Do maternal effects influence phenotypic traits in a cooperatively breeding mammal?

Animals within populations show considerable variation in physiological and behavioural traits. These phenotypic differences can be profoundly influenced by the ecological and social conditions experienced in early life. Usually, adverse early life conditions, such as low food availability, will constrain offspring development. However, recent studies suggest that in some contexts suboptimal developmental conditions may be associated with fitness benefits. In this thesis I consider the ultimate and proximate mechanisms underlying individual variation in a cooperatively breeding mammal. I use hormone samples and the long-term dataset from the banded mongoose research project to investigate the social factors that influence early life stress and how patterns of reproductive investment change with maternal age. In chapter one I review current theories on the function of glucocorticoid (GC) hormones and give an overview of maternal stress and its effect on offspring development and physiology. In addition, I discuss the influence of maternal effects on cooperatively breeding systems. In chapter two, I introduce the study population and provide a general outline of the methods used in data collection and analysis. In chapter three, I test how maternal effects, specifically maternal rank, influence GC concentrations in banded mongoose pups and whether elevated early life GC is associated with mortality risk. I also consider whether the amount of cooperative care received by offspring in early life affects their GC concentrations. In chapter four, I use a larger dataset to consider how maternal effects influence adult life- history traits. I examine the influence that maternal age has on offspring survival, longevity and reproductive success and consider whether any effects might be sex- specific. I also ask whether age-related optimal maternal investment strategies can influence offspring sex ratios. In chapter five, I synthesis my findings and attempt to draw on the wider implications and make suggestions for future study

Doctor of Philosophy

dissertationThis dissertation utilizes life history theory to describe traits that are derived in humans through comparisons with other primate species. Modern human life histories are unique in that they are slower, exhibiting distinctly long postmenopausal life spans and later ages at sexual maturity as a result of a reduction in adult mortality since the evolutionary split the last Pan-Homo ancestor. Faster reproduction with shorter than expected interbirth intervals and earlier weaning ages are likely the result of cooperative breeding featuring postmenopausal grandmothers. Life history traits are distinguished from life history related variables (LHRVs) which are used to makes inferences about life history variables in extinct taxa. Body mass LHRV is a strong predictive life history proxy, but brain size and dental development are only weakly associated and inferences using them should be made with caution. Age at first birth is a central variable in demographic life history models as it identifies the beginning of fertility. For most mammals, age at first birth is closely aligned with the timing of physiological maturity. Humans live in varying ecologies that influence maturation rates and have marriage institutions that can constrain sexual access to fecund females. With few exceptions, the floor of the range of human age at first birth is remarkably consistent at about 17-18 years. Women who experience their first births before this age suffer maternal and infant costs. Heterogeneity, the inherent variation in individual quality, may have an important impact on the timing of life history events. Individuals of lower quality in severe conditions are prone to culling, leaving a subset of robust individuals who thrive in measurable ways. A test of this heterogeneity hypothesis is conducted using a subset of historic vital records from the Utah Population Database. Results show that mothers of twins have a more robust phenotype with lower postmenopausal mortality, shorter average interbirth intervals, later ages at last birth, and higher lifetime fertility than their singleton-only bearing counterparts. Thus, bearing twins may be a useful index of maternal heterogeneity

Ageing through reproductive death in Caenorhabditis elegans

The nematode worm Caenorhabditis elegans has emerged as one of the premier model systems for ageing research. Its use has led to the discovery of many biological mechanisms of ageing conserved across species, including acceleration of ageing by insulin/IGF-1 signalling (IIS) and germline signalling. Ageing in C. elegans, however, is unusual in terms of the severity and early onset of senescent pathology, particularly affecting organs involved in reproduction. For example, in post-reproductive hermaphrodites, intestinal biomass is converted into yolk leading to intestinal atrophy and yolk steatosis. While late yolk production has long been viewed as futile, is it possible that it somehow promote fitness? Results in this thesis show that post-reproductive C. elegans hermaphrodites vent yolk that can be consumed by larvae, promoting their growth. Thus, post-reproductive mothers can contribute to reproductive fitness by converting their biomass into milk, suggesting that intestinal atrophy is a cost of lactation. This type of massive reproductive effort involving biomass repurposing leading to organ degeneration is characteristic of semelparous organisms (i.e. that exhibit only a single reproductive episode) ranging from monocarpic plants to Pacific salmon where it leads to rapid death (reproductive death). Results also show that lactation occurs in other hermaphroditic Caenorhabditis nematode species but not in females. Moreover, the latter do not exhibit intestinal atrophy or steatosis, and are longer lived suggesting that they do not undergo reproductive death. Furthermore, germline ablation strongly increases lifespan in hermaphrodites but not females; similarly, blocking sexual maturation e.g. by gonadectomy can greatly increase lifespan in other organisms that undergo reproductive death. IIS which accelerates C. elegans ageing is also shown to promote lactation. These results suggest that C. elegans hermaphrodites exhibit reproductive death, suppression of which increases lifespan. This has major implications in terms of what one can learn about human ageing from C. elegans

Survival of the Confucians: social status and fertility in China, 1400-1900

This paper uses the genealogical records of 35,691 men to test one of the fundamental assumptions of the Malthusian model. Did higher living standards result in increased net reproduction? An empirical investigation of China between 1400 and 1900 finds a positive relationship between social status and fertility. The gentry scholars, the Confucians, produced three times as many sons as the commoners, and this status effect on fertility was stronger in the post-1600 period than in the pre-1600 period. The effect disappears once I control for the number of marriages. Increased marriages among upper-class males drove reproductive success in Imperial China. The results add a demographic perspective to explain the lack of modern economic growth in Imperial China