Variation In Female Reproductive Success Research Articles

Individual reproductive success and effective population size in the greater white-toothed shrew Crocidura russula.

In order to investigate the determinants of effective population size in the socially monogamous Crocidura russula, the reproductive output of 44 individuals was estimated through genetic assignment methods. The individual variance in breeding success turned out to be surprisingly high, mostly because the males were markedly less monogamous than expected from previous behavioural data. Males paired simultaneously with up to four females and polygynous males had significantly more offspring than monogamous ones. The variance in female reproductive success also exceeded that of a Poisson distribution (though to a lesser extent), partly because females paired with multiply mated males weaned significantly more offspring. Polyandry also occurred occasionally, but only sequentially (i.e. without multiple paternity of litters). Estimates of the effective to census size ratio were ca. 0.60, which excluded the mating system as a potential explanation for the high genetic variance found in this shrew's populations. Our data suggest that gene flow from the neighbourhood (up to one-third of the total recruitment) is the most likely cause of the high levels of genetic diversity observed in this shrew's subpopulations.

Morphological variation and female reproductive success in two sympatric Trillium species: evidence for phenotypic selection in Trillium erectum and Trillium grandiflorum (Liliaceae)

I investigated the mating systems and phenotypic variation of two sympatric spring ephemerals, Trillium erectum and T. grandiflorum (Liliaceae), and phenotypic selection acting through female reproductive success for 11 morphological characters in five sympatric populations of the two species. I examined the degree of self-compatibility, pollinator-visitation rates, and pollen limitation of fruit and seed production in both species. Both Trillium species were self-compatible, but outcrossed flowers produced more successful fruits and seeds than self-pollinated flowers. Pollinator-visitation rates to the two species were low compared to other insect-pollinated spring ephemerals. In addition, both T. erectum and T. grandiflorum experienced pollen limitation in fruit and/or seed production; however, levels of fecundity in both species may be influenced by resource availability as well. I found significant phenotypic variation in 11 morphological characters within and among the five study populations. The sizes of all morphological characters were positively correlated. In general, larger T. erectum and T. grandiflorum produced more seeds. Phenotypic selection analysis revealed that direct and indirect selection acted on the size of morphological characters for both species. But there was no detectable selection acting on plant shape. This study reveals that variation in plant size exists within and among populations of both species, and this variation is associated with variance in female reproductive success. Spatial and temporal variation in pollinator and/or resource abundance may play a role in the phenotypic variation exhibited by both Trillium species.

Temporal genetic variation of mitochondrial DNA and the female effective population size of red drum (Sciaenops ocellatus) in the northern Gulf of Mexico.

We studied genetic drift of mitochondrial DNA (mtDNA) haplotype frequencies in a natural population of red drum (Sciaenops ocellatus) from the northern Gulf of Mexico (Gulf). The amount of genetic drift observed across temporally adjacent year classes (1986-89) was used to estimate variance effective (female) population size (Nef). Nef was estimated to be 14 308 and the ratio of female effective size to adult female census size was approximately 0.004, which is among the lowest value reported for vertebrate animals. Low effective size relative to census size among red drum in the northern Gulf may result from yearly fluctuations in the number of breeding females, high variance in female reproductive success, or both. Despite low genetic effective size relative to census size, the genetic effective population size of red drum in the northern Gulf appears sufficiently large to preclude potentially deleterious effects of inbreeding.

THE EFFECTIVE SIZE OF A HIERARCHICALLY STRUCTURED POPULATION.

A knowledge of the effective size of a population (Ne ) is important in understanding its current and future evolutionary potential. Unfortunately, the effective size of a hierarchically structured population is not, in general, equal to the sum of its parts. In particular, the inbreeding structure has a major influence on Ne . Here I link Ne to Wright's hierarchical measures of inbreeding, FIS and FST , for an island-structured population (or metapopulation) of size NT . The influence of FST depends strongly on the degree to which island productivity is regulated. In the absence of local regulation (the interdemic model), interdemic genetic drift reduces Ne . When such drift is combined with local inbreeding under otherwise ideal conditions, the effects of FIS and FST are identical: increasing inbreeding either within or between islands reduces Ne , with Ne = NT /[(1 + FIS )(1 + FST ) - 2FIS FST ]. However, if islands are all equally productive because of local density regulation (the traditional island model), then Ne = NT /[(1 + FIS )(1 -FST )] and the effect of FST is reversed. Under the interdemic model, random variation in the habitat quality (and hence productivity) of islands act to markedly decrease Ne . This variation has no effect under the island model because, by definition, all islands are equally productive. Even when no permanent island structure exists, spatial differences in habitat quality can significantly increase the overall variance in reproductive success of both males and females and hence lower Ne . Each of these basic results holds when other nonideal factors are added to the model. These factors, deviations from a 1:1 sex ratio, greater than Poisson variance in female reproductive success, and variation in male mating success due to polygynous mating systems, all act to lower Ne . The effects of male and female variance on Ne have important differences because only females affect island productivity. Finally, it is noted that to use these relationships, FIS and FST must be estimated according to Wright's definition (and corrected to have a zero expectation under the null model). A commonly used partitioning (θ, θg ) can be biased if either island size or the number of islands is small.

Longitudinal patterns of reproduction in wild female siamang (Hylobates syndactylus) and white-handed gibbons (Hylobates lar)

I present the 6- year reproductive histories of three wild female siamang (Hylobates syndactylus)and four white-handed gibbons (Hylobates lar)at the Ketambe Research Station (Sumatra, Indonesia). Reproductive output varied considerably among females. Two females failed to gestate: both were nulliparous young adult H. lar,one of which remained unpaired for 4 years after dispersing from her group, while the other lost her recently acquired mate to another female. Only one- (a white-handed gibbon)- gave birth more than once, yielding interbirth intervals of 22 and 31 months. Pair bond stability or reduced interspecific feeding competition or both factors may have contributed to the brevity of these intervals. The other females- one H. lar,and three H. syndactylus-each gave birth once, suggesting minimum interbirth intervals exceeding 4–5 years (H. lar)and 3 years (H. syndactylus)in these individuals. Even given the pronounced variation observed among H. lar,these data suggest that interbirth intervals may often exceed the 2- to 3- year interval commonly attributed to these two species. Sources of reproductive failure were 1) maternal abandonment of the neonate due to impaired ability to provide maternal care (H. syndactylus,),(2) premature or stillbirth (H. syndactylus,),and (3) pregnancy termination (H. lar).These data and a review of information on longevity and age at menarche suggest that the actual lifetime reproductive output of a siamang or white-handed gibbon female may often fall far short of the 10 offspring/lifetime originally proposed for these species. Indeed, females may rear as few as five offspring to weaning in a lifetime, which is a figure reminiscent of the reproductive potential of some pongids. Finally, variance in female reproductive success is higher than expected in these monogamous species, which suggests that females (and males) are under strong selective pressure to exert mate choice, possibly through acquisition of (new) mates and extrapair copulations. Future research must clarify the availability of opportunities for paired adults to engage in these sociosexual behaviors.

Reproductive Ecology of the Puerto Rican Frog Eleutherodactylus coqui

We studied the reproductive phenology and ecology, and patterns of male and female reproductive success of Eleutherodactylus coqui, a frog with terrestrial reproduction, direct development, and male parental care. Reproductive activity (calling and oviposition) occurred during every month of our two-year study, although it was distinctly seasonal, with the lowest activity during the dry season and the highest during the wet season. Male chorus density and female clutch deposition were temporally associated with temperature and rainfall. Female breeding was asynchronous, with at least one female mating on 63% of all wet season nights. The mean interclutch interval was about 8 weeks during the wet season. Clutch size was correlated with female body size, but also showed a significant seasonal effect with clutch size increasing in the wet season. In contrast, ovipositing female body size declined from dry to wet season. Male mating success was variable, with some males mating up to five times over one six- month period. Mating success was not correlated with male body size and we found no evidence of size- assortative mating. Mating success was correlated with calling effort measured as chorus attendance. Variance in hatching rate comprised most (89%) of the total variation in female reproductive success, measured as number of hatchlings per clutch. Our results suggest that female coquies should be strongly selected to exercise mate choice based on variables that correlate with hatching rate, i.e., male parental care quality and nest site quality.

Subsociality and female reproductive success in a mycophagous thrips: An observational and experimental analysis

Oviparous females of the haplodiploid, facultatively viviparous thrips Elaphrothrips tuberculatus(Thysanoptera: Phlaeothripidae) guard their eggs against female conspecifics and other egg predators. The intensity of maternal defense increases with clutch size. Field and laboratory observations indicate that cannibalism by females is an important selective pressure favoring maternal care. Experimental removals of guarding females showed that egg guarding substantially increases egg survivorship and that the survivorship of undefended eggs is higher in the absence of nonguarding female conspecifics than in their presence. The fecundity of viviparous females increases with the number of eggs cannibalized. The reproductive success of oviparous females increases with body size and local food density and decreases with local density of breeding females. Social behavior may not have advanced beyond maternal care in Elaphrothrips tuberculatusbecause, relative to Hymenoptera, capabilities for helping relatives are few or nonexistent, and the causes of variation in female reproductive success are not influenced easily by cooperation among females.

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.

Evolution of Female Choice and Male Parental Investment in Polygynous Species: The Demise of the "Sexy Son"

Males of many polygynous animal species contribute nutrition, nest sites, parental care, or other material benefit to their mates in addition to their gametes. Variation in female reproductive success may be caused by variation in the quantity or quality of the male contribution, and in such cases a female's mate choice affects her reproductive success. I present here polygenic and haploid two-allele models for the joint evolution of a male secondary sexual trait and a female mating preference for that character. The male trait affects male survivorship, mating success, and the number of offspring produced per mating. A female's reproductive success is consequently affected by the type of male she mates. This selection on fecundity is assumed to operate in one of two ways. Under the assumption of limited male reproductive potential, the fecundity per mating is a declining function of the number of females to which a male is mated. Under the alternative assumption of unlimited male reproductive potential, the fecundity per mating is independent of the number of females mated, but does depend directly on the male's trait. The evolutionary equilibrium under both forms of fecundity selection is found to maximize immediate female reproductive success. The equilibrium, however, may or may not maximize male survivorship: when male reproductive potential is unlimited, the equilibrium for the male trait may lie far from the point that is optimal for male viability, and so average male survival can decrease under this form of sexual selection. The models also show that it is possible for the evolutionary equilibrium to become unstable, in which case the population will evolve rapidly away from the equilibrium in a maladaptive runaway process. These models address the "sexy son" hypothesis of Weatherhead and Robertson (1979), which contends that selection can create an equilibrium at which females on the average mate with certain attractive types of males that give them inferior material resources and therefore decrease the females' immediate reproductive success. The logic of this argument is that the females are compensated for their fecundity deficit by the mating success of their sons, which bear their fathers' genes and so are attractive to females. Results from the models developed here show that no such evolutionary balance is possible, and the sexy son hypothesis is not supported. Female fecundity is maximized at the evolutionary equilibrium, and is not offset against the fitness of descendants.