Sexual conflict and the energetic costs of mating and mate choice in water striders

ABSTRACT Analyses of intersexual conflicts of interest over courtship, mating, or mate guarding require an understanding of the physiological costs of sexual interaction. Repeated respirometric measures of energetic expenditure were taken on female Aquarius remigis while unladen and while carrying a mating male, a small metal weight, or a euthanized male. Unladen "cruising" locomotion consumed an average of 334.6 muW of energy ([Formula: see text]); this estimate of the cost of locomotion coincides with measures from voluntarily locomoting arthropods of similar mass and represents the first energetic measure of skating on a water surface. Cruising females carrying males or metal weights consumed 24% and 28% more energy than unladen females, respectively. Females engaged in "escape" locomotion consumed 43% more energy while carrying a male than while unladen. Further, our study shows that premating struggles, and therefore selective mating deccisions, are energetically costly. Struggling females consumed an average of 936.6 muW, a 126% increase compared to cruising, nonstruggling females, and 64% more than mating females engaged in escape locomotion. We develop a quantitative model showing that at a certain harassment rate threshold, accepting superfluous matings becomes the "best of a bad job" for females.     

Role of male-male competition and female choice in the development of breeding coloration in pupfish (Cyprinodon pecosensis)

The function of male breeding coloration and the conditionsunder which it is expressed were investigated using two experimentalprotocols that differed in the degree of control of social factors.In tank experiments, focal males were separated by opaque orclear partitions or allowed to interact freely without partitions.In lake experiments, males were introduced into opaque or clearenclosures in the lake from which they were collected. Thisdesign tested the relative importance of breeding substrate,social interactions with other males, and presence of femalesand juveniles on the expression of breeding coloration and agonisticand mating behavior in pupfish. Breeding coloration was notexpressed in the absence of conspecifics, even in the presenceof suitable breeding substrate, or in the presence of juveniles.Breeding coloration was poorly developed when only males werepresent, was more intense in the presence of females, and darkenedsignificantly after a spawning bout Males in clear enclosuresin their natural lake habitat were in visual contact with conspecificsof both sexes and developed more intense blue breeding colorationthan males in the tank experiments that were in visual contactonly with other males. Uncon-fined males with territories inthe lake developed the most intense breeding coloration. Breedingcoloration of male pupfish is a facultative trait that is welldeveloped when breeding females are present and plays a majorrole in the attraction and courtship of mates. However, maximumdevelopment of breeding coloration by territorial males suggeststhat intrasexual and intersexual selection act concordantlyon this trait.[Behav Ecol 7: 431-437 (1996)]     

条纹短攀鲈的雄性个体大小、雌性偏好以及雄性之间的竞争

本研究考察了条纹短攀鲈(Trichopsis vittata)雄性个体的身体大小对雌性个体性偏好的影响以及对雄性之间竞争的影响。本研究设立了两种处理来研究雌性个体的性偏好以及雄性之间的竞争。在一种处理中,放入同样大小的雄性个体,而在另一种处理中,放入大小不同的雄性个体。结果表明,雄性个体的大小不影响雌性条纹短攀鲈的性偏好。在对雄性之间竞争的研究中,发现个体较大的个体获胜的次数比较多。在个体大小相同的处理中,雄性之间打斗的持续时间比另一种处理中的打斗持续时间长。对这两种处理中不同对抗行为的比较表明,个体大小相同处理组中的发声、咬、以及总体对抗行为发生得更为频繁。对个体大小不同处理组中的大、小个体的进一步观察也揭示出体型大的个体比体型小的个体有更多的追逐行为和更少的逃脱行为。这些结果表明,在雄性个体比雌性个体大的物种中,体型的性两型可能与雄性之间的竞争有关,而不是与雌性偏好有关。     

Male-male competition and female choice in brown trout

In some salmonid species, the females have been assumed to choose their mates on the size of the male's adipose fin. This hypothesis was tested in a stream water aquarium, in which 19 brown trout, Salmo trutta, females were allowed to choose between two males of the same body size but with different adipose fin sizes. The two males were separated from each other in cages. After the female had started to prepare her nest close to one of them, the males were released and allowed to fight each other for the opportunity to spawn. Out of 19 females, 14 prepared a nest closest to the male with the larger adipose fin. However, only six of the 14 females spawned with this male. Males that spawned were more dominant (i.e. were more likely to win fights). When the female spawned with the male she chose, he was less aggressive towards her than when she spawned with the other male. There were no significant differences in the plasma levels of testosterone (T) and 11-ketotestosterone (11-KT) between the chosen males and those not chosen. However, the dominant males had significantly higher plasma levels of T and 11-KT both before and after the experiment. The results support the view that female brown trout exhibit mate choice, but their choice is overruled by male-male competition. Copyright 1999 The Association for the Study of Animal Behaviour.     

Sexual selection in a lekking bird: the relative opportunity for selection by female choice and male competition

Leks are classic models for studies of sexual selection due to extreme variance in male reproductive success, but the relative influence of intrasexual competition and female mate choice in creating this skew is debatable. In the lekking lance-tailed manakin (Chiroxiphia lanceolata), these selective episodes are temporally separated into intrasexual competition for alpha status and female mate choice among alpha males that rarely interact. Variance in reproductive success between status classes of adult males (alpha versus non-alpha) can therefore be attributed to male-male competition whereas that within status largely reflects female mate choice. This provides an excellent opportunity for quantifying the relative contribution of each of these mechanisms of sexual selection to the overall opportunity for sexual selection on males (I males). To calculate variance in actual reproductive success, we assigned genetic paternity to 92.3% of 447 chicks sampled in seven years. Reproduction by non-alphas was rare and apparently reflected status misclassifications or opportunistic copulations en route to attaining alpha status rather than alternative mating strategies. On average 31% (range 7-44%, n=6 years) of the total I males was due to variance in reproductive success between alphas and non-alphas. Similarly, in a cohort of same-aged males followed for six years, 44-58% of the total I males was attributed to variance between males of different status. Thus, both intrasexual competition for status and female mate choice among lekking alpha males contribute substantially to the potential for sexual selection in this species.     

The importance of sperm competition risk and nest appearance for male behavior and female choice in the sand goby, Pomatoschistus minutus

To test if an increased sperm competition risk affects malebehavior and mating decisions of both sexes, we performed twoexperiments using the sand goby, Pomatoschistus minutus, a nest-buildingfish with exclusive paternal care. In our first experiment,a nest-holding male, with a confined female, was sequentiallyexposed to a vial with a sneaker male or an empty vial. Whilemale courtship, nest building, displacement fanning, and timeoutside the nest were unaffected, individual males showed ahigher mucus preparation effort inside the nest in the presenceof a sneaker male than when alone. We found such mucus to containsperm, thus clearly suggesting an importance in sperm competition.In our second experiment, a female was free to spawn with twodifferent males, one of which was exposed to a confined sneakermale. Male mating success was not affected by the presence ofa sneaker male. However, the volume of sand the male had puton his nest was positively associated with female spawning decision,while nest-opening width was not. In a partial correlation offive traits thought to attract females (nest-opening width,sand volume, male courtship display, displacement fanning, andmale size), males that fanned well were found to also buildlarge nests or display intensely, but not both. This indicatesthat rather than being jacks-of-all-trades, individual malesfocus on a subset of traits for attracting females.     

An experimental test of condition-dependent mating behavior and habitat choice by water striders in the wild

Individual condition is expected to be an important determinantof many behaviors, including mating dynamics and habitat choice.In this study we experimentally investigated the linkages betweenindividual condition, habitat use, and mating dynamics in thewild. We manipulated recent feeding history of the water strider,Aquarius remigis, and then quantified the habitat use and matingactivity of males and females. Females could choose from threehabitats (refuge, near shore, and open). On the water surface(open and near-shore habitats), in contrast to refuge, femalescan forage, but they are exposed to predation and sexual harassmentby males. We tested three main hypotheses. First, we predictedthat single females that were fed, relative to those that werenot, would reduce their exposure to predators and male harassmentby increasing their use of refuge. Similarly, we predicted thatfed females that were mating would spend more time in refugethan those that were not fed. Our results support these twopredictions. Fed single and mating females significantly increasedtheir use of refuge. Third, we predicted that mating activity(proportion of time spent mating) of fed females would be reducedrelative to starved females, because of reduced exposure tomales while in refuge, and perhaps because of decreased receptivityto male mating attempts while on the water surface. Mating activityof fed females was about one third that of starved females.The decrease in mating activity could not be accounted for byany change in female receptivity, but could be accounted forby change in habitat use. The decrease in mating activity mayhave resulted from decreases in both mating frequency and matingduration. Our estimates of minimum mating frequency indicatea large and significant decrease, but we were unable to assessmating duration. We found no significant effect of our manipulationon habitat use or mating activity of males.[Behav Ecol 7: 474–479(1996)]     

Sexual conflict and the evolutionary ecology of mating patterns: water striders as a model system

Two core ideas in the study of mating systems and sexual selection are (1) the existence of a conflict between the sexes over mating decisions, and (2) that variation in ecological conditions drives the evolution of adaptive mating strategies and the diversification of mating systems. A recent burst of experimental studies of mating behavior and sexual selection in water striders has focused on the interaction of these ideas and led to new insights into the evolutionary ecology of mating systems and sexual selection.     

Wake up and smell the conflict: odour signals in female competition

Odour signals used in competitive and aggressive interactions between males are well studied in the context of sexual selection. By contrast, relatively little is known about comparable signals used by females, despite current interest in the evolution of female ornaments and weaponry. Available evidence suggests that odour signals are important in competitive interactions between female mammals, with reductions or reversals of male-biased sexual dimorphism in signalling where female competition is intense. Scent marking is often associated with conflict between females over access to resources or reproductive opportunities. Female scent marks may therefore provide reliable signals of competitive ability that could be used both by competitors and potential mates. Consistent with this hypothesis, we report that aggressive behaviour of female house mice is correlated with the amount of major urinary protein (MUP) excreted in their urine, a polymorphic set of proteins that are used in scent mark signalling. Under semi-natural conditions, females with high MUP output are more likely to produce offspring sired by males that have high reproductive success, and less likely to produce offspring by multiple different sires, suggesting that females with strong MUP signals are monopolized by males of particularly high quality. We conclude that odour signals are worthy of more detailed investigation as mediators of female competition.     

The relative importance of predation and competition in two reef fishes

Competition and predation may both strongly influence populations of reef fishes, but the importance of these processes relative to one another is poorly understood. I quantified the effects of predation and competition on the growth and survival of two temperate reef fishes, Lythrypnus dalli and Coryphopterus nicholsii, in field experiments in which I manipulated the densities of the two species and the abundance of predators (using exclosure cages) on small replicate patch reefs. I also evaluated the influence of predators on the behavior of the two species to help interpret the mechanisms of any predatory influences on growth or survival. Predation was much more important than competition (inter- or intraspecific) in Lythrypnus. For Coryphopterus, neither competition nor predation were particularly important. Behaviorally, both species responded to predators by reducing foraging rate and hiding. This altered behavior, however, had no repercussions for growth or survival of Coryphopterus. In contrast, Lythrypnus grew more slowly and suffered greater mortality when exposed to predators. Interspecific competition did not significantly influence either species. Intraspecific competition did not affect the growth of Coryphopterus, but survival tended to be lower at high densities. Growth of Lythrypnus was depressed by intraspecific competition, but survival was not, except that, in the presence of predators, survival was density dependent. In contrast to the historical emphasis placed on the role of competition, this study indicates that predation can be more important than competition in determining patterns of abundance of some reef fishes. For example, predators not only influenced foraging of both Lythrypnus and Coryphopterus, but they also reduced growth and survival of Lythrypnus, and therefore appear to help maintain the marked habitat segregation between the two species. Received: 16 June 1997 / Accepted: 3 December 1997     

Sexual conflict and the evolution of female mate choice and male social dominance

Conflicts between the sexes over control of reproduction are thought to lead to a cost of sexual selection through the evolution of male traits that manipulate female reproductive physiology and behaviour, and female traits that resist this manipulation. Although studies have begun to document negative fitness effects of sexual conflict, studies showing the expected association between sexual conflict and the specific behavioural mechanisms of sexual selection are lacking. Here we experimentally manipulated the opportunity for sexual conflict in the cockroach. Nauphoeta cinerea and showed that, for this species, odour cues in the social environment influence the behavioural strategies and fitness of males and females during sexual selection. Females provided with the opportunity for discriminating between males but not necessarily mating with preferred males produced fewer male offspring than females mated at random. The number of female offspring produced was not affected, nor was the viability of the offspring. Experimental modification of the composition of the males' pheromone showed that the fecundity effects were caused by exposure to the pheromone component that makes males attractive to females but also makes males less likely to be dominant. Female mate choice therefore carries a demographic cost but functions to avoid male manipulation and aggression. Male-male competition appears to function to circumvent mate choice rather than directly manipulating females, as the mate choice can be cryptic. The dynamic struggle between the sexes for control of mating opportunities and outcomes in N. cinerea therefore reveals a unique role for sexual conflict in the evolution of the behavioural components of sexual selection.     

Effect of sex,hunger and relative body size on the use of ripple signals in the interactions among water striders Gerris latiabdominis

Water striders use ripple signals in aggressive interactions between individuals for access to food. We asked whether water striders produce ripple signals more frequently when they are hungrier and when the value of food resources is higher. We also asked if and how the use of signals depends on the size difference between interacting individuals. We found that females used ripple signals more often than males did. The experiment suggested that use of aggressive ripple signals is affected by hunger in females – the sex with high demands for food resources. Among females, but not males, we found out that the probability of using signals in response to the approaching intruder depended both on the degree of hunger and on the size of the focal animal relative to the size of the intruder. Before starvation, the probability of a female using a signal in an interaction with an intruder was higher when the individual's size was larger relative to the intruder. After starvation, the focal individuals were more likely to signal when their size was smaller relative to the intruder. The results are consistent with the idea that these signals may reveal information about the signalers weight or hunger level, and specific hypotheses are suggested for the future studies.     

Male mating competition,female choice and dominance in a free ranging group of Japanese macaques

Focal and ad libitum samples of high and middle ranked males in a group of free ranging Japanese macques were taken in order to examine rank related differences in male mating strategies. Males tended to have like ranked females as consort partners, with high rank males showing more consort activity, over all, than middle rank males. High rank males tended to interfere in consorts and middle rank males tended to have their consorts disrupted. Consorts involving high rank females were most subject to interference. With one exception, proximity between partners in consorts involving high rank males was due to male actions while proximity in consorts involving middle ranked males was due to female actions. The two highest ranked males were never observed copulating. Their mating failure may have been due to avoidance by females who had known them since immaturity. High rank males were somewhat more likely than middle rank males to have consorted with females during the period of likely conception. There was some evidence that frequent consort partners joined the same subgroup during a group fission. Males appeared to use the advantage conferred by high rank mainly in competition for high rank females. Females showed some indications of preference for mates likely to retain or attain high rank in the future.     

Male competition and female choice interact to determine mating success in the bluefin killifish

Whether male competition and female choice act in concert, independently,or in opposition is a critical issue for understanding sexualselection. In complex social systems, the outcomes of pairwiseinteractions may not be accurate indicators of how sexual selectionemerges. We investigated how female choice and male competitioninteract in the bluefin killifish, Lucania goodei, in a 3-stagedexperiment where 1) females could choose between 2 males, 2)those males could interact in the presence of that female, and3) females and males could freely interact and spawn. In thepairwise stages (1 and 2), females displayed pronounced preferencesbetween males and male competition produced a distinctly dominantindividual. None of the morphological traits, including color,measured in males were associated with either female preferenceor male dominance. When all 3 fish interacted (stage 3), maleactivity level was the sole predictor of spawning success. Maleswith elevated activity levels were more aggressive toward malesand females, exhibited intensified courtship, and obtained morespawns. Female preference did not predict the number of spawnswith a male, but it did predict her latency to spawn; femalesspawned more quickly with preferred males. Thus, male competitionand female choice interact to determine reproductive success,but there is evidence for conflict and a cost to females ofassociating with dominant males. Reproductive success in thisspecies is not easily predicted from simple measures of morphologyor female preference and is influenced by complex social interactions,both between males, and between males and females.     

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators. Received: 26 August 1996 / Accepted: 12 June 1998     

Interacting effects of predation risk and male and female density on male/female conflicts and mating dynamics of stream water striders

We used a factorial experiment to examine interacting effectsof male density, female density, and sunfish (predation risk)on mating dynamics of the stream water strider (Aquarius remigis).Many of our results corroborated earlier studies on the isolatedeffects of each factor on mating behavior. The effect of eachfactor, however, depended on the other factors. For example,in low density pools, predation risk decreased male generalactivity, male/female harassment rates, mating activity, andmating duration and increased the large male mating advantage.At higher densities, however, water striders apparently enjoyed"safety in numbers" and did not alter their mating dynamicsin response to the presence of predators. Female activity showeda particularly complex response to male density and fish. Whenmales were scarce, fish caused females to reduce their activity.However, when males were abundant, fish increased female activity,probably because fish decreased male activity thus releasingfemales from harassment by males. The three treatment factorsalso had interacting effects on male mating success. In theabsence of fish, when females were scarce, increased male densityresulted in a decrease in mean male mating success; however,when females were abundant, increased male density enhancedmean male mating success. In contrast, in the presence of fish,male density had little effect on male mating success. Manyof the observed mating patterns can be explained by the effectsof ecological and social factors on male/female conflicts; thatis, on male harassment of females and female reluctance to mate.