Adaptive superparasitism in solitary parasitoids: marking of parasitized hosts in relation to the pay-off from superparasitism

Abstract.

• 1 The pay-off from an egg laid in a parasitized host is an important parameter in models on adaptive superparasitism in solitary insect parasitoids.
• 2 For Leptopilina heterotoma, a parasitoid of larval Drosophila, the pay-off from a second egg laid in a host is 0.43 offspring when the interval between the two ovipositions is less than 3h. For longer intervals, this pay-off decreases to almost zero for an interval of 24 h.
• 3 When a female encountering a parasitized host is able to estimate the interval since the first oviposition, it is expected that she will take this into account in her host selection decisions. This is, however, not in the direct interest of the female that lays the first egg, and marks the host.
• 4 We studied whether superparasitism in hosts containing a young egg is more common than in hosts containing an older egg, when searching in a patch containing once-parasitized and unparasitized hosts.
• 5 The acceptance/encounter ratio of parasitized hosts increased for intervals longer than 6h, as predicted when the interests of the marking female and the longevity of the mark are taken into account.
• 6 Superparasitism occurred more often when parasitoids had previously searched a host patch 7 days before the experiment compared to when parasitoids had searched a patch 1 day before, a phenomenon predicted by dynamic optimal diet models.

     

Female and male Texas cichlids (Herichthys cyanoguttatum) do not fight by the same rules

Selection usually acts differently on males and females duringintrasexual competition for resources and/or mates. Nevertheless,agonistic behavior has been examined both theoretically andempirically mostly in males. Our research questions whethermales and females follow the same rules of engagement in intrasexualcontests as predicted by the sequential assessment model (SAM).The SAM predicts negative correlations between contest intensityand duration and the magnitude of asymmetry in resource holdingpower (RHP) between the contestants, such that the most escalatedcontests are those between similarly endowed individuals. Westaged male and female intrasexual contests with varying degreesof body size asymmetry under a round robin design using themonogamous Texas cichlid fish (Herichthys cyanoguttatum) asa study case. We used Mantel's matrix analysis to compare howthe behavioral content, duration, structure, and outcome ofmale and female contests were affected by the relative bodysize of the contestants. In the case of males, relative sizein each contest predicted outcome, duration, and frequency ofconventional and escalated behaviors according to prevailingtheory. Female contest structure and outcome, however, werenot predicted by the relative size of contestants. We discussour results in terms of other asymmetries that might be importantin structuring female contests, and we propose potential approachesto study female–female aggression.     

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.     

Factors that affect hatching asynchrony in the chinstrap penguin (Pygoscelis antarctica)

We examined if laying intervals and hatching asynchronies are related in a chinstrap penguin (Pygoscelis antarctica) population in the South Orkneys. The lack of association between the two variables, as well as data on brood patch development, indicated that incubation did not begin immediately after the first egg was laid. This suggests that longer laying intervals may be compensated by longer delays in the onset of full incubation. Hatching asynchrony increased with within-clutch egg size asymmetry, decreased with breeding date, and was related to the laying order of the eggs according to size. However, only egg-size asymmetry remained significant when controlling for the other variables. We conclude that more asymmetric clutches were more asynchronous, although a large part of the variation in hatching asynchrony remains unexplained.     

Visual information, resource value, and sequential assessment in convict cichlid (Cichlasoma nigrofasciatum) contests

We staged contests between convict cichlids (Cichlasoma nigrofasciatum)that were matched for size and gender to test the influenceof prior information and resource value on the duration andstructure of fights. The contestants were separated before thecontest by either clear or opaque dividers to allow or preventvisual assessment, respectively. Contests were shorter in the"clear" than in the "opaque" treatment, suggesting that visualassessment occurred. The duration of lateral display, a noncontactdisplay, was shorter in the clear than in the opaque treatment,but the treatments did not differ significantly in the durationof three contact displays (biting, mouth wrestling, and circling).These results are consistent with the hypothesis that lateraldisplay provides primarily visual information, probably aboutbody size, whereas the other behavior patterns provide primarilynonvisual information, probably about strength. Second contestsbetween the same pair of fish were shorter than first contests,suggesting that the information acquired during the first contestmade it easier to resolve the second. After the subordinatefish from the second contest was given access to a mate, itfought more persistently so that third contests were longerthan second contests. Our results support the predictions ofthe sequential assessment model.     

Contest behavior in territorial male butterflies: does size matter?

Although intrasexual contests generally favor bigger or stronger individuals, the relevance of body size to war of attrition-typedisputes between weaponless animals such as butterflies isunclear. In this study I aimed to investigate the significanceof size in this context by studying territorial contests inHypolimnas bolina (L.), a species that exhibits consistentseasonal plasticity in body size. In this species adult ageis positively correlated with large size in spring but withsmall size in autumn. This shift allowed independent evaluationof the relevance of each variable (size and age) to intrasexualcontest success. Observation of a population of marked individualsindicated that only age appeared important, with the winnersof pair-wise contests significantly older than losers in both seasons, and with contests lasting longer when the age differencebetween the combatants was small. Age was also linked to residency;residents won 99% of all contests. This study suggests thatsize does not matter in these aerial disputes, but age andresidency do. It is not yet possible to determine whether olderbutterflies are intrinsically better competitors, or whether they simply have greater opportunity to find a vacant territory.     

Female-specific color is a signal of quality in the striped plateau lizard (Sceloporus virgatus)

Recent theoretical and empirical studies confirm that male matechoice and/or female–female mate competition can be expressedin the absence of sex-role reversal. Such reproductive patternsmay select for the evolution of female traits that indicatefemale phenotypic or genotypic quality among non–role-reversedspecies. Although attention to the evolution and function offemale ornaments is increasing, additional focus is needed onfemale-specific ornaments (those not expressed in conspecificmales) and on nonavian systems in order to gain a broad understandingof how selection acts directly on ornamentation of female animals.In the striped plateau lizard, Sceloporus virgatus, only femalesdevelop orange throat patches during the reproductive season.The color peaks in expression near the time of ovulation andappears to stimulate male courtship. Here, I examine whetherthis female-specific ornament can be used by males to reliablyassess female phenotypic quality. Using multivariate regressionanalyses, I show that the area of the orange color patch predictsbody condition and mite load, the chroma (i.e., saturation)of the color patch predicts body size, and both patch area andchroma reliably predict average egg mass. Thus, female reproductivecolor may function as a condition-dependent signal, indicatingphenotypic quality to potential mates.     

Habitat assessment by parasitoids: mechanisms for patch use behavior

Animals foraging for patchily distributed resources may optimizetheir foraging decisions concerning the patches they encounter,provided that they base these decisions on reliable informationabout the profitability of the habitat as a whole. Females ofthe parasitoid Lysiphlebus testaceipes exploit aphid hosts,which typically aggregate in discrete colonies. We show herehow between-colony travel time and the number of aphids in previouslyvisited colonies affect parasitoid foraging behavior. We firstassumed that parasitoids use travel time and previous colonysize to estimate a mean rate of fitness gain in the habitatand derived quantitative predictions concerning the effect ofthese two variables on patch residence time and patch-leavingrate of attack. We then tested these theoretical predictionsin laboratory experiments in which female parasitoids were allowedto visit two successive colonies. As predicted, the observedresidence time in the second colony increased with increasingtravel time and decreasing size of the first colony. Patch-leavingrate of attack decreased with increasing travel time but wasnot affected by previous colony size. These results suggestthat parasitoids use these two variables to assess habitat quality.However, discrepancies between the data obtained and quantitativepredictions show that the effect of travel time on patch usemay be more complex than assumed in our model.     

Evolution of fighting behavior under asymmetric competition: an experimental test with juvenile salmonids

Large-dominant and small-subordinate species engaging in asymmetricinterference competition may optimize behavior under differenttrade-offs between the chance of winning and the cost of fighting.If fighting behavior is heritable and under selection, theorysuggests that large-dominant and small-subordinate species shouldevolve aggressive and passive fighting behaviors, respectively.To test this prediction, I manipulated the size and competitiveasymmetry of juveniles from sympatric populations of large-dominantcoho salmon (Oncorhynchus kisutch) and small-subordinate steelheadtrout (O. mykiss) and asked whether differences in fightingbehavior persisted independently of competitive ability. I observedfighting behavior during dyadic contests in two habitats, mutuallypreferred pools and energetically demanding riffles, under eachof three size treatments: natural size asymmetry, asymmetryremoved, and reversed size asymmetry. The results supportedthe prediction. Competitive ability depended primarily on size;large individuals of both species dominated smaller heterospecifics,and neither species dominated when size matched. Fighting behaviordepended primarily on species identity; coho salmon used a higherproportion of aggressive chases, whereas steelhead trout useda higher proportion of passive displays. Large individuals weremore likely to chase, and small individuals were more likelyto display. As evidence that asymmetric competition is associatedwith behavioral divergence, these results complement previouswork on morphological divergence under asymmetric competitionand provide a richer context for other features of the coho–steelheadsystem.     

Aggression and RHP in the Northern Swordtail Fish, Xiphophorus cortezi: The Relationship Between Size and Contest Dynamics in Male–Male Competition

Current theory predicts that contest outcome, as well as decisions on whether to initiate a contest, escalate during a contest or retreat are decided by asymmetries in resource holding potential (RHP) and/or expected payoffs between contestants. In this investigation, dyadic contests were staged between male swordtail fish (Xiphophorus cortezi) where individuals were paired based on cumulative fight records and were ranked at the end of the trials in order to approximate RHP. Size was the only asymmetry that I did not attempt to control for and as a result, I was able to determine the relationships between size, contest initiation, escalation and outcome. Individuals changed their contest initiation strategy based on their size relative to that of their opponents, and contrary to predictions, the smaller of the two males in each contest was more likely to initiate the conflict than was the larger male. However, the larger of the two males was more likely to win and standard length proved to be a moderate predictor of an individual's final rank. Regardless of size, initiators fared poorly, winning only 31% of the contests. In instances where the smaller males won the contests, they were no more likely to initiate the encounter than was the larger male. However, when small males did win, fights lasted longer, suggesting that in some cases smaller males may be able to outlast their opponents.     

挥发性信息化合物对玉米螟赤眼蜂寄主选择行为的影响

在实验室条件下,利用“Y”型嗅觉仪测定了源于寄主亚洲玉米螟Ostrinia furnacalis (Guenée) 鳞片、卵表以及不同生理阶段的雌蛾附腺的不同浓度的提取物对玉米螟赤眼蜂Trichogramma ostriniae Pang et Chen寄主选择行为的影响,并对其引诱作用大小进行了比较。结果表明： 亚洲玉米螟鳞片的正己烷提取物在0.5 mg/mL和 1 mg/mL浓度时对玉米螟赤眼蜂有明显的引诱作用; 寄主卵的正己烷提取物在5块卵/mL和10块卵/mL浓度时对玉米螟赤眼蜂有显著的引诱作用,而在40块卵/mL浓度时对玉米螟赤眼蜂有极显著的驱避作用; 玉米螟赤眼蜂对亚洲玉米螟交配未产卵和产卵后前期的雌蛾附腺提取液有反应,而对处女蛾和产卵后期的附腺提取液没反应。卵表提取物和附腺提取物比鳞片提取液对赤眼蜂引诱作用强,两者对赤眼蜂的引诱作用没有明显差异。     

Self and conspecific superparasitism by the solitary parasitoid Antrocephalus pandens

Abstract.

• 1 The fitness consequences of superparasitism for a solitary parasitoid depend on whether the host was first parasitized by itself (‘self-superparasitism’) or a different individual (‘conspecific superparasitism’). Self superparasitism is usually expected to be avoided.
• 2 A.pandens females showed no difference in their probability of superparasitism between self-parasitized and conspecifically-parasitized hosts. The probability of superparasitism decreased as time from the laying of the first egg in a host increased, from about 0.29–0.46 at a time interval of 1 h to 0.10–0.14 at 72 h.
• 3 The egg distribution of wasps foraging alone on a patch showed significant avoidance of superparasitism, but that of wasps foraging in the presence of conspecifics was not significantly different from a random distribution. This suggests that wasps switch from avoidance of superparasitism when alone to acceptance of all hosts when in a group.
• 4 When wasps foraged in a group, the hosts had many more ovipositor puncture marks than when wasps foraged singly. This suggests that either hosts were attacked several times per encounter, or that the wasps' encounter rate with hosts was much higher when in a group. If the latter is true, it is possible that, although the egg distribution suggested a higher rate of superparasitism when wasps foraged in a group, the ratio of acceptances to contacts of parasitized hosts may in fact have been lower.

     

The impact of patch encounter rate on patch residence time of female parasitoids increases with patch quality

Abstract.  1. For animal species that forage on patchily distributed resources, patch time allocation is of prime importance to their reproductive success. According to Charnov's marginal value theorem (MVT), the rate of patch encounter should influence negatively the patch residence time: as the rate of patch encounter decreases, the patch residence time increases. Moreover, the MVT predicts that animals should stay longer in high quality patches.
2. Using the aphid parasitoid Aphidius rhopalosiphi (Hymenoptera: Aphidiinae), the effects of these two factors (patch encounter rate and host density) were combined in order to test if the increment in patch residence time for a given decrease in patch encounter rate was larger for high quality patches than for low quality patches.
3. The results show a significant effect of the interaction between the two factors. In high host density patches, parasitoids spent more time if they experienced a low patch encounter rate, while in low host density patches, patch encounter rate had no significant effect on the patch residence time. This suggests that the response of A. rhopalosiphi females to patch encounter rate varied with host density in the patch. Moreover, the same interaction effect was observed for the number of ovipositor contacts on aphids.
4. Parasitoid females can use patch encounter rate to estimate patch density in the habitat but the effect of this estimate on their patch residence time is modulated by patch quality. Staying longer in a patch when patches are rare is more advantageous when the fitness gained by doing so is large. In low quality patches, the expected fitness gain is small and the female may gain more by leaving and taking her chance at finding another patch.     

Fighting in Males of the Autumn Spider,Metellina segmentata: Effects of Relative Body Size,Prior Residency and Female Value on Contest Outcome and Duration

Males of the autumn spider, Metellina segmentata (***Araneac: Metidae), compete for access to mates by guarding the orb webs of mature females. We investigated the influences of relative male righting ability and resource value on fighting behaviour by staging interactions in the field on webs occupied by females. In these contests, the larger male nearly always defeated its opponent when it was at least 10% greater in size. For smaller size asymmetries between opponents, the male previously resident on the female's web usually won the contest. Contest duration decreased exponentially with increasing size asymmetry between opponents, as predicted if each male assessed its relative size and adjusted its fighting strategy according to its likelihood of winning. Contest duration was also greater when the prior resident was the lighter opponent, or when size and residency asymmetries favoured different opponents as winners. Prior residents fought longer over larger, more fecund females, indicating an adjustment of fighting effort according to assessments of resource value. In contrast, intruders did not increase their fighting effort over larger females, suggesting an inability to assess female size quickly and accurately. Assessment appears to reduce the costs of settling conflicts, but imperfect information can result in inaccurate assessments and unexpected outcomes. Assessment strategies are used by other types of spiders to resolve contests, but this appears to be the first evidence for such strategies among orb-web-building spiders.     

The kinds of traits involved in male--male competition: a comparison of plumage, behavior, and body size in quail

I compared the role of ornate plumage, behavior, and body sizeduring male—male competition in two species of New Worldquail. Gambel's quail (Callipepla gambelii) is a highly ornateand dichromatic species, whereas scaled quail (C. squamata)is unornamented and monochromatic. During paired contests betweenunfamiliar males, high rates of testosterone-mediated behaviors(tidbitting, calling) and large body size (mass, tarsus, andtail length) corresponded to winners. In the highly ornate Gambel's quail, male head plumes also influenced the outcomeof contests. Plume enhancement made Gambel's quail more likelyto win contests, whereas plume removal made males more likelyto lose. Plume position also reflected male status. Winningmales erected plumes, whereas losers frequently flattened them.Some plumage ornaments, such as belly patches, did not playa primary role during male contests. Unlike static ornaments,head plumes are highly modifiable and likely signal immediateinformation regarding a male's intent, similar to a coverablebadge. Combined, intrasexual selection favored dynamic traits(fast display rates, modifiable ornaments) and static traits(body size) as indicators of male condition or motivation.In scaled quail only, male size was favored both by male—malecompetition and female choice. Accordingly, the degree of sizedimorphism (tarsus length) is greater in scaled than in Gambel'squail. The frequency of overt aggression (chases, pecks, displacement)also differed between species. Gambel's quail were very aggressive,and subordinates often challenged their opponents. In contrast, scaled quail were less aggressive, and subordinates rarely disputedrank. Interspecific comparison indicated differences in themaintenance of male status and possibly in the honesty of signaling.Both appear to be related to differences in social system.     

Influences of sex, size, and symmetry on ejaculate expenditure in a moth

Although sperm fundamentally function to fertilize eggs, forcesarising from both sexes select for optimal ejaculate composition.Sperm competition is one recognized agent in the evolution ofsperm and ejaculate structure. Few studies, however, have examinedhow female factors influence ejaculate structure, despite somebehavioral evidence for male mate choice. Male Plodia interpunctella(Lepidoptera, Pyralidae) accrue all resources for reproductionas larvae. Adults emerge with a limited sperm complement andare therefore under intense selection to optimize gamete allocation.I detected no effect of male body weight on ejaculate size.However, female reproductive potential (ovary masses) was dictatedby body weight In addition, heavier females had greater spermathecalvolumes, but there was no such relationship with bursal size.Finally, heavier females showed a higher mating frequency. Ifound that mating males were sensitive to female size and producedlarger ejaculates when mating with heavier females. Males mayejaculate more sperm into larger females either because it paysthem to "spend" more reproductive resources on matings thatprovide greater reproductive potential, or because heavier (longerlived and more attractive) females mate more frequently andhave larger spermathecal volumes. Alternatively, females maycontrol spermatophore formation and "accept" an appropriateejaculate to maximize fertility. Males may therefore be alsoselected to ejaculate more sperm into larger females to counteractgreater risks of sperm competition associated with heavier females.There was no association between male or female femur asymmetryand ejaculate size. P.interpunctella may be selected to exercisemodulation of ejaculate size because males invest paternally,sperm for the single reproductive episode are limited, and femalefecundity and mating pattern vary between individuals and areassociated with body weight. More obvious variability in malereproductive behavior and choice may therefore be paralleledat the cryptic gametic level by plasticity in ejaculate allocation.     

Reproductive senescence and dynamic oviposition behaviour in insects

We explore the relationship between clutch size behaviour and the likelihood of egg limitation in insects with parasitoid-like life histories. We compare the incidence of egg limitation in insects that produce clutch sizes consistent with maximizing fitness per host (i.e. the single host maximum, or Lack clutch size) with insects exhibiting two types of dynamic behaviour. In the first case, insects produce clutch sizes according to a rule that specifies a negative relationship between egg complement and clutch size; in the second case, clutch size is determined by both egg complement and the host encounter rate using a dynamic state-variable model. The analyses are an extension of models presented by Rosenheim (1996), which predicted a strong positive relationship between the risk of egg limitation and the host encounter rate in the absence of dynamic oviposition behaviour. Our results are consistent with Rosenheim's, but they show that dynamic behaviour can greatly reduce th e risk of egg limitation and weaken the relationship between the host encounter rate and egg limitation.     

Optimal patch residence time in egg parasitoids: innate versus learned estimate of patch quality

Charnovs marginal value theorem predicts that female parasitoids should exploit patches of their hosts until their instantaneous rate of fitness gain reaches a marginal value. The consequences of this are that: (1) better patches should be exploited for a longer time; (2) as travel time between patches increases, so does the patch residence time; and (3) all exploited patches should be reduced to the same level of profitability. Patch residence time was measured in an egg parasitoid Anaphes victus (Hymenoptera: Mymaridae) when patch quality and travel time, approximated here as an increased delay between emergence and patch exploitation, varied. As predicted, females stayed longer when patch quality and travel time increased. However, the marginal value of fitness gain when females left the patch increased with patch quality and decreased with travel time. A. victus females appear to base their patch quality estimate on the first patch encountered rather than on a fixed innate estimate, as was shown for another egg parasitoid Trichogramma brassicae. Such a strategy could be optimal when inter-generational variability in patch quality is high and within-generational variability is low.     

Fiery red heads: female dominance among head color morphs in the Gouldian finch

Although the evolution of genetic color polymorphisms has receivedmuch theoretical interest, few empirical studies have investigatedthe adaptive function of alternative color morphs. Furthermore,most studies have focused almost exclusively on the evolutionand adaptive expression of male coloration, leaving the roleof conspicuous female coloration largely unknown. Using thecolor polymorphic Gouldian finch (Erythrura gouldiae), thisstudy experimentally tests the status signaling function ofhead color (red, yellow, and black) among the 3 female colormorphs. In standardized dominance contests between unfamiliarfemales of different head colors, red-headed females dominatedboth black- and yellow-headed females. During contests betweenthe morphs, red-headed females passively displaced black- andyellow-headed opponents, whereas interactions between red-headeddyads were particularly aggressive and more frequent than interactionswithin dyads of the other color morphs. This effect of red dominancefurther persisted when head color was experimentally altered;red-manipulated females (of the other morphs) dominated bothblack- and yellow-headed females, whereas blackened red-headedfemales were dominated by naturally red-headed birds. Togetherwith similar dominance-related differences among male morphs,these results suggest that the 3 color morphs may display alternativestrategies in dominance behavior.     

Effects of Aging and Mate Retention on Reproductive Success of Captive Female Peregrine Falcons

SYNOPSIS. Separating ecological (extrinsic) factors affectingreproduction from physiological and experiential (intrinsic)factors can be problematic in free-ranging animals. This studyexamined age-related changes in six measures of reproductivesuccess (clutch size, fertility, hatchability, brood size athatching, survivorship of nestlings, and brood size at fledging)for captive female peregrine falcons (Falco peregrinus) whereecological factors (i.e., access to mates, nesting sites, andfood) were not limiting and were similar across years and betweenindividuals. Mean nestling survivorship increased throughoutthe lifespan of the female while all other measures of reproductionpeaked at about seven years of age and decreased thereafter.Birds with prior breeding experience had higher productivitythan inexperienced birds of the same age. Productivity increasedwith increasing experience of the pair. Productivity droppedby an average of 53% when females experienced a change of mate,and then subsequently increased over a period of several years.Productivity was not affected when birds were moved to differentbreeding sites. There was no evidence that early improvementsin reproduction were related to chronological aging in the absenceof experiential differences. Birds that began breeding earlierproduced more fledglings during their lifetimes as a resultof higher annual productivity. Lifetime reproduction was notcorrelated with longevity because birds with higher maximalegg production had shorter breeding lifespans. Birds retainingmates produced more fledglings during their lifetime than birdsthat changed mates, but birds that changed mates more than oncedid not have lower lifetime reproduction than birds that hadonly one mate change. These results suggest that 1) age-relatedchanges in reproduction are not necessarily resource-mediated,2) in the absence of resource limitation, experience of thepair is a primary factor determining annual reproductive success,3) benefits of increasing experience may be offset by the onsetof senescence, 4) the cost of present egg production on futurereproductive potential supports a "pleiotropic" theory of aging,and 5) costs associated with mate changes may encourage selectionfor low "divorce" rates (i.e., lifetime monogamy) in this species.