cooperative breeding system otherwise driven primarily by direct breeding benefits.     

Testosterone, cuckoldry risk and extra-pair opportunities in the Seychelles warbler

In male birds, testosterone (T) plays an important role in aggressive and mate-attraction behaviour. In the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, extra-group copulations (EGCs) occur frequently, but are not accompanied by sexual courtship displays as in within-pair copulations. Paternity is nearly always gained by primary males. We investigated whether T levels and sperm storage capability (cloacal protuberance (CP)) in adult primary and subordinate males were related to timing of egg laying, levels of cuckoldry and extra-group paternity (EGP) opportunities. During the sexually active period before egg laying, T levels and CP were only elevated or enlarged (respectively) in primary males, and some suggestion was found that subordinate males do not invest in elevated T levels. The peak in T occurred during the fertile period of the female partner and corresponded to the peak period of male sexual displays and mate guarding, but was independent of cuckoldry risk (density of neighbouring primary males). CP was also enhanced during this period; however, CP but not T remained elevated after egg laying by their mates, and CP but not T was positively related to EGP opportunities (density of neighbouring fertile females). We conclude that T is involved in sexual courtship displays and mate guarding, but not in gaining EGCs. These findings contrast with those in other species where EGP involves elaborate sexual displays.     

Daughters on request: about helpers and egg sexes in the Seychelles warbler

The Seychelles warbler (Acrocephalus sechellensis) was an endangered endemic of the Seychelles islands where, until 1988, the entire population of ca. 320 birds was restricted to the one island of Cousin Island (29 ha). Additional breeding populations were successfully established on the islands of Aride (68 ha, 1988) and Cousine (26 ha, 1990) and now with the existence of ca. 2000 warblers on three islands the conservation status of the warbler has improved from endangered to vulnerable. Emigration from the island is extremely rare, so birds that disappeared were known to have died. Almost every bird on Cousin Island has been individually colour ringed and monitored throughout all breeding attempts during a 17-year period (1985-2002; total ca. 2400 birds). These birds were also blood sampled for molecular parentage and sex analyses. Therefore the lifetime reproductive success of many birds is known. Although warblers can breed independently in their first year, some individuals remain in their natal territory as subordinates, and often help by providing nourishment to non-descendant offspring. The frequency of 'helping' is affected by habitat saturation and variation in territory quality (insect prey availability). The long-term benefits of helping are higher for daughters than for sons, and it is therefore no wonder that most helpers are daughters from previous broods. Furthermore, on low-quality territories breeding pairs raising sons gain higher fitness benefits than by raising daughters, and vice versa on high-quality territories. Female breeders adaptively modify the sex of their single-egg clutches according to territory quality: male eggs on low quality and female eggs on high quality. The Seychelles warbler is a beautiful example of behavioural and life-history adaptations to restricted circumstances.     

Parent presence, delayed dispersal, and territory acquisition in the Seychelles warbler

The presence of parents in the natal territory may play an important,but often overlooked, role in natal dispersal and the consequentacquisition of a territory. Living with parents in a territorymay confer a fitness advantage to subordinates through, forexample, the nepotistic behavior of the parents or indirectbenefits gained by helping to raise nondescendent kin. Whena parent is replaced by a stepparent, such advantages are reducedor disappear and, as a result, subordinates may disperse. Subordinatesthat disperse after parent replacement may be constrained intheir timing of dispersal, which could have negative fitnessconsequences. In the cooperatively breeding Seychelles warbler,we show that when a parent was naturally replaced or experimentallyremoved and subsequently replaced by a stepparent from outsidethe territory, subordinates were more likely to disperse thanwhen both parents remained in the natal territory. Furthermore,subordinates dispersing from territories in which one or bothparents had been replaced were less likely to acquire a breederposition than subordinates dispersing when both parents werestill on the natal territory. Our findings suggest that thepresence of parents in the natal territory may promote delayeddispersal and facilitate the eventual acquisition of a breederposition outside the natal territory. Our results support theidea that the prolonged parental care, which long-lived speciesare able to provide, may have selected for family living.     

Natal dispersal patterns are not associated with inbreeding avoidance in the Seychelles warbler

In this study, we test whether patterns of territory inheritance, social mate choice and female-biased natal dispersal act as inbreeding avoidance mechanisms in the cooperatively breeding Seychelles warbler. Our results show that Seychelles warblers do not reduce the likelihood of inbreeding by avoiding related individuals as mates. The occurrence of natural and experimentally induced territory inheritance did not depend on whether the remaining breeder was a parent of the potential inheritor or an unrelated breeder. Furthermore, dispersing individuals were no less related to their eventual mates than expected given the pool of candidates they could mate with. The female bias in natal dispersal distance observed in the Seychelles warbler does not facilitate inbreeding avoidance because, contrary to our prediction, there was no sex difference in the clustering of related opposite sex breeders around the natal territories of dispersers. As a result, the chance of females mating with relatives was not reduced by their greater dispersal distance compared with that of males.     

Mate guarding in the Seychelles warbler is energetically costly and adjusted to paternity risk

Males may increase their fitness through extra-pair copulations (copulations outside the pair bond) that result in extra-pair fertilizations, but also risk lost paternity when they leave their own mate unguarded. The fitness costs of cuckoldry for Seychelles warblers (Acrocephalus sechellensis) are considerable because warblers have a single-egg clutch and, given the short breeding season, no time for a successful replacement clutch. Neighbouring males are the primary threat to a male's genetic paternity. Males minimize their loss of paternity by guarding their mates to prevent them from having extra-pair copulations during their fertile period. Here, I provide experimental evidence that mate-guarding behaviour is energetically costly and that the expression of this trade-off is adjusted to paternity risk (local male density). Free-living males that were induced to reduce mate guarding spent significantly more time foraging and gained significantly better body condition than control males. The larger the reduction in mate guarding, the more pronounced was the increase in foraging and body condition (accounting for food availability). An experimental increase in paternity risk resulted in an increase in mate-guarding intensity and a decrease in foraging and body condition, and vice versa. This is examined using both cross-sectional and longitudinal data. This study on the Seychelles warbler offers experimental evidence that mate guarding is energetically costly and adjusted to paternity risk.     

H-Y antigen in the study of sex determination and control of sex ratio

It has been proposed, on the basis of widespread phylogenetic conservation, that H-Y antigen is the inducer of primary sex, causing the undifferentiated XY gonad to become a testis in male heterogametic species such as the human and bovine. That proposition has withstood extensive testing in vivo and in vitro. Freemartin gonads are H-Y⁺, for example, and masculinization of the freemartin gonad has been attributed to soluble H-Y, borne and transmitted in the serum of the bull twin, and bound in ovarian receptors of the female. We have applied monoclonal H-Y antibodies to the identification of gender in embryos of the bovine. Our preliminary results imply presence of H-Y in bovine embryos of the morula and blastocyst stages recovered at about 6–12 days of gestation. Assignment of H-Y phenotype -- positive in males and negative in females -- allows selective implantation of male and female during embryo transfer. Thus in an early study, we correctly identified gender in 6 of 7 calves born healthy at term, after transfer of 8 blastocysts.     

Heterozygosity-fitness correlations in a bottlenecked island species: a case study on the Seychelles warbler

We used capture-mark-recapture models to investigate the effects of both individual and parental heterozygosity, measured at microsatellite loci on the survival of Seychelles warblers (Acrocephalus sechellensis), an endemic island species which went through a severe population bottleneck in the middle of the last century. We found that an individual's survival was not correlated with multilocus heterozygosity, or with heterozygosity at any specific locus. However, maternal, but not paternal, multilocus heterozygosity was positively associated with offspring survival, but only in years with low survival probabilities. A nestling cross-fostering experiment showed that this was a direct maternal effect as there was an effect of the genetic mother's, but not of the social mother's, heterozygosity. Heterozygosity-fitness correlations at microsatellite markers were generally assumed to reflect genome-wide effects. Although this might be true in partially inbred populations, such correlations may also arise as a result of local effects with specific markers being closely linked to genes which determine fitness. However, heterozygosity at the individual microsatellite loci was not correlated and therefore does not seem to reflect genome-wide heterozygosity. This suggests that even in a small bottlenecked population, heterozygosity-fitness correlations may not be caused by genome-wide effects. Support for the local effects hypothesis was also equivocal; although three specific loci were associated with offspring survival, including all single-locus heterozygosities as independent predictors for the variation in survival was not supported by the data. Furthermore, in contrast to the local effects hypothesis, the loci which contributed most to the heterozygosity-survival relationship were not more polymorphic than the other loci. This study highlights the difficulties in distinguishing between the two hypotheses.     

Evidence that helping at the nest does not result in territory inheritance in the Seychelles warbler

In an environment that has a shortage of territories, helping to rear younger siblings ('alloparenting') is proposed to facilitate territory acquisition in two ways: (i) through group augmentation that leads to an increase of the territory with subsequent partial inheritance (budding); and (ii) through site dominance that leads to greater success when competing for the natal or a nearby territory after the death of the territory owner (complete territory inheritance). Most young Seychelles warbler (Acrocephalus sechellensis) males either show alloparenting or budding behaviour. Future budders had significantly more aggressive interactions with neighbours and assisted their parents more with territory defence than similarly aged future alloparents or non-helpers. This led to an increase of the natal territory of future budders before actual budding took place, whereas the natal territories of future alloparents remained constant in size. Alloparents never became budders and vice versa, refuting partial inheritance as an advantage of alloparenting. Natural male breeding vacancies were never inherited by alloparents born on vacant or other territories, but were inherited by budders born on the vacant territory or, if these were absent, predominantly by budders from neighbouring territories. We offer explicit experimental evidence against the 'helping at the nest to inherit' hypothesis. Experimentally created male breeding vacancies, with both a male alloparent and a similarly aged sibling budder present simultaneously in the vacant territory, were filled by budders only. Site dominance over territory inheritance is linked to budding and not to alloparenting.     

蜜蜂性别决定与性比调控机理研究

叙述了 4个主要蜜蜂性别决定机理的假说 :即性位点假说、基因平衡假说、蜜蜂性别决定综合假说和性基因数量决定假说。然后就蜜蜂性比由蜂王操纵 ,或是由工蜂操纵进行了论述 ,并对蜜蜂性比调控机理研究提出了一些建议     

Explicit experimental evidence for the role of mate guarding in minimizing loss of paternity in the Seychelles warbler

Extra-pair copulations (EPCs) (copulations outside the pair bond) resulting in extra-pair fertilizations (EPFs) are widespread in birds. To increase reproductive success, males should not only seek EPCs, but also prevent their females from having EPFs. Male Seychelles warblers (Acrocephalus sechellensis) follow their partner closely during the period when these females are most receptive (fertile period). The Seychelles warbler is the first species to offer explicit experimental evidence that mate guarding functions as paternity guarding: in territories where free-living males were induced to stop mate guarding during the pair female''s fertile period, the rates of intrusions by other males and successful EPCs (male mounting female) were significantly higher than those observed in the control group and in the absence of mate guarding the frequency of successful EPCs increased significantly with local male density. Male warblers do not assure their paternity through frequent copulations to devalue any sperm from other males: males do not copulate with their partners immediately following a successful EPC obtained by their partners, the frequency of successful within-pair copulations does not increase with the frequency of successful EPCs and females initiate all successful copulations and are capable of resisting copulation attempts.     

Parasitic infection and oxidative status are associated and vary with breeding activity in the Seychelles warbler

Parasites can have detrimental effects on host fitness, and infection typically results in the stimulation of the immune system. While defending against infection, the immune system generates toxic oxidants; if these are not sufficiently counteracted by the antioxidant system, a state of oxidative stress can occur. Here, we investigated the relationship between parasitic infection-using malarial infection as a model-and oxidative status in a natural population of the Seychelles warbler, while taking into account potentially interacting environmental covariates. We found that malaria is associated with increased susceptibility to oxidative stress, but this depends on the breeding stage: only during the energetically demanding provisioning stage did infected birds have higher oxidative stress susceptibility than non-infected birds. The imbalance in oxidative status was caused by a marked increase in oxidant levels observed only in infected birds during provisioning and by an overall reduction in antioxidant capacity observed in all birds across the breeding cycle. This finding implies that higher workload while dealing with an infection could aggravate oxidative repercussions. Malarial infection was not associated with body condition loss, suggesting that even when conditional effects are not directly visible, detrimental effects may still manifest themselves over the longer term through the oxidative consequences.     

Breeding in the monsoon: semi-annual reproduction in the Seychelles warbler ( Acrocephalus sechellensis )

Despite the absence of pronounced changes in day length, there is considerable climatological seasonality in the tropics. Its expression can be complex like in the monsoon climate of the Indian Ocean Islands. The land mass distribution on both sides of the equator leads to seasonal changes in prevailing wind direction and seasonal patterns in food supply, which are distinct between different coasts. Here we assess that territory quality of the Seychelles warbler, endemic to the small island of Cousin peaks during the South East monsoon (April to September) in most of the island. The Southeastern wind-exposed side is exceptional by being generally poor and having slightly increased food supply in the opposite season (NW monsoon; October to March). In response, the warblers in most territories breed primarily during the SE season, while on the SE shores they tend to breed in both seasons. The result is a semi-annual periodicity in breeding. Individual pairs can breed in 6 months intervals. In the poorer territories on the SE shore most of them indeed do this, while in the richer central habitats most pairs restrict themselves to the best (SE) season. The frequency of breeding thereby increases with territory quality in the SE season, and decreases with quality in the NW season. Withholding from breeding in the NW season despite the high food supply may have long-term advantages for the birds occupying the richer habitat. Such flexible adjustment of seasonality to local circumstances requires not only an immediate response mechanism but also access to accurate information about time of year. The subtle sensitivity of tropical songbirds to photoperiod, demonstrated so convincingly by Eberhard Gwinner (Biological rhythms, Hokkaido University Press, Sapporo, pp19–40, 2005), may well have profound adaptive meaning in taking temporally optimized reproductive decisions in response to both food and time of year.

Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment

Reproductive success of the cooperative breeding Seychelleswarbler (Acrocephalus sechellensis) increases with age. Thisage effect is not due to differential survival or increasedreproductive effort, but to accumulated helping and breedingexperience. In their first year of breeding, reproductive performanceof inexperienced warblers with neither helping nor breedingexperience was significandy lower than that of warblers of thesame age with either previous helping or breeding experience.Reproductive performance was the same for primiparae with helpingexperience and for birds with breeding experience. Female primiparaewith helping experience or breeding experience built betternests and spent more time incubating than inexperienced females,which led to increased hatching success. Male primiparae withhelping experience or males with breeding experience guardedthe clutch better than inexperienced males, which led to reducedegg predation. Even-aged warblers with different previous experienceswere transferred to unoccupied islands, where birds startedbreeding immediately in high-quality territories. The experimentshowed that birds with helping experience produced their firstfledgling as fast as experienced breeders, and significandyfaster than inexperienced birds. Breeding performance did notimprove further with experience after the first successful breedingattempt. Only birds with previous breeding experience who pairedwith inexperienced birds, were likely to change mate. The otherpair combinations remained stable. Thus, primiparous birds withhelping experience have greater lifetime reproductive successthan inexperienced primiparae of the same age. This experimentshows that helping behavior has not only been selected for inthe context of promoting an individual's indirect fitness, butalso in the context of gaining helping experience which translatesinto improved reproductive success when a helper becomes a breeder.[Behav Ecol 7: 326-333 (1996)]