Structural coloration and sexual selection in the barn swallow Hirundo rustica

Structural coloration has been hypothesized to play a role insexual selection, and we tested whether this was the case ina field study of the barn swallow Hirundo rustica. The dorsaliridescent plumage of barn swallows has a strong reflectancein the ultraviolet (UV) region, with adult males on averagereflecting 8-9% more than adult females, as revealed by a 2-yearstudy in southwestern Spain. The correlation between structural coloration (described by the reflectance in the UV part of thespectrum, UV chroma and blue chroma) and three other secondarysexual characters significantly associated with male matingsuccess (tail length, tail asymmetry, and red facial coloration)was weak and generally nonsignificant. Nor was there a significantrelationship between color parameters and body condition. Wetested for an association between structural coloration of the dorsal plumage and sexual selection in a number of independenttests. Arrival date of males was not significantly relatedto color; there was no significant relationship between colorationand probability of survival or age; mated males did not havestronger reflectance than unmated males; and the duration ofthe premating period was not significantly related to color.Reproductive success was not significantly correlated withplumage coloration in males, nor was the feeding rate of offspringby brightly colored males higher than that of males with lessbright plumage. Given that sample sizes were large, and the power of statistical tests high, we conclude that current sexualselection on the coloration of the dorsal plumage in the barnswallow is, at best, weak.     

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.     

Biological observations on small cyclopoid copepods in the Red Sea

The breeding activity, sex ratio and variation in body lengthof a number of cyclopoid copepod species from the upper 450mof the Red Sea were analyzed. In most species the body lengthof adult specimens was in the lower range or less than thosereported from other pelagic regions. In the central Red Seamost species had some individuals actively breeding, duringboth autumn and winter. To the north, breeding was observedin fewer species. Only two species, Paroithona sp. and Agetuslimbatus, however, seemed to find conditions for reproductionbetter in the north than in the central area. The sex ratioswere quite consistent for all species within the families Sapphirinidaeand Corycaeidae (high percentage of males) and Oithonidae (lowpercentage of males). Within the Oncaeidae, the percentage ofmales varied from a few percent to >60% of adults. Thereappears to be a relationship between sex ratio and size. Populationsof the very small Oncaea species, O.ivlevi, O.prendeli and O.zernovi,include far fewer males than those of their larger congeners.The available information on the biology of Oncaea is summarized.Peculiarities in their life histories and reproduction are discussed.     

A Review of Size at Maturity in Male Tanner (Chionoecetes bairdi) and King (Paralithodes camtschaticus) Crabs and the Methods Used to Determine Maturity

This paper reviews existing studies on the size of sexual maturityfor male Tanner or snow crab (Chionoecetes bairdi), a brachyuran,and the anomuran red king crab (Paralithodes camtschaticus).In this report the term sexual maturity is denned as the abilityto reproduce. A variety of indirect and direct methods thathave been used to determine maturity are reviewed. Examiningthe vas deferens for the presence of spermatophores was usefulin determining the size at which males first become mature.Breeding experiments in the laboratory demonstrated that mostmales, from both species, that produced spermatophores couldbreed with soft-shelled mates. Males of both species can breedat smaller sizes than do females. Morphometric techniques basedon reproductive tract weights and chela morphometry overestimatedthe sizes at which males mature in both species. Previous experimentsfor Tanner crab, which have internal fertilization, suggestsmall mature males can fertilize two to five females. Breedingexperiments showed recently matured red king crab do not appearto be able to fertilize more than one female per breeding season,while males nearing harvestable size can fertilize more thanone female. Breeding experiments and in situ observations of grasping pairsappear to be the most feasible methods for identifying malesize at maturity for these species. The value of morphometricestimations for determining when males mature is questionable.     

The influence of changing temperature on the life history of Daphniopsis ephemeralis

We present evidence which suggests that for Daphniopsis ephemeralischanges in water temperature may directly cue changes in sexratio, ephippial and parthenogenetic egg production and indirectlydetermine female body size. Field data suggested that over anannual cycle, a population of D. ephemeralis comprised animalswith three distinct life history phases. The first was madeup of exephippial females which hatched in the fall. They produceda second group made up of large parthenogenetic females whichcould produce more than one overwintering generation. In thespring these parthenogenetic females gave rise to a third lifehistory phase which comprised males and small ephippial females.Field enclosure experiments suggested that the small size ofspring females was not related to predation. Laboratory experimentssuggested that ephippial egg production at small body sizeswas associated with ‘increasing’ temperature. Theseexperiments also suggested that the fall reproductive pattern(parthenogenetic eggs and large body sizes) was associated with‘decreasing’ temperatures. We conclude that directionalityof seasonal temperature change may be the prime factor responsiblefor D. ephemeralis life history changes observed in the field.     

Female affiliative preference depends on reproductive state in the African cichlid fish, Astatotilapia burtoni

Hormones play a pivotal role in reproductive behavior and havebeen implicated in mediating mate choice decisions. Here weasked whether the differences in female reproductive state dependenton changes in hormone levels correspond to changes in femaleaffiliation with males. In the African cichlid fish, Astatotilapiaburtoni, males shift between reproductive (territorial; T) andnon-reproductive (non-territorial; NT) states depending on socialcontext while females alternate between gravid (egg bearing;G) and non-gravid (NG) reproductive states independent of socialconditions. Moreover, the brain-pituitary-gonadal axis controllingreproduction and reproductive hormones is substantially remodeledin both males and females depending on reproductive state. Tomeasure affiliative preference, gravid and non-gravid femaleswere given the choice of associating with T or NT males. Gravidfemales preferentially associated with T males, whereas non-gravidfemales showed no preference. To discover whether gravid femalesuse male size independent of dominance status as a cue for theirchoice, gravid females were given a choice between territorialmales of different sizes. Gravid females preferred the smallerof two T males, but the smaller T males were significantly moreactive. Our results show that associative change could be animportant behavioral mediator between hormonal cues and reproductivesuccess, and that females use a hierarchy of cues in decision-making,preferring to affiliate with T over NT males and, among T males,preferring more active animals.     

Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contestsover breeding burrows and is waved to attract females. We recentlydiscovered that males of the red-jointed fiddler crab, Uca minax,also use the claw to kill smaller-sized fiddler crabs, U. pugnaxand U. pugilator, with which they co-occur in Atlantic coastsalt marshes. Large U. minax males use walking legs or the enlargedclaw to capture prey feeding on moist sand flats. On sand flats,small U. minax males and females are much less common than largemales, suggesting that large males move onto sand flats to seekprey. Males of prey species use the major claw against attackingpredators and, consequently, are more likely than females toescape. In laboratory experiments, large U. minax males weremore likely to attack and kill small-clawed males and femalesthan large-clawed males, consistent with a preference for morevulnerable, less threatening prey. The size of the major clawis a positive allometric function of body size. The allometricfunction varies little among species. Also, the mechanical advantageand indices of closing speed and closing force of the majorclaw, when corrected for body size, are not consistently greaterin U. minax relative to prey species. Thus, predation by U.minax males may reflect the opportunity afforded by larger bodysize and positive allometric growth, which result in a majorclaw that is more massive than the prey it is directed against.     

The influence of water velocity on the display behavior of male guppies, Poecilia reticulata

The objective of this experiment was to determine if differentwater velocities during ontogeny affect male physical condition,male signal intensity, and female mating preferences in theguppy (Poecilia reticulata). Guppies were raised in aquariain either high or low water velocities and fed in excess. Analysisof 72 males from four replicates indicated that high velocitymales had longer mean displays and spent more total time displayingthan low velocity males. These males also had significantlyfaster swimming speeds, wider caudal peduncles, and were moreattractive to females than low velocity males. There were nodifferences in display rates, body widths, standard lengths,or copulation attempts between high and low velocity males.These results indicate that water velocity conditions duringontogeny act as a proximal factor that influences the displaybehavior of guppies. Water velocity caused an indirect increasein the power of the male display to produce a female sexualresponse. The morpho-metric measurements suggest that the proximalmechanism behind the increased display intensity was the increasedmuscle development of males raised in high water velocities,which resulted in longer prolonged swimming speeds and moreintense displays. Therefore, display behavior of guppies maybe a general indicator of overall male physical condition.[BehavEcol07: 272–278 (1996)]     

Male body length influences mate-choice copying in the sailfin molly Poecilia latipinna

Females can choose a male independently of other females' matepreferences, or they can copy the mate choice of other females.Alternatively, mate-choice copying and independent mate choicecan interact if females assess male traits when deciding whetheror not to copy. We investigated how mate-choice copying interactswith a preference for large males in the sailfin molly (Potecilialatipinna). Sailfin molly females exhibited a preference forlarger males. They also copied the mate choice of other femaleswhen males were of similar body length. Females did not copy,however, when males differed substantially in body length. Ourresults show that conspecific mate copying occurs in the sailfinmolly but does not override a preference for larger males.     

Territorial male bullfrogs (Rana catesbeiana) do not assess fighting ability based on size-related variation in acoustic signals

Some animals use communication signals to assess their opponent'ssize and fighting ability during aggressive conflicts. Malefrogs assess their opponent's size based on the fundamentalfrequency (pitch) of advertisement calls, which is negativelycorrelated with body size, an important determinant of fightingability in frogs. I conducted a field playback experiment to investigate whether territorial male bullfrogs assess the sizeof opponents based solely on size-related variation in fundamentalfrequency. I repeatedly broadcast synthetic bullfrog advertisementcalls to three groups of males. Playback stimuli simulateda large male (n = 24), a small male (n = 24), or an acousticallysize-matched male (n = 34). Neither the simulated size of theopponent, the subject's own size, nor the degree of size asymmetrybetween the subject and simulated intruder had significanteffects on the magnitude of responses during the playback testor on the rate of habituation that occurred with repeated stimulation.Post-hoc analyses of effect sizes and statistical power indicatedthat the effects in this study were quite small compared toprevious studies in other frogs. More important, power analysesindicated that this study had high power (1 - ß >0.90) to detect the magnitude of effect sizes observed in previous studies. Thus, territorial male bullfrogs do not appear to assessan opponent's fighting ability based solely on the fundamentalfrequency of acoustic signals. These results contrast starklywith theoretical predictions and previous empirical work withfrogs.     

Mate preference for multiple cues: interplay between male and nest size in the sand goby, Pomatoschistus minutus

Female mating preferences are often based on more than one cue.In empirical studies, however, different mate choice cues aretypically treated separately ignoring their possible interactions.In the current work, we studied how male body size and sizeof the male's nest jointly affect mate preferences of femalesand gobies, Pomatoschistus minutus. The females were givena binary choice between males that differed either in body sizeor size of their nest or both. We found that neither body sizenor size of the nest alone affected male attractiveness, buttogether these 2 cues had a significant effect. Specifically,large males were more popular among females when they had alarge nest than when they occupied a small nest. The resultssuggest that if interaction effects between multiple mate choicecues are not considered, there is a danger of ignoring or underestimatingthe importance of these cues in sexual selection by female choice.     

Contrasting life histories of the predatory cladocerans Leptodora kindtii and Bythotrephes longimanus

The predatory cladocerans, Leptodora kindtii (Focke, 1844) andBythotrephes longimanus (Leydig, 1860), express markedly differentlife-history traits. Leptodora produce small-bodied neonatesthat mature at small sizes but continue to grow throughout life.Bythotrephes produce larger neonates in both relative and absoluteterms that grow rapidly to a large size at maturity whereuponthey curtail somatic growth and divert resources mainly to reproduction.Despite their remarkable differences, the sets of life-historytraits of both species appear to be solutions to the same basicselection pressures imposed by visually discriminating gape-limitedfishes and foraging constraints imposed by prey size. Leptodorastresses pre-contact (transparency) while Bythotrephes stressespostcontact (caudal spine) modes of morphological defense againstfishes. Mounting these disparate modes of defense has consequencesfor selection on timing and allocation to body growth that mayunderlie competitive imbalance between the species. Owing tothe production of large-bodied neonates that grow rapidly, Bythotrephesquickly attain body sizes that both admit them to a broaderprey base in size and taxonomic variety, and allow shorter preyhandling times, in comparison to Leptodora. This provides Bythotrepheswith a wider and more exploitable prey base from an earlierage and may explain why Leptodora has declined in density followingBythotrephes invasion into some North American lakes. The divergentsets of life-history traits expressed by Leptodora and Bythotrephesparallel two dominant life-history strategies evolved by phytoplanktivorousspecies of the order Cladocera.     

Allometry and sexual selection of male weaponry in Wellington tree weta, Hemideina crassidens

Both male and female Wellington tree weta, Hemideina crassidens,use cavities in trees as diurnal shelters. That these galleriesare often limiting in nature offers males the opportunity toincrease their reproductive success by monopolizing galleriesand the females residing in them. Male H. crassidens, can matureat either the 8th, 9th, or 10th instar, whereas females matureat the 10th instar only, and male head (and mandible) size positivelycovaries with ultimate instar number. It has been suggestedthat males fight for control of galleries by using their enlargedmandibles as weapons, and males with larger mandibles controlgalleries with more females. In the present study, I presenta statistical examination of sexual dimorphism, showing thattraits related to head size are on average significantly largerin males, whereas traits related to body size are on averagesignificantly larger in females. I tested three predictionsaddressing the hypothesis that sexual selection is driving megacephalyin male H. crassidens. First, as predicted, traits related tohead size show a positive allometric relationship with bodysize in males but not in females. Second, adapting a novel statisticaltechnique based on maximum likelihood and bootstrapping revealedthat males, but not females, exhibit a multimodal distributionin head and body size traits. This is likely a consequence ofmales maturing at one of three instars, which results in positivecovariance between the ultimate instar number and morphologicaltraits. Third, as predicted, single adult males with largerheads reside in galleries housing larger groups of adult females.     

Sexual cannibalism, competition, and size dimorphism in the orb-weaving spider Nephila plumipes Latreille (Araneae: Araneoidea)

The degree and direction of sexual dimorphism varies widely,but in several taxa of orb-weaving spiders, including Nephila,males may be less than one-tenth the size of females. This differenceis commonly attributed to selection through precopulation sexualcannibalism: females may either fail to detect very small males,or ignore them as potential prey items. However, there is oftenthe potential for male-male competition in these species becauseseveral males can be found on the web of a single female. Weinvestigated experimentally the effects of sexual cannibalismand male-male competition on male body size and hence sexualdimorphism in the Australian golden orb-weaver (Nephila plumipes).Small males were less likely to be detected and cannibalizedthan larger males. However, larger males excluded small malesfrom the central hub of the web, where mating takes place. Theconflicting effects of sexual cannibalism and male-male competitionmay be responsible for the relatively large variation in malebody size in this species.     

Hydrolysis of sex pheromone by antennal esterases of the cabbage looper, Trichoplusia ni

Esterases were isolated from chemosensory sensilla on the antennaeof Trichoplusia ni (Hübner). The disc gel electrophoreticpatterns of these esterases from males and females were similar;however, more bands were observed in the antennae than in 8other tissues examined. Most of the esterases detected in the100,000 g supernatant of the antennal preparation could be dissociatedfrom the 100,000 g membrane pellet. Esterases from both maleand female antennae hydrolyzed the sex attractant, (Z)-7-dodecen-l-olacetate, more rapidly than did the legs, fat body or Malpighiantubules. The enzymes primarily responsible for pheromone catabolismwere less sensitive to paraoxon, eserine and p-(hydroxymercuri)benzoatethan those hydrolyzing 1-napthyl acetate. This suggested thata major portion of the observed pheromone-hydrolytic activitywas due to acetylesterases. Measurement of pheromone hyrolysisin sections of disc gels that contained separated antennal orabdominal body wall esterases revealed 2 peaks of activity withboth tissues; however, the rate of pheromone hydrolysis by theabdominal esterases was slower than that of the antennae. Thesignificance of these findings is discussed in relation to thepossibility of antennal esterases having a functional role inthe olfactory process of males of T. ni.     

Pheromonally mediated mate attraction by males of the burying beetle Nicrophorus orbicollis: alternative calling tactics conditional on both intrinsic and extrinsic factors

Male burying beetles attract females using a pheromonal signaland can provide parental care and a food resource, vertebratecarrion, for their developing offspring. But males attempt toattract females even when they have no carrion. We examinedthe factors that influence male behavior directed toward findingor attracting mates in both field-caught and laboratory-reared Nicrophorusorbicollis, a North American burying beetle. We investigatedwhether male behavior differed based on both intrinsic (size)and extrinsic (resources held) differences among males. Further,we examined repeatability of individual behaviors and the effectof holding or lacking resources on these repeatabilities. Field-caughtand laboratory-reared individuals differed in overall activitybut not in their behavioral repertoire, making studies of laboratory-rearedmales relevant. The behavior of individual males was very consistentwithin a condition, but plastic between resource conditions.The frequency of calling (adopting a posture that indicatespheromone release to attract females) depended on male sizewhen males did not hold resources, but this relationship disappearedwhen males held resources. Without carrion, smaller males calledmore frequently than did larger males. When holding carrion,smaller males reduced their calling, whereas larger males significantlyincreased the frequency with which they attempted to attractfemales and reduced the amount of time they spent searching.Thus, calling behavior of males was conditional on not only intrinsicand extrinsic factors, but also an interaction between them.We suggest that the changes in calling represent alternativetactics based on the costs and benefits of attracting both potentialmates and competitors, which differ for males of different sizes.     

Seasonal variations in copepod size: effects of temperature, food abundance, and vertebrate predation

Seasonal dry weights of female and male Cyclops bicuspidatusthomasi, Diaptomus ashlandi and Diaptomus minutus were studiedin southeastern Lake Michigan during 1975–1981. Smallestanimals occurred during summer and early fall, and largest animalsin winter and spring, a pattern reported for other copepods.The range of weights for the species and sexes decreased fromwinter to summer, and converged in summer to a value of approximately2 µg/animal. Surface water temperature and abundancesof young-of-the-year (YOY) fish were inversely correlated withweights of males and females of the three copepod species. Standingstocks of important phytoplankton groups, especially pennatediatoms and flagellates, were positively correlated with copepodweight. Seasonal change of copepod body size appears to be morethan simply a function of temperature; seasonal predation byYOY fish may be a factor influencing sizes of adult copepods.     

Sequential mate encounters: female but not male body size influences female remating behavior

Whether and how individuals choose sequentially among matesis an important but largely neglected aspect in sexual selectionstudies. Here, we explore female remating behavior in the cellarspider Pholcus phalangioides. We focus on body size as one ofthe most important traits involved in mate choice. Large andsmall females (n = 216) were double mated with large or smallmales in all eight possible combinations. All females copulatedwhen virgin, but only 82% accepted a second male. The chanceof a female remating was not significantly predicted by thebody size of the second or first male or by the size differencebetween the two. In contrast, a previous study demonstrateda male size effect in that larger males monopolized femalesuntil egg laying when two males of different sizes were present.We suggest that sequential encounters are more common undernatural conditions than male monopolization of females becauseestimates of concurrent multiple paternity together with observationsin a natural population do not favor mate guarding as the predominantmating strategy in this species. It follows from our study thatthe intensity of sexual selection on male size may be greatlyoverestimated when using a competitive laboratory setting fora species in which females generally encounter mates in a sequentialfashion. Female remating probability was significantly predictedby female size, with large females remating with higher probabilitythan small females. Thus, when mating with large females, malesmay gain higher fertilization success through increased femalefecundity but also face a higher sperm competition risk.     

Male tactics and reproductive success in the harem polygynous bat Saccopteryx bilineata

Alternative tactics in reproductive behavior enable individualsto maximize their fitness in relation to competitors in thesame population. In many taxa, territoriality is a common tacticof males to increase their reproductive success. In the batSaccopteryx bilineata, territorial males defend roosting areasfor females against other males and court females throughout the year. Peripheral males in the same colonies do not defendterritories but compete with territorial males for reproductionwith females. In this study, we monitored the behavior of themales in a natural colony over three reproductive seasons.We compared morphological and age data and measured the reproductiveoutput of males adopting the territorial or peripheral tactic.No differences in body size or weight were detected betweenmale types, but the probability of adopting a tactic seemedto be age dependent. Peripherals were often young males andreplaced territorials in several cases, whereas the oppositecase was not observed. Peripherals were not excluded from reproduction,but territorials were more likely to reproduce. Variation in reproductive success was high within both male tactics, andthe reproductive success of some peripherals was comparableto territorials, but, on average, the reproductive successof territorials was more than twice as high. Therefore, behavioraltactics do not seem to be equally profitable in general butmay represent different phases in the reproductive life of manyS. bilineata males.     

Linear and threshold-dependent expression of secondary sexual traits in the same individual: insights from a horned beetle (Coleoptera: Scarabaeidae)

Elaborate horns or horn‐like structures in male scarab beetles commonly scale with body size either (a) in a linear fashion with horn size increasing relatively faster than body size or (b) in a threshold‐dependent, sigmoid fashion; that is, males smaller than a certain critical body size develop no or only rudimentary horns, whereas males larger than the threshold size express fully developed horns. The development of linear vs. sigmoid scaling relationships is thought to require fundamentally different regulatory mechanisms. Here we show that such disparate regulatory mechanisms may co‐occur in the same individual. Large males of the south‐east Asian Onthophagus (Proagoderus) watanabei (Ochi & Kon) (Scarabaeidae, Onthophagini) develop a pair of long, curved head horns as well as a single thoracic horn. We show that unlike paired head horns in a large number of Onthophagus species, in O. watanabei the relationship between head horns and body size is best explained by a linear model. Large males develop disproportionately longer horns than small males, but the difference in relative horn sizes across the range of body sizes is small compared to other Onthophagus species. However, the scaling relationship between the thoracic horn and body size is best explained by a strongly sigmoid model. Only males above a certain body size threshold express a thoracic horn and males smaller than this threshold express no horn at all. We found a significant positive correlation between head and thoracic horn length residuals, contrary to what would be expected if a resource allocation tradeoff during larval development would influence the length of both horn types. Our results suggest that the scaling relationship between body size and horn length, and the developmental regulation underlying these scaling relationships, may be quite different for different horns, even though these horns may develop in the same individual. We discuss our results in the context of the developmental biology of secondary sexual traits in beetles. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83 , 473–480.