Settlement patterns of female barn swallows Hirundo rustica across different group sizes: access to colorful males or favored nests?

In most avian species, individuals are faced with two critical decisions at the start of a breeding season: choosing a breeding site and a mate. An analysis of these decisions in light of population-level patterns, such as group size variation in social breeders, can illuminate the causes and patterns of habitat selection behavior. Group sizes are variable in barn swallows; however, no clear and consistent benefits of group breeding have been found in this species, and it is puzzling as to why individuals breed socially. Previous analyses demonstrated that individuals aggregate to gain access to nests that were constructed during previous seasons; however extra-pair matings are also prevalent in this species, raising questions about the mate-selection strategies of females across different group sizes. In this paper, I address the question of how females make their first site-selection decisions in terms of two features known to be causally related to seasonal reproductive success: (1) colorful males or (2) old nests. Using experimental and observational data, I tested, but found no support for, the hypotheses that propose female settlement decisions are a function of (1) the prevalence of colorful males or (2) the increased opportunity for extra-pair matings at group sites. Instead, it is apparent that female settlement patterns are strongly tied to the availability of old nests at a site. Extra-pair fertilizations are equally common across all group sizes in this population, suggesting that females do not face a trade-off between old nests and the possibility of extra-pair mating decisions when making settlement decisions.     

Early nutritional conditions, growth trajectories and mate choice: does compensatory growth lead to a reduction in adult sexual attractiveness?

It is well documented that poor early nutrition can have profound negative effects on adult life-history traits. However, it has also been demonstrated that organisms can undergo compensatory resource allocation strategies (such as an accelerated growth rate) if food availability improves, so as to mitigate the effects of the poor early conditions. Previous research has indicated that elevated growth rates can incur costs in the longer term, such as an increased rate of senescence and shorter lifespan. We tested whether a phase of compensatory growth after a period of reduced food availability earlier in life affected the sexual attractiveness of adult male green swordtails Xiphophorus helleri, a species in which it has previously been documented that females prefer larger-bodied and longer-tailed males. The experiment compared the attractiveness of size-matched brothers that had experienced contrasting growth trajectories as juveniles; the experiments were initially conducted in the middle of a male’s sexually reproductive life and were then repeated towards the very end of life. At both ages, males that had undergone compensatory growth were equally as attractive as their brothers that had grown normally. These results suggest that the growth compensation benefits males through an increase in their attractiveness over that which they would have had if they had remained on their original growth trajectory. The lack of change in relative attractiveness with age indicates that the compensation does not cause greater deterioration in secondary sexual characters at older ages than in continuously well-fed males.     

Specificity of odour-mediated avoidance of competition in Drosophila parasitoids

Although there are many examples of the role of volatile infochemicals in interactions between trophic levels of insect communities, surprisingly little is known of volatile interactions between species within the third trophic level. Recently it was found that Leptopilina heterotoma, an endoparasitoid that attacks Drosophila larvae, avoids one type of patches (decaying stinkhorn mushrooms) when parasitoids of another species (L. clavipes) are present on these patches. L. heterotoma is able to smell the presence of L. clavipes from a distance (Fig. 1). In this paper we investigate the source of the odour that induces avoidance behaviour, by varying the host species and parasitoid species present on stinkhorn mushrooms, and by using another type of patch (sap-fluxes of wounded trees). L. heterotoma was found to avoid stinkhorn patches with conspecific as well as heterospecific parasitoids (Fig. 2). Hosts had to be present in the patch to elicit avoidance, but avoidance behaviour was also found with another host species present in the patch (Fig. 3). No avoidance behaviour was found with sap-flux patches with hosts and parasitoids on them (Fig. 4). Avoidance of stinkhorn patches only occurred when the parasitoids present on the patch were able to contact hosts (Figs. 5 and 6). The exact source of the odour that elicits avoidance is still unclear, so that one can only speculate on the function of the signal. However, there is a clear benefit to the receiver, because it is able to avoid superior competitors. Avoidance can lead to non-aggregated parasitoid distributions. The importance of avoidance behaviour for population dynamics and stability of parasitoid-host systems is discussed.     

Fighting success and attractiveness as predictors of male mating success in the black field cricket, Teleogryllus commodus: the effectiveness of no-choice tests

Females are generally assumed to prefer larger, more dominant males. However, a growing number of studies that control for male-male competition have shown no correlation between dominance and attractiveness. Aggressive males can interfere with female mate preference either by physically coercing females into mating or by driving submissive males away and restricting mate choice. The most common method of assessing female mate choice is by using simultaneous two-choice tests. These control for male-male interactions, but usually interfere with physical and chemical cues involved in mate selection or alter male behaviour. They are therefore unsuitable for many study species, especially insects. Another method is the no-choice test that measures a females latency to mating when placed with a single male as an indication of male attractiveness. No-choice tests control for male-male aggression while allowing full contact between pairs (they allow actual mating to be directly observed rather than to occur based on a correlated behaviour). So far, however, no study has confirmed that males that entice females to mate sooner actually enjoy increased longer-term mating success. As such, the accuracy of no-choice tests as a method of examining mate choice remains untested. Here, we used no-choice tests on the black field cricket, Teleogryllus commodus, to show that (1) females did not prefer males that won fights (dominant males), and (2) latency to mating predicts actual mating success. We have clearly demonstrated the usefulness of no-choice tests and, considering the advantages of this method, they should be more often considered for a wider variety of taxa.Communicated by D. Gwynne     

The dynamics of brood desertion among communally breeding females in the treehopper, Publilia concava

In species that exhibit extended parental care, females sometimes lay eggs communally in order to redistribute the costs of offspring care. Communal egg-laying often involves redundancy in female effort, such that the number of females contributing to reproduction is more than is needed to provide adequate parental care. As a result, a subset of females will often abandon the communal brood, with the time of departure ranging from immediate abandonment after egg-laying (brood parasitism) to delayed abandonment with prolonged care (cooperative breeding). In this paper I approach the parental care dynamics of female-female broods as a desertion game similar to that of mate desertion in species with bi-parental care. I describe a field study of the insect Publilia concava (Hemiptera: Membracidae), a species that exhibits communal oviposition and a full range of egg guarding. This species exhibits full redundancy in female care, with no difference in survival between singly and doubly guarded broods. I find that double guarding is extremely rare in the population, with most communal broods having only one female guard. While this guard was usually the female that initiated the brood, these same females were more likely to abandon when secondary females arrived and when secondary females exhibited longer guarding durations. Paradoxically, the secondary females usually abandoned the broods they visited, resulting in up to 50% of broods with double abandonment. These unguarded broods suffered a 50% reduction in hatching success, reflecting an important risk for primary females that abandon egg masses to secondary females. Overall, P. concava exhibits desertion dynamics similar to mate desertion in vertebrates and it is likely that the theoretical work in this area will be useful for future work that addresses the allocation of parental care among communal breeders.     

Strike-induced chemosensory searching is absent in the cordylid lizard,Cordylus cordylus,a scleroglossan ambush forager

Summary Strike-induced chemosensory searching (SICS) was not detected experimentally in the cordylid lizard,Cordylus cordylus. Both components of SICS, a post-strike elevation in tongue-flick rate (PETF) and searching movements for attacked and released prey, were absent. The findings are consistent with previous data showing that PETF and/or SICS are lacking in all lizard families yet studied that forage primarily by ambush, but are present in actively foraging scleroglossan families and the herbivorous iguanian family Iguanidae. It is suggested that foraging behavior is a primary determinant of the presence or absence of SICS in lizards. Nevertheless, in most families in the two major clades, Iguania and Scleroglossa, the plesiomorphic foraging mode is retained. The findings agree with the prediction that SICS is absent in families lacking lingually mediated prey chemical discrimination (PCD), presumably due to selection against movement by ambush foragers that avoid being detected by either prey or predators because they remain motionless. Although PETF and SICS were absent, labial-licking and lingual movements similar to those observed after swallowing increased after biting prey, suggesting that the functions of these lingual movements may have been related to grooming. Locomotory movements did not increase following biting and appeared to represent avoidance of the experimenter.     

Mate choice tactics and swarm size: a model and a test in a dance fly

In the dance flyEmpis borealis (L.) (Diptera: Empididae) females gather to swarm and males visit swarms for mating. A model was constructed, based on previously published data, simulating how males may choose among females of different sizes in swarms of different sizes. The focal question was, what influences the number of individuals in the swarm in this and possibly other swarming insects? The relationships between original swarm size and both the number of males arriving per minute and the proportion of males mating are both logarithmic. The model predicted that if these relationships were linear, or if males were able to judge absolute female size, the mean swarm size should increase and be at least four times as large as those found in the field. The only type of male mate choice strategy that gave rise to very large swarms (>25) was size-related choice (if males are able to assess the size of a female in relation to the entire population and not merely to the swarm). Furthermore, no swarming behaviour would occur if males mate independently of swarm size. Thus, the numbers of females attending a given swarm site are influenced by male arrival pattern, male preference for larger swarms, the inability of males to judge the absolute body size of females, and female polyandry. Males searching for mates seem to prefer larger swarms than females searching for a swarm to join, but the mean swarm size is primarily set by the swarm size preference of females. Optimal swarm size predicted from the model was 4.68±0.53 females. In order to test model predictions, 69 natural swarm sites were studied during one season. The mean swarm size was 4.85±4.54 females (median 4.03), and about 90% of swarms consisted of 11 females or fewer. Predicted and observed swarm size did not differ significantly.     

Search costs influence the spatial distribution,but not the level,of extra-pair mating in tree swallows

Costs of searching for a mate are an important component of models of sexual selection, yet they have rarely been examined in wild populations of vertebrates. In this paper, we report an experiment in which we handicapped female tree swallows by clipping some flight feathers. This manipulation increased the costs of flight and searching for extra-pair mates. Despite these costs, handicapped females had the same level of extra-pair mating (percentage of extra-pair young, percentage of broods with extra-pair young, and the number of extra-pair sires per brood) as control females. However, handicapped females were more likely to have young sired by extra-pair males that lived closer to her nest than control females. This change in the distribution of extra-pair mating was most likely due to female choice rather than male coercion, and it suggests that extra-pair mating has significant benefits to females. One important implication of our study is that ecological and social factors that influence search costs could affect the spatial distribution of extra-pair sires and, consequently, the intensity of sexual selection. These effects may have been overlooked in previous studies that did not identify extra-pair sires.     

An experimental test of female choice relative to male structural coloration in eastern bluebirds

Several experimental studies have shown that female birds use ornamental melanin and carotenoid plumage coloration as criteria in mate choice. Whether females choose mates based on natural variation in structural coloration, however, has not been well established. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet (UV)-blue plumage coloration on their head, back, wings, and tail, which is positively correlated with condition, reproductive effort, and reproductive success. We experimentally tested the hypothesis that female eastern bluebirds prefer as mates males that display brighter structural coloration by presenting breeding-condition females with males of variable coloration. We conducted two types of mate-choice experiments. First, females chose between males whose coloration was manipulated within the natural range of variation in the population; feathers were either brightened with violet marker or dulled with black marker. Second, females chose between males with naturally dull or bright plumage coloration. In both manipulated and unmanipulated coloration trials, female choice did not differ significantly from random with respect to structural coloration. We found no support for the hypothesis that the UV–blue coloration of male eastern bluebirds functions as a criterion in female mate choice.     

Ornamentation predicts reproductive success in female pipefish

In the sex-role reversed pipefish Syngnathus typhle females compete for access to males and males are choosy. Females develop a temporary ornament when competing over mates with other females and when performing nuptial dances with males. This ornament is an amplification of the normal striped pattern in these fishes. We here show experimentally that (1) the contrast of this normal pattern forecasts the extent to which the ornament is shown, (2) contrast and ornamentation honestly signal female quality (egg numbers), (3) contrast and ornamentation accurately predict female mating success, (4) contrast is a phenotypically plastic trait specifically exaggerated under situations of female – female competition, and (5) neither contrast nor ornament are energetically expensive to the females (i.e., they are independent of short-term nutritional status). Hence, as predicted in sex-role reversed species, ornament design is constrained by costs to female fecundity: an energetically demanding ornament would impair on a female's ability to produce eggs. The type of ornament described here is the expected one, costly for reasons other than being energetically expensive to produce. Received: 4 April 1996 / Accepted after revision: 27 October 1996