Age-specific Sex Ratios Research Articles

Age-specific survival and fecundity and their effects on the intrinsic rate of increase of Aphelinus gossypii (Hym., Aphelinidae), a parasitoid of Aphis gossypii (Hom., Aphididae)

Abstract: The age-specific survival and fecundity of female adults of the aphidophagous parasitoid, Aphelinus gossypii Timberlake (Hym., Aphelinidae), were determined at a host density of 50 Aphis gossypii Glover (Hom., Aphididae) per leaf of Ageratum houstonianum Mill each day at 25°C. The age-specific mummy production, emergence rate and sex ratio of progeny were calculated. The implication of these results in terms of potential population growth of A. gossypii and related species is discussed. The age-specific survival curve (lx) of females exhibited a Type I pattern, which resulted in little difference between Σmx and Σlx mx. The lx curve of males exhibited a Type II pattern, and their survival time was much shorter than that of females. Each mated female produced on average 598.9 ± 64.0 aphid mummies, and preyed on 87.9±6.2 aphids. Most of the biological performance parameters were not different significantly between mated and virgin females. The highly female-biased sex ratio of offspring produced by mated females soon after their emergence resulted in a high fecundity rate (mx) during the early reproductive period. Therefore, although the sex ratio of progeny was male-biased during the entire reproductive period, the intrinsic rate of increase estimated using age-specific sex ratios was larger than that estimated using a constant sex ratio of 0.5. These results indicate that the high survival and the adaptation of producing mostly female offspring during the early reproductive period contribute much to the population increase potential of this parasitoid.

Secular movements in sex ratios of adults and of births in populations during the past half-century.

There is some evidence for a small overall negative correlation across populations between sex ratio (proportion male) at birth and adult sex ratio. There seems to be no systematic correlation within populations across time of sex ratio at birth with adult sex ratio during the past 50 years. So even if adult sex ratios play some part in determining the overall level of the sex ratio at birth, they apparently have played little role in the recent widespread secular changes in sex ratio at birth. It is shown here that there is a strong cohort effect in adult sex ratios: if a woman is in a marriage squeeze (i.e. in a cohort with a relative abundance of women) at age 15 years, she will remain in such a squeeze for the rest of her reproductive life. In England and Wales, the maternal age-specific sex ratios at birth moved roughly in parallel across time during the years 1950-1995. This suggests that sex ratio at birth is not a cohort phenomenon (as it would be if it were affected by adult sex ratio) but is subject to some agents which change with time and affect women (parents) of all ages roughly equally.

Comparative Analysis of the Reproductive Attributes of Three Commercially-Produced Trichogramma Species (Hymenoptera: Trichogrammatidae)

The reproductive attributes of commercially-produced Trichogramma platneri, T. minutum and T. pretiosum reared from eggs of both Ephestia kuehniella and Sitotroga cerealella were monitored at 25 C. The age-specific fecundity, longevity and progeny sex ratio and adult size were determined for individual females from all six combinations of Trichogramma and rearing host, using E. kuehniella as the experimental host. Rearing host had a significant influence on the lifetime fecundity, 3-day fecundity and longevity of all three Trichogramma species. In general, the performance of T. minutum and T. pretiosum was better when reared from S. cerealella, but that of T. platneri was superior when reared from E. kuehniella. The lifetime fecundity of the Trichogramma species was linearly related to longevity and the ranking between species was T. pretiosum > T. minutum > T. platneri. The age-specific pattern of oviposition for T. platneri was distinctly precocious, with 40% of its lifetime fecundity oviposited on the first day, in contrast to 17-24% for the other two species. Progeny sex ratio over the lifetime of the Trichogramma females was slightly male biased and differed significantly from 0.5 for T. minutum and T. platneri. Daily sex ratio for parasitoids reared from the most productive rearing host was female biased only for the first day of oviposition for T. platneri in contrast to the first 5-6 days of oviposition for the other two species. There was little evidence that any of the reproductive attributes of these Trichogramma species, reared from small host eggs, was dependent on the size of the adult females.

Colon cancer prevention update.

C OLORECTAL cancer is second only to lung cancer in mortality in the United States. 1 A review of the epidemiology of colorectal cancer shows interesting findings in general trends, in geographic distribution, in migrant populations, in ethnic variation, and in location of cancer within the colon itself. 2-4 In the United States, colon cancer mortality has remained flat over the last 40 years. Incidence peaked during 1985, with increased screenings after President Reagan's diagnosis of colon cancer that same year ) Since 1985, incidence rates for colorectal cancer have been lower, but they show marked geographic and racial disparity. The highest rates of colorectal cancer occur in the United States, other Anglo-Saxon countries, and western and northern Europe. 4 Africa, China, Japan, and India have low incidence rates, especially in the nonurban areas. 4 Migrants from areas with low colorectal incidence to areas with higher incidence rates show an increase in colorectal cancer. 5'6 Similarly, migrants from areas with higher incidence rates to areas with lower incidence rates evidence a decrease in risk. 7 From 1973 to 1988, the National Cancer Institute Surveillance, Epidemiology, and End Results (SEER) Program reported an increase in age-adjusted colon cancer incidence rates of 30.6% for blacks and only 5.4% for whites. 2 Colorectal cancer occurs in the proximal colon, the distal colon, and the rectum. These divisions of the colon are significant for cancer development and, potentially, for cancer prevention. In areas of the world where the incidence of colorectal cancer is low, for example, colorectal cancer occurs predominantly in the proximal or transverse colon. 4 In geographic areas that have a high incidence of colorectal cancer, the cancer occurs more frequently in the distal colon. 8 Areas with a high incidence of colon cancer appear to have a high risk of rectal cancer. Aneuploidy, K-ras mutations, and allelic losses of chromosomes 5, 17, and 18 vary with location in the colon. 9 Rectal cancer occurs with higher frequency in men than in women, is positively associated with alcohol consumption, and is less common among persons of Jewish ancestry than is colon cancer, j~ There are also differences in age-specific sex ratios for various parts of the large bowel, as exemplified by older women who show an increase in right-sided colorectal cancer. 8 The reasons for these differences are not clear and may be attributed to variations in fecal contents, differences in the colonic and rectal mucosa, or differences in length of fecal contact. 10

The effective population size of an age-structured population with a sex-linked locus

Let a population have the same age distribution and age-specific sex ratios at times 0, 1, 2, …, and let M, F, and L, respectively, be the numbers of males and females in the youngest age group and the generation interval. It can then be shown that if there is a sex-linked locus the fixation probabilities of a neutral allele are respectively 1⧸3 LM or 1⧸3 LF if the allele first appears in one newborn male or in one newborn female. The effective population size can then be derived. It is the same as for a population with discrete generations having the same means, variances, and covariances of male and female progeny during a lifetime and the same number of individuals entering the population per generation.

Complete Estimates of Repreductive Success in a Closed Population of Smallmouth Bass (Micropterus Dolomieui)

We conducted a 4—yr study of 154 parental male smallmouth bass (Micropterus dolemieui) and 982 nests in a closed natural population (a temperate seepage lake in Wisconsin). Our goal was to test common assumptions and hypothesis about reproduction in natural populations. Mark—recapture techniques were used to measure changes in the demography, age—specific sex ratios, and mortality rates of the whole population. An area—density method was used to estimate the number of eggs spawned in each sampled males's nest, and we volumetrically estimated the number of larvae produced by sampled males. The lake is closed to migration, allowing us to accurately estimate total population size and individual variance in reproductive success (RS) for each year without having to assume a demographically stable population. We found that a large proportion of non—breeding adults of both sexes were present in each year. We compared estimates of variance in RS among subsets of males and females in the population to examine the effect on variance when mature non—breeding individuals are included in the analysis. Excluding non—breeders resulted in the true variation in RS being underestimated and the mean RS overestimated, both by a factor of °3 for males and 2 for females. Most male bass spawned only once in their lifetimes, and size at age 3 yr appeared to determine whether a male spawned or postponed reproduction. Males that bred at age 3 yr were among the largest of their cohorts, and males that bred at age 5 yr were among the slowest growing. There may be greater selection on adults for opportunity to breed than there is selection in RS among breeding adults in this population.

Further studies on skin melanomas apparently dependent on female sex hormones.

In the British Isles the incidence of mortality rates from malignant melanoma of the skin have been shown to be higher in females than in males in the later years of reproductive life. In populations with higher rates for malignant melanoma of skin (Scandinavia, Australasia etc), and also in Japan, where the rates are low, the ratio of female to male deaths is lower than in the British Isles. However, a similar pattern of age-specific sex ratios (risks for females compared with males peaking at some age in the latter half of reproductive life, and relatively low in middle age) is found in these populations. This 'gynaecologic' factor is not an artifact of interactions between age and year of birth effects, and appears to act multiplicatively with other aetiologic factors. These findings have implications for studies of the aetiology and pathogenesis of malignant melanoma, as well as for the descriptive mathematical modelling of the rates for purposes of environmental legislation.

On the Proportion of the Chesapeake Bay Stock of Striped Bass That Migrates into the Coastal Fishery

Past observations of year-class patterns and racial characteristics of striped bass (Morone saxatilis) indicate that the Chesapeake Bay spawning grounds provide the principal support of the Atlantic coastal fishery for this species. Past tagging studies have suggested that only a few percent of the striped bass leave the bay, but a few percent of the Chesapeake Bay stock could not support the coastal fishery. To resolve this contradiction, I reexamined tagging studies conducted during 1957–1961 in the Potomac River using recent data on age-specific sex ratios at several positions in the river. For 3-year-olds, the proportions of tag returns from outside the bay were linearly related to the female proportion of the stock in the tagging area. The analysis indicates that few young males leave the bay and approximately one-half of the 3-year-old females migrate to coastal waters. A similar estimate that 50% of age-III females migrate from the bay was independently derived from age-specific sex ratios in Chesapeake Bay in fall 1976, coupled with sex-specific estimates of spring fishing mortality. Smaller proportions of age-II and age-IV females leave the bay. This level of migration could support the majority of the coastal striped bass fishery.

Demographic Differences in Contiguous Populations of White-Tailed Deer

Over 5,000 deer (Odocoileus virginianus) were collected from 2 contiguous areas in South Carolina between 1965 and 1971. Ageand sex-specific survival rates were about equal in the 2 populations, but fawn production, age structure, and sex ratios differed markedly despite close proximity of the herds. Carrying capacity, population pressure, and degree of environmental stability were probably responsible for most of the observed demographic differences. Age-specific dispersal was not a factor in herd dynamics. The study shows that contiguous, localized populations under different environmental conditions may operate independently of each other. J. WILDL. MANAGE. 43(4):889-98 Despite a great volume of literature on the demography of white-tailed deer, there have been few long-term, multifaceted studies. Deer from the Savannah River Plant (SRP) in South Carolina have been the subject of numerous published studies for over a decade, and the studies have included both demographic and genetic approaches (Urbston 1967, 1972; Manlove et al. 1975, 1977; Smith et al. 1975; Baccus et al. 1978; Johns et al. 1978; Ramsey et al. 1979). On 14 December 1952, the Atomic Energy Commission closed to the public 816 square kilometers of the Savannah River Project near Aiken, South Carolina. In 1952, most of the upland areas were overworked farmlands that supported few deer. Most of the remaining area was in bottomland hardwood which evidently supported only a relatively small number of deer (Jenkins and Provost 1964, Urbston 1967). With nearly complete protection from hunting, the herd rapidly increased and spread to adjacent uplands. By 1965, deer had become a highway hazard, and a policy of herd reduction through public hunting was initiated. Because of the brief time factor (1952-65), the extremely small size of the founding population, and the reasonably small area involved, one might expect to find a relatively homogeneous population across the SRP. However, as early as 1965, pronounced demographic differences were evident between 2 contiguous geographic areas (Urbston 1967). Since 1971, demographic, but not genetic differences have lessened, and both populations have shown reduced production of young. In this paper, we present data on fetal and postnatal sex ratios, age structure, age-specific natality, and survival between 2 contiguous populations. This information is correlated with population pressure as we address several questions pertinent to the history of the SRP deer: how and why populations became demographically distinct, what caused them to become demographically similar after 1971, and why productivity declined after 1971. Comparisons of productivity are made between SRP and other areas, and explanations for sexand herd-specific fawn survival are offered. This research was supported by a contract (EY-76-C-09-0819) between the University of Georgia and U.S. Energy Research and Development AdministraJ. Wildl. Manage. 43(4):1979 889 This content downloaded from 207.46.13.129 on Sat, 25 Jun 2016 06:35:27 UTC All use subject to http://about.jstor.org/terms 890 DEMOGRAPHIC DIFFERENCES IN DEER POPULATIONS * Dapson et al. Table 1. Fetal sex ratios (M:100 F) according to maternal age, and age-specific postnatal sex ratios. Data from 1965 to 1971 were pooled. Swamp and upland data were combined for fetal sex ratios. Numbers in parentheses represent ranges among years. Asterisks indicate departures from a 1:1 sex ratio: * = P < 0.05, ** = P -< 0.01.

The Evolution of Behaviors Regulating Density and Age-Specific Sex Ratios in a Primate Population

The Evolution of Behaviors Regulating Density and Age-Specific Sex Ratios in a Primate Population

The Adaptive Significance of Sequential Hermaphroditism in Animals

In some cases, sequential hermaphroditism can convey a selective advantage to an individual by increasing its reproductive potential relative to nontransforming members of the population. This is because age-specific fecundity in many populations is not distributed in the same way for males and females. By functioning as that sex which has the higher fecundity in a particular age span, an individual could increase its reproductive potential relative to lifetime males or females. The magnitude of the reward that comes from such sex changes depends on both the demography of the population and its spawning habits. Protandry may be selected for in populations where female fecundity increases with age and where individuals mate at random. Factors which favor the evolution of protogyny are those which tend to depress male fecundity values at early ages, such as inexperience, territoriality, or female mate selection. Selection for protogyny can also exist when female fecundity decreases with age, although this situation seems rare in the field. It appears that the nature of the fecundity schedule for females has a greater effect on the magnitude of the selection pressure for sequential hermaphroditism than does the age structure of the population. If sequential hermaphroditism becomes genetically fixed in some members of a population with differential age-specific fecundities, the frequency of the hermaphrodites should increase over time until they eliminate the gonochoristic individuals. The pattern of sexual transformation eventually reached in a population is predictable through the use of a simulation model developed here. The pattern depends on the age structure, female fecundity schedule, and mating characteristics of the population. For two examples, the predictions appear consistent with what is seen in nature. At equilibrium, the simulated populations consist of individuals which changed sex at one of two ages, and each type has essentially the same reproductive potential. This suggests that we should see a rather sharp break in age-specific sex ratios in protogynous or protandrous populations. Some hermaphroditic populations exhibit such sharp changes, but others do not. The expected pattern would tend to be obscured by differing growth rates among individuals.

Sex ratio in twin births.

1. Data on more than 2 1/2 million twin births suggest that the regression of sex ratio in twins on maternal age does not decline monotonically like that of singletons, but, like the incidence of dizygotic twinning, seems to rise and then fall with maternal age. 2. Accordingly it is hypothesized that the sex ratio in monozygotic twins is lower than that in dizygotic twins or that in singletons. This would account also for the low overall sex ratio in twins. 3. The data are consistent with the hypotheses that the monozygotic twin sex ratio is constant for all maternal ages at a value of about 0.496, and that the dizygotic twin maternal age-specific sex ratios are the same as the singleton sex ratios for the same maternal ages. 4. The hypothesized low sex ratio in monozygotic twins is reminiscent of that in some congenital malformations: possibly some aetiological factor is common to monzygotic twins and such congenital malformations. 5. It is suggested that the total-birth sex ratio of triplets is underestimated in national vital statistics records.