The influence of maternal care in shaping human survival and fertility

The influence of maternal care on child survival has evolved throughout human history due to variation in altriciality, allocare, and maternal behaviors. Here, we study the impact of these factors on the force of selection acting on age-specific survival and fertility (measured with elasticity analysis) in a model that incorporates the dependence of child survival on maternal survival. Results reveal life-history changes that cannot be elucidated when considering child's survival independent of maternal survival: decrease of late fertility and increase of late survival, and concomitant decrease of early and late fertility. We also show that an increase of child altriciality in early humans might explain the main human life-history traits: a high life expectancy and postreproductive life; a long juvenile period and a higher, and narrowed, fertility at the peak of the reproductive period.     

The life-history trade-off between fertility and child survival

Evolutionary models of human reproduction argue that variation in fertility can be understood as the local optimization of a life-history trade-off between offspring quantity and ‘quality’. Child survival is a fundamental dimension of quality in these models as early-life mortality represents a crucial selective bottleneck in human evolution. This perspective is well-rehearsed, but current literature presents mixed evidence for a trade-off between fertility and child survival, and little empirical ground to evaluate how socioecological and individual characteristics influence the benefits of fertility limitation. By compiling demographic survey data, we demonstrate robust negative relationships between fertility and child survival across 27 sub-Saharan African countries. Our analyses suggest this relationship is primarily accounted for by offspring competition for parental investment, rather than by reverse causal mechanisms. We also find that the trade-off increases in relative magnitude as national mortality declines and maternal somatic (height) and extrasomatic (education) capital increase. This supports the idea that socioeconomic development, and associated reductions in extrinsic child mortality, favour reduced fertility by increasing the relative returns to parental investment. Observed fertility, however, falls considerably short of predicted optima for maximizing total offspring survivorship, strongly suggesting that additional unmeasured costs of reproduction ultimately constrain the evolution of human family size.     

Disease diversity and human fertility

The existence of parasitic constraints on the evolution of life-history traits in free-living organisms has been demonstrated in several plant and animal species. However, the association between different diseases and human traits is virtually unknown. We conducted a comparative analysis on a global scale to test whether the diversity of human diseases, some of them responsible for high incidences of morbidity and mortality, were associated with host life-history characteristics. After controlling for direct confounding effects exerted by historical, spatial, economic, and population patterns and their interactions, our findings show that human fertility increases with the diversity and structure of disease types. Thus, disease control may not only lower the costs associated with morbidity, but could also contribute directly or indirectly to reductions in human population growth.     

Variation in human fertility

Data on family size distribution and inheritance are presented. Variance in sibship size is in general greater than mean sibship size. The inherited component in the variation appears in general to be small, though not in a Scottish sample examined in more detail. Changes in family size distribution for social (or other) reasons are considered, together with some of their effects on the change in gene frequency in small populations.     

Variability in human fertility

Edited by Lyliane Rosetta and C.G.N. Mascie-Taylor. 1996. New York: Cambridge University Press. 225 pp. ISBN 0-521-49569-5. $54.95 (cloth).     

Phosphoglucomutase genetic polymorphism and human fertility

Abstract

We studied the phosphoglucomutase phenotype in relation to fertility parameters in a consecutive series of 204 women who had delivered a normal live‐born child in Rome. A highly significant association was found between age of the women and phosphoglucomutase phenotype, suggesting a reduced rate of reproduction among women of phosphoglucomutase Type 1. Previous spontaneous abortion appears related to both age and phosphoglucomutase enzymatic type. An increased incidence of abortion in women of older ages was observed only in phosphoglucomutase Type 1. Gestational duration and fetal intrauterine growth rate are also significantly associated with maternal phosphoglucomutase phenotype. The pattern is complex, but also in this instance the influence of maternal age was evident. Considered altogether, the data suggest that phosphoglucomutase may have an important role in zygote development and survival through the whole span of intrauterine life.     

Factors influencing heifer survival and fertility on commercial dairy farms

The average dairy cow survives only three lactations, reducing the availability of replacement heifers. Prenatal losses occur due to early embryonic mortality (about 40%), later embryo loss (up to 20% in high-yielding herds) or abortion (about 5%). A recent survey of 19 UK herds showed that 7.9% of calves were born dead and 3.4% died within 1 month. During the rearing phase, 6.7% of animals were lost before reaching first service at 15 months due to disease or accident and another 2.3% failed to conceive. Many potential replacements therefore never enter the milking herd. This severely limits opportunities for on-farm selection of breeding cows in addition to presenting a welfare issue and causing economic loss. The most profitable animals once lactation is reached combine good milk production with a regular calving pattern. Some aspects of performance are related to age at first calving (AFC), which in turn is influenced by heifer growth rates. Poorly growing animals required more services to conceive, calved later and subsequently performed badly. Optimum fertility and maximum yield in the first lactation were associated with an AFC of 24 to 25 months. However, heifers calving at 22 to 23 months performed best in terms of total milk yield and survival over the first 5 years, partly because good heifer fertility was associated with better fertility later. We have investigated some possible juvenile predictors of future performance. Low-birth-weight calves were more likely to come from either primiparous mothers or older dams (3+ lactations) with higher peak milk yields, suggesting that the uterine environment may limit prenatal calf growth due to competition for nutrients with maternal growth or milk production. Linear trait classification scores for frame size show genetic correlations with longevity. The skeletal measures of height and crown rump length in 1-month-old calves was correlated to subsequent stature, and frame size was correlated to weight at 15 months. It may thus be possible to predict performance from simple size measurements as juveniles. Neither endogenous nor stimulated growth hormone (GH) release in 6-month-old calves were related to milk yield in the first three lactations, but size of a stimulated GH peak was positively related to milk energy values in the first lactation. Cows with delayed ovulation (>45 days) in the first lactation had a higher GH pulse amplitude and lower IGF-I as a juvenile. Cows that partition excess energy into milk in their first lactation may suffer reduced longevity.     

Allometry of human fertility and energy use

The flux of energy and materials constrains all organisms, and allometric relationships between rates of energy consumption and other biological rates are manifest at many levels of biological organization. Although human ecology is unusual in many respects, human populations also face energetic constraints. Here we present a model relating fertility rates to per capita energy consumption rates in contemporary human nations. Fertility declines as energy consumption increases with a scaling exponent of ?1/3 as predicted by allometric theory. The decline may be explained by parental trade‐offs between the number of children and the energetic investment in each child. We hypothesize that the ?1/3 exponent results from the scaling properties of the networked infrastructure that delivers energy to consumers. This allometric analysis of human fertility offers a framework for understanding the demographic transition to smaller family sizes, with implications for human population growth, resource use and sustainability.     

Endocannabinoids,hormone-cytokine networks and human fertility

Anandamide (N -arachidonoylethanolamine, AEA) is a major endocannabinoid, shown to impair mouse pregnancy and embryo development and to induce apoptosis in blastocysts. Here, we review the roles of AEA, of the AEA-binding cannabinoid (CB) receptors, of the selective AEA membrane transporter (AMT), and of the AEA-hydrolyzing enzyme fatty acid amide hydrolase (FAAH), in human gestation. In particular, we discuss the interplay between the endocannabinoid system and the hormone-cytokine array involved in the control of human pregnancy, showing that the endocannabinoids take part in the immunological adaptation occurring during early pregnancy. In this line, we discuss the critical role of FAAH in human peripheral lymphocytes, showing that the expression of this enzyme is regulated by progesterone, Th1 and Th2 cytokines, which also regulate fertility. Moreover, we show that AEA and the other endocannabinoid, 2-arachidonoylglycerol, inhibit the release of the fertility-promoting cytokine leukemia inhibitory factor from human lymphocytes. Taken together, low FAAH and consistently high blood levels of AEA, but not CB receptors or AMT, can be early (<8 weeks of gestation) markers of spontaneous abortion, potentially useful as diagnostic tools for large-scale, routine monitoring of gestation in humans.     

The impact of anaesthetic technique on survival and fertility in Drosophila

Abstract The consequences of ice and carbon dioxide anaesthetics on the survival and reproductive success of Drosophila simulans and Drosophila melanogaster were investigated after observations of high levels of mortality in D. simulans, possibly associated with brief chill coma. The association between brief chill coma and death is confirmed in female D. simulans but there is no mortality in male D. simulans or D. melanogaster of either sex. Mortality is unlikely to be associated with a strain specific cold intolerance because two geographically distinct populations of D. simulans were examined. In addition to the effect of ice anaesthesia, anaesthetizing newly‐eclosed male D. simulans with CO₂ causes a reduction in fertility, which is evident 9–13 days after anaesthesia. This finding is important given that CO₂ anaesthesia is a standard technique used in Drosophila and other insect cultures, and may have important consequences for studies of male fertility and sperm competition.     

Phosphoglucomutase genetic polymorphism and human fertility.

We studied the phosphoglucomutase phenotype in relation to fertility parameters in a consecutive series of 204 women who had delivered a normal live-born child in Rome. A highly significant association was found between age of the women and phosphoglucomutase phenotype, suggesting a reduced rate of reproduction among women of phosphoglucomutase Type 1. Previous spontaneous abortion appears related to both age and phosphoglucomutase enzymatic type. An increased incidence of abortion in women of older ages was observed only in phosphoglucomutase Type 1. Gestational duration and fetal intrauterine growth rate are also significantly associated with maternal phosphoglucomutase phenotype. The pattern is complex, but also in this instance the influence of maternal age was evident. Considered altogether, the data suggest that phosphoglucomutase may have an important role in zygote development and survival through the whole span of intrauterine life.     

Role of prostaglandins in human fertility

One function of prostaglandins in semen may be suppression of an anti-sperm immune response.     

Biology of sperm chromatin structure and relationship to male fertility and embryonic survival

Embryonic mortality in mammals is typically thought to result from 'female factor' infertility. There is growing evidence, however, that the status of sperm chromatin (DNA) at the time of fertilisation can also influence embryonic survival. During the final stages of spermatogenesis (spermiogenesis) a number of unique biochemical, morphological and physiological processes take place that are associated with marked changes in the structure of sperm chromatin. In early stages of spermatogenesis, sperm DNA is associated with histone nucleoproteins and structured into classical nucleosome core particles similar to other somatic cells. As spermiogenesis proceeds, the histone nucleoproteins are replaced by transition proteins which are subsequently replaced by protamines. At the completion of spermiogenesis the chromatin of mature sperm has a toroidal structure that is tightly compacted and resistant to denaturation. The compaction is necessary to protect sperm chromatin during transit through the epididymis and female reproductive tract. Disruption to chromatin remodelling during spermiogenesis results in chromatin that is susceptible to denaturation. Inappropriate chromatin structure has been shown in a number of mammalian species to be related to male infertility, and specifically the failure of embryonic development. A range of techniques are available to assess chromatin status in sperm but arguably the most informative is the sperm chromatin structure assay (SCSA). The SCSA is a flow cytometric assay that uses the metachromatic properties of acridine orange to measure the susceptibility of sperm chromatin to acid-induced denaturation. A relationship has been demonstrated, primarily in men, between the SCSA outcome and the probability of continued embryonic development and the establishment of pregnancy after fertilisation. The contribution of sperm chromatin instability to reproductive wastage in both natural mating and assisted reproduction warrants further investigation as it may prove valuable as a means of decreasing the incidence of embryonic mortality. In this regard, it is possible that 'male factor' infertility may emerge as an even more important component in embryonic development.     

Genetics of Darwinian fitness. II. Pleiotropic selection on fertility and survival

A non-overlapping generation model is proposed which links Wright's adaptive topography concept to the rather inexact notion of Darwinian fitness as survival and reproduction. In general, evolution is seen to weight proportionate increases in survival twice as greatly as proportionate increases in fertility. Certain special cases are also delineated in which measurements of survival and fertility receive equal weight in the fitness equations. In addition, some implications of the present study for Wright's shifting balance theory and models based on Fisher's reproductive value are discussed.     

Evolution of masting with intermittence and synchronization under the enhancements of fertility and survival

Mast seeding is a reproductive mode in plants characterized by intermittence and intra- or interspecific synchronization. Several mechanisms have been proposed to explain the evolution of mast seeding, but the relative importance of each is still unclear due to the complex interactions among the various factors involved, i.e., the two components of masting (intermittence and synchronization), two potential advantages of masting (enhancement of fertility and survival), and the intensities of interactions among individuals through enhancement effects. Several masting studies have claimed that independent selective forces may operate to drive the evolution of intermittence and synchrony, although a theoretical framework for the action of these independent selective forces has not yet been established. In the present study, we investigated the relationships among these factors by analyzing a mathematical model and conducting computer simulations. We found that intense interactions among plant individuals, through the enhancement of fertility or survival, promoted synchronous reproduction while concomitantly suppressing evolution of intermittence. We also demonstrated that enhancement of either fertility or survivorship alone may be insufficient for the evolution of masting, whereas a combination of the two effects can significantly promote it. This suggested a complementary relationship between two well-known hypotheses for the origin of masting, namely, the pollen/pollinator hypothesis and the predator satiation hypothesis.