Operational sex ratios and sperm limitation in populations of Drosophila pachea

Males of the cactophilic fruitfly, Drosophila pachea, produce relatively few but very large sperm, and partition their limited gamete numbers among successive mates. The present study found that males take 10 days longer than females, post-eclosion, to become sexually mature. The pattern of testes development suggests that the need to produce testes long enough to manufacture the giant sperm is the cause of the delayed male maturity. These findings generate the prediction that the operational sex ratio (OSR) of populations will be female-biased. The size, sex ratio, and OSR of natural populations were examined. In general, local populations tended to be small and sex ratios tended to be slightly male-biased. However, as predicted, the OSR of populations, at least in one season, tended to be female-biased, with an average of 2.3 receptive females for each sexually active male. Results of laboratory experiments to determine the relationship between female remating frequency and fitness, and between population OSR and productivity, suggest that natural populations with female-biased OSRs are sperm-limited. The origin and maintenance of sperm gigantism and the unusual sperm-partitioning behavior of males are discussed with respect to population structure.     

Female encounter rate and the calling behavior and mating propensity of male Requena verticalis (Orthoptera: Tettigoniidae)

Mating effort, the energy exerted in finding and persuading a member of the opposite sex to mate, may be influenced by how frequently potential mates are encountered. Specifically, males that frequently encounter females may reduce calling effort and be less eager to mate than males that infrequently encounter females. An experiment was set up to test this hypothesis, using the tettigoniid Requena verticalis. We examined the song structure, calling activity and mating propensity of individual males exposed to one of five different encounter rates with virgin females. Song structure and calling effort were significantly altered by an encounter with a female. After an encounter, males significantly increased chirp rate and decreased variability in interchirp interval. Encounters also stimulated a male to call and to continue to call for up to two hours. The elapsed time since mating affected mating propensity but not calling activity. Mating propensity asymptotically increased to reach a maximum by day 17 since last mating. However, neither the frequency of encounters, nor the number of previous encounters experienced by a male, influenced calling activity or the propensity of a male to mate. The significance of changes in song structure and calling activity following an encounter, and of increasing male mating propensity over time, are discussed. Correspondence to: G.R. Allen     

Operational sex ratios and behavioural sex differences in a pipefish population

In the pipefish Syngnathus typhle, only males brood embryos in specially developed brood pouches, supplying oxygen and nutrients. Laboratory studies have shown that this elaborate paternal care has led to sex-role reversal in this species: males limit female reproductive rate, females are the primary competitors for mates and males exercise greater selectivity in accepting mates. In the first field study of this pipefish, we describe mating behaviour in the wild and test the hypothesis that temporal variations in the operational sex ratio (OSR) determine sex differences in mating behaviour. Our study comprised two reproductive seasons of two sequential mating periods each, the latter separated by a lengthy interval of male brooding. During mating periods, females displayed to all males without wandering and males moved about searching for females, without reacting to all females. The OSR was least female-biased (or even male-biased) at the onset of the breeding season, when most pipefish were simultaneously available to mate, but became strikingly female-biased as males' pouches were filled. The OSR remained substantially female-biased during the second mating period, because few males became available to remate at any one time. As hypothesised, female-biased OSRs resulted in more female-female meetings. As well, females were above the eelgrass more often than brooding males, thus exposing themselves to conspecifics and/ or predators. In the second year, males arrived earlier than females on the breeding site and male pregnancies were shorter, because of higher water temperatures, so rematings occurred earlier. Males met more often during that year than the previous one, but male competitive interactions were still not observed. The field results support laboratory studies and demonstrate that behaviours associated with female-female competition are more prominent when the OSR is more female-biased. Correspondence to: A. Vincent     

Sex ratios and intrasexual competition for mates in a sex-role reversed shorebird,Wilson's phalarope (Phalaropus tricolor)

Summary Operational sex ratio (OSR) was proposed by Emlen and Oring (1977) as an empirical measure of the intensity of sexual selection. Few studies, however, have examined the link between OSR and levels of intrasexual competition, which may influence selection. We studied the seasonal relationship between OSR and female-female competition for mates in Wilson's phalarope (Phalaropus tricolor), a sex-role reversed, non-territorial shorebird. Positive correlations between four measures of OSR (Scan Ratio, Day Ratio, Focal Ratio, and Chase Ratio) indicated seasonal changes in the availability of mates for females. Changes in mate availability resulted from the interaction between paternal care and female emancipation, asynchronous spring arrival schedules of the sexes (Reynolds et al. 1986), and the effect of clutch failure on renesting opportunities. Measures of intrasexual competition (courtship chases, percent males defended, rate and intensity of mate defense, and female-male proximity) varied significantly within and among years. Univariate and multivariate correlations indicated general agreement between measures of OSR and estimates of intrasexual competition. Our results suggest that OSR may provide a useful estimate of the opportunity for sexual selection, especially in species with matedefense mating systems.     

Female preferences for male calling bout duration in a field cricket

Summary The calls of male field crickets attract sexually receptive females. In Gryllus integer, males differ from one another in their durations of uninterrupted calling (calling bout lengths). Tape recordings of the calls of 50 wild-caught males revealed that 14 males spent most of their calling time in short bouts (Fig. 1A), 18 in both short and long bouts (Fig. 1B), and 18 in long bouts (Fig. 1C). Re-recordings of 32 males after 3 weeks showed that calling bout lengths of individual males are stable with time (age) (Fig. 2). Three phonotaxis experiments investigated whether calling bout lengths of males affect female preferences. They demonstrated that (1) females can discriminate among conspecific males on the basis of calls alone; (2) females are preferentially attracted to males with long calling bout lengths; and (3) calling bout length is the specific factor responsible for preferential attraction. These results precisely identify a criterion that females use to discriminate among potential mates of their own species.     

Evidence for seasonal mate limitation in populations of an intertidal amphipod, Corophium volutator (Pallas)

Potential rates of reproduction (PRR) differ between the sexes of many animal species. Adult sex ratios together with PRR are expected to determine the operational sex ratio (OSR) defined as the ratio of fertilizable females to sexually active males at any given time. OSR is expected to determine the degree to which one sex competes for another—the limiting sex. We explored the potential for mate limitation in an intertidal amphipod, Corophium volutator (Pallas). Males have higher PRR than females, but males may be limiting because of extreme female-biased sex ratios observed in this species. Consistent with this idea, late season females were less likely to be ovigerous and had smaller size-specific clutches, both of which were associated with seasonal declines in availability of males of reproductive size. Seasonal changes in ovigery could not be explained by seasonal changes across sites in other factors (e.g., female body size or phenology of breeding). Smaller females were less likely to become ovigerous later in the season at three of four sites. Seasonal reductions in clutch size also occurred among small females expected to be reproducing for their first time. In complimentary laboratory experiments, reduced likelihood of ovigery and reduced fecundity occurred when the number of receptive females was increased relative to availability of a reproductively active male. Our results suggest male mate limitation can occur seasonally in this species and that male limitation is regionally widespread and may affect recruitment.     

Role-reversal in katydids: Habitat influences reproductive behaviour (Orthoptera: Tettigoniidae,Metaballus sp.)

Summary Mating behaviour of the katydid Metaballus sp. varies. At sites QS and DB (in 1982) females competed for access to calling males and males chose mates by mating with heavier, more fecund females. At another site (BR) there was no evidence of role-reversal in reproductive behaviour, and males were observed to compete for mates. This species has a large spermatophore, a product of male reproductive glands, that is eaten by the female after mating. Males at the DB site had small reproductive glands. This suggests that some aspect of the QS and DB environments decreases spermatophore production; spermatophores become a limiting resource for females resulting in the reversal in reproductive roles observed at these sites. A field experiment that involved moving individuals from site BR to QS in 1983 determined that mating system was influerced by site (Table 1). At BR, males produced a continuous calling song, a third of the males observed attracted mates, and called for about 30 min before the female arrived; courtship duration was short. Males that were moved from BR to QS encountered a higher density of receptive females as all males attracted females after an average of just 3 min of calling. They changed their behaviour by producing short periodic bursts of song (zipping), and by courting females for long periods of time. The long courtship period may function as as a mate-assessment period for males. The reproductive behaviour of BR males moved to QS differed from that of native QS males only in the length of time spent in copula.     

Sex differences in the foraging behavior of squirrel monkeys in a seasonal habitat

Summary The effects of sex and seasonal changes in food abundance on foraging behavior was studied in squirrel monkeys (Saimiri oerstedi) in Costa Rica over an eleven-month period. Females searched for and ate food at significantly greater frequencies than did males throughout the study. The frequency of the specific foraging techniques used occasionally differed significantly within seasons, but not across the study period. Few differences were found in the foraging behaviors of nonreproductive sexually mature females compared to females that were pregnant or lactating. The major exception was that during the month following parturition reproductive females foraged for flowers and fruits more frequently than did non-reproductive females. The reduction of time spent by males in foraging activities gives them more time for other activities, especially anti-predator vigilance. Foraging techniques and the proportions of different food types in the diet changed seasonally. Foraging for arthropods was most frequent in the season when arthropod abundance was lowest, resulting in the amount of time spent eating arthropods to vary less across the seasons. Fruits and flowers were not eaten in a direct relationship to availability, but were used more than expected relative to availability when arthropod abundance was reduced. Individuals were more dispersed when foraging compared to other activities. Overall, there was little evidence of any direct foraging benefits for a squirrel monkey from being social.     

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     

Independent effects of male and female density on sexual harassment, female fitness, and male competition for mates in the western mosquitofish Gambusia affinis

Operational sex ratio (the ratio of sexually active males to fertilizable females) has a major influence on male competition for mates and male–female interactions. The contributions of male and female density per se to mating system dynamics, however, are rarely examined, and the fitness consequences are often inferred rather than quantified. Male mosquitofish (Gambusia affinis) compete aggressively and frequently harass females for copulations, a behavior thought to reduce female fitness. Female fitness can also be reduced by increases in female density, which may affect food availability, cannibalism rates, and chemical interactions between females. I manipulated male and female densities of G. affinis to measure their effects on male–male aggression, male harassment toward females, and female fitness. I found that males chased rivals more often and attempted fewer copulations when female density decreased, but surprisingly male density had no significant effect on the frequency of these male behaviors. In contrast, males’ agonistic displays toward other males increased with male density, but display behavior was unaffected by female density. These results suggest that male and female density do not always contribute equally or at all to the patterns of behavior we observe. Female fitness declined as female density increased, the opposite pattern expected if male harassment is costly to females. This suggests that a strong, negative effect of female density overwhelmed any potential costs of male harassment. Sources of female density dependence and the consequences of changes in male and female density to patterns of male behavior are discussed.     

Dynamic female preference for multiple signals in <Emphasis Type="Italic">Rhinogobius brunneus</Emphasis>

Many species base their choice of mates on multiple signals which provide them with different kinds of information. Choosers may assess the signals together to evaluate the overall quality of potential mates, but individuals often pay attention to different signals in different contexts. In Rhinogobius brunneus, a fish displaying exclusive male parental care, females generally prefer males showing larger first dorsal fins (FDF) and more active courtship displays as mates. Females choosing a mate usually initially assess the FDF and later utilize courtship for the final decision. In our experiments, females with different hunger states used different signals when selecting mates. Females in both hunger states preferred males with larger FDF in the first stage. In the second stage, well-fed females showed highly repeatable choice, whereas poorly fed females responded only to variation in the courtship activity of males. The males preferred by poorly fed females exhibited significantly higher offspring survival than nonpreferred males. Under conditions of food shortage, males allocate more energy to future reproduction at the expense of the present brood, and females may prioritize signals predictive of offspring survivorship over signals reflecting other aspects in male quality to minimize the losses in direct benefits. We conclude that R. brunneus females may employ information from both signals but dynamically adjust their prioritization of each signal to current conditions to ensure the choice that is currently most adaptive.     

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     

Resource quality or harem size: what influences male tenure at refuge sites in tree weta (Orthoptera: Anostostomatidae)?

Rather than seeking females directly, males in many animal species locate and defend sites that contain spatially limited resources essential for female survival and reproduction. In these cases, resident males successfully repelling conspecific rivals will mate with sexually receptive females that seek to use the resident’s resources. Theory predicts that if resources are limiting in nature, are costly to procure, and if residency at the resource site increases male reproductive success, each site should be monopolised by a single adult male. Moreover, if females aggregate at these sites, males should be sedentary and monopolise harems. This predicts that males should reside at resource sites longer than females and male tenure should be positively correlated with harem size. I address these hypotheses using a wild population of the Wellington tree weta, Hemideina crassidens, a sexually dimorphic insect in which males use mandibular weapons in fights to control resource sites (galleries in trees) required by females for diurnal refuge. In a longitudinal study using artificial galleries, I show that male tenure in a gallery is positively related to harem size and, contrary to prediction, not gallery size per se. Moreover, contrary to prediction, females are more sedentary than males: males appear to move frequently between galleries to search for novel mates. I show experimentally that galleries represent mating sites to males: males prefer galleries housing adult females whereas females prefer unoccupied galleries. Females likely avoid male-occupied galleries because they incur injuries when interacting with males.     

Intersexual food transfer among orangutans: do females test males for coercive tendency?

Tolerated transfer of food among adults is rare among primates, except in humans. Here, we present data on a consistent pattern of tolerated intersexual transfer of food (held in hand, foot, or mouth by the owner) among adult orangutans, in two different natural populations (Pongo abelii and Pongo pygmaeus wurmbii), based on ca. 9,000 h of focal observation per site. Although rare, intersexual food transfers were disproportionately from males to sexually active females and involved food that was equally available to both sexes. There was no evidence for direct trading of food for social favors (mating, grooming, or agonistic support) or for sharing under pressure of harassment. However, females frequently protested with loud screams when males, especially unflanged ones, attempted to take food they possessed, and also when males responded aggressively to their taking attempt. Since associations ended sooner when the female emitted noisy calls, a male who did not allow a female to take food from him risked losing the association. These findings support the hypothesis that by taking food, a sexually active female may test the male's tendency toward violence. Thus, intersexual food taking in orangutans is based on female leverage, resulting in a species-wide female entitlement to male “generosity”. The inhibition of food defense required for this kind of transaction may also form the basis for sharing patterns among species in which nutritional benefits have become important, such as chimpanzees and perhaps human foragers.     

Reproductive rates,operational sex ratios and mate choice in St. Peter’s fish

Operational sex ratio (OSR) theory predicts that sexual differences in potential reproductive rates (PRRs) create biases in the OSR and thus determine the relative strength of sexual selection (competition and choice) operating on each sex. Although this theory is well accepted, empirical studies that quantify it are still lacking. This paper presents such a study. I measured the natural OSR of Galilee St. Peter’s fish (Sarotherodon galilaeus) in the field (Lake Kinneret) and examined the direction of mate choice in the laboratory. The OSR in Lake Kinneret was male biased. Both a male-biased sex ratio and higher male reproductive rates (twice as fast as females) contributed to the skew in the OSR, but the sexual differences in PRR were shown to be the main factor causing variation in the OSR. Females, the sex with the lower PRR, were more selective for mates. The faster male reproductive rate may explain why females are more selective for mates despite varying less in quality. Received: 29 May 1995/Accepted after revision: 13 April 1996     

Mating and its effect on acoustic signalling behaviour in a primitive orthopteran,Cyphoderris strepitans (Haglidae): the cost of feeding females

Summary Males of the primitive orthopteran, Cyphoderris strepitans, provide their mates with two types of nuptial food gift during mating: 1) females feed while coupled on the fleshy metathoracic wings of the male and the resultant flow of haemolymph and 2) the spermatophore transferred by the male includes a gelatinous spermatophylax which the female eats after mating. During the peak breeding interval, virgin males secure significantly more matings than their numbers relative to non-virgin males would predict. We tested the hypothesis that non-virgin males, having lost a substantial portion of available energy through previous investment in females, call significantly less than virgin males. Reduced calling should result in the attraction of fewer females and/or a greater risk of intrusion from competing males and consequently, a lowered mating success. Calling activity of male C. strepitans of varying mating status was monitored with a sound activated relay apparatus for two consecutive nights following their capture. Males were of three experimental groups, virgin, freshly wounded males (mated on the same night of capture), and old-wound males (mated at least one night prior to capture). Our cata showed a significant short-term reduction in signalling activity as a consequence of mating. Whereas there was no difference in the time spent calling by virgins on the first and second nights following capture, freshly wounded and oldwound males called significantly less on the first night than on the second. Furthermore, the proportion of males calling on both nights was significantly greater for the virgin and old-wound groups than for freshly wounded males. When the duration over which males called on the first night was compared with that of the second, virgin males showed no difference whereas old-wound males called for significantly shorter intervals on the first night. These results indicate that the cost of a large nutrient investment in females lowers the energy level in males below the threshold required for a prolonged signalling period. After a refractory interval during which males feed and replenish their energy reserves, calling levels equivalent to those which occur prior to mating are regained. Active female choice may also contribute to the reduced mating success of non-virgins, but this possibility remains untested.     

The costs of male parental care and its evolution in a neotropical frog

Summary Parental care is practiced exclusively by males of the Puerto Rican frog, Eleutherodactylus coqui. Males brood clutches of direct-developing eggs in non-aquatic nest sites and defend eggs against cannibalistic nest intruders. Here, I report on energetic and mating costs incurred by males that provide parental care, and suggest how these proximate costs affect male fitness and the evolution of male parental care in this species. Energetic costs are small for brooding males in comparison to non-brooding, calling males. Brooding males had a higher frequency of empty stomachs and lost small, but significant, fractions of their initial body mass during parental care. Abdominal fat bodies of brooding males during the middle third of parental care were significantly smaller than those of calling males; those of males brooding eggs in earlier or later stages were not different. The mating cost of parental care is greater. Most brooding males cease calling during parental care. However, gravid females are available (i.e., known to mate) on most nights during the principal breeding season; hence non-calling males miss potential opportunities to mate. A mating cost was estimated by calculating nightly mating probabilities for calling males in a plot where nightly calling male densities and daily oviposition schedules were known. On average, a male exhibiting normal calling behavior would be expected to obtain a new mate once every 35.7 days. Hence a brooding male that ceased calling for a 20-day parental care period would miss, on average, 0.56 additional mates. Males that were more successful than average in attracting mates could miss up to 1.63 matings. A marginal value model (Fig. 1) is used to analyze the net effect on male fitness of parental care benefits and costs in E. coqui (Fig. 3). The model indicates that males garner the highest reproductive success by providing care from oviposition through hatching. There is no stage during the pre-hatching period at which a desertion strategy would yield higher reproductive success. In fact, the model suggests that males should provide full parental care even in the face of much higher mating costs than currently obtain in the system.     

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     

Mate choice,fecundity and sexual dimorphism in two pipefish species (Syngnathidae)

Summary In order to understand the causes of sexual dimorphism, mate choice and size-related fecundity were studied in two pipefish species, Syngnathus typhle and Nerophis ophidion. Sexual dimorphism is more pronounced in N. ophidion; females are larger, have sexual colourings, and are more active during courtship. In S. typhle the sexes are alike in all these respects. Males brood their offspring in both species. In N. ophidion fecundity was positively correlated with both body size and the amount of sexual colouring in females. In males no correlation between body size and fecundity, or between body size and embryo size existed. Predictably, in mate choice experiments with equal-sized females, males chose females with more extensive sexual colourings. We explain sexual dimorphism in this species as a consequence of both natural selection (fecundity increases with size in females but not in males) and sexual selection (males prefer larger females). We argue that sexual size dimorphism did not evolve by selection minimizing overlap in food niches between the sexes, because food production is high in the Zostera beds where the fishes live, and no size dimorphism was found in the sympatrically occurring S. typhle. Furthermore, in N. ophidion dimorphism is not greater in a particular mouth character than in overall body size. In S. typhle egg size and the average number of eggs transferred per spawning were positively correlated with female body size. Apparently more energy per offspring was provided by larger males than by smaller males, and larger males also carried more offspring. As predicted, large mates were preferred by both sexes in mate choice experiments. This is explicable in terms of both natural selection (fecundity increases with size in both sexes) and sexual selection (both sexes prefer large mates). As a consequence of selection acting in the same direction in both sexes, sexual dimorphism is absent in S. typhle.     

Song rates of dark-eyed juncos do not increase when females are fertile

Frequency of singing by birds may vary with reproductive stage in ways that reflect variation in the functions of song in intersexual and intrasexual communication. In dark-eyed juncos (Junco hyemalis) high-amplitude song is produced only by males. To investigate the function of this song, we tested whether fertility of females affected singing by their mates or by neighboring males. Using focal observations, song censuses, and radiotracking data, we determined whether song production varied between and among periods when females were fertile and non-fertile. Our findings show that males do not increase song production when their mates are fertile, nor do they increase song production when neighboring females are fertile. These results suggest that male juncos do not signal their intent to defend territories (or mates) more when females are fertile and that they do not use song to advertise to specific potential participants in extra-pair fertilizations. Received: 13 February 1997 / Accepted after revison: 2 May 1997