Helping behaviour in facultatively eusocial hover wasps: an experimental test of the subfertility hypothesis

A candidate explanation for the evolution of eusociality is that helpers are physiologically constrained such that helping is their only realistic option. We tested this subfertility hypothesis in a species of facultatively eusocial hover wasp (Hymenoptera, Stenogastrinae: Liostenogaster flavolineata) by seeing whether helpers that were forced to nest on their own were able to mature their own eggs. One focal helper was left alone on each of 22 nests, from which all other adult wasps (including the dominant) were permanently removed. After 18 days, all but one of the 19 focal helpers that remained on their nests had ovarian development and insemination status characteristic of dominants, and the majority had probably laid eggs. This was in striking contrast to the reproductive status of other helpers removed from the same nests at the start of the experiment. These results provide convincing experimental evidence that females do not become helpers because of some unconditional physiological constraint. There is currently no unequivocal support for the subfertility hypothesis in facultatively eusocial Hymenoptera lacking morphological castes. Copyright 1999 The Association for the Study of Animal Behaviour.     

Do female spotless starlings Sturnus unicolor adjust maternal investment according to male attractiveness?

In birds, female egg allocation patterns have a strong influence in offspring development and differential investment in egg size and composition has been shown to respond to male attractiveness. In this study we experimentally manipulated the perceived attractiveness of male starlings Sturnus unicolor by increasing the amount of green material in some nests (a male courtship display in this species). We predicted that, if female investment before laying is related to male attractiveness, experimental females would increase their reproductive investment in response to the addition of plants in their nests when compared to control females. We found that our manipulation caused variations in female reproductive investment in a way that seems to influence offspring quantity but not offspring quality: Females laid larger clutch sizes but not larger eggs when green plant material was added. However, yolk androgens contents were not related to the experimental manipulation. Contrary to expectations, females breeding in experimental nests laid eggs with smaller amounts of eggshell pigments. Interestingly, we found that eggs laid later in the sequence had higher testosterone levels and showed more intense egg colouration than eggs laid earlier in the sequence. These differences at the within-clutch level suggest that selection has favoured compensatory strategies for hatching asynchrony. Alternatively, since nest sabotages by other females are most common at the beginning of laying, this could be seen as female strategy to minimise losses due to nest sabotages. As far as we know, this is the first study to show that an external egg characteristic such as blue-green colouration reflects yolk androgen concentration.     

A molecular genetic analysis of the communal nesting of the ostrich (Struthio camelus)

The ostrich breeding system is complex and unique; communal clutches are laid by several females, although only one female, the major female, and the resident territorial male provide parental care. More eggs are laid in the nest than can be incubated and the major female ejects surplus eggs from the incubated central clutch. Microsatellite markers were used to analyse the parentage of communal nests in Nairobi National Park. This revealed that major females contributed a disproportionate number of fertile eggs to the central, incubated clutch and that multiple paternity and maternity within a nest were common; 68.9% of all incubated eggs on a nest were not parented by both the resident territorial male and the major female of that nest. All the males fertilized eggs on the clutches of neighbouring males. Unexpectedly, every major female with her own nest was also simultaneously a minor female with incubated eggs on neighbouring clutches. The relatedness between females laying in the same nest was not significantly different from the population average and significantly less than that between chicks hatched from the same nest.     

Intraspecific nest parasitism in the European starling Sturnus vulgaris

Biochemical genetic markers were used along with conventional methods (abnormal laying sequence/clutch size, unusual egg shape/pigmentation) to identify intraspecific nest parasitism at two British nestbox colonies of the European starling. Between 11 and 37% of first clutches were parasitized during 1977–1979. Parasitic females probably comprised all of the following categories: (1) paired females contesting a nestbox occupied by another pair; (2) previously paired females who had laid a clutch but had been unsuccessful; (3) unpaired females who had copulated with males that already had a mate and nest site; and (4) ‘professional’ nest parasites who distributed at lest some of their eggs in one or more nests other than their own. Although parasitized nests had higher clutch sizes, parasitism led to fewer host young fledging per egg laid, mainly through the eviction of eggs and subsequent nest desertion. Number of parasitic young fledged per egg laid was highest when eggs were laid synchronously with the host, when host clutches were larger, or a smaller number of parasite eggs were added to a nest, thus favouring parasites that distribute their eggs amongst a number of nests. A greater pressure on nest sites may have accounted for the higher levels of parasitism at the Aberdeen colony and for the greater number of parasite eggs laid in a nest. Although most parasitic female starlings appeared to be much less successful than non-parasitic ones, nest parasitism in the starling might evolve directly when one or more of the following advantages are present. (1) There are no constraints on the number of eggs a female may lay but there are constraints on the number of young she may feed adequately. (2) Female survival is increased by having fewer or no eggs/young to care for. (3) Current feeding conditions favour the survival of more young than would be produced by the most common clutch size. Intraspecific nest parasitism is considered to be a first stage in the evolution of interspecific nest parasitism.     

Resource defence on exploded leks: do male little bustards, T. tetrax, control resources for females?

In lekking species, female choice of particular males for mating is based on male characters that are not related to immediate gains such as access to resources. This is, however, unclear in exploded-lekking species, as male territories may contain resources for females; it is thus the degree to which males regulate female access to these resources to obtain matings that should distinguish lekking from nonlekking species. We investigated the relationship between resources for females, male distribution and male attractiveness in little bustards. One parameter determining resource quality for females was invertebrate availability: females laid eggs in grassland fields (grass, alfalfa and ryegrass) with significantly higher invertebrate biomass than others of similar cover but nests were in areas that did not have higher invertebrate biomass, at a scale of 1 km². Males set up territories and females laid eggs in areas that had a high proportion of alfalfa, but not high mean invertebrate biomass. Thus males set up territories at resources potentially used by females, but that appeared not to be critical for breeding. In addition, females did not use the amount of resources defended as a criterion for mate choice, as male attractiveness was not related to various territorial resources. Locating the lek in areas attractive to females might be a way of increasing the probability of encounter between the sexes. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Mate Choice and Mating Pattern in a Stream Goby of the Genus Rhinogobius

The mate choice and mating pattern of a benthic goby Rhinogobius sp. CB (cross band type) were investigated in the Kamo River, Shikoku, Japan. During the breeding season, gravid females assumed a nuptial color and either males or females initiated a courtship display. Males preferentially courted a female of similar size to lead her to his nest, whereas females courted more frequently when they encountered a large male. Eggs in any one nest were always at the same developmental stage. Sampling data of nesting males and females indicated that, in more than half the nests, males gathered more than one female before spawning. In some nests with eggs, two or three females had spent ovaries, indicating that the eggs were laid by multiple females within a short span of time. However, a comparison between the total number of eggs which females would spawn in one nest and the number of eggs actually deposited suggested that eggs were contributed by one female in most nests. This low level of polygyny in spite of multiple female availability is attributed to a limited available spawning area of the nest.     

不同性比对小菜蛾繁殖及田间种群数量的影响

不同雌雄比对小菜蛾Plutella xylostella(L.)繁殖的影响以及性诱剂对田间种群的控制效果研究试验结果表明:随雌雄比的提高,雌蛾的产卵量减少,孵化率降低,在1∶1,4∶1状态下平均单雌产卵量分别为79.33粒和76.03粒,平均孵化率为89.03%和78.44%,在13∶1状态下平均单雌产卵量为36粒,平均孵化率仅为46.05%,全雌状态下所产的卵无一孵化。田间应用性诱剂也可有效降低虫口数量。     

Maternal androgens in eggs of communally breeding guira cuckoos (Guira guira)

Variation of maternal androgens in avian eggs may be a mechanism of maternal influence on offspring development, growth, and/or behavior. We studied yolk androgen concentrations in eggs of guira cuckoos (Guira guira) to understand how females might enhance the success of offspring in a complex communal breeding system. We measured concentrations of androstenedione, 5alpha-dihydrotestosterone, and testosterone in yolks and identified eggs and clutches of individuals in joint nests by yolk protein electrophoresis. Androstenedione had the highest yolk concentration, at least 10 times higher than that of testosterone and 5alpha-dihydrotestosterone. The first eggs of individual females that laid two or three eggs in a joint nest had lower androstenedione concentrations than their second and third eggs, the latter having a lower probability of being ejected from the nest. This implies that guira cuckoo females may influence offspring survival and competitiveness in communal nests by means of differential allocation of androstenedione and laying tactics. There was significant variation in yolk androstenedione among females, but the order in which females entered laying in the communal clutch had no effect on the concentrations. Androstenedione yolk concentrations increased with communal clutch size, which may indicate that higher levels of competition in larger groups lead to higher yolk androgen concentrations. Finally, androstenedione concentrations were higher in clutches in the later wetter periods of the rainy season than during the earlier drier period. This may be explained by the high frequency of large clutches in the later periods, with more females contributing to a joint clutch.     

Male dominance determines female egg laying rate in crickets

A key prediction of theories of differential allocation and sexual conflict is that male phenotype will affect resource allocation by females. Females may adaptively increase investment in offspring when mated to high quality males to enhance the quality of their offspring, or males may vary in their ability to manipulate female investment post-mating. Males are known to be able to influence female reproductive investment, but the male traits underlying this ability have been little studied in taxa other than birds. We investigated the relationship between male dominance and female oviposition rate in two separate experiments using the field cricket, Gryllus bimaculatus. In both experiments, females mated to more dominant (but not larger) males laid more eggs. This reveals that either females allocate more effort to reproduction after mating with a dominant male or that dominance status is associated with male ability to manipulate their mates. This is the first evidence that dominance, rather than male attractiveness, has a post-copulatory effect on reproductive investment by females.     

Social behaviour ofRopalidia fasciata (Hymenoptera: Vespidae) females on satellite nests and on a nest with multiple combs

The social behaviour ofRopalidia fasciata females on 3 satellite nests and on a nest with multiple, independent combs was observed. Many females often visited satellite nests, and even dominant foundresses, who usually spent more of their time on the nest, became active when satellite nests were being constructed. Three foundresses laid eggs on a satellite nest and eggs laid by 2 of these (who performed foraging) survived. On another satellite nest, only the dominant foundress laid an egg which survived. On a nest consisting of 2 independent combs, females frequently shifted between the 2 combs even after the emergence of many adults. Constructions of satellite nests and of multiple independent combs are considered to be adaptive for this species living in Okinawa, where most of nests are destroyed by frequent typhoons or abandoned after predation by ants.     

Consequences of prenatal androgen exposure for the reproductive performance of female pheasants (Phasianus colchicus)

Maternal hormones in vertebrate eggs can mediate important forms of maternal effects. However, the function of hormone transfer to the eggs is still debated, especially because long-term fitness consequences have been little studied. We investigated the effect of prenatal exposure to physiologically elevated yolk testosterone (T) levels on reproduction of female pheasants (Phasianus colchicus) in captivity. We found that females hatching from T-injected eggs (T-females) had a lower egg-laying rate than controls, and their eggs were more frequently infertile than those laid by control females. There were no effects of prenatal maternal treatment on egg size and yolk T concentration, but eggs carrying a female embryo laid by T-females had smaller yolks than eggs with a male embryo, while there was no sex difference in yolk size among the eggs laid by control females. Progeny sex ratio was unaffected by maternal treatment. These findings suggest that the transfer of high androgen levels to the eggs by the mother is constrained by complex trade-offs between direct effects on her daughters' reproduction and by trans-generational differential consequences on male and female descendants.     

Forced Copulations and Intra-specific Parasitism: Two Costs of Social Living in the White-fronted Bee-eater

White-fronted bee-eaters are colonially breeding birds that exhibit highly developed helping-at-the-nest. Through long-term studies of an individually-marked population, we have documented two costs of social living: 1) harassment of mated females by extra-pair males, and 2) intra-specific parasitism by females who lay eggs in the nests of others. Breeding females are sexually chased and, occasionally, forceably mated by males other than their mates. Focal-sampling of females throughout their period of receptivity revealed that the average female is involved in 5 to 8 sexual chases and is forceably copulated 0.15 to 0.23 times per breeding season. This risk to females would be much greater were it not for the behavior of male mates who remain close to, and actively defend, their partners. Such mate-guarding is highly effective — females entering and leaving the colony in consort with their mates are sexually harassed only 1/10 as often as females travelling alone. Although sexual harassment of females is common at bee-eater colonies, the risk of paternity uncertainty arising from forced copulations is thought to be low. The reason is that females copulate repeatedly with their male mates on all days immediately prior to as well as during egg laying. This point has been overlooked in previous reports and has led to an exaggeration of the paternity risks associated with forced sexual chases. We conclude that sexual chasing of extra-pair females is a low yield reproductive tactic employed primarily by monogamously mated males whose presence at the colony is required to allofeed and mateguard their own egg-laying females. Female white-fronted bee-eaters lay eggs in nests other than their own. This intraspecific parasitism constitutes a greater threat to certainty of parentage than does forced copulation. Over four years of study, 16% of nests were parasitized and 7 % of all eggs were laid by a female other than the breeder (Table 2). Parasitizing females come primarily from two sources: (1) members of mated pairs whose own breeding attempt is disrupted at the time of egg laying, and (2) single females who opportunistically add an egg at the nest of their parents (or parent plus step-parent). In each case of kin-parasitism, the “parasitic” female remained socially integrated with the host group and helped in the rearing of the young. In contrast, 9 of 10 females that parasitized the nests of non-relatives had no other interactions with the hosts (Table 3). Parasitizing females exhibited two specialized behaviors that enhanced their reproductive effectiveness: (1) they spent many hours observing, investigating, and testing the defenses of potential host nests, and (2) they preferentially laid in hosts' nests at the appropriate chronological stage of development. Breeding females also exhibited counterbehaviors against being parasitized. These included: (1) remaining sequestered in their nest chambers for 64%-65% of the daylight hours and 94 % of the pre-roost hours during their days of egg laying, (2) aggressively defending their nest entrances against all investigating (potentially parasitic) females, and (3) actively removing any eggs laid in their nests prior to the initiation of their own clutch. These tactics and countertactics suggest a long evolutionary history of parasitic opportunities and risks among white-fronted bee-eaters.     

Within-nest spawning-site preferences of female bluefin damselfish: the effect of early-stage eggs

In some fish species with exclusive paternal care females prefer to deposit their eggs in nests containing early-stage eggs and avoid those containing only late-stage eggs. Such female preference may also be shown when it comes to deciding where to lay within a given nest; that is, females may prefer to lay contiguously to younger rather than to older eggs. To understand if females benefit from such preferences by obtaining higher offspring survival, we conducted field studies with the bluefin damselfish, Abudefduf luridus. We investigated (1) female between-nest and within-nest spawning-site preferences relative to brood characters and (2) egg-hatching success related to male filial cannibalism. We found evidence that males entirely ate some of their clutches, especially the eggs received near the end of the breeding season. These were also more susceptible to being eaten when laid in nests containing only eggs in a late stage of development. Theoretically, these patterns of clutch loss should result in adaptive female preference for nests containing early-stage eggs, but we found no evidence for such a preference in female A. luridus. However, once females had chosen a nest they strongly preferred to lay contiguously to early-stage eggs than to late-stage eggs. Moreover, females benefited from this decision by obtaining higher survival rates for their clutches. We conclude that within-nest spawning-site preference of the female for early-stage eggs may have evolved as an adaptive response to a dilution effect on the risk of egg loss.     

栎旋木柄天牛生殖行为研究

为了阐明栎旋木柄天牛Aphrodisium sauteri Matsushita生殖行为的方式和特点,寻找有效的防治手段,本试验观察了栎旋木柄天牛的生殖行为,并在林间尝试了成虫诱集。结果表明:雌雄成虫之间表现出较强的吸引作用,近距离时雌雄虫可相互吸引,而远距离时雌虫引诱力更强。室内试验中求偶时间会随着成虫的衰弱而增加。成虫交配主要发生于白天。羽化出孔后1～14日龄雌虫和2～13日龄雄虫均可多次交配。成虫一次完整交配平均需时70.12min,交配后保护平均仅用时3.86min。一次完整交配过程中,不同日龄成虫的交配历时、交配间隔历时及交配次数都存在差异,各日龄成虫的交配间隔历时均大于交配历时。交配或产卵经历对雄虫交配行为的影响明显大于雌虫。雌虫产卵前期平均为30.71h。雌虫每产1粒卵平均需时99.58s,并随日龄增长显著加长。每头雌虫平均每日产卵量和总产卵量分别为7.89和26.20粒。雌虫多在白天产卵,用产卵器触探树皮表面寻找合适的产卵部位。卵主要产于树皮裂缝和枝条疤痕内,一般每次产卵1粒,卵表面无覆盖物。     

An experimental analysis of joint nesting in the salamander Hemidaetylium scutatum (Caudata: Plethodontidae): the effects of population density

Three nesting behaviour patterns are documented in the plethodontid salamander Hemidactylium scutatum. A female may lay eggs (1) in a solitary nest and brood them, (2) in a joint nest and brood them as well as eggs of other females, or (3) in a joint nest that is brooded by another female. The hypothesis that population density was positively associated with joint nesting was tested by following two populations for 5 years and by experimentally manipulating the population density of nesting females in artificial habitats for the latter 2 years. The proportion of joint nests did not vary with density, although joint nests tended to contain eggs of more females at the high population density. Joint nests were usually brooded by one female; thus, most females that laid eggs in joint nests did not brood them at high density. The reproductive success, as measured by survival of embryos, of solitary and joint nesters was equivalent. Joint nests were deserted less often, however, which decreased the probability of catastrophic mortality. The number of days of brooding was significantly positively correlated with loss of body mass of females, suggesting a cost to brooding behaviour. Joint nesting with solitary brooding is not explained by aggressive usurpation of nests or by brood parasitism.     

ON MULTIPLE BROODS AND THE BREEDING STRATEGY OF ARCTIC SANDERLINGS

Sanderlings on Bathurst Island in the Canadian arctic have two patterns of incubation. At some nests the eggs are covered soon after the fourth egg has been laid and at others incubation is delayed for 5–6 days. Because the delay is about the same time required to lay a second clutch and because a single individual alone incubates at any one nest, we suspected that Sanderlings might normally lay two clutches in a season, the male caring for one brood and the female for the other.
Histological and gross examination of the ovaries of two females taken as the birds began incubation showed eight freshly ovulated follicles in each female. The size gradation and histological appearance of the follicles indicated that two clutches of four eggs each had been laid within 8–10 days by a single female. The ovary of one female had additional large yolky follicles, suggesting a physiological capability of further ovulations.
Field conditions in the arctic summer are highly variable, and the small eggs and the rapid sequence of broods of Sanderlings may be breeding adaptations that permit them to multiply the traditional wader clutch of four eggs by 2 or 3 in favourable years. Selection for mating systems characterised by brief pair bonds and by polyandry is expected in precocial birds where some broods are incubated and cared for by the male, but further field work is required to determine the precise mating system of Sanderlings.     

Costly reproductive competition between females in a monogamous cooperatively breeding bird

In many cooperatively breeding societies, only a few socially dominant individuals in a group breed, reproductive skew is high, and reproductive conflict is common. Surprisingly, the effects of this conflict on dominant reproductive success in vertebrate societies have rarely been investigated, especially in high-skew societies. We examine how subordinate female competition for breeding opportunities affects the reproductive success of dominant females in a monogamous cooperatively breeding bird, the Southern pied babbler (Turdoides bicolor). In this species, successful subordinate reproduction is very rare, despite the fact that groups commonly contain sexually mature female subordinates that could mate with unrelated group males. However, we show that subordinate females compete with dominant females to breed, and do so far more often than expected, based on the infrequency of their success. Attempts by subordinates to obtain a share of breeding impose significant costs on dominant females: chicks fledge from fewer nests, more nests are abandoned before incubation begins, and more eggs are lost. Dominant females appear to attempt to reduce these costs by aggressively suppressing potentially competitive subordinate females. This empirical evidence provides rare insight into the nature of the conflicts between females and the resultant costs to reproductive success in cooperatively breeding societies.     

Egg Colour Covaries with Female Expression of a Male Ornament in the Spotless Starling (Sturnus unicolor)

The sexually selected egg colour hypothesis (SSECH) proposes that egg colouration is as a post-mating sexually selected signal of female phenotypic quality, maintained by a higher allocation of paternal care. Similarly, some female traits can reflect genetic quality or condition and males could use this information in mate choice or in modulating parental investment. In our study, we examined the correlation of individual variation in egg colouration with female expression of a male ornament and how male feeding covaried with these two female traits in the spotless starling, in which egg colour varies widely between clutches and where both sexes possess showy throat feathers that are age dependent and that may signal individual quality. According to the SSECH, high-quality females (females with longer throat feathers) are expected to lay more colourful eggs than low-quality females and males should modify their feeding behaviour accordingly. By means of a principal component analysis, we found that most of the variation in egg colouration was due to brightness differences, and in a lower proportion to chromatic variation. Chromatic variation reflected a ultraviolet (UV) vs. greenness trade-off and was positively associated with throat feather length: females with larger throat feathers laid eggs with higher UV and lower green reflectance. However, egg brightness was not related to female feather length, as the SSECH would predict. Male feedings were positively related to female throat feather length and negatively related to chromatic variation, meaning that males contributed more to nests of females with long throat feathers who laid eggs with higher UV and lower green reflectance. In conclusion, our data provide mixed support for the SSECH: although egg chromatic variation was related to female expression of a male ornament and male parental care, we found no evidence that egg brightness was involved in these processes.     

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests?

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.     

Parentage assignment and extra-group paternity in a cooperative breeder: the Seychelles warbler (Acrocephalus sechellensis)

We describe the development and initial application of a semiautomated parentage testing system in the Seychelles warbler (Acrocephalus sechellensis). This system used fluorescently labelled primers for 14 polymorphic microsatellite loci in two multiplex loading groups to genotype efficiently over 96% of the warbler population on Cousin island. When used in conjunction with the program CERVUS, this system provided sufficient power to assign maternity and paternity within the Seychelles warbler, despite the complications associated with its cooperative breeding system and a relatively low level of genetic variation. Parentage analyses showed that subordinate 'helper' females as well as the dominant 'primary' females laid eggs in communal nests, indicating that the Seychelles warbler has an intermediate level of female reproductive skew, in between the alternative extremes of helper-at-the-nest and joint nesting systems. Forty-four per cent of helpers bred successfully, accounting for 15% of all offspring. Forty per cent of young resulted from extra-group paternity.