Predicting polygynous settlement while incorporating varying female competitive strength

The rationale behind the polygyny threshold model is that thebreeding situation quality (BSQ) males have to offer femalesvaries, and that differences in BSQ offset females' costs ofsharing with other females, thus favoring polygynous settling.It predicts that the first chosen territories become polygynousfirst, and that breeding success of secondary and contemporarymonogamous females is similar. This is not generally found.Testing of the polygyny threshold model (PTM) assumes that femalesare equal competitors and distribute ideally free around availablebreeding resources, a condition probably not often met. If sharinga male is costly, and competitors differ in quality, weakerindividuals should experience degrees of competitive exclusion.Setting female competitive abilities proportional to arrivalorder, we use an individual-based interference-competition modelto examine settlement patterns. Shifts in the ratio of variancein interfemale competitiveness to interterritorial differencesin BSQ result in various settlement patterns, with differentpredictions concerning settlement order and fitness returns.We find support for the novel predictions from data on northernlapwing (Vanellus vanellus), starling (Sturnus vulgaris), piedflycatcher (Ficedula hypoleuca), and blue tit (Parus caeruleus).We suggest that before testing polygyny predictions, an evaluationof the settlement sequence should be made, which may help togenerate more accurate predictions. We argue that violationof the "equal female" assumption may explain much of the discrepancybetween predictions and empirical findings in previous testsof the PTM, and that secondary females in general have lowersuccess than do monogamous breeders because they are of lowerquality.     

Integrating individual behavior and population ecology: the potential for habitat-dependent population regulation in a reef fish

We used the predictions of the ideal free and ideal despoticdistributions (IFD and IDD, respectively) as a basis to evaluatethe link between spatial heterogeneity, behavior, and populationdynamics in a Caribbean coral reef fish. Juvenile three-spotdamselfish (Stegastes planifrons) were more closely aggregatedin patch reef habitat than on continuous back reef. Agonisticinteractions were more frequent but feeding rates were lowerin the patch versus the continuous reef habitat. Growth rateswere lower in patch reef habitat than on the continuous reef,but mortality rates did not differ. A separate experiment usingstandard habitat units demonstrated that the patterns observedin natural habitat were the result of the spatial distributionof the habitat patches rather than resource differences between habitats. Our results do not follow the predictions of simpleIFD or IDD models. This deviation from IFD and IDD predictionsmay be the result of a number of factors, including lack ofperfect information about habitat patches, high movement costs,and higher encounter rates of dispersed patches. Our resultsdemonstrate that behavioral interactions are an integral partof population dynamics and that it is necessary to considerthe spatial organization of the habitat in both behavioraland ecological investigations.     

Interference and the ideal free distribution: oviposition in a parasitoid wasp

The interference ideal free distribution (IFD) model of Sutherlandmakes a number of predictions that have yet to be tested andthat have implications for the validity of subsequent extensionsto the theory. We tested these predictions in a study usingdifferent densities of the parasitoid wasp, Venturia canescens,foraging on patches containing different densities of its host,Plodia interpunctella. Our results support a number of the interferenceIFD model's general predictions. Gain rate decreased becauseof increased interference at higher density. Although gain rateson the two patches differed slighdy, this would be expectedallowing for some sampling behavior and perceptual constraints.Early in each experiment when patch assessment is likely tooccur, wasp movement was higher and gain rates lower. However,the more specific prediction of Sutherland's model, that proportionalpatch use should be constant and independent of density, wasnot upheld. Contemporary IFD models use only one of severalequally valid potential relationships between gain rate, interference,and competitor density. The results of this study provide supportfor the additive model developed by Tregenza et al. (companionarticle).     

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.     

Sexual conflict over copula timing: a mathematical model and a test in the yellow dung fly

Sexual conflict over mating occurrence, timing, or durationis common in animals. This explains conspicuous female materejection behavior in many species, often involving shaking,fighting, and occasional forced copulations. We present a simplemodel that generates predictions about whether and when copulationoccurs in such conflict situations and how much female rejectionbehavior should be observed. Predictions depend on 2 underlyingparameters affecting female resistance and male persistence.We supply 2 qualitative tests of the model using the yellowdung fly Scathophaga stercoraria (Diptera: Scathophagidae).We manipulated adult age, body size (large and small), and adultfood availability (low and high), independently in males andfemales, staging replicate pairings of all treatment combinations.In agreement with predictions of our model, shaking durationfirst increased to a maximum at intermediate age, when the averagefemale copulated, and then decreased again. Contrary to expectation,body size did not affect copulation timing, female resistance,or male persistence. As predicted, adult food limitation delayedsexual maturity and hence prolonged female resistance, resultingin later copulations after more shaking. However, although foodlimitation equally delayed the increase in male persistencewith age, copulation also occurred later after more shaking,opposite to the model prediction. We conclude that shaking isdriven primarily by female age and male responses to it. Althoughfemale shaking can initially successfully deter males in S.stercoraria, this behavior is subtle and has apparently shiftedfunction from an effective means of mate choice to a signalof nonreceptivity, though its importance in nature remains unclear.     

Parasites and Bright Male Plumage in the Satin Bowerbird (Ptilonorhynchus violaceus)

Results from our field studies of the satin bowerbird (Ptilonorhynchusviolaceus) suggest that females choose males as mates basedon their level of infection with the ectoparasite (Myrsideaptilonorhynchi: Menoponidae). We evaluated predictions fromthree hypotheses for why this pattern of choice might evolve.The bright male and correlated infection models both suggestthat females choose parasite free males because these malesare more likely to sire parasite resistant offspring. The brightmale hypothesis suggests that females are able to gauge infectionbased on plumage brightness. The correlated infection hypothesisclaims that females assess resistance to endoparasites througha correlated effect on ectoparasites. In the parasite avoidancemodel female choice is shaped by the proximate benefits of avoidinginfection. Six predictions from these models were tested usinginformation on patterns of infection in satin bowerbirds. Ofthese models the parasite avoidance model was best supportedby the available data.     

Use, misuse and extensions of "ideal gas" models of animal encounter

Biologists have repeatedly rediscovered classical models from physics predicting collision rates in an ideal gas. These models, and their two-dimensional analogues, have been used to predict rates and durations of encounters among animals or social groups that move randomly and independently, given population density, velocity, and distance at which an encounter occurs. They have helped to separate cases of mixed-species association based on behavioural attraction from those that simply reflect high population densities, and to detect cases of attraction or avoidance among conspecifics. They have been used to estimate the impact of population density, speeds of movement and size on rates of encounter between members of the opposite sex, between gametes, between predators and prey, and between observers and the individuals that they are counting. One limitation of published models has been that they predict rates of encounter, but give no means of determining whether observations differ significantly from predictions. Another uncertainty is the robustness of the predictions when animal movements deviate from the model's assumptions in specific, biologically relevant ways. Here, we review applications of the ideal gas model, derive extensions of the model to cover some more realistic movement patterns, correct several errors that have arisen in the literature, and show how to generate confidence limits for expected rates of encounter among independently moving individuals. We illustrate these results using data from mangabey monkeys originally used along with the ideal gas model to argue that groups avoid each other. Although agent-based simulations provide a more flexible alternative approach, the ideal gas model remains both a valuable null model and a useful, less onerous, approximation to biological reality.     

An evolutionary explanation for seasonal trends in avian sex ratios

We present an extensive set of data for five species of raptorialbirds to demonstrate that some raptor species produce an excessof daughters early in the season and an excess of sons in latenests, while others show the reverse. By means of a simulationmodel we investigate an evolutionary explanation for this phenomenonin terms of sex-specific differences in the relation betweenage at first breeding and date of birth. The model predictsthat that gender should be produced first in the season whoseage of first breeding is more strongly accelerated by an earlybirth date. We argue that this tends to be the male gender inraptor species, such as the common kestrel (Falco tinnunculus),which tend to breed early in life, while it is the female genderin larger species with later onset of breeding, such as themarsh harrier (Circus aeruginosus). The empirical evidence isqualitatively consistent with this hypothesis. Our model isquite general in that it makes no assumptions about the mechanism(primary sex-ratio bias at egg laying or secondary sex-differentialmortality before fledging) by which the bias is generated. Yetit is able to create quantitative predictions for species wheresufficient demographic and life-history data are available.From the available data set in the common kestrel we derivea quantitative prediction for the seasonal trend in brood sexratio. The observed trend is in good agreement with this prediction.[BehavEcol 7: 426–430 (1996)]     

Elaborate Nests in a Weaverbird: A Role for Female Choice?

Studies on sexual selection have focused on behaviour and morphology, but several groups of animals build elaborate structures associated with acquiring a mate. I investigated female choice for nests built by male baya weavers (Ploceus philippinus). Nest choice by females should be strong, as nests are obvious direct benefits provided by males. I used a field experiment supplemented with correlational information to ask whether females appear to base mate choice decisions on male behaviour, nest architecture, and nest location. When the nests of highly visited males were exchanged with those of poorly visited males, female visits remained highest at the original male and location. I found no relationship between female choice and male display or other behaviour. Correlational analyses show that nest location was a better predictor of female choice than was nest architecture. These data suggest that current female choice is driven more by access to safe nesting sites rather than to well‐built nests, possibly because all males are able to build nests of adequate quality. However, nest architecture is unlikely to be irrelevant to females, and its role deserves further investigation.     

Why female crested tits copulate repeatedly with the same partner: evidence for the mate assessment hypothesis

That repeated copulation with the same partner within a singlefertile period is beneficial to the male is generally accepted,but why it should be adaptive to the female is controversialand clear evidence supporting any hypothesis is lacking. Hunteret al. (1993) presented seven hypotheses explaining repeatedmating from the female perspective. Four of them are consistentwith the occurrence of male refusal to copulate: females mighttrade copulations for (1) immediate and or (2) future materialbenefits, or use mating as a mechanism for (3) mate-guardingand or (4) mate-assessment. To test these hypotheses in a populationof crested tits Parus cristatus, we collected data on variationin female solicitation rate, proportion of male refusal, andextra-pair paternity. We found that (1) female solicitationrate was independent of male condition, (2) the proportion ofmale refusal was higher in poor-condition males and (3) femalespaired to poor-condition males sought extra pair paternity.These findings agree with predictions stemming from the mateassessment hypothesis. Therefore, it is suggested that, in crestedtits, male response to female copulation solicitation reflectsmale condition and is used by females to assess male quality     

Differential Responses to Same and Opposite Sex Odors by Adult House Mice Are Associated with Anogenital Distance

Intrauterine position (IUP) of female and male fetuses in litter-bearing mammals can affect their physiology, morphology and behavior. The relationship between anogenital distance (AGD) and IUP was used as a bioassay for the degree of exposure of female and male fetuses to hormones in utero . Based on laboratory work in several rodent species, the following predictions were made for house mice ( Mus musculus domesticus ): (1) female mice should prefer odors from males with larger AGDs because such males are more aggressive, could protect more resources, and are better parents than males with smaller AGDs; (2) male mice should prefer odors from females with smaller AGDs because these females produce more offspring and are better parents than females with larger AGDs. We also tested the prediction that within sexes, mice should avoid odors from mice with larger AGDs because such mice are more aggressive. Responses to odors in traps were used to test these predictions for house mice living in outdoor enclosures using odor-baited traps. Both predictions were confirmed. Furthermore, mice of both sexes tended to avoid odor cues from individuals of the same sex that had larger AGDs, probably to decrease chances of an aggressive encounter that could result in injury.     

Resource Acquisition and Alternative Mating Strategies in Water Striders

Behavioral polymorphisms occur among male and female water striders,Gems remigis, when competing for food and mates. Individualsof both sexes vie for positions in the fastest flowing portionsof streams. Here prey capture rates are highest, as are thoseof swimming and aggression. Only the largest females, and maleswith the largest first appendages, can regularly maintain positionsin these areas. The remaining females are arranged along theflow gradient according to their size with the smallest holdingpositions in pools of slow moving water. For the remaining malesneither overall size, nor the size of the first appendages,appears to determine which males swim near the edge of streams,or which males swim as satellites behind those occupying thefast flowing productive areas. Preliminary data show that matingsuccess of edge and satellite males are about equal, but significantlyless than that of the centrally positioned males with the largestfirst appendages. Thus although it appears that morphologicalphenotype influences male competitive behavior, when the absolutesize of the critical trait is small males adopt behavior afterassessing the actions of others. For these "subordinate" males,behavioral assessment appears to produce an "ideal free" spatialdistribution.     

Lek size variation and its consequences in the ochre-bellied flycatcher, Mionectes oleagineus

Several hypotheses suggest that the costs and benefits of displayin aggregations of different sizes play a major role in boththe evolution of leks and in the distribution of males acrossleks of different sizes. We examined the consequences of variationin lek size for both males and females in a study of the ochre-belliedflycatcher, Mionectes oleagineus. We observed 41 solitary displaysites and leks, ranging in size from 1 to 5 mates, over 3 breedingseasons. Although mean visitation rate by females was positivelycorrelated with lek size, female visitation rate per male remainedconstant across lek sizes. The rate at which females visitedthe male who had the highest female visitation rate at eachlek was positively correlated with lek size as predicted bythe hotshot hypothesis. Neither mean nor per capita intrusionrates were correlated with lek size. For the top-ranked male,however, there was a significant correlation between intrusionrates and the size of the lek at which he displayed. Intrusionat leks may be costly, as 28% of female visits were interruptedby intruders. Solitary mates suffered no such interference.Females show no preferences for larger leks, visiting and matingat solitary sites as well as at leks. However, females preferentiallyvisit males with high singing rates, and this male trait maydetermine visitation patterns. Our data argue that preferencesfor larger leks are not important in the evolution of lekkingin this species, nor do they affect lek size. Instead, the dataare in accordance with the predictions of both the hotshot andhot-spot models. These processes may be operating simultaneouslyin this species     

Tests of the mate-guarding hypothesis for social monogamy: male snapping shrimp prefer to associate with high-value females

Social monogamy without biparental care has evolved in manytaxa, and a number of hypotheses have been developed to explainthis phenomenon. Several authors have suggested the importanceof male mate-guarding behavior in the evolution of social monogamy,although empirical support for this hypothesis is lacking. Inthe caridean shrimp genus Alpheus, social monogamy may resultfrom selection on males for long-term guarding of females becausemating is temporally restricted to a short time after the female'smolt. I used Alpheus angulatus to test two predictions of theextended mate-guarding hypothesis: Males should (1) be physiologicallycapable of predicting the timing of female sexual receptivity,and (2) prefer to associate with (guard) females that are closerto sexual receptivity. Data from a Y-maze experiment testingfor distance chemical communication showed that males of A.angulatus were attracted to water treated by exposure to premoltfemales, repulsed by water treated by exposure to intermoltmales and females, and did not appear to respond in either directionto water treated by exposure to premolt males. In mate choiceexperiments, significantly more males paired with premolt femalesthan with postmolt females. These data suggest that males ofA. angulatus engage in precopulatory mate-guarding behavior.Other factors (population density, sex ratio) may have playeda role in the temporal extension of mate guarding to socialmonogamy.     

Unequal competitor ideal free distribution in fish?

Key predictions of unequal competitor ideal free distribution models were tested using a continuous input situation. Ten individually identifiable cichlid fish competed for food items at either end of their tank. Their distribution fitted the predictions of the equal competitor, continuous input ideal free model almost perfectly. However, examination of individual intakes revealed significant variation in individual success and relative competitive ability between patches. Contrary to expectations, fish did not exclusively use the patch where their intake was higher, although individuals experiencing greater differences in intake rate between patches were more selective. We found no evidence for a truncated distribution or even a correlation between competitive ability and patch quality. Changing the input regime to reduce competition did not produce a decrease in the range of intake rates between individuals. This study indicates the value of future empirical and theoretical work on how relative competitive ability varies with the nature of the foraging environment.     

Runaway sexual selection without genetic correlations: social environments and flexible mate choice initiate and enhance the fisher process

Female mating preferences are often flexible, reflecting the social environment in which they are expressed. Associated indirect genetic effects (IGEs) can affect the rate and direction of evolutionary change, but sexual selection models do not capture these dynamics. We incorporate IGEs into quantitative genetic models to explore how variation in social environments and mate choice flexibility influence Fisherian sexual selection. The importance of IGEs is that runaway sexual selection can occur in the absence of a genetic correlation between male traits and female preferences. Social influences can facilitate the initiation of the runaway process and increase the rate of trait elaboration. Incorporating costs to choice do not alter the main findings. Our model provides testable predictions: (1) genetic covariances between male traits and female preferences may not exist, (2) social flexibility in female choice will be common in populations experiencing strong sexual selection, (3) variation in social environments should be associated with rapid sexual trait divergence, and (4) secondary sexual traits will be more elaborate than previously predicted. Allowing feedback from the social environment resolves discrepancies between theoretical predictions and empirical data, such as why indirect selection on female preferences, theoretically weak, might be sufficient for preferences to become elaborated.     

Insect Choice and Floral Size Dimorphism: Sexual Selection or Natural Selection?

In considerations of sexual floral size dimorphism, there is a conflict between sexual selection theory, which predicts that larger floral displays attract more pollinators, and optimality theory—particularly the ideal free distribution—which predict that pollinators' visits should match nutritional rewards. As an alternate explanation of this dimorphism, Müller reported that pollinators tend to visit larger male flowers before visiting smaller female flowers, thereby promoting effective pollination. To investigate optimality predictions, I offered pollinators a choice between smaller, less numerous, but more rewarding flowers; and larger, more numerous, but less rewarding flowers. Foragers initially favored the larger and more numerous flowers, but rapidly shifted preferences to conform with the predictions of the ideal free distribution. To test Müller's hypothesis, I offered pollinators choices between larger and smaller corollas of equal caloric reward. Results showed that although pollinators tended to visit larger corollas first, they did not visit them more often. These experiments highlight the need for further investigation into the tradeoff between natural and sexual selection, and their respective influences in pollination ecology.     

Intersexual spatial relationships in a lekking species: blue-crowned manakins and female hot spots

Leks offer an intriguing evolutionary problem: why do malesaggregate when this apparently leads to fitness costs? Aggregationcosts can be balanced if males settle on patches where theyare more likely to encounter females (hot-spot hypothesis).We evaluated whether female hot spots can account for patternsof lek structure in the blue-crowned manakin (Lepidothrix coronata)by modeling female distribution patterns relative to lek locationsin two 100-ha plots. Individual females were mapped based onnest locations and capture points and had their home ranges(HRs) modeled based on radiotelemetry data. The number of femalesthat lekking males can be expected to encounter was estimatedas the number of individual female HRs overlapping each maleterritory; hot spots were defined as patches where more femalesare found than average. We investigated how changes in femaleHR size and devaluation effects (decrease in female availabilitydue to the presence of neighboring males) influence male accessto females. Both factors strongly influenced the expected ratesof female encounter, but the hot-spot hypothesis was not supported:most male territories consistently overlapped fewer or justas many female HRs as expected by chance. Leks were not closerto hot spots than similar-sized nonlek sites. A proportion ofmales were, indeed, settled at hot spots, but, contrary to predictionsof the hot-spot hypothesis, they belonged to smaller leks thanmales located outside hot spots. Our results indicate that thislack of spatial correlation between males and females resultspartly from differences in sex-specific habitat preferences.     

Novel cost of a sexually selected trait in the rubyspot damselfly Hetaerina americana: conspicuousness to prey

Conspicuousness to predators frequently has been invoked asa cost of sexually selected traits, but conspicuousness to preyhas not We tested for the latter using rubyspot damselflies(Hetaerina americana) as the predator. Previous work on thisspecies showed that the red spots on male wings are intrasexuallyselected and reduce survival. Since female wings lack red spots,we first compared male and female weight gain rates per unithunting time. Females gained weight significandy faster thanmales in both mg per hour and relative to body weight. We thencompared the weight gain rates of females painted with red wingspots to those of control females painted with clear ink ornot manipulated. Controls gained weight significandy fasterthan red-painted females. Behavioral observations revealed thatred females attempted to capture prey at normal rates and experiencednormal rates of agonistic interference from conspecifics ofboth sexes. Nevertheless, red females captured fewer prey perminute and per capture attempt than did sham-manipulated andunmanipulated controls. We infer that the red spots reducedfemale weight gain rates by increasing their visibility to prey.Close similarity between male and red female weight gain ratesrelative to unmanipulated females suggests that red spots mayalso be a hunting handicap for males. [Behav Ecol 7: 465–473(1996)]     

Local resource competition and sex ratio in the ant Cataglyphis cursor

The local resource competition (LRC) hypothesis predicts thatwherever philopatric offspring compete for resources with theirmothers, offspring sex ratios should be biased in favor of thedispersing sex. In ants, LRC is typically found in polygynous(multiple queen) species where foundation of new nests occursby budding, which results in a strong population structure anda male-biased population-wide sex ratio. However, under polygyny,the effect of LRC on sex allocation is often blurred by theeffect of lowered relatedness asymmetries among colony members.Moreover, environmental factors, such as the availability ofresources, have also been shown to deeply influence sex ratioin ants. We investigated sex allocation in the monogynous (singlequeen) ant Cataglyphis cursor, a species where colonies reproduceby budding and both male and female sexuals are produced throughparthenogenesis, so that between-colony variations in relatednessasymmetries should be reduced. Our results show that sex allocationin C. cursor is highly male biased both at the colony and populationlevels. Genetic analyses indicate a significant isolation-by-distancein the study population, consistent with limited dispersal offemales. As expected from asexual reproduction, only weak variationsin relatedness asymmetry of workers toward sexual offspringoccur across colonies, and they are not associated with colonysex ratio. Inconsistent with the predictions of the resourceavailability hypothesis, the male bias significantly increaseswith colony size, and investment in males, but not in females,is positively correlated with total investment in sexuals. Overall,our results are consistent with the predictions of the LRC hypothesisto account for sex ratio variation in this species.