LIFETIME REPRODUCTIVE SUCCESS AND THE OPPORTUNITY FOR SELECTION IN A NONTERRITORIAL DAMSELFLY (ODONATA: COENAGRIONIDAE)

Major components of male and female lifetime reproductive success (LRS) were quantified for a damselfly that exhibits “scramble competition” for mates. The opportunity for selection on male reproduction was potentially 2.9 times that for females. Differential fertility/clutch and survivorship each accounted for about half of the total variation in female reproductive success. Variation in fertilization efficiency accounted for 7% of the total opportunity for selection on males. Although differences in survivorship and mating efficiency each contributed to about a third of the total opportunity for selection on male reproduction, both components appeared to be influenced by random factors. Survivorship was age-independent, and the mating distributions among males with equal mating opportunities were indistinguishable from those expected if matings were random with respect to male phenotype. Because the proportion of the standarized variance (I) in LRS that was attributed to sexual selection depended on the way the selective episodes were defined, the sample of individuals included in the partitioning analysis, and the degree of sexual selection on mated males that could be detected, my results caution against drawing conclusions about the dynamics of sexual selection on populations based on a superficial comparison of I values.     

Reproductive behavior of the dragonfly,Orthetrum japonicum (Odonata: Libellulidae)

The reproductive behavior of the dragonfly,Orthetrum japonicum, is described. Behavioral processes of turnover of territorial males, simultaneous guarding of 2 females by a male, and copulation by non-territorial males are described. The males with longer hind wings won the territorial conflicts more frequently. The total duration of territorial residence of a given male was correlated with the number of his matings, but not correlated with the length of his abdomen or hind wings. The territorial site with the lower degree of vegetation cover was occupied by males more consistently. Males in more consistently occupied territorial sites did not have longer abdomen and hind wings than males in less consistently occupied sites. The territorial site where the larger number of copulations was observed was not occupied more consistently. Selection episode analysis using the method of Arnold & Wade (1984a, b) showed that direct selection on the hind wing length favored the short wing and that direct selection on the abdomen length favored the long abdomen during mating.     

THE EVOLUTION OF SEXUAL DIMORPHISM BY SEXUAL SELECTION: THE SEPARATE EFFECTS OF INTRASEXUAL SELECTION AND INTERSEXUAL SELECTION

Libellula luctuosa, a pond dragonfly found in eastern North America, is apparently sexually dimorphic. Previous studies of the mating behavior in this species suggested that both male-male competition and female mate choice are important influences. Males compete for territories, where they attract females and where mating occurs. Female behavior influences both the copulation success and the fertilization success of males. Because of temporal and spatial separation of these episodes of sexual selection, multivariate and nonparametric statistical techniques could be used to investigate the influence of components of sexual selection on various sexually dimorphic traits. Sexual dimorphism in L. luctuosa was first quantified; then the direct effects and the form of selection were estimated. Sexually dimorphic wing size, body size, wing coloration, and body coloration are distributed either continuously or discontinuously between the sexes in L. luctuosa. These traits have apparently diverged between the sexes as a result of directional sexual selection. Body size is further influenced by stabilizing selection. Intrasexual selection (success in gaining access to a territory) and intersexual selection (success in copulation and fertilization) can influence the same or different sexually dimorphic characters. Body size is influenced by directional selection during the intrasexual phase of sexual selection and is also influenced by stabilizing selection during intersexual selection. The size of the brown wing patch is influenced by directional selection, primarily during the intersexual phase of sexual selection. There is directional selection on the white wing patch during both phases. Thus, the different proximate mechanisms of sexual selection may jointly or separately affect the evolution of sexually dimorphic characters. Further empirical and theoretical investigations into the differences in the effects of intrasexual selection and intersexual selection are needed to clarify the circumstances leading to separate consequences of these two mechanisms of sexual selection.     

NATURAL SELECTION ON BEAK AND BODY SIZE IN THE SONG SPARROW

We documented temporal patterns of natural selection on beak and body traits in a song sparrow population. We looked for evidence of selection in association with reproduction and overwinter survival in order to identify the conditions under which size in beak and body traits is adaptive. We also attempted to identify the specific traits most closely associated with fitness under these conditions. Selection was observed in association with both survival and reproduction. Patterns of selection differed between the sexes. Selection on males was weak and stabilizing in association with overwinter survival. Selection on females was strong, was both stabilizing and directional, and was associated with both survival and reproduction. In females, traits that enhanced juvenile survival also reduced reproductive success; i.e., there was a trade-off between survival and reproduction. Patterns of selection in the song sparrow parallel those reported for the Galápagos finch, Geospiza fortis. However, in song sparrows, selection occurred mainly on tarsus length and beak length, and not on beak depth or width as in G. fortis. This difference may occur because most North American sparrows partition food resources by habitat, while most Galápagos ground finches partition food by seed size.     

Sex differences in survival selection in the serin,Serinus serinus

Natural selection is demonstrated in most natural populations which suggests that populations are dispatched from their adaptive peaks as a result of selection on correlated characters, or conflicting selection between the sexes. We analysed patterns of survival selection in a population of serins (Serinus serinus) outside Barcelona over a period of 13 years. There was directional selection for increased wing length in males and females accompanied by strong disruptive selection on both tail and wing length in males and a selection against a positive correlation between the two characters in males. In females there was directional selection for increased bill width but decreased bill depth, which should be contrasted to the stabilizing selection acting on bill depth in males. There were conflicting selection on the characters within a sex and conflicting selection of the same characters between sexes, which constrain the rate of access to the nearest adaptive peak.     

Hind Wing Shape Evolves Faster than Front Wing Shape in Calopteryx Damselflies

Wing shape has been shown in a variety of species to be influenced by natural and sexual selection. In damselflies, front- and hind wings can beat independently, and functional differentiation may occur. Males of Calopteryx damselflies show species-specific nuptial flights that differ in colour signalling with the hind wings. Therefore, hind wing shape and colour may evolve in concert to improve colour display, independent of the front wings. We predicted that male hind wing shape evolves faster than front wing shape, due to sexual selection. Females do not engage in sexual displays, so we predicted that females do not show differences in divergence between front- and hind wing shape. We analysed the non-allometric component of wing shape of five European Calopteryx taxa using geometric morphometrics. We found a higher evolutionary divergence of hind wing shape in both sexes. Indeed, we found no significant differences in rate of evolution between the sexes, despite clear sex-specific differences in wing shape. We suggest that evolution of hind wing shape in males is accelerated by sexual selection on pre-copulatory displays and that this acceleration is reflected in females due to genetic correlations that somehow link the rates of wing shape evolution in the two sexes, but not the wing shapes themselves.     

The combined effects of pre‐ and post‐copulatory processes are masking sexual conflict over mating rate in Gerris buenoi

In polygynandrous animals, post‐copulatory processes likely interfere with precopulatory sexual selection. In water striders, sexual conflict over mating rate and post‐copulatory processes are well documented, but their combined effect on reproductive success has seldom been investigated. We combine genetic parentage analyses and behavioural observations conducted in a competitive reproductive environment to investigate how pre‐ and post‐copulatory processes influence reproductive success in Gerris buenoi Kirkaldy. Precopulatory struggles had antagonistic effects on male and female reproductive success: efficiently gaining copulations was beneficial for males, whereas efficiently avoiding copulations was profitable for females. Also, high mating rates and an intermediate optimal resistance level of females supported the hypothesis of convenience polyandry. Contrary to formal predictions, high mating rates (i.e. the number of copulations) did not increase reproductive success in males or decrease reproductive success in females. Instead, the reproductive success of both sexes was higher when offspring were produced with several partners and when there were few unnecessary matings. Thus, male and female G. buenoi displayed different interests in reproduction, but post‐copulatory processes were masking the effects of copulatory mating success on reproductive success. Given the high mating rates observed, sperm competition could easily counter the effect of mating rates, perhaps in interaction with cryptic female choice and/or fecundity selection. Our study presents a complex but realistic overview of sexual selection forces at work in a model organism for the study of sexual conflict, confirming that insights are gained from investigating all episodes in the reproduction cycle of polygynandrous animals.     

SEXUAL SELECTION AFFECTS THE EVOLUTION OF LIFESPAN AND AGEING IN THE DECORATED CRICKET GRYLLODES SIGILLATUS

Recent work suggests that sexual selection can influence the evolution of ageing and lifespan by shaping the optimal timing and relative costliness of reproductive effort in the sexes. We used inbred lines of the decorated cricket, Gryllodes sigillatus, to estimate the genetic (co)variance between age‐dependent reproductive effort, lifespan, and ageing within and between the sexes. Sexual selection theory predicts that males should die sooner and age more rapidly than females. However, a reversal of this pattern may be favored if reproductive effort increases with age in males but not in females. We found that male calling effort increased with age, whereas female fecundity decreased, and that males lived longer and aged more slowly than females. These divergent life‐history strategies were underpinned by a positive genetic correlation between early‐life reproductive effort and ageing rate in both sexes, although this relationship was stronger in females. Despite these sex differences in life‐history schedules, age‐dependent reproductive effort, lifespan, and ageing exhibited strong positive intersexual genetic correlations. This should, in theory, constrain the independent evolution of these traits in the sexes and may promote intralocus sexual conflict. Our study highlights the importance of sexual selection to the evolution of sex differences in ageing and lifespan in G. sigillatus.     

BREEDING COMPETITION IN A PACIFIC SALMON (COHO: ONCORHYNCHUS KISUTCH): MEASURES OF NATURAL AND SEXUAL SELECTION

In the breeding system of Pacific salmon, females compete for oviposition territories, and males compete to fertilize eggs. The natural selection in females and sexual selection in males likely has been responsible for their elaborate breeding morphologies and the dimorphism between the sexes. We quantified direct-selection intensities during breeding on mature coho salmon (Oncorhynchus kisutch), measured for seven phenotypic characters, including three secondary sexual characters. Wild and sea-ranched hatchery coho were used to enhance the range of phenotypes over which selection could be examined. The fish were allowed to breed in experimental arenas where we could quantify components of breeding success as well as estimate overall breeding success. We found that without competition, natural selection acts only on female body size for increased egg production; there is no detectable selection on males for the phenotypic distribution we used. Under competition, the opportunity for selection increased sixfold among females. Natural selection favored female body size and caudal-peduncle (tail) depth. Increased body size meant increased egg production and access to nesting territories. The caudal peduncle, used in burst swimming and nest digging, influenced both successful egg deposition and nest survival. Increasing density increased competition among females, though it did not significantly intensify natural selection on their characters. In males, competition increased the opportunity for selection 52-fold, which was nine times greater than for females. Sexual selection favored male body size and hooked snout length, both characters directly influencing male access to spawning opportunities. Selection on male body size was also affected significantly by breeding density. The ability of large males to control access to spawning females decreased at higher densities reflecting an increase in the operational sex ratio. Further, the relative success of small males, which could sneak access to spawning females, appeared to increase as that of intermediate-sized males decreased. Such disruptive selection may be responsible for the evolution of alternative reproductive tactics in salmon.     

Differences in Patterns of Reproductive Allocation between the Sexes in Nicrophorus orbicollis

Organisms are selected to maximize lifetime reproductive success by balancing the costs of current reproduction with costs to future survival and fecundity. Males and females typically face different reproductive costs, which makes comparisons of their reproductive strategies difficult. Burying beetles provide a unique system that allows us to compare the costs of reproduction between the sexes because males and females are capable of raising offspring together or alone and carcass preparation and offspring care represent the majority of reproductive costs for both sexes. Because both sexes perform the same functions of carcass preparation and offspring care, we predict that they would experience similar costs and have similar life history patterns. In this study we assess the cost of reproduction in male Nicrophorus orbicollis and compare to patterns observed in females. We compare the reproductive strategies of single males and females that provided pre- and post-hatching parental care. There is a cost to reproduction for both males and females, but the sexes respond to these costs differently. Females match brood size with carcass size, and thus maximize the lifetime number of offspring on a given size carcass. Males cull proportionately more offspring on all carcass sizes, and thus have a lower lifetime number of offspring compared to females. Females exhibit an adaptive reproductive strategy based on resource availability, but male reproductive strategies are not adaptive in relation to resource availability.     

SEXUAL SIZE DIMORPHISM AND SELECTION IN THE WILD IN THE WATERSTRIDER AQUARIUS REMIGIS: LIFETIME FECUNDITY SELECTION ON FEMALE TOTAL LENGTH AND ITS COMPONENTS

Darwin's fecundity advantage model is often cited as the cause of female biased size dimorphism, however, the empirical studies of lifetime selection on male and female body size that would be required to demonstrate this are few. As a component of a study relating sexual size dimorphism to lifetime selection in natural populations of the female size-biased waterstrider Aquarius remigis (Hemiptera: Gerridae), we estimated coefficients for daily fecundity selection, longevity selection, and lifetime fecundity selection acting on female body size and components of body size for two consecutive generations. Daily fecundity was estimated using females confined in field enclosures and reproductive survival was estimated by twice-weekly recaptures. We found that daily fecundity selection favored females with longer total length through direct selection acting on abdomen length. Longevity selection favored females with smaller total length. When daily fecundity and reproductive longevity were combined to estimate lifetime fecundity we found significant balancing selection acting on total length in both years. The relationship between daily fecundity and reproductive longevity also reveals a significant cost of reproduction in one of two years. We relate these selection estimates to previous estimates of sexual selection on male body size and consider the relationship between contemporary selection and sexual size dimorphism.     

Strong sexual selection in males against a mutation load that reduces offspring production in seed beetles

Theory predicts that sexual reproduction can increase population viability relative to asexual reproduction by allowing sexual selection in males to remove deleterious mutations from the population without large demographic costs. This requires that selection acts more strongly in males than females and that mutations affecting male reproductive success have pleiotropic effects on population productivity, but empirical support for these assumptions is mixed. We used the seed beetle Callosobruchus maculatus to implement a three‐generation breeding design where we induced mutations via ionizing radiation (IR) in the F₀ generation and measured mutational effects (relative to nonirradiated controls) on an estimate of population productivity in the F₁ and effects on sex‐specific competitive lifetime reproductive success (LRS) in the F₂. Regardless of whether mutations were induced via F₀ males or females, they had strong negative effects on male LRS, but a nonsignificant influence on female LRS, suggesting that selection is more efficient in removing deleterious alleles in males. Moreover, mutations had seemingly shared effects on population productivity and competitive LRS in both sexes. Thus, our results lend support to the hypothesis that strong sexual selection on males can act to remove the mutation load on population viability, thereby offering a benefit to sexual reproduction.     

THE EFFECT OF BIRTH DATE ON FITNESS OF FEMALE DWARF PERCH,MICROMETRUS MINIMUS (PERCIFORMES: EMBIOTOCIDAE)

Lifetime reproductive success may vary considerably with birth date. I measured phenotypic selection on female birth date in a viviparous teleost fish (Embiotocidae: Micrometrus minimus) by sampling birth-date cohorts over time in Tomales Bay, California. Four episodes of selection were measured: survival from birth to first reproduction, reproductive success in the first breeding season, survival to second reproduction, and reproductive success in the second season. Birth date had a significant impact on fitness in the first two episodes. Early born females were more successful in their first breeding season than late born females (directional selection on birth date), but early born females were less likely to survive the period between birth and first reproduction, relative to females born in the middle of the season (stabilizing selection on birth date). The final two episodes of selection had no detectable effect on birth date. Because of the relationship between birth date and survival in the first year, overall selection on female birth date was stabilizing.     

Impacts of diet quality on life‐history and reproductive traits in male and female armed beetle,Gnatocerus cornutus

1. The energy available for reproduction is usually limited by resource acquisition (i.e. condition). Because condition is known to be strongly affected by environmental factors, reproductive investments also vary across heterogeneous environments. 2. Although the condition dependence of reproductive investment is common to both sexes, reproductive traits may exhibit sexually different responses to environmental fluctuation due to sex‐specific life‐history strategies. However, few direct experimental studies have investigated the condition dependence of reproductive investments in both sexes. 3. We investigated the condition dependence of life‐history and reproductive traits of males and females in the beetle Gnatocerus cornutus Fabricus by manipulating larval and adult diet quality. We found that male and female life‐history traits exhibited similar responses to environmental fluctuations. 4. By contrast, the sexes exhibit different patterns of condition dependence in reproductive traits (i.e. the adult nutritional environment has a strong impact on the female lifetime reproductive success, whereas larval nutritional environment strongly affects the secondary sexual trait in males). 5. This difference in the plasticity of reproductive traits may lead to different selection pressures for each sex, even if both sexes develop and/or live in the same environment.     

THE EVOLUTION OF ALTERNATE MORPHOLOGIES: FITNESS AND WING MORPHOLOGY IN MALE SAND CRICKETS

Many organisms show distinct morphological types. We argue that the evolution of these alternate morphologies depends upon both fitness differences between morphs within each sex and the genetic correlation between sexes. In this paper, we examine the evolution of alternate morphologies using wing dimorphism in insects as a model system. Many insect species are wing dimorphic, one morph having wings and being capable of flight, the other lacking functional wings. While there is a well established trade-off in females between macroptery and reproduction, there are few data on the possible costs in males. We examine trade-offs between macroptery and life-history traits in male sand crickets, Gryllus firmus, and estimate the genetic correlation of wing dimorphism between the sexes. Macropterous males develop faster than micropterous males and are either larger or the same size depending upon rearing conditions. There is no difference in absolute or relative testis size at eclosion or 7 d thereafter. Finally, there is no difference between macropterous and micropterous males in relative success at siring offspring. Thus, with respect to the above traits, there are no costs associated with being winged in male G. firmus. It is possible that there may be a trade-off between calling rate and macroptery. A comparison of the relative frequency of macroptery between males and female across different orders of insects supports this hypothesis. The genetic correlation of wing dimorphism between the sexes is high (r₈ = 0.86), and hence the frequency of macroptery in males may be strongly influenced by selection acting on females.     

BODY SIZE AND HAREM SIZE IN MALE RED-WINGED BLACKBIRDS: MANIPULATING SELECTION WITH SEX-SPECIFIC FEEDERS

We experimentally manipulated the strength of selection in the field on red-winged blackbirds (Agelaius phoeniceus) to test hypotheses about contrasting selective forces that favor either large or small males in sexually size dimorphic birds. Selander (1972) argued that sexual selection favors larger males, while survival selection eventually stabilizes male size because larger males do not survive as well as smaller males during harsh winters. Searcy (1979a) proposed instead that sexual selection may be self limiting: male size might be stabilized not by overwinter mortality, but by breeding-season sexual selection that favors smaller males. Under conditions of energetic stress, smaller males should be able to display more and thus achieve higher reproductive success. Using feeders that provisioned males or females but not both, we produced conditions that mimicked the extremes of natural conditions. We found experimental support for the hypothesis that when food is abundant, sexual selection favors larger males. But even under conditions of severe energetic stress, smaller males did not gain larger harems, as the self-limiting hypothesis predicted. Larger males were more energetically stressed than smaller males, but in ways that affected their future reproductive output rather than their current reproductive performance. Stressed males that returned had smaller wings and tails than those that did not return; among returning stressed males, relative harem sizes were inversely related to wing and tail length. Thus, male body size may be stabilized not by survival costs during the non-breeding season, nor by energetic costs during the breeding season, but by costs of future reproduction that larger males pay for their increased breeding-season effort.     

Effect of non‐lethal sampling on life‐history traits of the protected moth Graellsia isabelae (Lepidoptera: Saturniidae)

Abstract 1. Non‐lethal genetic surveys in insects usually extract DNA from a leg or a piece of wing. Although preferable to lethal sampling, little is known about the effect of leg/wing non‐lethal sampling on fitness‐related traits. 2. Graellsia isabelae (Graells, 1849) is a European moth protected by the Habitats Directive and the Bern Convention. Conservation genetics surveys on this species should therefore use non‐lethal sampling. 3. The present study aimed to (1) quantify the effects of both leg and hind‐wing tail sampling on survivorship and reproductive behaviour of adult males and females, and (2) assess the quality and quantity of DNA obtained from those tissues. 4. Both hind‐wing tails and mid‐legs proved to be good sources of high quality nuclear and mitochondrial DNA. DNA concentration was significantly higher when extracted from a large (130 mm²) piece of the hind‐wing tails than from about half of the mid‐leg. Using mark–release–recapture experiments with adults, it was found that neither mid‐leg nor hind‐wing tail sampling significantly reduced male survivorship or total number of matings. However, although mid‐leg sampling did not significantly affect female survivorship, it had a negative effect on female mating success. 5. Wing‐tail clipping on males appeared to be the best non‐lethal sampling procedure for G. isabelae. It is a fast procedure, similar to natural wing impairment, and did not significantly affect survival or mating behaviour.     

SEXUAL SELECTION AND FITNESS VARIATION IN A POPULATION OF SMALLMOUTH BASS,MICROPTERUS DOLOMIEUI (PISCES: CENTRARCHIDAE)

Monogamy is often presumed to constrain mating variance and restrict the action of sexual selection. We examined the reproductive patterns of a monogamous population of smallmouth bass (Micropterus dolomieui), and attempted to identify sources of within-season fitness variation among females and known-age males. Many males did not acquire a nest site, and many territorial males were unsuccessful in acquiring a mate. The likelihood that territorial males mated depended on several aspects of nest sites. Mated males of age three were larger than the average size of age-three males in the population. The mean sizes of age-four and age-five mated males were not different from the average of same-age males in the population. Thus, selection resulting from the acquisition of a mate favored large size among only age-three males. Timing of nest construction and breeding among territorial males was negatively related to male size and did not depend on male age after taking male size into account. Indirect evidence (numbers of eggs deposited in nests) suggests that the timing of spawning among females was also negatively related to female size. Fertility selection favored early reproduction within the season by males of all ages, but large male size was favored among only age-four males. The combined early breeding of fecund females and female mate choice of large males may explain the positive correlation between the size of age-four males and the number of eggs acquired. Despite large differences of female fecundity, however, the variance of relative mate number contributed about two times more than the variance of relative fertility among females to the total variance of relative fitness within each sex.     

Is There Adaptive Value to Reproductive Termination in Japanese Macaques? A Test of Maternal Investment Hypotheses

Evolutionary biologists often argue that menopause evolved in the human female as the result of selection for a postreproductive phase of life, during which increased maternal investment in existing progeny could lead to enhanced survivorship of descendents. Adaptive theories relating menopause to enhanced maternal investment are known as the mother (first-generation) and grandmother (second-generation-offspring) hypotheses. Although menopause—universal midlife termination of reproduction—has not been documented in primates other than humans, some researchers have argued that postreproductive alloprimates also have a positive impact on the survivorship of first and second generation progeny. We tested the maternal investment hypotheses in Japanese macaques by comparing the survivorship of offspring, final infants, and great-offspring of females that terminated reproduction before death with females that continued to reproduce until death. SURVIVAL analyses revealed no significant difference in the survivorship of descendents of postreproductive and reproductive females, though final infants of postreproductive females were 13% more likely to survive than final infants of females that reproduced until death were. We also explored possible differences between these two groups of females, other than survivorship of progeny. We found no difference in dominance rank, matrilineal affiliation, body weight, infant sex ratio, age at first birth, fecundity rate or lifetime reproductive success. However, postreproductive females are significantly longer-lived than reproductive females and as a result experienced more years of reproduction and produced more infants in total. Apart from final infants, offspring survival is marginally lower in postreproductive females. Since offspring survival is not significantly enhanced in postreproductive females, the greater number of infants produced did not translate into greater lifetime reproductive success. Our findings fail to support the maternal investment hypotheses and instead suggest that reproductive termination in this population of Japanese macaques is most closely associated with enhanced longevity and its repercussions.     

高体鳑鲏的性选择问题研究

以高体鳑鲏为研究对象,分别设置个体大小不同的雌鱼和雄鱼进行性选择实验并验证性选择理论,对雌鱼的怀卵量和雄鱼的空间学习记忆能力也进行了检测。雄鱼对雌鱼的性选择实验结果表明,雄鱼对大个体雌鱼有明显的选择偏好;雄鱼选择雌鱼的次数及停留时间与雌鱼的体长、体高、产卵管长度等特征呈正相关。推测雄鱼是基于直接收益的角度,即最大限度的获得较多的后代及较高的后代存活率对雌鱼进行选择,因为大个体雌鱼拥有长的产卵管及绝对多的处于Ⅴ时相的成熟卵粒,可以提供较多的后代及较高的后代存活率。雌鱼对雄鱼的性选择结果表明,雌性高体鳑鲏对大个体雄鱼也表现出了明显的偏好;雌鱼选择雄鱼的次数及停留时间与雄鱼的体长、体高呈正相关;大个体雄鱼的空间学习记忆能力比小个体强,能更快捷地找到并占领质量好的河蚌而成为领域鱼。从直接收益的观点看,雌鱼选择大个体的雄鱼可能是因为大个体雄鱼可以更加容易、迅速和便捷地寻找到质量好的河蚌引领雌鱼进行产卵,从而保证子代的存活率。