3. Variation and life history Flashcards
what is the term ‘father absence’ generally taken to indicate?
generally taken to indicate that an individual has spent some or all of their childhood with a nonresident biological father. While this can be due to paternal death, the majority of research concentrates on cases of nonresident fathers who are separated from (or were never pair bonded with) the individual’s mother
why is there an interest in puberty onset?
There is extensive variation in the timing of puberty across human populations, raising questions about the causes of these differences
what has father absence been shown to increase in terms of development. And what does this result in?
Early father absence (potentially among other stressors) may accelerate pubertal development and the onset of sexual behavior in daughters as a response to environments which were unstable and/or unlikely to support viable pair-bonds. As such, maximizing reproductive success might best be achieved, for girls, by starting reproduction earlier (Belsky et al. 1991)
study support for absent father causing menarche
Webster et al. (2014) published a meta-analysis of this literature suggests that approximately 5.6 % of variance in age of menarche may be explained by the presence or absence of the biological father from the home in early childhood.
evidence for absent father causing earlier sexual activity
Alongside earlier menarche, father absence in Western samples is also typically associated with earlier sexual activity and earlier reproduction in women (e.g., Ellis et al. 2003; Boothroyd et al., 2013)
what is the current explanation for stress/ paternal absence on menarche
The dominant current explanation for these phenomena focuses on the influence of stress on the maturation of the endocrine system (see, e.g., Ellis 2013)
what is the key factor from either perspective model of fitness benefits post stress/ father absence
The key factor from either perspective is that father absence may trigger earlier menarche and earlier reproduction as a means of maximizing reproductive success when faced, in some form, with harsh environments.
why is the timing of reproductive maturity so important to study?
the timing of reproductive maturity is considered a key developmental decision point, since it is the transition at which an organism’s priorities shift from growth to reproduction (Hill & Kaplan, 1999).
what is life history theory?
a branch of evolutionary theory developed to explain the ex-tensive variation in life cycle timing and reproductive strategies across mammalian taxa (Stearns, 1992).
• The study of how and why organisms vary in their prioritisation of different goals in energy expenditure
what does LHT predict about mortality and what would we expect from this?
Because high extrinsic (unavoidable) mortality increases the risk of dying before reproducing and leads to discounting of future fitness, prominent LHT models assume that mortality is a key determinant of variation in the optimal age of maturity across species
evidence for LHT and mortality
As a particularly salient cue of harshness, direct experience of a heightened mortality risk—such as through the death of a parent or sibling—have also been shown to predict accelerated maturation and earlier onset of reproduction (Chisholm, 1993; Ellis, et al, 2009). These findings have been interpreted as evidence that individuals can selectively adopt a “faster” life history when local conditions signal an uncertain or predictably harsh future
what is the big issue with many studies into menarchy and LHT
Although many studies generally support the expectations of this LHT-inspired model, it is notable that the majority of studies investigating links between psychosocial stress, environmental risk, and pubertal timing have focused on populations living in high-income, low-fertility countries in which the maturation-accelerating effect of over-nutrition and overweight are common (Anderson, 2015; Sohn, 2017).
what has been suggested may play a more vital role than mortality/ paternal absence?
nutrition and calorific intake
talk about menarche in northern european countries
Studies have demonstrated a roughly four to five year decline in age at menarche in northern European countries with good historical records, from a mean of approximately 17 years of age in the mid 1800s to a recent mean of 12.5 in wealthy nations (Konner,2010)
menarche in lower and middle income countries when does earlier menarcheal age occur?
documented in many lower and middle-income countries, and earlier menarcheal age is also characteristic of individuals of higher socioeconomic status within populations (Parent et al., 2003)
what have findings on menarcheal age changing in terms of wealth indivate
Traditionally, these findings have been interpreted as a reflection of improvements in nutrition and control of common early life infections, which led to a more rapid pace of growth and an earlier onset of puberty and reproductive maturation
what have studies which have studied the effects of family instability and psycho social stress in populations with less abundant nutrition
studies in populations faced with less abundant nutrition generally find that psychosocial stress and familial instability are not predictive of earlier maturity for females (Anderson, 2015; Sheppard et al., 2014)
main study on nutrition and life history theory what did they do?
Kyweluk et al., 2018
Used cohort of women from Cebu Longitudinal Health and Nutrition Survey – following a large group in the Phillipines from birth into middle adulthood.
what did the main study on LHT find?
Kyweluk et al., 2018
Found that measures of early life adversity or familial instability—as reflected in paternal absence, maternal absence or sibling death—did not predict maturational tempo. Instead, measures reflecting favourable infancy and childhood nutrition and growth were strong predictors of maturational acceleration.
what do the findings or Kyweluk et al., 2018 run counter to?
findings run counter to the expectations of psychosocial models of maturational acceleration derived from life history theory, and suggest that in contexts like Cebu nutritionist likely the primary determinant of pubertal timing, with more favourable nutrition leading to earlier menarche
conclusion on nutrition and LHT
the conclusive nature of work in this field suggests that nutrition may serve as more of a cue than stress. And whilst something to do with stress may be going on what seems clear is that when psychosocial stress is accompanied by some degree of nutritional stress, the maturation-slowing effect of poor nutrition will overcome any more modest accelerating effect of psychosocial stressors and lead to an overall delay in maturation in high stress settings, much as we see at Cebu.
what can be said about LHT put into modern societies in terms of menarche and sex
From the perspective of human evolution, the co-occurrence of caloric abundance alongside stress related to ecological harsh-ness is likely a recent phenomenon unique to the rise of wealthy countries with pronounced income inequality; such environments, in which major psychosocial stressors may occur in the absence of energetic growth constraint, have the potential to inflate the apparent im-portance of developmental stressors as accelerators of maturational timing (Kuzawa & Bragg, 2012)
father absence in males for reproduction and puberty
- Reproduce earlier in western samples (Jaffee et al., 2001)
- Results are decidedly mixed- some have found father absence to predict later puberty in males (e.g. Sheppard et al., 2015)
what are males with no fathers likely to go on to be?
• Father absent boys are more likely to go on to be absent fathers themselves (e..g Jaffee et al., 2001)
what have researchers argued about sex differences in father absence for males
Several authors have argued that a sex difference in responses to father absence is consistent with sex differences in predictors of reproductive success, with males perhaps benefiting most from building competitive advantage rather than accelerating reproduction in harsh and/or low-paternal-investment environments (e.g. James et al. 2012).
what does behavioral ecology focus on?
offspring outcomes in terms of survival and fertility- drivers of parenting behaviour rather than drivers of child development
does father absence or presence have an effect on offspring fertility?
Seems that father absence (or presence) often makes little difference to offspring at all
Wide-ranging systematic review (Sear and Coall, 2011)
Who conducted a review of father absence in fertility and what did they find?
Sear and Coall, 2011
- In WEIRD (low-fertility) samples the presence of fathers appeared to have an inhibitory effect on fertility (indexed by less teenage pregnancy, later first birth)
- fathers in high-fertility samples had a facilitatory or no effect on offspring fertility.
- Where there were significant associations, present/living fathers were typically associated with earlier first birth and a greater number of children born, the former of which is directly at odds with the data from Western samples.
what seems the most likely explanation for Sear and Coall’s findings?
Perhaps the most likely explanation for these results is that in small-scale and high-fertility populations, the most critical contribution of secondary caregivers (as fathers tend to be) is to supplement the child’s nutritional or social status in a manner which makes a much greater contribution to maturation and/or marriageability than any variation induced by psychosocial stressors.
why should we raise the question on how much we can infer from Weird samples
Indeed the simple fact that girls in WEIRD samples display considerably earlier menarche than in small-scale societies suggests that considerable nutritional constraints on menarche have been lifted in the West and, as such, raises the question of how much these WEIRD samples can tell us about ancestral selection pressures on reproductive development
what unites both behavioral ecology and developmental psychology?
Mortality and morbidity outcomes in offspring –> interest in how paternal care/ stress may influence offspring viability and condition
support that father absence influences well being of offspring
There are a number of large epidemiological studies in the West which find an association between parental divorce (usually synonymous with father absence in these data) and poorer adult health, shorter adult height, and shorter life expectancy in offspring (Sheppard et al. 2015).
In some samples, the associations between family structure and later offspring outcomes were mediated by stressors inherent in, or other sequelae of, the separation (e.g., Maier and Lachman 2000), which is consistent with other evidence suggesting relational stress in childhood may be associated with health outcomes in later years.
