Weight lifting and health status in the black wheatear

Male black wheatears Oenanthe leucura demonstrate an exaggerated sexualdisplay by carrying many heavy stones (on average 3.1 kg perseason for this 35-g passerine) to cavities inside caves beforethe start of each clutch. The energetic cost of this displayis mainly determined by the size of stones, which gives riseto the largest power output of a muscle ever recorded for any species(400 W / kg), rather than the total number of stones carried,which only amounts to 1% of daily energy expenditure for a blackwheatear male. We tested whether stone carrying reflected malehealth status by removing two primaries from a group of males,but not from controls. Experimental manipulation did not significantlyaffect stone carrying. Similarly, male health status measuredas hematocrit, leukocyte concentration, and T-cell responseto an injection with phytohemagglutinin (a lectin) was not affected byexperimental manipulation. Male black wheatears that carriedvery heavy stones, each on average more than a quarter of theirown body mass, had a stronger T-cell response to injection witha lectin than males that carried light stones. Furthermore,males that carried heavy stones had a reduced hematocrit level,which may have arisen from the effects of the extremely heavywork load. Rate of stone carrying and mass of stones was not significantlyrelated to the abundance of two species of ectoparasites. Male blackwheatears with high T-cell responses were significantly morelikely to survive than males with low responses, but other measuresof male performance did not differ significantly between survivorsand nonsurvivors. The male stone-carrying display thus reflectsmale health status as demonstrated by a component of immunocompetence.     

Great tits (Parus major) reduce body mass in response to wing area reduction: a field experiment

Flight performance is crucial in determining whether a smallbird will survive an attack by a predator. Given the importanceof body mass in determining flight performance, it has beensuggested that birds should strategically regulate body massas a response to predation risk. However, all experiments upto now have been carried out with captive birds, comparing experimental to control birds. Here we present data from thefirst experiment in the field using a within-individuals experimentaldesign. The wing area of wild great tits, Parus major, wasreduced by reversibly taping primaries five to seven. Thisallowed for the same individual to alternatively act as controlor experimental bird. Great tits reduced body mass (but not pectoral muscle width) during episodes of wing area reduction,lending support to the view that the reduction in body massexperienced by birds during molt is a strategy rather thanthe result of energetic stress. Theoretical models establishingthe different trade-offs that determine optimal body mass should therefore take into account this important life-history episode.     

Butterfly contests and flight physiology: why do older males fight harder?

The males of many butterfly species compete for territoriesvia conspicuous aerial wars of attrition, in which the determinantsof persistence ability are largely unclear. Flight performancefeatures, such as stamina, acceleration, and maneuverability,are often assumed to be important in this context, yet thereis no direct evidence by which to evaluate these possibilities.Recent research has indicated that competitive ability increaseswith age in a notably territorial species, Hypolimnas bolina,which could arise from lifetime morphological or physiologicalchanges that directly affect flight performance. I evaluatedthis hypothesis by investigating how size-independent variancein body composition, energy stores, flight muscle ratio (FMR),and wing condition change with age in this species. Males infive age categories (spanning the functional life span of territorialindividuals) were sampled from encounter sites in tropicalAustralia. Analysis of body composition with respect to anestimate of eclosion mass (forewing length) indicated thattotal body mass, abdomen mass, and wing area decrease throughoutan individual's lifetime, but thorax mass remains unchanged.Wing loading (the ratio of wing area to body mass) is lowestin intermediately aged individuals, but FMR and energetic statusremain largely similar regardless of age. On average, therefore,the energetic cost of sustained flight should first decrease,then increase, with age in a male H. bolina (of standardizedbody size), while available energy reserves decline slightly. Acceleration and maneuverability should remain relatively constant.These results, coupled with the fact that body size is unrelatedto contest success in this territorial butterfly, fail to supportthe idea that age-related competitive ability is mediated simplyby energetics or flight performance.     

褐飞虱翊型分化遗传规律的研究

以褐飞虱Nilaparvata lugens (Stal)长翅型(macroptery,简称M)、短翅型(brachyptery,简称B)的遗传纯系为实验材料,进行亲本、稻株生育期、虫口密度三因子交互实验。结果表明：(1)在环境条件(指稻株生育期、虫口密度等)一致时,B♀×B♂、B♀×M♂、M♀×B♂、M♀×M♂四种亲本组合的F1代短翅型成虫百分率分别为：98%、92%、64%、29%,各亲本组合间差异极显著;(2)亲本相同时,将F1代褐飞虱初孵若虫多头词养(多于l0头/株)在黄熟期稻株上,其长翅型雌、雄成虫百分数均高于灌浆期稻株上;(3)单头饲养实验中发现,不论亲本组合、稻株生育期如何,雌虫绝大多数分化为短翅型,而雄虫则几乎全为长翅型。这表明褐飞虱的翅型分化遗传由一个受多种因子影响的调控体系决定,且调控作用与性别有关。     

The scaling of primary flight feather length and mass in relation to wing shape, function and habitat

The mass and length of each primary flight feather was measured in 120 species of birds (347 individuals) representing 37 families and 15 orders. The scaling relationship between mass and length was determined using the mass of each primary as a proportion of total primary feather mass for that individual and, similarly, length as a proportion of total length. This eliminated errors due to intra- and interspecific differences in absolute size. In every species there was a highly significant constant scaling relationship (log mass/log length) for all of the primary feathers proximal to the feather that formed the wing tip. This relationship was allometric and varied between 1.80 in Rooks Corvus frugilegus and 4.87 in Winter Wrens Troglodytes troglodytes . The mean scaling relationship for 120 species was 2.41 ± 0.42 sd, which was significantly less ( P  < 0.0001) than isometry (i.e. 3.00). In most species (117 of 120) the primary feather forming the wing tip and all feathers distal to it had a different scaling relationship, and had a greater mass than expected from their length. The greater relative mass of the outer primaries may reflect a protective role against physical abrasion, or an aerodynamic role in that each of these feathers provides a leading edge to the wing. Thus, there were two scaling relationships that pivoted about the feather forming the wing tip, resulting in a characteristic 'signature' for each species. Scaling relationships can be related to flight characteristics and habitat, rather than to phylogeny. Closely related species often had widely varying scaling relationships. In general, species exploiting dense vegetation had greater scaling relationships than more aerial species. However, species with a high scaling relationship did not have a greater mean feather mass, so the increased relative mass of the distal primaries was at the expense of proximal primary feather mass.     

Breeding biology of the Madagascar Paradise Flycatcher,Terpsiphone mutata,with special reference to plumage variation in males

This study was undertaken to understand the migratory strategies of the Dunlins (Calidris alpina) caught in Eilat, Israel, before and after they accomplish the crossing of the combined ecological barrier of the Sinai, Sahara and Sahel deserts. Between 1999–2001, a total of 410 adults and 342 juveniles were banded. The significant difference in mean wing length between birds caught in autumn and spring reflects the degree of abrasion of the outer primaries during over-wintering in Africa. Dunlins caught in Eilat in autumn and early winter had a mean wing length 1.4–1.9mm longer than in the spring. The rate of body mass increase was comparatively high and the mean body mass of the heaviest 10% of Dunlin at Eilat was 56.2g (SE ± 0.6, N = 80). The heaviest birds from Eilat carried on average about 10g of fat with a lipid index (fat mass as a percentage of total body mass) of 18%. These reserves allow a flight of approximately 1 000km, which is probably sufficient for continued migration to more southerly wintering grounds.     

On the sex-specific mechanisms of butterfly flight: flight performance relative to flight morphology, wing kinematics, and sex in Pararge aegeria

Many evolutionary ecological studies have documented sexual dimorphism in morphology or behaviour. However, to what extent a sex-specific morphology is used differently to realize a certain level of behavioural performance is only rarely tested. We experimentally quantified flight performance and wing kinematics (wing beat frequency and wing stroke amplitude) and flight morphology (thorax mass, body mass, forewing aspect ratio, and distance to centre of forewing area) in the butterfly Pararge aegeria (L.) using a tethered tarsal reflex induced flight set-up under laboratory conditions. On average, females showed higher flight performance than males, but frequency and amplitude did not differ. In both sexes, higher flight performance was partly determined by wing beat frequency but not by wing stroke amplitude. Dry body mass, thorax mass, and distance to centre of forewing area were negatively related to wing beat frequency. The relationship between aspect ratio and wing stroke amplitude was sex-specific: females with narrower wings produced higher amplitude whereas males show the opposite pattern. The results are discussed in relation to sexual differences in flight behaviour.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 675–687.     

Annual survival of Gurney's Sugarbird,Promerops gurneyi

Seasonal variation in body mass and wing length, and the onset and duration of primary moult, were investigated for Chestnut Weavers from northern Namibia. Body mass of adult males was 31.2g (SD 2.6), and adult females weighed 27.4g (SD 1.9). Body mass declined from March to April, and started increasing after August (i.e. near the end of moult) in males and females. Wing length in adult males with new primaries (Oct–Feb) was 80.7mm (SD 2.7) and for adult females (Oct–Feb) 76.8mm (SD 2.6). For both sexes wing length declined during and after the breeding season, due to extensive feather wear. Adult males started primary moult significantly earlier than females (9 April vs 30 April) and moult lasted longer (206 days vs 189 days). The peak summer rainfall and the start of primary moult was earliest in 2000 and latest in 2004 for males and females. Individual primary feathers took 11–18 days to grow.     

Why make daughters larger? Maternal sex-allocation and sex-dependent selection for body size in a mass-provisioning wasp, Trypoxylon politum

Mass-provisioning wasps package maternal investment into broodcells, sealed structures that contain all the provisions necessaryfor an offspring's growth and development. Optimal sex-allocationtheory predicts that if maternal provisions determine the sizeof each offspring, and the amount of provisions available toeach offspring varies, females should allocate well-stockedbrood cells to the sex that benefits most from being large.I tested this hypothesis using observations of organ-pipe wasps,Trypoxylon politum, and dissections of their nests. A Mississippipopulation of T. politum was intensively studied from 1993 to1995. This population fit the assumptions of optimal sex-allocationmodels by Green and Brockmann and Grafen. Female weight at emergencewas 1.29 times that of males, and wing length was 1.15 timesthat of males. This discrepancy in size occurred because thevolume of parental provisions strongly influenced adult bodysize, and better-stocked brood cells were preferentially allocatedto daughters. Brood-cell volume correlated with both wing lengthand weight at emergence in both sexes, and the chance that agiven brood cell contained a female offspring increased withincreasing brood-cell volume. Fitness was positively relatedto body size for females, but I found no evidence of an advantageto large males. Although there was evidence of stabilizing selectionfor male wing length in one year, there was no evidence of anincreasing relationship between body size and fitness (directionalselection) for males in either 1993 or 1994. Female fecunditywas positively related to body size in both years, indicatingthat larger females have increased reproductive success. Therate at which females provisioned brood cells was also correlatedwith body size. Observed patterns of investment in brood cellsare quantitatively consistent with the predictions of optimalsex-allocation theory, but certain aspects of female provisioningbehavior suggest females are not following a single "optimal"strategy. Patterns of provisioning were variable among differentfemales at the study site during the same year. Large femalestended to produce larger offspring. Although Brockmann and Grafen'smodel predicts a single, population wide "switchpoint" fromthe production of male to female offspring, there was no evidencefor such a switchpoint     

The behavioral ecology of threshold evolution in a polyphenic beetle

Facultative expression of alternative male morphologies is thoughtto allow individual males to select the phenotype with the highestfitness gain given their competitive status relative to othermales with which they compete for females. Choice of, or switchingbetween, morphs commonly relies on developmental threshold responses.Evolutionary changes in developmental threshold responses arethought to provide an important avenue for phenotypic diversificationand the evolution of morphological and behavioral novelties.However, the extent to which alternative male phenotypes andtheir underlying threshold responses actually evolve in naturalpopulations is unclear. Likewise, the ecological factors thatshape the evolution of threshold responses in natural populationsare unexplored for most organisms, as are the consequences ofsuch modifications for patterns of morphological diversity.I examined the ecological basis of rapid threshold evolutionin exotic populations of the horn-polyphenic dung beetle Onthophagustaurus. Male O. taurus vary continuously in body size as a functionof larval feeding conditions. Only males that exceed a criticalthreshold body size develop a pair of long horns on their heads,whereas males below this threshold remain hornless. Populationsin two exotic ranges of this species, the eastern United Statesand western Australia, have diverged in the mean threshold bodysize, which has resulted in the evolution of highly divergentand novel horn length–body size allometries in these populations.Populations in a third and previously unstudied exotic rangeof O. taurus in eastern Australia exhibit threshold body sizesroughly intermediate between the eastern U.S. and western Australianpopulations. I tested three hypothesis to explain how differencesin ecological and demographic factors can drive allometric divergencesbetween populations, using data derived from comparative, standardizedsampling of a large number of populations in each exotic range.Results suggest that differences in the intensity of both intra-and interspecific competition have contributed to the evolutionof divergent thresholds in these populations. My results donot support the hypothesis that shifts in threshold body sizesto larger body sizes are a consequence of increases in the meanbody size of competing males. I discuss my results in the contextof Onthophagus mating systems and the evolutionary implicationsof threshold evolution.     

Phenotypic quality and molt in the barn swallow, Hirundo rustica

Phenotypic quality may determine the development and expressionof secondary sexual characters. We studied the relationshipbetween molt and several measures of phenotypic quality in thesexually size-dimorphic barn swallow (Hirundo rustica) in itswinter quarters in Namibia. Males were in a more advanced stageof molt than females and juveniles, and the speed of molt asdetermined from the residual of the regression of the size ofthe gap in wings caused by missing and growing feathers on wingmolt score (residual wing raggedness) was also higher in malesthan in females and juveniles. Male barn swallows with longand symmetric tail feathers had a more advanced stage of moltand molted at a higher speed than males with short and asymmetrictails. Long-tailed females had a delayed molt, and females withasymmetric tails had less advanced molt and lower rates of feathergrowth than females with symmetric tails. Molt of secondariesin juveniles also appeared to be less advanced if they had longtails. Adult barn swallows molted their tail feathers in anirregular sequence with the longest, outermost tail featherusually replaced before the second or the third outermost feathers.Good body condition was positively associated with a high moltscore for some feather tracts and a rapid wing molt in adultfemales and tail molt in juveniles. Mallophaga were only weaklynegatively associated with primary and secondary molt scorein adult females and speed of wing molt in adult males. In conclusion,phenotypic quality of adult male barn swallows as reflectedby the expression of their secondary sexual character duringthe previous molt reliably reflected stage and speed of currentmolt.     

Low frequency of extrapair paternity in the polygynous great reed warbler, Acrocephalus arundinaceus

We carried out DNA fingerprinting on 553 young (130 broods)great reed warblers (Acrocephalus arundnaceus) in 1987–1991.In the study population, where 40% of the males become polygynous,there was a low frequency of extrapair fertilizations (EPF).When data from all five years were pooled, 3.1% of the youngwere sired by extrapair males (EPF-males) and 5.4% of the broodscontained extrapair young. We found no cases of extrapair maternity;young with 6–17 mismatched DNA bands (n= 17) had highband sharing with their putative mothers (range = 0.52–0.72)but low band sharing with their putative fathers (range = 0.24–0.40).In broods exposed to EPF, on average 53% of the young were siredby EPF-males. We found the genetic father to each of the illegitimateyoung. In all cases the same EPF-male sired all extrapair youngin a brood. Broods containing EPF-young tended to be initiatedlate during the breeding season. Breeding attempts were ratherevenly distributed over two months, thus this breeding asynchronywould have facilitated EPFs. There was no difference in EPFfrequency between broods where the pair males had left theirfemales unguarded during parts of their fertile periods andbroods where males guarded throughout the fertile periods. Nestswith extrapair young had significantly shorter mean distanceto the closest male neighbor and more male neighbors within100 m than nests without extrapair young. We found no indicationthat females engaged in EPF to get parental care from the EPF-males,or because they were forced to copulate with extrapair males.The low frequency of EPF suggested that females did not seekgenetic diversity to their brood. We cannot rule out the possibilitythat females engaged in EPF to insure fertility. However, datasupporting this hypothesis were weak. Instead, our data supportthe conclusion that females engaged in EPF to increase the geneticquality of their offspring, and that females may have used malesong repertoire size as a cue when choosing EPF partners.     

Costs of loading associated with mate-carrying in the waterstrider, Aquarius remigis

When one sex carries the other during some phase of courtshipor mating, the associated loading may entail a significant costto the carrier. This paper presents a series of laboratory experimentsdesigned to identify the costs of mate-carrying in Aquariusremigis. Female A. remigis mate repeatedly and carry each matefor several hours. Dead males and lead weights were used tosimulate normal mating and loading associated with mate-carrying,respectively. Females carrying weights equivalent to the weightof an average male showed no detectable reductions in survival,lipid reserves, or foraging success, and maintained themselveson the water surface for more than 10 days without access toresting sites. Weights equivalent to two males were supportedfor 6.1 ± 4.9 days. Thus, female A. remigis appear tobe very well adapted to carrying their mates and are unlikelyto be near their load limits when carrying a single mate. However,females carrying males or equivalent weights suffered a significantreduction in maximum mobility (stride length and speed), andan increased risk of predation by frogs (Rana clamitans). Femalescarrying weights were more susceptible to predation than unburdenedfemales but were less susceptible than females carrying males,suggesting that loading contributes significantly to, but doesnot fully explain, the increased predation risk. This risk probablyresults from both reduced mobility due to loading and greatervisibility (size). Possible influences of the costs of loadingon mating behavior and sexual size dimorphism are discussed.     

Exploring individual quality: basal metabolic rate and reproductive performance in storm-petrels

Despite evidence that some individuals achieve both superiorreproductive performance and high survivorship, the factorsunderlying variation in individual quality are not well understood.The compensation and increased-intake hypotheses predict thatbasal metabolic rate (BMR) influences reproductive performance;if so, variation in BMR may be related to differences in individualquality. We evaluated whether BMR measured during the incubationperiod provides a proximate explanation for variation in individualquality by measuring the BMRs and reproductive performance ofLeach's storm-petrels (Oceanodroma leucorhoa) breeding on KentIsland, New Brunswick, Canada, during 2000 and 2001. We statisticallycontrolled for internal (body mass, breeding age, sex) and external(year, date, time of day) effects on BMR. We found that maleswith relatively low BMRs hatched their eggs earlier in the seasonand that their chicks' wing growth rates were faster comparedto males with relatively high BMRs. Conversely, BMR was notrelated to egg volume, hatching date, or chick growth rate forfemales or to lifetime (23 years) hatching success for eithersex. Thus, for males but not for females, our results supportthe compensation hypothesis. This hypothesis predicts that animalswith low BMRs will achieve better reproductive performance thananimals with high BMRs because they have lower self-maintenancecosts and therefore can apportion more energy to reproduction.These results provide evidence that intraspecific variationin reproductive performance is related to BMR and suggest thatBMR may influence individual quality in males.     

Correlates of male quality in the field cricket, Gryllus campestris L.: age, size, and symmetry determine pairing success in field populations

Sexual selection in field crickets (Gryllidae) is well documented.Several studies have identified male traits, such as age andbody size, that influence pairing success in the field. HereI show how these traits covary with male quality by examiningthe degree of fluctuating asymmetry in male Gryllus campestris.Older males were both larger and more symmetrical. Principalcomponents analysis suggests that these three variables wereessentially measures of the same trait, male quality. A comparisonof paired and calling males showed that males of high qualitywere more successful in obtaining mates; paired males were older,larger, and more symmetrical. The area of the harp covariedwith morphological traits and determined the carrier frequencyof the male's call. Information related to male quality wastherefore available for female discrimination.     

Black holes, mate retention, and the evolution of ungulate leks

In some ungulates living in unstable herds, females in estrusleave their usual groups and join males defending mating territories.During the 12-24 h before mating, females commonly move severaltimes between males. These movements are commonly caused byharems being disrupted by young males or by overenthusiasticcourtship by the territorial male. When females leave a territorythey typically move to its nearest neighbor. Under these circumstances,clusters of territories can retain estrous females until theymate and leave the lek. This paper develops a model to investigatethe benefits of defending clustered versus dispersed territoriesto males and the consequences of variation in the rate of femalemovement between territories (P_m) and the tendency for femalesto move from one territory to a neighboring territory (a). Wherefemales move between territories at least once every 24 h (P_m< 0.04) and usually move to neighboring territories (a <0.5), the mating success of males is inversely related to thedistance from their territory to its nearest neighbor, and malesdefending clustered territories have higher mating rates thanthose defending dispersed territories. This process may be importantin the initial evolution of ungulate leks, which may resembleblack holes, attracting and retaining estrous females untilthey mate and their estrus ceases. It provides one possibleexplanation of the evolution of ungulate leks that does notrely on female preferences for mating with particular phenotypiccategories of males.     

Constraint versus restraint in the body mass dynamics during moult of a species with precocial young

Body mass declines during wing moult in numerous, but not all, populations of Anatidae. We assessed two leading hypotheses for body mass dynamics during wing moult: (1) body mass dynamics are adapted to attain a target body mass at the end of wing moult (restraint hypothesis) vs. (2) body mass dynamics reflect environmental constraint on the nutrient–energy balance during wing moult (constraint hypothesis). We used regressions of mass of breeding female Black Brant Branta bernicla nigricans on ninth primary length (a measure of moult stage) for each of 16 years to assess mass dynamics during wing moult and used regression equations to predict mass at the beginning and end of wing moult each year. We also included gosling mass at 30 days (an indicator of forage availability) in models of adult mass to assess how mass dynamics varied as a function of foraging conditions. Predicted body mass (± 95% CI) at the start of wing moult (ninth primary = 0 mm) varied significantly among years from 1032 ± 52 to 1169 ± 27 g. Similarly, predicted mass in late wing moult (ninth primary = 142 mm) ranged from 1048 ± 25 to 1222 ± 28 g. The rate of mass gain was significantly related to gosling mass at 30 days: interaction between adult ninth primary length and gosling mass = 0.0031 ± 0.0020 (P = 0.003). Females initiated wing moult at lower body masses, gained mass more rapidly and ended with wing moult heaviest when goslings were heaviest. Body mass dynamics of female Black Brant during wing moult were consistent with the constraint hypothesis. The positive association between gosling mass and rate of body mass gain by adult females during wing moult was also consistent with the constraint hypothesis.     

Vertical bars on male Xiphophorus multilineatus: a signal that deters rival males and attracts females

We examined the function of the vertical bar pattern on maleswordtails (Xiphophorus multilinneatus) as a signal in bothmale-male competition and female choice. This pattern had previouslybeen described as an aggressive signal because males intensifiedthe bars during male-male encounters in the laboratory. Ourfield observations supported this observation and also showedthat bars intensified when males courted females. The intensityof bars was correlated with access to females in the field.Within the size range of males that have bars, however, neitherbar number nor male size appeared to influence access to females.We used freeze-branding to remove the bars from males in thelaboratory so that we could control for characters correlatedwith bar intensity, and tested males and females separatelyso that we could separate the influence of these two componentsof sexual selection. We compared the responses of males andfemales to males that had their bars removed and control malesfreeze-branded between the bars. Test males responded more aggressivelyto males without bars as compared to control males. In addition,females showed a preference for control males over males thathad their bars removed. These results suggest that the barsmay function as a signal that deters rival males and attractsfemales.     

Bellows, copulations, and sexual selection in bison (Bison bison)

We provide data on the timing and frequency of bellowing episodesand 261 copulations in North American bison (Bison bison) extractedfrom more than 8500 h of observation to examine Darwin's ideathat by using breeding-season vocalizations the male "endeavoursthus to charm or excite the female." Copulatory status affectedvocalization rates; after mating, the frequency of male bellowingdropped to 16.1% of precopulatory rates (p < .0001). Malesbellowed at a higher rate on days when females were in estrus(p < .025) than when they were in anestrus, but females'probability of offspring production in the next year was notcorrelated with bellowing rate. For estrous females, bellowingwas positively correlated with number of attendant males (p< .0001) and inversely related both to the number of daysindividual males had participated in the rut (p < .05) andto the date during the 6-week breeding season (p < .003).Body size also influenced bellowing rate: with other factorsheld constant, small males bellowed more than large males (p< .03). With respect to Darwin's idea that vocalizationsserve as a male display to females, (1) males bellowed neitherbefore nor after copulating when rivals were absent but (2)they bellowed both before and after copulating when rivals werepresent. Overall, these results suggest that bison bellows donot serve as advertisements to females but function as intrasexualdisplays. [Behav Ecol 1991; 2: l-6]     

Conflict between sexes in the water strider, Gerris lacustris: a test of two hypotheses for male guarding behavior

We studied the effect of operational sex ratio on female reluctanceand male persistence to mate as well as on the length of copulationand postcopulatory guarding in Gerris lacustris by adding fivesurplus males or females to the basin with a pair in tandem.In the control treatment, a pair alone was tested. Accordingto the copulatory guarding hypothesis (CGH), males should prolongmating and guard females in the presence of surplus males. Accordingto the convenience polyandry hypothesis (CPH), females shouldshow lower levels of resistance to prolonged mating in the presenceof surplus males because the mating male protects the femaleagainst harassment from other males. As expected on the basisof both the CGH and CPH, mating (copulation + guarding) averagedlonger in the male-biased treatment. The behavior of males andfemales during mating suggested that both hypotheses hold true:females showed less resistance to prolonged mating (as predictedfrom CPH), and male behavior suggested stronger efforts to stayon the female when surplus males were present (as predictedfrom CGH). Comparisons of the treatment with surplus femaleswith the results from the mating pair without surplus individualssuggested that the capabilities of water striders in tandemto assess the sex of nearby nonmating striders are limited.