Complex courtship in a bimodal grasshopper hybrid zone

Grasshoppers of the Chorthippus albomarginatus-group, which is outstanding with respect to its complex courtship song, were studied at fifteen localities in the Ukraine and Moldova. The analysis of the courtship songs revealed two species: C. albomarginatus in north-eastern Ukraine and Chorthippus oschei in the south-western Ukraine and Moldova. In a belt about 200 km wide, not only were one or the other pure species found, but also males with intermediate song characters. C. albomarginatus and C. oschei were hybridised in the laboratory, and F1 hybrid males as well as F2 hybrid males produced intermediate song patterns, quite similar to those recorded in the field. We defined a "hybrid song score" for intermediate songs. The score showed a bimodal distribution with most songs resembling one or other parental type, but with only a few intermediates. At several localities, where hybrids with songs similar to one of the parental species dominated, some individual males sang more similarly to the other species. In one locality, two hybrid populations only 3 km apart had different parental types. Hybrid songs can contain novel elements, even more complex than the parental ones, which may offer a new starting point for sexual selection. We suggest that genetic introgression occurs between the two sibling species C. albomarginatus and C. oschei within a wide hybrid zone stretching over a distance of several hundred kilometres, but with a patchy spatial distribution. Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material. Communicated by D. Gwynne     

Developmental stress affects the attractiveness of male song and female choice in the zebra finch (Taeniopygia guttata)

Developmental stress has recently been shown to have adverse effects upon adult male song structure in birds, which may well act as an honest signal of male quality to discriminating females. However, it still remains to be shown if females can discriminate between the songs of stressed and non-stressed males. Here we use a novel experimental design using an active choice paradigm to investigate preferences in captive female zebra finches (Taeniopygia guttata). Nine females were exposed to ten pairs of songs by previously stressed and non-stressed birds that had learned their song from the same tutor. Song pairs differed significantly in terms of song complexity, with songs of stressed males exhibiting lower numbers of syllables and fewer different syllables in a phrase. Song rate and peak frequency did not differ between stressed and non-stressed males. Females showed a significant preference for non-stressed songs in terms of directed perching activity and time spent on perches. Our results therefore indicate that developmental stress affects not only the structure of male song, but that such structural differences are biologically relevant to female mate choice decisions.     

Vocal interactions in common nightingales (<Emphasis Type="Italic">Luscinia megarhynchos</Emphasis>): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.     

Reproductive success of male bank voles (<Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>): the effect of operational sex ratio and body size

The operational sex ratio (OSR) may influence the intensity of competition for mates and mate choice and is therefore thought to be a major factor predicting the intensity and direction of sexual selection. We studied the opportunity for sexual selection, i.e., the variance in male reproductive success and the direction and intensity of sexual selection on male body mass in bank vole (Clethrionomys glareolus) enclosure populations with experimentally manipulated sex ratios. The opportunity for sexual selection was high among male-biased OSRs and decreased towards female-biased OSRs. Paradoxically, selection for large male body mass was strongest in female-biased OSRs and also considerable at intermediate OSRs, whereas at male-biased OSRs, only a weak relationship between male size and reproductive success was found. Litters in male-biased OSRs were more likely to be sired by multiple males than litters in female-biased OSRs. Our results suggest that the intensity and direction of sexual selection in males differs among different OSRs. Although the direction of sexual selection on male body mass was opposite than predicted, large body mass can be favored by sexual selection. Naturally varying OSRs may therefore contribute to maintain variation in male sexually selected traits.     

Asymmetric sexual conflict over parental care in a biparental cichlid

Theoretical models predict that parents should adjust the amount of care both to their own and their partner’s body condition. In most biparental species, parental duties are switched repeatedly allowing for repeated mutual adjustment of the amount of care. In the mouthbrooding cichlid Eretmodus cyanostictus, terms are switched only once with females taking the first share. The timing of the shift of the clutch between mates strongly determines both partners’ brooding period and thereby their parental investment. Females signal their readiness to transfer the young several days before the male finally takes them, suggesting sexual conflict over the timing of the shift. In a lab experiment, we reduced the body condition of either the female or the male of a pair to test whether energy reserves affect the timing of the shift and whether female signalling behaviour depends on energetic state. Males with a lowered condition took the young later and incubated for a shorter period, which prolonged the incubation time of their female partners. When female condition was lowered, female and male incubation durations remained unchanged, although females signalled their readiness to shift more intensely. Our results suggest that males adjust their parental investment to own energy reserves but are unresponsive to their mate’s condition. Females appear to carry the entire costs for the male’s adjustment of care. We propose that intrinsic asymmetries in the scope for mutual adjustment of parental investment and the costs of negotiation crucially influence solutions of the conflict between sexes over care.     

Zebra finches,<Emphasis Type="Italic"> Taeniopygia guttata</Emphasis>, with elongated tails delay taking flight during group take-offs

Zebra finches with tail-elongations left perches during induced take-offs consistently later than unmanipulated controls. Latency in take-off was the same for both a short pintail-elongation (27% of finch long-axis) and an extreme pintail-elongation (49% of finch long-axis). There was no evidence for artificial pintails increasing energy expenditure, as predicted by flight aerodynamics models. Neither flight metabolic rate determined using the doubly labelled water technique, nor resting metabolic rate or food intake rates were affected. Hence, the energetic costs of elaborate pintails seem likely to be small and perhaps biologically trivial. In contrast, pintails appear to confer a manoeuvrability cost, as found in previous studies of long tails. Latency in take-off may be induced by an increased risk of collision with conspecifics during group take-offs and landings. Alternatively, latency may represent a change of predator-avoidance strategy, because taking off within the group does not minimise predation risk when handicapped by tail-elongation. The effects of tail-elongation are likely to be context-dependent and may differ between solitary species and more gregarious species that fly or forage in flocks.Communicated by W. Wiltschko     

Fitness consequences of sexual cannibalism in female<Emphasis Type="Italic"> Argiope bruennichi</Emphasis>

The evolution of sexual cannibalism as the most extreme form of nuptial feeding is still poorly understood. Although increasing evidence suggests that female aggressiveness is related to other aspects of foraging behaviour, it is not clear whether the nutritional value of a male is sufficient to provide an adaptive significance for sexual cannibalism. A widely cited though rarely tested explanation is based on a paternal investment model, and predicts that consumption of a male results in increased female fecundity. The available evidence is either correlational or restricted to species with relatively large and potentially nutritious males, and different studies have come to different conclusions. Here we present a test of the paternal investment hypothesis using the very cannibalistic and highly size-dimorphic spider Argiope bruennichi. After a preset schedule, we had females consume none, one or two males independent of the female's cannibalistic behaviour. Consumption of male bodies did not result in any detectable fitness benefit for the female: neither the number of clutches, nor clutch size or hatching success were affected by consumption of males. The frequency of cannibalism was around 80%, independent of the female mating status. We did not observe male complicity, but cannibalism was associated with prolonged copulation. This suggests a sexually selected benefit of cannibalism for males. We conclude that the paternal investment hypothesis does not explain the existence of sexual cannibalism in A. bruennichi and probably not in other spider species with a pronounced sexual size dimorphism.Communicated by L. Simmons     

Sex ratio dynamics and fluctuating selection pressures in natural populations of the Trinidadian guppy,<Emphasis Type="Italic"> Poecilia reticulata</Emphasis>

In many species, population sex ratios have far-reaching consequences for a wide variety of population-level and behavioural processes and can directly influence sexual selection through differential effects on male and female mating behaviour. Although sex ratios are often treated as more or less stable population characteristics, recent theoretical evidence suggests that sex ratios fluctuate under many conditions, and that the amplitude of these fluctuations can be considerable. Few studies have attempted to quantify this variation in systems with prominent, sex ratio-dependent sexual conflict. One of the species with the greatest potential to integrate these factors in the wild is the Trinidadian guppy, Poecilia reticulata. In this study, we quantified natural sex ratio variation both as detailed longitudinal studies of focal guppy populations and as snapshot estimates across a range of freshwater habitats. In line with theoretical predictions, we expected to detect significant sex ratio variation over time. We also investigated the association between juvenile and adult sex ratios to quantify a possible compensatory feedback implied in standard models of sex ratio evolution. Our results confirm that population-level sex ratios in wild guppy populations have a range of dynamic features, with all four focal populations showing significant variation in sex ratio over time. The survey showed that juveniles were generally close to equal (50:50) sex ratios whereas 7 out of 11 adult sex ratios differed significantly from equality. We found no evidence that a surplus of juveniles of the locally rarer sex had been produced. The results indicate that sex ratios and hence the balance between sexual selection and sexual coercion is normally fluctuating in nature, despite juvenile ratios being close to equality.Communicated by J. Krause     

Mating behavior of female rock shrimp Rhynchocinetes typus (Decapoda: Caridea)—indication for convenience polyandry and cryptic female choice

While studies on a wide diversity of organisms have demonstrated the importance of female behavior during matings, in crustacean studies, a strong bias towards male mating behavior prevails. Reproductively mature rock shrimp (Rhynchocinetes typus) exist as several ontogenetic stages that differ in their morphological and physiological capacities. In natural populations, the majority of males are in early ontogenetic stages (termed typus), many are in intermediate stages (intermedius), and few are in the terminal molt stage (robustus). Dominant robustus males, which have already demonstrated their biological fitness by surviving to this stage, have previously been shown to have a higher potential than subordinate typus males to defend receptive females against other males, and fertilize the entire clutch of a female. While females should thus show a preference for robustus males, they nevertheless frequently receive sperm from typus males. These observations suggested that females might have mechanisms to discriminate against sperm from subordinate males. In laboratory experiments, we observed that females avoided being seized by typus males for longer time periods in the absence of robustus males than in their presence. Following seizure, females that were initially held by typus males, required more time to initiate spawning than those held by robustus males. Many typus males transferred spermatophores to females before these started to spawn while robustus males waited until females began to spawn before they transferred spermatophores. Females manipulated spermatophores received from typus males for long time periods (minutes), but not those they received from robustus males. By accepting sperm from subordinate typus males, females may avoid further harassment (convenience polyandry), but they subsequently may discriminate against these subordinate males by delaying spawning and removing their sperm. These observations suggest that female behavior influences the outcome of matings, favoring fertilization of eggs by sperm from dominant males. Convenience polyandry and cryptic female choice may be common in other crustaceans as well.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Communicated by T. Czeschlik     

Magpies' tails: damage as an indicator of quality

Quality-indicating sexually selected traits may have their honesty maintained by their costs or by an inherent “revealing” nature. Long tails in birds are usually considered to be costly “handicaps”, but may have additional potential as revealing indicators through the incidence of breakage. Magpies Pica pica with unbroken and less abraded tails paired earlier, but did not nest or fledge young earlier than pairs with tails in poorer condition. Pairs mated assortatively by tail quality, and magpies with very broken tails remained unmated. Pairs in which both members had almost undamaged tails fledged more offspring than pairs with poorer tails. Tail quality did not correlate with the extent of any habitat type in the territory. Tail damage thus honestly indicated a magpie's reproductive potential, and the data are consistent with its having a role in mate choice, as a revealing element of tail morphology. Received: 28 March 1996 / Accepted after revision: 9 December 1996