Reconnaissance for future breeding sites by spotted sandpipers

We studied the sex-role-reversed, polyandrous spotted sandpiper(Actitis macularia) from 1974 to 1990 in northern Minnesota,USA. After peak arrival of breeding birds and before peak departureat the end of the breeding season, there were many short-termvisitors (transients) to the study site. Stepwise discriminantfunction analysis (DFA) was used to determine the importanceof absolute sex ratio (males/female), sex of the transient bird,number of nests, and number of breeding males and females duringthe week of visit in predicting whether a visiting bird wouldreturn the following year. In addition, multiple regressionwas used to determine how much variability in the number oftransient birds returning in subsequent yearscould be explainedby annual values during the year of transience for numbers ofbreeding males and females, numbers of eggs laid and hatched,and absolute sex ratio. Annual recruitment of foreign adultsranged from 1 to 20 birds, of which 0–56% were seen visitingin previous years. Female recruits were more likely than malesto have been observed previously as transients. Twenty-two chickshatched at our study site returned and bred for the first timemore than 1 year after hatching. Of these, 9 (41%) were seenas transients between the year of hatch and breeding. The DFAshowed that transient females returned more often than transientmales and that the number of transients returning in subsequentyears was positively associated with the absolute sex ratioduring the week visited. When the sexes were analyzed separately,none of the weekly variables significantly discriminated femalereturn, but sex ratio was positively associated with male return.Regression showed that the number of transient birds returningin subsequent years was positively associated with the numberof male breeders at our study site during the year a bird visited.Percentage return the year after transience was positively associatedwith the number of eggs laid at our study site during the yeara bird visited. When sexes were analyzed separately, the higherthe number of female breeders during the year a bird visited,the greater the number of males returning in subsequent years,and greater numbers of breeding males were positively associatedwith transient female return. Based on our results, we suggestthat transient birds were searching for better breeding areasfor future breeding and that intrasexual competition made thisinformation more important to females than to males.     

Sexual cannibalism and sperm competition in the golden orb-web spider Nephila plumipes (Araneoidea): female and male perspectives

Mating systems are frequently shaped by conflicts over reproductive interests between males and females. Sexual cannibalism canbe an especially dramatic manifestation of such conflicts.However, the resolutions of this conflict differ among sexuallycannibalistic spider species. Cannibalism may be in the interestof both sexes when females consume males as a foraging decisionto improve fecundity and/or males sacrifice their bodies toincrease fertilization success. In other species, females exertsequential choice of partner by selectively terminating copulationthrough cannibalism while males fail to obtain a paternityadvantage. Here, we investigate the adaptive value of cannibalismin the orb-web spider Nephila plumipes where 60% of males donot survive copulation. Virgin females in poor condition aremore frequently cannibalistic and more likely to kill largemales, but the frequency of cannibalism among mated femalesis not influenced by these factors. Instead, males that matewith mated females increase their fertilization success bybeing cannibalized. Cannibalized males generally mate for longer,but longer copulations correspond with increased paternity onlyin mated females. The amount of sperm from particular malesthat a female stored was not influenced by any of the measuredvariables. The number of sperm stored was not related to paternity,nor was there any detectable reduction in sperm number afterfemales had reproduced. Our data suggest that the conflict between the sexes differs between virgin and mated females.Females should always cannibalize a male, but males only gainfrom cannibalism when mating with mated females, not when matingwith virgin females. Interestingly, the frequencies of cannibalismare not different in matings with virgin or mated females.     

Female defense polygyny in the territorial triggerfish <Emphasis Type="Italic">Sufflamen chrysopterum</Emphasis>

The occurrence of polygyny requires specific environmental conditions such as female aggregation or patchy resource distribution. However, it is difficult to determine the factors responsible for polygyny in species in which the territories of both sexes overlap. To overcome this, we performed female removal experiments in the polygynous triggerfish Sufflamen chrysopterum (Balistidae) in the Okinawa coral reef. Both sexes defended their territories exclusively against consexuals of the same species, and female aggregation was absent. Each male territory included 1–3 female territories, and nonterritorial males were significantly smaller than territorial males. Further, the body size of territorial males was positively correlated with that of the largest female in their territories, and larger males tended to mate with more females. The results of the female removal experiments (n = 10 females) indicated that females competed for better territories rather than larger mates. In contrast, males abandoned the territories once the females emigrated. These results strongly suggest that males defend females rather than sites and compete for larger and a greater numbers of females. Thus, in S. chrysopterum, female defense polygyny occurs in the absence of female aggregation.     

A state-based model of sperm allocation in a group-breeding salamander

We developed a dynamic program of optimal sperm allocation for group-breeding species. Using the small-mouthed salamander,Ambystoma texanum, as a model organism, we considered how spermatophoredeposition is affected by sperm reserves, male and female numberin breeding aggregations, and time during the breeding season.Parameters for part of the model were based on field data ofbreeding-pond arrival times for both sexes and on laboratoryspermatophore deposition data. Our model included simulationsof three different seasonal patterns of female arrival rate: decreasing (as in A. texanum), increasing, and uniform. General predictions are (1) Increased male competitor numbers at breedingaggregations should cause a reduction in spermatophore allocation.(2) Increased female numbers at breeding aggregations shouldincrease spermatophore allocation. (3) The effect of currentsperm reserve levels on sperm allocation depends on the seasonaldistribution of the mean number of females per male during the breeding season: (3a) If relative female availability decreasesover time, males with low sperm reserves should limit allocationearly in the season but should deposit maximal sperm loadslate in the season; (3b) if female availability increases overtime, males with low sperm loads should limit allocation throughoutthe entire breeding season; and (3c) if female availabilityis constant, sperm reserves are predicted to have little effect on spermatophore allocation tactics. We discuss model predictionsin the context of current sperm allocation theory.     

Sex Ratios and Resource Allocation among Sexually Reproducing Plants of Rubus chamaemorus

Sex ratios and patterns of size variation and resource allocationwere investigated in the dioecious species Rubus chamaemorus.Sex ratios among flowering ramets varied from 6% to 40% of females.Female ramets were slightly, although not significantly, tallerthan males. It appeared that population effects (including bothgenetic population and environmental site effects) on plantsize and allocation patterns at flowering are considerably greaterthan sex effects. If both flowering and fruit production areconsidered, then female allocation to reproduction clearly exceedsmale allocation. In females, no significant relationship wasdetected between the mass of reproductive and vegetative tissues,while males did exhibit such a relationship. Reproductive effortwas less for tall males than for small males. Despite the occurrence of sexual reproduction, the main modeof reproduction in R. chamaemorus is vegetative propagation,which is the best strategy for reproduction in the unpredictableclimate of high latitudes but which leads to skewed sex ratios.As a consequence of vigorous vegetative reproduction, individualclones can grow to be large. The results of electrophoreticstudies show that the numbers of clones per population are low.Copyright1994, 1999 Academic Press Rubus chamaemorus, cloudberry, sex ratios, resource allocation, clonal structure, electrophoresis     

Congruence between the sexes in preexisting receiver responses

Preexisting receiver biases have been shown to affect how femalesdetect and respond to new conspecific traits in a mate choicecontext. Although preexisting biases have often been discussedin the context of female mate choice, these biases need notbe sex limited. In the genus Xiphophorus, swordtail males possessa sexually selected trait, the sword. Here I consider evidencethat the state of a bias favoring sworded conspecifics maybe generally shared by the sexes in taxa in which the sword has not arisen. In three unsworded species of poeciliid fishes,both males and females prefer members of the other sex withswords. In a fourth species, males and females share the absenceof a response to a sword. This congruity between the sexessuggests that response biases may not be sex limited and thatthe sexes could historically share common mechanisms producingshared mating responses. Alternatively, selection may tendto result in parallel changes in biases in the sexes. Thiswork expands our understanding of receiver biases by usinga phylogenetic approach to examine whether biases are historicallyshared by the sexes and suggests that there can be general congruence between the sexes in such biases.     

Early development, adult mass, and reproductive success in bighorn sheep

Despite considerable empirical and theoretical work on the individualand population consequences of early development, little isknown about the correlations between early mass and adult sizeor lifetime reproductive success of free-ranging mammals. Usinga 26-year study of bighorn sheep (Ovis canadensis), we examinedhow mass as a lamb and mass gain as a yearling affected adultmass for both sexes, horn length of males and lifetime reproductivesuccess of females at different population densities. Mass asa 3-week-old lamb was either weakly or not correlated withadult mass, horn length of adult males, or the number of lambsweaned over a ewe's lifetime. Weaning mass was correlated withmost of these variables when the number of ewes in the populationwas taken into account. When weaning mass was controlled throughpartial correlation, mass as a yearling was correlated with adult mass of ewes but not with ewe reproductive success orwith adult mass or horn length of rams. Lamb mass and numberof ewes explained more of the variance in adult characteristicsfor males than for females. Our results suggest that mass gainduring lactation, possibly but not necessarily related to theamount of maternal care received, affects adult mass and reproductive success. Females appear better able than males to compensatefor poor early development, likely by postponing their firstreproduction. Mass gain over several years and the number ofewes in the population strongly affect adult mass of both sexesand therefore can have profound effects on reproductive successof this long-lived species with a multi-year growth period.     

Cross‐sex genetic correlations and the evolution of sex‐specific local adaptation: Insights from classical trait clines in Drosophila melanogaster

Natural selection varies widely among locations of a species’ range, favoring population divergence and adaptation to local environmental conditions. Selection also differs between females and males, favoring the evolution of sexual dimorphism. Both forms of within‐species evolutionary diversification are widely studied, though largely in isolation, and it remains unclear whether environmental variability typically generates similar or distinct patterns of selection on each sex. Studies of sex‐specific local adaptation are also challenging because they must account for genetic correlations between female and male traits, which may lead to correlated patterns of trait divergence between sexes, whether or not local selection patterns are aligned or differ between the sexes. We quantified sex‐specific divergence in five clinally variable traits in Drosophila melanogaster that individually vary in their magnitude of cross‐sex genetic correlation (i.e., from moderate to strongly positive). In all five traits, we observed parallel male and female clines, regardless of the magnitude of their genetic correlation. These patterns imply that parallel spatial divergence of female and male traits is a reflection of sexually concordant directional selection imposed by local environmental conditions. In such contexts, genetic correlations between the sexes promote, rather than constrain, local adaptation to a spatially variable environment.     

ECOLOGICAL ASPECTS OF SEX EXPRESSION IN SUBDIOECIOUS SCHIEDEA GLOBOSA (CARYOPHYLLACEAE)

The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.     

Mate sampling by female barking treefrogs (Hyla gratiosa)

Despite intense interest in mate choice, relatively little isknown about how individuals sample prospective mates. Indeed,a key issue is whether females sample males or simply matewith the first male encountered. We investigated mate samplingby female barking treefrogs (Hyla gratiosa). Females choosingmates in natural choruses did not move between males but insteadmated with the first male they approached closely. Most femalesmated with the male closest to them at the start of their mate-choiceprocess, and females were more likely to mate with the closest male when the distance to other males was large. These observationsare consistent with the hypothesis that females do not samplepotential mates but instead mate with the first male they distinguishfrom the rest of the chorus. To test this initial detectionhypothesis, we conducted a playback experiment in which weoffered females a choice between two calls, one of which was detectable above the background chorus sound at the female'srelease point, and one of which became detectable only as femalesmoved toward the initially detectable call. Females did notprefer the initially detectable call, thus ruling out the initialdetection hypothesis and implicating sampling of potentialmates by females. Based on the behavior of females in natural choruses, we hypothesize that females approach the chorus, moveto locations where they are able to detect the calls of severalmales simultaneously, and choose a mate from among these malesat some distance from the males. Such simultaneous samplingmay be common in lekking and chorusing species, which havebeen the subjects of many studies of sexual selection.     

Decoupling of reproductive rates and parental expenditure in a polyandrous butterfly

Current theory postulates that the operational sex ratio (OSR)determines the relative degree of mating competition in thetwo sexes and is in turn influenced by a sexual difference inthe potential reproductive rate (PRR) denned as 1/time out,where time out is the time an individual must spend recoveringfrom a bout of mating activity and/or caring for offspring.In bushcricket mating systems where males provide females witha nuptial gift, relative energy expenditure in offspring influencesthe PRR of males and females and underlies a diet-mediated shiftin the OSR. Here we investigated if there is a similar positiverelationship between relative parental nutrient expenditurein offspring and PRR in the polyandrous butterfly Pieris napi,where female fecundity is strongly dependent on male nuptialgifts at mating. By varying the amount of nutrients femalesreceive at mating and relating this to number of offspring produced,we show that male P. napi have, on average, a nutrient expenditurein offspring equaling that of females. In spite of this, themale reproductive rate is 8–13 times higher than thatof females. Hence the relative degree of parental expenditurein offspring is largely decoupled from the degree of matingcompetition in P. napi. Two alternative explanations are advancedto account for the difference between the butterfly and thebushcricket mating systems.     

Mechanisms and consequences of sexual conflict in garter snakes (Thamnophis sirtalis, Colubridae)

Red-sided garter snakes (Thamnophis sirtalis parietalis) courtand mate in early spring around large communal overwinteringdens in central Manitoba. Emerging females are immediately coveredby dozens or hundreds of vigorously-courting males, potentiallyimposing significant costs to the female. By manipulating numbersof courting males (both directly and by applying anticourtshippheromones), we quantified the degree to which female dispersalfrom the den is hindered by courtship. Courted females dispersedonly about half as fast as did solitary females. Blood lactatelevels were higher in mating than in courting or noncourtingsnakes of both sexes; the high levels of lactate in mating femalessupport the idea that courtship is physiologically stressfulto these animals, perhaps via constraints to female respiration.In arena trials, females that were exercised to exhaustion beforecourtship mated with smaller males than did control females.The spatial distribution of snakes around the den exhibits substantialheterogeneity, with densities often varying markedly betweenadjacent areas. Arena trials mimicking this heterogeneity showedthat unmated females avoided parts of the enclosure containingscent cues from males. Our data support the hypothesis thatcourtship in T. s. parietalis confers significant costs to females,and that female behaviors have evolved to reduce those costs.     

Food affects the potential reproductive rates of sand goby females but not of males

The interspawning interval of female sand gobies, Pomatoschistusminutus, a batch-spawning fish with paternal care, was significantlyshorter when the fish were fed daily than when they were fedevery fourth day. The incubation time of males was not affectedby feeding, nor was the interbrood interval Males have an equalor higher potential reproductive rate than females. As femalesreproduce more slowly when food is scarce than when it is abundant,and males do not, the difference between the sexes in potentialreproductive rate increases when there is food shortage. Becauseof this difference, both male bias in operational sex ratioand intensity in male-male competition for mates are predictedto increase as food availability decreases. Furthermore, a tradeoffbetween current and future reproduction is demonstrated to operateonly when resources are limited, because the correlation betweenegg number of the first and second clutch was positive amonghigh-food females but negative among low-food females. The numberof eggs per female clutch did, however, not differ between treatmentsin first or second dutch. I conclude that operational sex ratioand sexual selection are expected to vary within and betweensand goby populations in accordance with prey availability     

Sexual Dimorphism in Cirsium arvense (L.) Scop

Differences between males and females of Cirsium arvense (L.)Scop. in morphology and sexual performance are described. Nofunctional pollen was observed in females, and no pollen-sterileanthers were observed in males. The difference between the sexesin ovule functions is not absolute however: some males produceoccasional fruits. The number and distribution of fruits onboth sexes are described. The fruits borne on males are smallerand germinate less well than those borne on females, but theseedlings from the two sexes grow equally well. On the average,males contribute only 5.3 x 10^–4 as many genomes via ovulesas do females. The average male constancy, the proportion ofmale genes transmitted via pollen, is estimated as 0.9989. Theprimary sex index, a measure of the functional approach to strictdioecy (represented as 1.00) is 0.9995. C. arvense is describedas near-dioecious and varying from dioecy in a gynodioeciousmanner. The opinion expressed by Correns that the near-dioecyof C. arvense has been derived from hermaphroditism via gynodioecyis upheld.     

Mating systems and sexual division of foraging effort affect puddling behaviour by butterflies

Abstract. 1. Foraging effort can vary among age classes and between the sexes. In many Lepidoptera, young males feed from mud, dung or carrion in a behaviour known as 'puddling', whereas females rarely puddle. In at least one species, males transfer sodium gained from puddling to females at mating for use in egg production.
2. Here we examine sex- and age-specific puddling patterns in seven montane butterfly species. We also test the hypothesis that among species in which young males predominate at puddles, differences in age- and sex-specific puddling patterns for a given species are related to mean female lifetime mating numbers.
3. For five species, young males fed proportionately more at puddles than other sex and age classes. Two species showed anomalous feeding patterns. In one, young females predominated at puddles; in the other, butterflies were rarely found at flowers.
4. As predicted, among the five species in which young males feed proportionately more at puddles, mean number of lifetime matings by females was negatively correlated with frequency of mud puddling by older females. A second prediction that mean number of lifetime matings by females is positively correlated with frequency of mud puddling by older males was not supported.
5. The results provide support for interspecific variation in division of responsibility between the sexes for resource acquisition for female reproduction, indicating close coordination between the sexes of foraging and life-history tactics.     

The use of extrapair copulations for mate appraisal by razorbills, Alca torda

Razorbills (Alca Undo) engaged in extrapair copulations (EPCs)during two phases of their breeding cycle when fertilizationof eggs was not possible, suggesting that EPCs provide nongeneticbenefits. Females actively pursued extrapair mountings afterthey completed egg laying, the first monogamous species reportedto do so. Mountings were performed in mating arenas outsideof the breeding colony, where attendance by postlaying femalesindicated that they sought encounters with extrapair males whiletheir mates were incubating. Postlaying females always successfullyresisted insemination yet positioned themselves to receive mountings.These findings support the hypothesis that resistance to inseminationis a ploy used by females to appraise males. At the end of thebreeding cycle, when males escorted the fledgling to sea, femalesremained at the colony where they consorted and sometimes copulatedwith other males. Nonfertilizable extrapair copulations mayserve two social functions for razorbills: female appraisalof males for future fertilizable EPCs and the appraisal andacquisition of new mates by both sexes.     

Habitat segregation in ungulates: are males forced into suboptimal foraging habitats through indirect competition by females?

Sex differences in habitat use (termed `habitat segregation') are widespread in sexually dimorphic ungulate species. They are a puzzling phenomenon, particularly when females use better foraging habitats than males. It has been suggested that males, owing to their larger body size and higher forage requirements, are inferior in indirect competition to females and are forced by female grazing pressure into marginal habitats (`indirect-competition hypothesis'). This hypothesis has been widely cited and has important theoretical and practical implications. However, evidence for it is inconclusive. The present paper presents the results of the first experimental test of the indirect-competition hypothesis. We manipulated female and male numbers of red deer (Cervus elaphus L.) on a large scale on the Isle of Rum, Scotland, and tested the influence of this manipulation on deer habitat use. We predicted that where female numbers were reduced, male use of preferred habitat should increase and sex differences in habitat use should decrease, while a reduction in male numbers should not have such effects. In contrast, we found that the manipulation of female and male numbers did not affect habitat use, and conclude that the indirect-competition hypothesis does not explain habitat segregation on Rum. Received: 19 August 1998 / Accepted: 13 February 1999     

Brood desertion in Kentish plover: sex differences in remating opportunities

To understand the evolution of parental care, one needs to estimatethe payoffs from providing care for the offspring and the payoffsfrom terminating care and deserting them. These payoffs arerarely known. In this study we experimentally estimated therewards from brood desertion in a species that has a variablepattern of parental care. In particular, either the female or themale parent may desert the brood in Kentish plover Charadrius alexandrinus,so some broods are attended by one parent of either sex, whereasin other broods both parents stay with the brood until the chicks fledge.We created single males and single females by experimentallyremoving the other parent and the clutch. The expected rematingtime of males was significantly higher (median: 25.4 days) thanthat of the females (5.3 days, p <.0001). The expected rematingtime tended to increase over the breeding season in both sexes,although the increase was significant only in females. The newnest of remated males was closer to their previous territory (mean± SE, 46 ± 8 m) than that of the remated females(289 ± 57 m, p <.001). Hatching success of new nestswas not different between remated males and females. Our resultsdemonstrate that the remating opportunities are different formale and female Kentish plovers and these opportunities varyover the season. We propose that the remating opportunitieswere influenced by the male-biased adult sex ratio and the seasonaldecrease in the number of breeders. However, we stress thatmeasuring remating times is a more direct measure of matingopportunities than calculating the operational sex ratio.     

Ecological disjunction across the sexes in an alloparasitoid: Host requirements,sex ratios and mate localization (Hymenoptera: Aphelinidae)

The larval females of Coccophagus sp. nr gurneyi Compere are primary parasitoids of lantana mealybugs, whereas males develop hyperparasitically through other parasitoids (never their own females), so the species is alloparasitic. Males are seldom even reared from lantana mealybugs (<0.3%, n = 4,212), and have not yet been reared from any other host. Adults were sampled in the field to establish that this species is sexual (by assessing female spermathecal content), and to quantify relative abundance of the sexes around host infestations. Adult males were scarce above hosts (3%, n = 314), but were attracted in relatively high numbers to caged virgin females within those infestations. Caged females outside infestations did not attract males, suggesting that mate attraction requires environmental signals other than those from females. Most females collected in the field above host infestations had sperm in their spermathecal capsules. They presumably had mated with males that developed elsewhere (so mate localization might involve searching across substantial distances). Virgin females were present only early in the day and evidently mate soon after eclosion. Evidence of sperm depletion in mated females was not found. The spatial scale of male and female movements needs to be quantified, but the ongoing movement of individuals (as a consequence of their sex‐related host relationships) seems to be a regular aspect of their ecology. The spatial and temporal dynamics across the sexes illustrates that their abilities to localize one another for mating leaves the sexes free to diverge ecologically, and their sex ratios to vary.     

Temperature affects male and female potential reproductive rates differently in the sex-role reversed pipefish, Syngnathus typhle

The differences in potential reproductive rate between the sexescan be used to predict the operational sex ratio and the patternsand intensity of mating competition and hence sexual selectionin a population. This article describes how one environmentalcomponent, temperature, affects potential reproductive ratesof the two sexes in the paternally brooding, sex-role reversedpipefish (Syngnathus typhle). Males brooded embryos much longer(on average 58 days) in cold water (about 10°C) than inwarmer water (35 days at about 15°C). As a consequence,the potential reproductive rate (number of eggs brooded perday) of males was significantly higher in warm water. In females,however, potential reproductive rate, i.e., number of eggs producedper day given an unlimited access to mates, was not significantlydifferent between temperatures. In both sexes, potential reproductiverate was positively related to body size. At both temperatures,females had the potential to reproduce faster than males. Asa result, the operational sex ratio will become female biasedand sex-roles reversed, as is the case in this species. Sincetemperature differently influenced the potential reproductiverates of males and females, with the sexual difference largerat lower temperatures, more intense female-female competitionis predicted at low temperatures.