A test on social behavior as a cause of dispersal of spruce grouse

Summary Social behavior is one hypothesized cause for dispersal. We tested this hypothesis on spruce grouse (Dendragapus canadensis) in two years by removing adults in summer and monitoring emigration of juveniles in autumn. Despite the absence of adults, juveniles emigrated from the natal range at rates comparable to juveniles on control areas. Data on density and dispersal from earlier spruce grouse studies are reviewed, which show that emigration is constant despite substantial differences in density, and that a clear female sex bias occurs across a wide range of densities. Results do not support the hypothesis that social behavior influences dispersal nor do they support a prediction of the Oedipus hypothesis that no sex bias occurs in polygynous birds. Offprint requests to: D.M. Keppie     

Sexual selection and mating system in Zorotypus gurneyi Choe (Insecta: Zoraptera)

Body size is clearly an important factor influencing the outcome of agonistic contests, but is often weakly correlated with dominance ranks in Zorotypus gurneyi Choe (Insecta: Zoraptera). The study of the development and dynamics of dominance relations using artificially constructed colonies show that age, or tenure within the colony, is the prime determinant of dominance among males. Dominance hierarchies become relatively stable within 2 or 3 days and males that emerge later normally begin at the bottom of the hierarchy regardless of size. Males interact much more frequently when they are simultaneously introduced to each other than when they are allowed to emerge at different times. In the latter case, males that emerge late appear to recognize relative dominance of older males and avoid direct contests. Considering the high correlation between dominance rank and mating success, there is a strong selective advantage to males that emerge earlier and such pressure of sexual selection may be responsible for the difference in life history strategies between Z. gurneyi and its sympatric congener, Z. barberi Gurney, in central Panama.     

Sexual selection and mating system in Zorotypus gurneyi Choe (Insecta : Zoraptera)

Summary Social behavior of a species in the little-known insect order Zoraptera is described for the first time. Zorotypus gurneyi Choe (Insecta: Zoraptera) is a wing-dimorphic species that lives colonially under the bark of rotting logs in central Panama. Males are larger than females in total body size and fight each other to gain access to females. Highly linear and stable dominance hierarchies exist among males. Higher-ranking males show such agonistic behavior as jerking, chasing, head-butting, hindleg-kicking, and grappling, whereas subordinates often try to avoid contacts. Higher-ranking males, the dominant males in particular, are well recognized by others and relatively free of injuries. Although the dominant males are often the largest, the correlation between body size and dominance rank is not always significant. The mating system of Z. gurneyi is an example of female defense polygyny in which the dominant males obtain the majority of matings (75% on average). Mating success among Z. gurneyi males is much more variable than that of some lekking species.     

Egg-mimicry as a mating strategy in the fantail darter,Etheostoma flabellare: females prefer males with eggs

Summary In some species of fishes with paternal care, females prefer to spawn with males already defending eggs. Such female preference appears to have resulted in adoption of unrelated eggs as a male mating strategy in several species. Page and Swofford (1984) proposed that such female preference may have also resulted in the evolution of male egg-mimics in several species of darters (Percidae); however, their hypothesis has not been tested. We examined female preference in the fantail darter (Etheostoma flabellare) and found that females preferred males with eggs over males without eggs, and males with egg-mimics over males without egg-mimics. Thus it appears that female preference for males already guarding eggs may have led to the evolution of specialized egg-mimicking morphology in males.     

Male size,mating success,and mating strategy in the mosquitofish Gambusia affinis (Poeciliidae)

Summary In laboratory choice experiments, receptive female western mosquitofish Gambusia affinis affinis (deprived of contact with males 30 days) preferred the larger of two males. When two males differing in size were placed with a receptive female, the larger was generally able to monopolize access to her, but not when the female was not receptive. In other experiments, a single male was placed with a receptive or non-receptive female. These experiments showed size-correlated differences in male mating behavior. Smaller males were more likely to chase non-receptive females and attempt to inseminate them forcibly, while larger males continued to court females even if they were non-receptive. Even when the female was receptive, there was some evidence of size-correlated differences in levels of courtship and chasing.     

Bumblebees as model organisms to study male sexual selection in social insects

Social-insect males are often regarded as being merely short-lived "flying sperm containers", which ignores their potential influence on females and paternity patterns as found in other animals. Consequently, social-insect males have received only marginal attention and sexual selection has almost never been studied in these species. Here I present a review of the mating biology of bumblebees (Bombus spp.), which are the best-studied social insects to date. I follow a male's pathway from his birth until he successfully contributes to the next generation, and show that males have evolved adaptations and behaviors to influence paternity patterns at various stages of their life, which are similar to those exhibited by males of non-social insects. By comparing the available bumblebee data with more sparse studies of male reproductive behavior in other social Hymenoptera, I argue that such male adaptations may indeed be widespread in social insects. I suggest that current paradigms on sexual selection should be challenged by using social insects as model systems, because they offer unique features, and a solid theoretical background in which clear predictions can be made and appropriate experimental tests of them can be designed.Communicated by A. Cockburn     

Sperm parasitism in ants: selection for interspecific mating and hybridization

Interspecific mating in eusocial Hymenoptera can be favored under certain conditions even if all hybrid offspring are completely infertile. This exploits two key features of the eusocial Hymenoptera: a haplodiploid genetic system and reproductive division of labor in females. Interspecifically mated queens can still produce viable sons that will mate intraspecifically. Apparent reduced fitness resulting from producing infertile daughter gynes can be also offset by advantages conferred by hybrid workers. An important advantage is likely to be superior ability at using marginal habitats. Interspecifically mated queens can nest in sites where intraspecific competition will be low. By mating interspecifically, a queen trades expected reproductive success through female offspring for a higher probability of achieving some reproductive success. Females that mate interspecifically can be considered "sperm parasites" on the males of the other species. I provide evidence that sperm parasitism is responsible for widespread hybridization in North America among two species of the ant subgenus Acanthomyops (genus Lasius), and review evidence for sperm parasitism in other hybridization phenomena in ants. Sperm parasitism in ants represents a novel form of social parasitism in ants and a dispersal polymorphism. It may also act as a precursor to the evolution of some other recently discovered phenomena, such as genetic caste determination.     

Multiple mating and the evolution of social behavior in the hymenoptera

Summary The data on the frequency of mating by queens of eusocial Hymenoptera are reviewed.It is pointed out that the issue of sperm clumping is probably irrelevant to the evolution of eusociality.The hypothesis is presented that multiple mating is an adaptation for maintaining large colonies. In ants there is a significant relation between the size of the colony and the frequency of mating.The effect of multiple mating on the spread of a gene for worker behavior is explored. If a female mates twice, the effective number of matings is less than two except in the case of identical sperm contribution by the males.Sperm bias is defined as the contribution of unequal amounts of sperm by the males that mate with a queen. Sperm bias can be produced as a sampling phenomenon, by inter-male competition for females and by sperm competition.The relation between the ergonomic efficiency of the workers at the production of reproductives and the number of matings that is consistent with the evolution of eusociality is derived. If workers are only about 10% more efficient at producing reproductives within a eusocial colony than they are solitarily, then two matings by the queen will still produce a selective advantage to eusocial behavior.     

“Philandering” — a mixed mating strategy in the splendid fairy-wren Malurus splendens

Summary In a population of the splendid fairy-wren Malurus splendens, we describe 202 intrusions by solitary adult males in full breeding plumage into the territories of conspecifics. Intrusions were not secretive and although silent they involved conspicuous flight above the vegetation and several characteristic display elements, including petal-carrying. Fifteen intrusions were observed in the 2 months before eggs were laid, but the rest occurred during the period of nest-building and egg-laying when the breeding female was presumed to be fertile, and were by males from adjacent territories. A high level of extra-pair fertilization has previously been determined in this species and we interpret many of these intrusions as attempts by males to obtain extra-pair copulations (EPC); we call this activity philandering. The costs and benefits of EPC for males and females are discussed and the promiscuous mating strategy of M. splendens is contrasted with the persistent social pairing in stable groups of this long-lived cooperatively breeding species.Offprint requests to: I. Rowley     

Genes and song: genetic and social connections in fragmented habitat in a woodland bird with limited dispersal

Understanding the processes leading to population declines in fragmented landscapes is essential for successful conservation management. However, isolating the influence of disparate processes, and dispersal in particular, is challenging. The Grey Shrike-thrush, Colluricincla harmonica, is a sedentary woodland-dependent songbird, with learned vocalizations whose incidence in suitable habitat patches falls disproportionally with decline in tree cover in the landscape. Although it has been suggested that gaps in tree cover might act as barriers to its dispersal, the species remains in many remnants of native vegetation in agricultural landscapes, suggesting that it may have responded to habitat removal and fragmentation by maintaining or even increasing dispersal distances. We quantified population connectivity of the Grey Shrike-thrush in a system fragmented over more than 120 years using genetic (microsatellites) and acoustic (song types) data. First, we tested for population genetic and acoustic structure at regional and local scales in search of barriers to dispersal or gene flow and signals of local spatial structuring indicative of restricted dispersal or localized acoustic similarity. Then we tested for effects of habitat loss and fragmentation on genetic and acoustic connectivity by fitting alternative models of mobility (isolation-by-distance [the null model] and reduced and increased movement models) across treeless vs. treed areas. Birds within -5 km of each other had more similar genotypes and song types than those farther away, suggesting that dispersal and song matching are limited in the region. Despite restricted dispersal detected for females (but not males), populations appeared to be connected by gene flow and displayed some cultural (acoustic) connectivity across the region. Fragmentation did not appear to impact greatly the dispersal of the Grey Shrike-thrush: none of the mobility models fit the genetic distances of males, whereas for females, an isolation-by-distance model could not be rejected in favor of the models of reduced or increased movement through treeless gaps. However, dissimilarities of the song types were more consistent with the model of reduced cultural connectivity through treeless areas, suggesting that fragmentation impedes song type sharing in the Grey Shrike-thrush. Our paper demonstrates that habitat fragmentation hinders important population processes in an Australian woodland bird even though its dispersal is not detectably impacted.     

Aggregation and social interaction in garter snakes (Thamnophis sirtalis sirtalis)

Behavioral Ecology and Sociobiology - Shared behavioural traits result from the interaction of two or more individuals, making it difficult to discern which individual is in control of the...     

The effects of mating system on male mate choice in a coral reef fish

Summary We show how mate limitation appears to be critical in determining whether or not males exercise mate choice among available females. Thalassoma bifasciatum is a Caribbean reef fish with two distinct mating patterns: group-spawning and pair-spawning. In both mating systems, female fecundity is variable and size dependent, and female availability is high. However, sperm competition among group-spawning males apparently limits the number of effective matings in which a male may engage, whereas territorial pair-spawning males have little or no such limitation. Group-spawning males should be discriminating in their choice of mates and our data confirm this: there is strong evidence for assortative mating in group-spawns, with more large males joining in mating groups around large females. In contrast, pair-spawning males show no indication of mate preferences, and spawn with all females who arrive at their territories.     

Sexual conflict in a polygynous primate: costs and benefits of a male-imposed mating system

Fundamental reproductive interests dictate that females generally benefit most from mate selectivity and males from mate quantity. This can create conflict between the sexes and result in sexual coercion: male use of aggression to garner mating success at a cost to females. Potential fitness costs of sexual coercion, however, can be difficult to measure. Here, we demonstrate benefits to males and costs to females of female defense polygyny in wild hamadryas baboons, cercopithecoid primates in which females are coercively transferred among social units by males, restricting both female choice and bonding among female kin. Of all coercive transfers (takeovers) of females with young infants, 67 % were followed by infant mortality, which was significantly more likely to occur after takeovers than at other times. As expected, infant mortality decreased time to subsequent conception but lengthened intervals between surviving infants. Following infant survival, whether a female had experienced a takeover after the previous birth was a significant predictor of subsequent interbirth interval, with interbirth intervals of females remaining with the same male between births being significantly shorter than those of females incurring takeovers between births. Together, these results reveal that takeovers increase the chance of infant mortality while delaying subsequent conception. Male-driven female defense polygyny in this species is thus costly to females in two ways. These results demonstrate that reproductive strategies benefitting males can evolve despite substantial costs to females. These costs may be mitigated over the long term, however, by female counterstrategies and protective behavior by males.     

Explaining long-distance dispersal: effects of dispersal distance on survival and growth in a stream salamander

Long-distance dispersal (LDD) may contribute disproportionately to range expansions, the creation of new evolutionary lineages, and species persistence in human-dominated landscapes. However, because data on the individual consequences of dispersal distance are extremely limited, we have little insight on how LDD is maintained in natural populations. I used six years of spatially explicit capture-mark-recapture (CMR) data to test the prediction that individual performance increases with dispersal distance in the stream salamander Gyrinophilus porphyriticus. Dispersal distance was total distance moved along the 1-km study stream, ranging from 0 to 565 m. To quantify individual performance, I used CMR estimates of survival and individual growth rates based on change in body length. Survival and growth rates increased significantly with dispersal distance. These relationships were not confounded by pre-dispersal body condition or by ecological gradients along the stream. Individual benefits of LDD were likely caused by an increase in the upper limit of settlement site quality with dispersal distance. My results do not support the view that the fitness consequences of LDD are unpredictable and instead suggest that consistent evolutionary mechanisms may explain the prevalence of LDD in nature. They also highlight the value of direct CMR data for understanding the individual consequences of variation in dispersal distance and how that variation is maintained in natural populations.     

Web tension and gravity as cues in spider orientation

Summary The bowl and doily spider, Frontinella pyramitela (Araneae, Linyphiidae), typically hangs at the lowest point on its concave upward, bowlshaped web (Fig. 1). It returns to the same area after prey capture, courtship, or repulsion of intruders, and naive spiders of either sex come to rest at the same center of preference.Because the center of preference is not visibly different from other parts of the bowl sheet, it is not obvious what cues the spiders use in establishing its location. Investigation of the roles of gravity, web curvature, thread density and web tension reveals the center of preference to be an area no different from others with respect to curvature and thread density but different with respect to tension. Tension in the web is locally high at the center, drops as one moves toward the periphery of the bowl, and rises again at the periphery (Fig. 6). The natural relationship between gravity and the orientation of the bowl is not a necessary condition in defining the location of the center of preference but gravity is involved in orienting the search for its location (Figs. 3, 4). Altering the pattern of web tension by cutting the tensor threads destroys the accord among spiders as to the location of the center of preference and confirms the role of web tension in the orientation of this spider.     

Task-dependent influence of genetic architecture and mating frequency on division of labour in social insect societies

Division of labour is one of the most prominent features of social insects. The efficient allocation of individuals to different tasks requires dynamic adjustment in response to environmental perturbations. Theoretical models suggest that the colony-level flexibility in responding to external changes and internal perturbation may depend on the within-colony genetic diversity, which is affected by the number of breeding individuals. However, these models have not considered the genetic architecture underlying the propensity of workers to perform the various tasks. Here, we investigated how both within-colony genetic variability (stemming from variation in the number of matings by queens) and the number of genes influencing the stimulus (threshold) for a given task at which workers begin to perform that task jointly influence task allocation efficiency. We used a numerical agent-based model to investigate the situation where workers had to perform either a regulatory task or a foraging task. One hundred generations of artificial selection in populations consisting of 500 colonies revealed that an increased number of matings always improved colony performance, whatever the number of loci encoding the thresholds of the regulatory and foraging tasks. However, the beneficial effect of additional matings was particularly important when the genetic architecture of queens comprised one or a few genes for the foraging task’s threshold. By contrast, a higher number of genes encoding the foraging task reduced colony performance with the detrimental effect being stronger when queens had mated with several males. Finally, the number of genes encoding the threshold for the regulatory task only had a minor effect on colony performance. Overall, our numerical experiments support the importance of mating frequency on efficiency of division of labour and also reveal complex interactions between the number of matings and genetic architecture.     

Delayed dispersal as a potential route to cooperative breeding in ambrosia beetles

Xyleborini are a species-rich tribe of ambrosia beetles, which are haplodiploid and typically mate among siblings within their natal brood chamber. Several characteristics of this tribe would predict the evolution of higher levels of sociality: high genetic relatedness within galleries due to inbreeding, high costs of dispersal and the potential benefit of cooperation in brood care within the natal gallery (e.g. by fungus gardening, gallery extension, offspring feeding and cleaning). However, information on the social system of these beetles is very limited. We examined the potential for cooperative breeding in Xyleborinus saxeseni by monitoring dispersal in relation to brood size and composition. Results show that adult female offspring delay dispersal despite dispersal opportunities, and apparently some females never disperse. The females’ decision to stay seems to depend on the presence of eggs and dependent siblings. We found no indication that female offspring reproduce in their natal gallery, as colonies with many mature daughters do not contain more eggs than those with few or no daughters. There is a significant positive relationship between the number of females present and the number of dependent siblings (but not eggs), which suggests that cooperative brood care of female offspring raises colony productivity by improving survival rates of immatures. Our results suggest that cooperative breeding is likely to occur in X. saxeseni and possibly other xyleborine species. We argue that a closer look at sociality within this tribe may yield important information on the factors determining the evolution of cooperative breeding and advanced social organization.     

Male mating competitiveness and age-dependent relationship between testosterone and social rank in bighorn sheep

In males, the acquisition and development of behavioral and morphological secondary sexual traits typically depends on testosterone and correlates with mating success. Testosterone level could affect competition for mates and thus be a target of sexual selection. We sought to relate testosterone levels to male mating competitiveness, by teasing apart the relationships between testosterone, behavior, and growth before the mating period. We monitored 24 adult bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, from 2008 to 2011. Using linear mixed models, we tested the relationships between testosterone metabolites in feces, social rank, and both growth and size of two sexually selected traits: horns and body mass. The correlation between testosterone and social rank varied with age. Testosterone and rank were weakly and negatively correlated for young rams, positively correlated for prime-aged rams, and negatively correlated for older rams. Although testosterone had an increasingly positive effect on total horn length until 8 years of age, we could not detect any effects on annual growth rate of horns or body mass. Testosterone may be related to male’s ability to compete for mates through its relationship with behaviors determining social rank, rather than by influencing the development of morphological traits. Differences in testosterone levels among competitors may be a proximate cause of variance in fitness.     

Positive association between social and extra-pair mating in a polygynous songbird, the dickcissel (Spiza americana)

In polygynous species, males appear to gain additional offspring by pairing with multiple females simultaneously. However, this may not be true if some females copulate outside of the social pair bond. Polygynous males could experience lower paternity because of trade-offs among gaining multiple social mates, guarding fertility with these mates, and pursuing extra-pair matings. Alternatively, polygynous males could simultaneously gain extra social mates and have high paternity, either because of female preferences or because of male competitive attributes. We tested four predictions stemming from these hypotheses in a facultatively polygynous songbird, the dickcissel (Spiza americana). Unlike most previous studies, we found that males with higher social mating success (harem size) also tended to have higher within-pair paternity and that the number of extra-pair young a male sired increased significantly with his social mating success. Females that paired with mated males were not more likely to produce extra-pair young. In contrast, extra-pair paternity was significantly lower in the nests of females whose nesting activity overlapped that of another female on the same territory. This pattern of mating was robust to differences in breeding density. Indeed, breeding density had no effect on either extra-pair mating or on the association between polygyny and paternity. Finally, nest survival increased with harem size. This result, combined with the positive association between polygyny and paternity, contributed to significantly higher realized reproductive success by polygynous male dickcissels.