This entry was written by Rebecca Johnston as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
The provision of parental care benefits offspring by increasing their likelihood of survival, however, providing such care comes at a cost for the parents. This cost can be divided into two categories, non-physiological and physiological, the former referring to costs related to resource acquisition, such as predators and parasites, whereas, the latter refers to costs associated with resource allocation (Alonso-Alvarez & Velando, 2012). While both sexes experience reproduction associated costs, females experience greater physiological costs due to pregnancy and lactation (Anderson & Starkweather, 2017; Ziomkiewicz et al., 2016). Within this essay, the physiological costs associated with maternal care will be explored, this will include a brief overview of life history theory, before energetic costs and oxidative stress are discussed.
Main Text
Life History Theory
According to life history theory, individuals face a trade-off between investing resources in somatic effort and reproduction, with the latter further divided into parental and mating effort. The resources involved in this process are limited and can only be spent once, thus, investing resources into parental duties, limits the amount that can be invested in self-maintenance, future reproductive opportunities and subsequent offspring (Anderson & Starkweather, 2017; Ziomkiewicz et al., 2016). Hence, providing parental care can be quite costly to a parent, particularly a mother who must provide two key stages of care that have high resource demands and physiological costs.
Energetic costs
Pregnancy and lactation are energetically expensive, requiring major increases in energy metabolism, around 20% during pregnancy and up to 50% during lactation (Ziomkiewicz et al., 2016). During these periods, energy expenditure rises with the cost of foetal development and milk production along with the energy requirements for physical activity and maintenance. Lactation in particular is costly, as while the production of milk offers reliable nourishment to offspring, producing such milk is energetically inefficient compared to delivering the food directly, as the nutrients need to be converted from food to milk, a process that is not perfect, thus energy is wasted at a cost to the mother (Sasha & Boyd, 2004). Therefore, lactation costs the mother approximately 2.7 MJ per day in the first six months (Sellen, 2007). To account for such demands, fat laid down prior and during pregnancy can be used, in addition, to increasing energy intake and reducing expenditure. However, even with such responses, the costs to the mother are estimated to be 0.9 MJ per day. This is substantial, as high energy expenditure and loss of energy stores are associated with reduced immunity and greater susceptibility to infection and injuries. Furthermore, this lost energy cannot be diverted to future offspring or survival and is particularly significant loss of time and energy if the reproductive effort fails (Alonso-Alvarez & Velando, 2012). Thus, the energy demands of gestation and lactation have significant energetic costs to the mother.
Oxidative Stress
Oxidative Stress
Oxidative stress (OS) is another physiological cost suggested to arise from the provision of care (Alonso-Alvarez & Velando, 2012; Ziomkiewicz et al., 2016). OS arises from an imbalance between the antioxidant protection and repair system, and the generation of reactive oxygen species (ROS), molecules that can damage macromolecules such as DNA, lipids and proteins (Vaanholt et al., 2016). The rise in energy requirements and metabolism during pregnancy and lactation is suggested to be one manner in which ROS can accumulate, leading to a greater risk of OS and subsequently accelerated aging and cell deterioration. Furthermore, OS is proposed to accumulate with consecutive pregnancies, with studies finding that OS is more than 50% higher in the blood of the placental cord of multiparous women compared to that of primiparous women. Likewise, it is proposed that maternal carbohydrate oxidation increases by approximately 30% during gestation, which in turn creates a greater burden on the oxidative/antioxidative balance, leading to increased OS. The consequences of this stress are significant, with OS suggested to have an essential role in the pathophysiology of several high mortality diseases including cardiovascular disease, diabetes, some cancers and Alzheimer’s. Thus, high reproductive effort can lead to OS, particularly given the limited resources that are allocated to somatic repair mechanisms during this period. This relationship is supported by correlative evidence found in mammals and birds, notably zebra finches, where parents were less resistant to ROS as offspring numbers increased. Furthermore, studies of OS and lactation in cows and red squirrels have found oxidative damage increased and antioxidant protection decreased during early lactation, this damage almost double when compared to non-lactation periods, an effect which intensified during the lactation of sequential offspring. However, conflicting findings for the relationship between OS and reproduction have also been found, for example OS was discovered to be reduced in reproductive mice and bank voles when compared to non-reproductive females (Vaanholt et al., 2016). Thus, it has been suggested that these contradictory results arise due to methodology, with the differences in experimental design, species, biological markers and tissues used resulting in the differing outcomes, in particular the biological marker or tissue used was found to change results within the same individual. Nonetheless, the selective investment of resources into reproduction, in conjunction with fewer available resources allocated to repair mechanisms can result in an increase in OS and subsequently oxidative damage, which in turn can reduce a mother’s survival and fecundity (Alonso-Alvarez & Velando, 2012; Ziomkiewicz et al., 2016).
Conclusion
Therefore, while the provision of care benefits offspring by increasing their likelihood of survival, it is evident that providing such care comes at a cost for the parents, particularly the mother who bears the high physiological costs of gestation and lactation. Physiological costs refer to those associated with the allocation of limited resources as per the life history theory, which states that any resources allocated to parental care cannot also be used for survival, future reproductive opportunities or future offspring. The two key areas of maternal care, gestation and lactation, are energetically expensive, which comes at a cost to the mother’s energy stores, which in turn increase her risk of injuries and disease. Likewise, maternal care, particularly gestation, is proposed to lead to the accumulation of reactive oxygen species which subsequently, result in oxidative stress. The consequence of this are substantial, given the link between OS, cell deterioration and high mortality diseases. Both physiological costs have been supported by studies in animals, although there are mixed results. Overall, it is evident that investing resources into the care of current offspring can reduce a mother’s survival and fecundity as result of the fewer resources that can be allocated to somatic repair, future reproduction and future offspring. Thus, providing maternal care while beneficial is not without significant physiological costs.
Literature Cited
Alonso-Alvarez, C., & Velando, A. (2012). Benefits and costs of parental care. The evolution of parental care: Oxford University
Anderson, K. G., & Starkweather, K. E. (2017). Parenting strategies in modern and emerging economies. Human Nature, 28(2), 133. doi:10.1007/s12110-017-9287-x
Sasha R. X. Dall, & Boyd, I. L. (2004). Evolution of mammals: Lactation helps mothers to cope with unreliable food supplies. Proceedings of the Royal Society of London. Series B: Biological Sciences, 271(1552), 2049-2057. doi:10.1098/rspb.2004.2830
Sellen, D. W. (2007). Evolution of infant and young child feeding: Implications for contemporary public health. Annual Review of Nutrition, 27(1), 123-148. doi:10.1146/annurev.nutr.25.050304.092557
Vaanholt, L.M., Milne, A., Zheng, Y., Hambly, C., Mitchell, S.E., Valencak, T.G., Allison, D.B. & Speakman, J.R. (2016). Oxidative costs of reproduction: Oxidative stress in mice fed standard and low antioxidant diets. Physiology & Behavior, 154, 1-7. doi:10.1016/j.physbeh.2015.11.009
Ziomkiewicz, A., Sancilio, A., Galbarczyk, A., Klimek, M., Jasienska, G., & Bribiescas, R. G. (2016). Evidence for the cost of reproduction in humans: High lifetime reproductive effort is associated with greater oxidative stress in post-menopausal women. PloS One, 11(1), e0145753.
