Females influence sperm storage and use in the yellow dung fly Scathophaga stercoraria (L.)

Summary The influence of the female on the process of sperm storage and use was examined. Copula duration, the condition of the female and whether or not a copula terminated naturally influenced the number of spermathecae (of three) in which once-mated females stored sperm. Females stored more sperm the larger their mate and the sperm from larger males were stored more unevenly amongst the spermathecae than were those from smaller males. Double-mated females had sperm in fewer spermathecae the larger the second of their mates and these spermathecae tended to be the ones which lay together within the female. The P2 values over three successive clutches were constant and sperm precedence was complete when the larger male was second to mate but began low and increased over subsequent clutches when the smaller male mated second. These results suggest females prefer, and are able, to use the sperm of larger males to fertilise their eggs. It is proposed that multiple spermathecae in Diptera have evolved to give females better control over offspring paternity.     

No male agonistic experience effect on pre-copulatory mate choice in female earwigs

Mating with dominant males may confer considerable benefits, but also incur significant costs, hence intrasexual competitiveness is a likely target of mate choice. In addition to established modes of mate assessment, females may use cues or signals associated with agonistic experience effects to assess the relative competiveness of males. Experience effects, where the outcome of a fight increases the likelihood of a similar outcome in subsequent fights, may result from an animal’s altered state after conflict, but can also arise from strategic rival use of information perceived about this altered state. While females may similarly use this information in mate choice decisions, this potential consequence of male–male conflict has largely been neglected. Here, we investigate the effects of experience on subsequent agonistic performance in the earwig Euborellia brunneri by imposing winning or losing experiences on males and rematching them with naïve, size-matched rivals. We reveal a strong loser effect in subsequent fights, with nearly all previous “losers” losing against new rivals. In contrast, we found no equivalent winner effect, with previous “winners” exhibiting no increased likelihood of winning. We subsequently test whether the effects of male agonistic experience extend to pre-copulatory female mate choice. We show that females, when allowed to choose between naïve males and “winners” or “losers”, do not discriminate between males based on their recent agonistic experience. Therefore, while fighting history can play an important role in male–male interactions, females may not attend to this information.     

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     

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.     

Competition for a mate restricts mate search of female pied flycatchers

Summary We studied the mate sampling behaviour of female pied flycatchers, Ficedula hypoleuca, in a 40-ha area containing 10–12 unmated males whose nestboxes were monitored with videocameras. The main results were: (1) The females undertook a restricted mate search. The females that mated in the area during three monitoring periods (n = 20, 12 females released by us and 8 females that arrived naturally) sampled 1–10 males (median 4.5). This was about 40% of the available mating options. (2) Search costs in terms of time and energy were low. The search period was short (median 5.1 h) and only a small proportion of the search period was spent at the nestboxes of males (median 4%). The females visited up to seven different males in 1 h, and the time elapsing between visits to different males was short (median 13 min). The minimum distance travelled during the search was also short (median 1.4 km). (3) There was competition between the females. We recorded seven cases of two females visiting the same male at the same time, including at least one case involving physical fighting. (4) Females that experienced a high level of competition had a more restricted mate search than females that experienced a low level of competition. (5) The search pattern of most of the females did not conform to the best-of-n-males rule nor to the threshold criterion rule, because they made repeated visits to many of the males sampled.     

Sperm competition or sperm selection: no evidence for female influence over paternity in yellow dung flies Scatophaga stercoraria

Recent studies of non-random paternity have suggested that sperm selection by females may influence male fertilization success. Here we argue that the problems originally encountered in partitioning variation in non-random mating between male competition and female choice are even more pertinent to interpreting patterns of non-random paternity because of intense sperm competition between males. We describe an experiment with the yellow dung fly, Scatophaga stercoraria, designed to partition variance in the proportion of offspring sired by the second male, P ₂, between males and females, and to control for sperm competition. Large males were shown to have a higher P ₂ than small males but P ₂ was independent of the size of the female’s first mate. This result might suggest an absolute female preference for large males via sperm selection. However, large males have a higher constant rate of sperm transfer and displacement. After controlling for this effect of sperm competition, large males did not achieve higher paternity than small males. We argue that a knowledge of the mechanism of sperm competition is essential so that male effects can be controlled before conclusions are made regarding the influence of sperm selection by females in generating non-random paternity. Received: 4 April 1995 / Accepted after revision: 17 October 1995     

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     

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     

An advantage to mate choice in the seaweed fly,Coelopa frigida

Summary All natural populations studied of the seaweed fly, Coelopa frigida, are polymorphic for the inversion system on chromosome I. It has previously been shown that heterokaryotypes have superior egg-to-adult survival than either homokaryotype, and that there is non-random mating with respect to inversion genotypes. We have inquired whether the production of fitter progeny in larger numbers is the possible reason for the existence of non-random mating behaviour. We measured the fertility, fecundity and progeny survival of adults that have either been allowed to exercise mate preference, or been given a single, randomly chosen, mate. The fertility (% of females producing progeny) and progeny survival (% of larvae surviving to the late 3rd instar stage) are both significantly greater in choice than nochoice females. There are also significant differences in fertility and progeny survival depending on whether the parents were of similar or dissimilar inversion genotypes. An effect of relative adult size on fertility was also observed. There appear to be no effects of mate choice on fecundity. Indirect evidence is presented that adults are choosing their mates not only on the basis of genotypes but on other criteria as well. It is suggested that the superior reproductive performance of animals allowed a choice of mates may account for the evolutionary origin and continued existence of mate discrimination. Furthermore, since over 25% of the genome of Coelopa is located within inversions, the observed pattern of negative assortative mating may maintain a substantial fraction of the genes in a polymorphic state.     

Copula duration and testes size in the yellow dung fly,Scathophaga stercoraria (L.) : the effects of diet,body size,and mating history

Summary Seasonal variation in mean hind tibia length and mean testes length is investigated in the yellow dung fly, Scathophaga stercoraria (L.). There is a cycle in mean hind tibia length and mean testes length over a season. The body size curve peaks later than the testes length curve, showing that there is no fixed relationship between the two variables. The causes of variation in testes size and its influence on copula duration are experimentally examined. Increasing the number of Drosophila eaten per day leads to increased mature testes length. Males with larger testes copulate for longer than males with smaller ones, and smaller males copulate for longer than do larger males. While testes shrivel with successive copulas, copula duration remains constant. The more females a male is prepared to copulate with in a day (up to five), the longer he copulates with each. The shrinkage of the testes of males collected throughout a day suggests that males copulate with an average of 4 females per day. The costs of sperm production are thus shown to have a significant influence on the copula duration.Offprint requests to: P.I. Ward at the second address     

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.     

An estimate of female mate searching costs in the lekking butterfly Coenonympha pamphilus

We estimated the cost to females of the lekking butterfly Coenonympha pamphilus of visiting males on leks instead of taking off and soliciting courtship from males passing them outside leks, as occurs in the non-lekking congener C. tullia. We followed released virgin females of C. pamphilus in the field until they mated. We estimated the time they lost by remaining perched and not approaching males that passed them, and observed how often and at what distance virgin females were passed by males in the field. We then calculated how much faster these females would have been detected if they had taken off and approached these males, using probabilities of detection as a function of passing distance derived from field observations on C. tullia females. C. pamphilus females on average lost 201 min by not approaching males on their way to leks. To estimate what this time loss meant in terms of fitness, we measured how the age at mating affected fecundity. Using field measures of fecundity and mortality, the time loss translated into an average 2.8% reduction in fecundity as a best estimate, and an average 1.3% reduction in fecundity as a lower estimate. This fitness cost is larger than has been reported earlier for a lekking bird, but is probably too small to eliminate the possibility of indirect benefits of mating with males on leks. Received: 15 August 1996 / Accepted after revision: 24 January 1997     

The role of sexual imprinting and the Westermarck effect in mate choice in humans

Positive sexual imprinting is a process by which individuals use the phenotype of their opposite-sex parent as a template for choosing mates and is suggested to play an important role in human mate choice. In contrast, negative imprinting, or ??The Westermarck Effect??, is characterized by individuals developing a strong sexual aversion to others with whom they lived closely in infancy and early childhood. In this review, we evaluate the literature on their effects on mate choice in humans. We find little evidence to support positive imprinting in humans because the studies either have serious design flaws, do not exclude effects of heritable mating preferences, or do not account for several possible alternative explanations. Instead, it seems that the opposite phenomenon, negative sexual imprinting, has some support from natural experiments which have found that individuals avoid mating with those with whom they lived closely in infancy and early childhood. However, it seems that early association does not produce a strong-enough aversion to completely annihilate sexual desire, probably because the mind uses multiple kinship cues to regulate inbreeding avoidance. Thus, it appears that the evidence for both types of imprinting is fairly weak in humans. Thus, more studies are needed to test the role of sexual imprinting on mate choice in humans, especially those measuring interactions between positive and negative imprinting.     

The effect of chorus organization on mate localization in the Painted Reed frog (Hyperolius marmoratus)

Summary Experiments with female Painted Reed frog (Hyperolius marmoratus) indicate that temporal separation of the calls of neighbouring males does not enhance their locatability. The ability of females to locate a sound source is not impaired when identical calls are broadcast simultaneously through two spatially separated loudspeakers (n=8). We suggest that call alternation may be important in the spatial distribution of males rather than in the ability of females to locate mates.     

The effect of postcopulatory male behaviour on ejaculate distribution within the female sperm storage organs of the fly,Dryomyza anilis (Diptera : Dryomyzidae)

Summary After sperm transfer, male Dryomyza anilis increase their fertilization success by tapping the female external genitalia with their claspers. In order to clarify the mechanism, we examined the effect of tapping movements on the distribution and fate of the male ejaculate within the female's sperm storage organs. The structure of the female internal genitalia is described and the volume of sperm found within the female is estimated. Experiments show that males place their sperm near the exit of the female's bursa copulatrix: most of the last male's sperm are expelled before oviposition and only 10–30% remain in the female. The results suggest that the mechanism by which male D. anilis gain last male sperm precedence is complex, and two possibilities are suggested. Offprint requests to: M. Otronen     

The influence of potential reproductive rate and variation in mate quality on male and female choosiness in the sand goby, Pomatoschistus minutus

Recent models of choosiness in mate choice have identified two particularly important factors: the potential reproductive rate (PRR) of the choosing sex relative to that of the chosen sex, and the variation in quality of potential mates. This experimental study tested how these factors affected choosiness in male and female sand gobies, Pomatoschistus minutus. We manipulated relative PRR by means of water temperature, and mate quality by means of body length. The choosing male or female was offered a choice between two mates with either a small or a large difference in body length representing a small or a large variation in mate quality. Choosiness was measured as (1) preference for the larger mate, and (2) as whether or not spawning occurred with the smaller mate, while the larger mate was visible but screened off. We found that females preferred large males, and that their level of choosiness was affected by variation in male quality, but not by their own relative PRR. Males, on the other hand, seemed unselective in all treatments and were in general more likely than females to spawn with their provided partner. This suggests that in the sand goby, variation in male mate quality has a greater influence than relative PRR on facultative changes in female choosiness. However, a general difference in PRR between males and females may be one important factor explaining the observed sex difference in choosiness. Received: 17 April 2000 / Revised: 24 June 2000 / Accepted: 17 July 2000