Experimental manipulation of mate choice by male katydids: the effect of female encounter rate

Summary This study tests the general prediction that discrimination among potential mates increases with the availability of potential mates. Specifically, we conducted two experiments that examined mate choice by male zaprochiline katydids in relation to their prior encounter rate with females. The probability of mate acceptance or rejection was measured for males given either frequent or no contact with females in the laboratory (experiment 1) and males taken directly from natural areas of either high or low female abundance (experiment 2). In both experiments, males with low female encounter rates were more likely to mate than males with high female encounter rates. In both cases, the decreased mating probability of males in the high encounter treatment resulted from their tendency to reject lighter (and less fecund) females. Despite the presumed advantage to males of selecting heavier females, field data indicate that, unlike females, males do not aggregate in rich food patches. Possible explanations for this finding are discussed. Offprint requests to: T.E. Shelly at the present address     

The effect of male coloration on female mate choice in closely related Lake Victoria cichlids (Haplochromis nyererei complex)

We studied the effect of male coloration on interspecific female mate choice in two closely related species of haplochromine cichlids from Lake Victoria. The species differ primarily in male coloration. Males of one species are red, those of the other are blue. We recorded the behavioral responses of females to males of both species in paired male trials under white light and under monochromatic light, under which the interspecific differences in coloration were masked. Females of both species exhibited species-assortative mate choice when colour differences were visible, but chose non-assortatively when colour differences were masked by light conditions. Neither male behaviour nor overall female response frequencies differed between light treatments. That female preferences could be altered by manipulating the perceived colour pattern implies that the colour itself is used in interspecific mate choice, rather than other characters. Hence, male coloration in haplochromine cichlids does underlie sexual selection by direct mate choice, involving the capacity for individual assessment of potential mates by the female. Females of both species responded more frequently to blue males under monochromatic light. Blue males were larger and displayed more than red males. This implies a hierarchy of choice criteria. Females may use male display rates, size, or both when colour is unavailable. Where available, colour has gained dominance over other criteria. This may explain rapid speciation by sexual selection on male coloration, as proposed in a recent mathematical model. Received: 11 April 1997 / Accepted after revision: 27 July 1997     

Association patterns among male and female spotted hyenas (Crocuta crocuta) reflect male mate choice

Although female animals tend to be choosier than males in selecting mates, sexual selection theory predicts that males should also be choosy when female fecundity varies. Reproductive success among female spotted hyenas varies greatly with social rank. Our goals were therefore to determine whether male hyenas preferentially associate with high-ranking females, and whether male preferences are affected by female reproductive state. Interactions between adult males and females were observed intensively, and association indices calculated for all male-female pairs, over a 7-year period in one population of free-living hyenas. Males initiated most affiliative interactions with females, and males associated most closely with females that were likeliest to be fertile. High- and middle-ranking males associated most closely with high-ranking females, but low-ranking males associated equally closely with females in all rank categories. We used molecular markers to determine the paternity of cubs born during the study period, and found that sires associated more closely with the mothers of those cubs than did non-sires, particularly during the last months before conception. These association data indicate that male spotted hyenas do indeed exhibit selective mate choice, and that they prefer females likeliest to maximize male reproductive success.     

Pheromone-based female mate choice and its effect on reproductive investment in a spitting spider

Numerous studies have focused on whether organisms can signal or perceive pheromones and use chemical signals in species and mate recognition. Recently, there have been an increasing number of studies investigating whether pheromones are used in mate choice. Yet, little attention has been paid in exploring the effects of pheromone-based mate choice on reproductive investment. We first tested this hypothesis by providing virgin Scytodes sp. females with a choice between two virgin males in the presence of chemical signals alone and found strong evidence of an odor-based mate preference. We then examined the consequences of the odor-based mate choice by allowing female Scytodes sp. that had previously made an odor-only mate choice to mate with preferred and non-preferred males, respectively. We measured the success of copulation, mortality of male, pre-oviposition interval, egg-sac weight, egg weight, fecundity, fertility, embryonic period, and size of offspring at hatching. Females that mated with the preferred males produced significantly heavier egg sacs that contained more and larger eggs with a greater fertility. Significantly more non-preferred males than preferred males were killed by spitting. However, pre-oviposition interval, embryonic period, and hatchling size were not affected by female mate choice. This study is the first to demonstrate that female spiders are able to regulate their highly valuable reproductive investment based solely on chemical signals.     

Variance in female quality, operational sex ratio and male mate choice in a bushcricket

Male bushcrickets, Kawanaphila nartee, exercise mate choice when nutrients are limited. Male mate choice is associated with a female-biased operational sex ratio (OSR) that arises from an increased relative paternal investment under nutrient limitation. However, increased male choosiness could be attributable to the fact that females vary more in fecundity, and consequently in mate quality, when nutrient limited. Our objective was to experimentally partition the influences of OSR (male or female bias) and variance in mate quality (high or low) and to assess their relative influence on the intensity of mate choice by male bushcrickets. Female quality was manipulated by controlled feeding regimes that directly affected female fecundity. We found that males and females engaged in sexual interactions sooner under a male-biased than a female-biased OSR. Males were more likely to reject females on their first encounter when variance in female quality was high. However, the effect of quality variance on the total number of rejections during a 4-h observation period was dependent on the perceived OSR. A male's prior experience of variance in female quality did not influence male choosiness. Our observed rates of mate rejection conformed well with those predicted from recent theoretical models of sexual differences in choosiness. In conclusion, our results show that the opportunity for selection via male mate choice is influenced by an interaction between OSR and the variance in mate quality that arises within nutrient-limited populations of females. Received: 5 January 1998 / Accepted after revision: 25 October 1998     

Measuring mate choice using correlation: the effect of female sampling behaviour

For intersexual selection to occur, it is necessary that females choose between males. It is now well appreciated that constraints exist, which preclude females sampling all the available males in a population. These constraints are likely to have caused the evolution of sampling rules (such as the “best-of-n” rule) by which females sample males. Here we investigate the impact of female subsampling of the male population, not on the evolution of sampling behaviour, but on the population-level correlation between a male trait and currencies such as reproductive success. This study is important as it illustrates when population-level correlations can be safely used to infer the presence and strength of sexual selection in the field. We find that the correlation between a male trait and a mate choice variable rises steeply as the number of males sampled by each female increases, flattening above seven to ten males sampled. This shape is found to be remarkably robust, and little affected by, for example, the mate choice variable used, by noise in assessment, by sampling behaviour depending on female quality, or by population size. The only variable found to have a large impact is male clumping according to their “quality”. If females are sampling about four males, the maximum correlation that can be found at the population level is in the range 0.4–0.6, perhaps as little as 0.1 if males are strongly clumped. A recent review of the literature suggests that four is the average number of males that females sample. Thus, the absence of a strong correlation cannot by itself be used to infer that sexual selection is weak, as it may be due to females sampling few males. Received: 18 May 1998 / Accepted after revision: 18 July 1998     

Sequential female choice and the previous male effect in sticklebacks

Summary Female choice, identified as a major force in sexual selection theory, has recently been demonstrated in a number of species. These tests concentrated on simultaneous choice situations although females have to compare males sequentially in most territorial species, which is the more demanding task. Here it is shown that female three-spined sticklebacks, Gasterosteus aculeatus L., rate sequentially presented males according to their brightness. With increasing costs of sampling the females become less choosy. Furthermore, a male's attractiveness has a significant effect on the female's rating of the next male; a given male is rated higher when preceded by a duller male than by a brighter one and vice versa. Female sticklebacks use a stochastic decision rule in sequential mate choice that is attuned to the attractiveness of the present and previously encountered male. This demonstration of a previous male effect not only indicates an efficient mechanism for finding the best of a number of males but also extends the applicability of sexual selection theory.Offprint requests to: T.C.M. Bakker     

Sperm competition risk affects male mate choice copying

Mate choice copying was mostly described as a strategy employed by females to assess the quality of potential mates, but also males can copy other males’ mate choice. An open question in this context is whether and how copying males evaluate sperm competition risk, as mating with a female that has already copulated with another male obviously sets the stage for intense sperm competition (i.e., in species with internal fertilization). Using the livebearing Atlantic molly (Poecilia mexicana) as a model, we asked (a) whether males of that species indeed copy other males’ choices, and if they do so, (b) whether copying males strategically adjust their behavior to sperm competition risk. We used an approach where focal males could first choose to associate with a large or a small stimulus female. Mate choice tests were then repeated after an “observation phase” during which either no model male was present (treatment 1, control) or the previously non-preferred female could be seen associating (treatment 2) or physically interacting (treatment 3) with a model male. We found that, after the observation phase, males spent considerably more time with the previously non-preferred female in treatment (2), i.e., they copied the model male’s choice. This effect was much weaker during treatment (3) where sexual interactions between the model male and the formerly non-preferred female were allowed. Males, therefore, seem to adjust their copying behavior strategically to the perceived risk of sperm competition.     

No evidence for female mate choice based on genetic similarity in the túngara frog Physalaemus pustulosus

In most sexually reproducing animals, the behavior of one or both sexes during courtship critically influences the success at mating of the opposite sex. This behavior is often interpreted as “mate choice,” and there is great interest in why such choices are exercised. The explanation for the evolution of mate choice that has received the most attention and generated the most controversy is based on assumed genetic effects. In this study, we investigated whether female túngara frogs, which choose mates based on acoustic cues, have a preference for genetically less related males. Specifically, we determine if there is disassortive mating based on microsatellite markers, if there is information in the advertisement call that could be used to assess genetic similarity, and if females exhibit acoustic-based mating preferences that would promote choice for genetic diversity. Using seven microsatellite markers, we found no correlation of male call similarity and male genetic relatedness. Female choice experiments showed no female preference for calls of less related males, and there was no evidence for inbreeding avoidance in the field. Our results do not support the hypothesis of mate choice based on information about genetic relatedness conveyed by acoustic signals in túngara frogs.     

Female mate choice in the zebra finch — the effect of male beak colour and male song

Studies of female mate preference in zebra finches (Taenopygia guttata) have shown that male beak colour and song rate are important. However, the two characters are correlated. Here the effect of beak colour and song rate on female choice are examined independently. In mate choice tests involving two males, beak colour was manipulated artificially using nail varnish. The results showed that females showed a significant preference for males with a high song rate, but not with a red beak. Females did not prefer males with a red beak if song rate was low and females preferred males with orange beaks who expressed a high song rate. Female preference for males with red beaks was not found when beak and song characters were no longer correlated.     

Effects of testosterone supplementation on chemical signals of male Iberian wall lizards: consequences for female mate choice

In some lizards, female mate choice is influenced by chemicals secreted by males, e.g., via the femoral glands. Secretions of the femoral glands are under direct androgenic control and vary seasonally with androgen production. However, whether increased testosterone (T) levels affect the concentration and chemical composition of secretions or their attractiveness to females is unknown. We manipulated T levels of male Iberian wall lizards, Podarcis hispanica, with silastic implants. Differential tongue-flick rates indicated that females detected and discriminated between femoral secretions of control and T-implanted males based on chemical cues alone. Females showed greater responses to secretions of T-males, which might suggest that T increased the concentration of chemical signals. Further analyses by gas chromatography–mass spectrometry indicated that T supplementation induced qualitative changes in lipid composition of femoral secretions. T-males had decreased relative proportions of cholesta-5,7-dien-3-ol in secretions, which might be related to the negative effects of T on the immune system and on lipid metabolism. However, experiments of choice of males’ scent showed that females neither preferred nor avoided the scent marks of T-males but preferred males that maintained higher proportions of cholesta-5,7-dien-3-ol in secretions, independently of the experimental manipulation. Thus, because this steroid is negatively affected by T, there might be trade-offs among increasing T levels to increase the production of chemical secretions, maintain metabolism, and attract females.     

Strategies of female mate choice: A theoretical analysis

Summary Females of many species face a difficult sampling problem while searching for a mate. How do they find the best male possible when the constraints of time, memory, and mobility prevent them from visiting all the males in the population?Five possible solutions are examined: random mating a fixed-threshold criterion, a fixed-threshold with last-chance option, an optimal one-step decision strategy, and the best-of-n-males strategy. Random mating is the worst strategy whenever the female gets more than one chance to mate. The two fixed-threshold strategies approach equal effectiveness as n increases but are always below the optimal one-step decision strategy. However, the best-of-n-males strategy always yields the highest expectation of fitness in a mate. The difference is especially great when n5.Plotting the average fitness of males chosen vs n, the number of males examined, yields a negatively accelerating curve. Since the cost of searching will be an increasing function of n, the two curves can be combined to yield an optimum n: the point where the difference between the curves is greatest.The one field study (Brown 1978) that addresses these problems in detail reveals that female mottled sculpins choose males on a relative, rather than absolute, basis, as theory suggests they should.     

Sexual voyeurs and copiers: social copying and the audience effect on male mate choice in the guppy

In most mating systems, males and females are commonly within signalling and receiving distance of conspecifics during courtship and mating activities. Although it is well known that females who observe sexual interactions between conspecifics will use public information obtained from these interactions when making their own mating decisions, much less is known about whether males use this type of information in making mating decisions. We used the Trinidadian guppy (Poecilia reticulata) to test whether males use public information to (i) copy the apparent mate choice of another male and (ii) modify their mating preference for a given female in the presence of one or two sexual rivals (potential copiers). We show that males use public information to copy the mate choice of other males and that males alter their mating preferences in response to the presence of an audience of sexual rivals, but find no evidence of a stronger audience effect when the number of sexual rivals increases. Collectively, these results indicate that males pay attention to their immediate social environment in making mating decisions and suggest that they avoid having another male copy their mate choice by weakening or even reversing their initial mating preference in the presence of eavesdropping male sexual competitors. Our findings highlight the importance of social context and public information in male mate-choice decisions and have implications for the evolution of male mating preferences and of social information use in populations.     

No relationship between long-distance acoustic mate attraction signals and male fertility or female preference in spring field crickets

To father offspring, a male must succeed at two processes of sexual selection: (1) mate with a female and (2) fertilize her eggs. We investigated the relationships between pre- and post-copulatory male traits and female mating responses in wild-captured and laboratory-reared spring field crickets, Gryllus veletis. The phenotype-linked fertility hypothesis suggests that females may receive a direct benefit, enhanced fertilization efficiency, by mating with males that signal attractively. We measured fine-scale components of male acoustic mate attraction signals as well as how much time males spent signalling, measured female preference for males in mating trials and then quantified sperm number and viability. We found no relationship between male signalling traits and male fertility or female preference, providing no evidence for the phenotype-linked fertility hypothesis. We also found no difference in sperm metrics between wild-captured and laboratory-reared males. While female crickets may receive benefits by choosing males based on acoustic signal characteristics, whether the benefits are a result of genetic quality, seminal fluid contents or some other male trait remains unknown.     

The effect of perceived female parasite load on post-copulatory male choice in a sex-role-reversed pipefish

The last several decades of research in behavioral ecology have resulted in a deeper appreciation of post-mating processes and sexual conflict in sexual selection. One of the most controversial aspects of sexual selection is cryptic mate choice. Here, we take advantage of male pregnancy in a sex-role-reversed pipefish (Syngnathus typhle) to quantify cryptic choice based on perceived parasite load and other sources of variance in female fitness. Studies have shown that S. typhle males preferentially mate with females with lower parasite loads and that a male’s perception of female parasite load can be altered by tattooing females. We manipulated the apparent parasite load of females in controlled mating experiments to test the hypothesis that post-copulatory sexual selection is dependent on a male’s perception of female parasite load in pipefish. Our results provided no evidence for cryptic male choice based on perceived female parasite load. However, we found evidence that eggs from larger females were more likely to result in viable offspring than eggs from smaller females and that the first female to mate with a male transferred more eggs per copulation on average. Overall, our results show that potential for post-copulatory sexual selection does exist in pipefish, but the male’s perception of female parasite load does not play a major role in this process.     

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.     

Determinants of male reproductive success in wild long-tailed macaques (Macaca fascicularis)—male monopolisation, female mate choice or post-copulatory mechanisms?

One of the basic principles of sexual selection is that male reproductive success should be skewed towards strong males in species with anisogamous sex. Studies on primate multi-male groups, however, suggest that other factors than male fighting ability might also affect male reproductive success. The proximate mechanisms leading to paternity in multi-male primate groups still remain largely unknown since in most primate studies mating rather than reproductive success is measured. Furthermore, little research focuses on a female’s fertile phase. The aim of this study was to investigate the relative importance of male monopolisation and female direct mate choice for paternity determination. We also investigated the extent to which paternity was decided post-copulatory, i.e. within the female reproductive tract. We used a combined approach of behavioural observations with faecal hormone and genetic analysis for assessment of female cycle stage and paternity, respectively. The study was carried out on a group of wild long-tailed macaques living around the Ketambe Research Station, Gunung Leuser National Park, Indonesia. Our results suggest that both male monopolisation and post-copulatory mechanisms are the main determinants of male reproductive success, whereas female direct mate choice and alternative male reproductive strategies appear to be of little importance in this respect. Female cooperation may, however, have facilitated male monopolisation. Since paternity was restricted to alpha and beta males even when females mated with several males during the fertile phase, it seems that not only male monopolisation but also post-copulatory mechanisms may operate in favour of high-ranking males in long-tailed macaques, thus reinforcing the reproductive skew in this species.     

Male moths undertake both pre- and in-copulation mate choice based on female age and weight

Variation in reproductive potential usually occurs among individuals of both sexes; for example, some individuals may carry more gametes or be able to continue to generate more gametes in their reproductive life than others. Therefore, to maximize their reproductive success, both sexes are expected to show adaptations for mate choice. However, most authors concentrated on how females choose their mates with the belief that females invest more in reproduction than males, and males are generally eager to pair with any female. In this paper, we report our work on a polygamous moth, Ephestia kuehniella Zeller (Lepidoptera: Pyralidae), with special emphasis on male pre- and in-copulation mate choice in relation to the reproductive quality and re-copulation potential of females. We show that in E. kuehniella, younger and heavier females have significantly higher reproductive value than older and lighter ones, and male sperm supply significantly decreases over successive copulations. Males exercise pre-copulation mate choice by selecting females with higher reproductive potential for copulation and in-copulation mate choice by allocating more sperm to females with higher reproductive potential. However, high-quality females are more likely to re-copulate than low-quality ones, and allocation of more sperm to females by males does not increase female reproductive outputs. It is suggested that the allocation of more sperm to high-quality females functions to increase sperm competitiveness against rivals.     

Mate sampling behaviour of female pied flycatchers: evidence for active mate choice

Summary This paper presents major new evidence for active mate choice of female pied flycatchers, Ficedula hypoleuca. Fifteen color-ringed females were released into a study area containing 23 unmated males defending one nestbox each. Through intensive surveillance, the behavior of the females was observed during 2 consecutive days. Twenty-two of the males received a total of 131 female visits. Six of the females settled in the study area, and their premating period lasted 1.3–2.5 days. The females were seen searching for mates for at least 6–32 h and were seen visiting at least 1–9 different males. Hence, some of the females rejected males before mating. Nevertheless, the females settled close to the site of release (range: 16–243 m), suggesting that they mated with one of the first males encountered. Females visited males most frequently in the morning, and the diurnal distribution of visits was significantly correlated with male song activity. Offprint requests to: S. Dale     

The effect of female arrivals on mate monopolization in the yellow dung fly

The degree of resource monopolization relates to the distribution of resources in space and time. In general, monopolization is predicted to be high when resources (food or mates) are clumped in space, dispersed in time, and predictable in space or time. Using the yellow dung fly, Scathophaga stercoraria (Diptera: Scathophagidae), we qualitatively tested a general model that predicts the distribution of mating success among competing males based on the temporal pattern of female arrivals relative to mating time and a ranking of males in priority of access to the resource (here by body size). In a laboratory experiment approximating the natural mating situation, a constant number of males of various sizes were allowed to compete for females. As predicted, mate monopolization decreased as the temporal clumping of female arrivals increased, mediated by either a decrease in the mean or an increase in the variance of female inter-arrival times, which were manipulated independently. Males appeared to adjust their behavior to variation in female arrivals in a manner consistent with the marginal value theory of Parker and associates: forcible take-overs of females were rarer, and copula durations shorter, when females arrived regularly at short intervals. Therefore, a complex interaction of variation in intrinsic characteristics affecting male resource holding potential, mating time and stochastic, extrinsic variables increasing temporal clumping of mates generally reduces the variance in mating success among competing males and thus ultimately the opportunity and intensity of sexual selection on traits influencing male success. This theory extends operational sex ratio theory at the mechanistic, behavioral level.