Testing early life effects frameworks: developmental constraints and adaptive response hypotheses do not explain fertility outcomes in wild female baboons

Abstract

In evolutionary ecology, two classes of explanations are frequently invoked to explain early life effects on adult outcomes. Developmental constraints (DC) explanations contend that the costs of early adversity arise from limitations adversity places on optimal development. Adaptive response (AR) hypotheses propose that later life outcomes will be worse when early and adult environments are poorly 'matched'. Here, we use recently proposed mathematical definitions for these hypotheses and a quadratic-regression based approach to test the long-term consequences of variation in developmental environments on fertility in wild baboons. We evaluate whether low rainfall and/or dominance rank during development predict three female fertility measures in adulthood, and whether any observed relationships are consistent with DC and/or AR. Neither rainfall during development nor the difference between rainfall in development and adulthood predicted any fertility measures. Females who were low-ranking during development had an elevated risk of losing infants later in life, and greater change in rank between development and adulthood predicted greater risk of infant loss. However, both effects were statistically marginal and consistent with alternative explanations, including adult environmental quality effects. Consequently, our data do not provide compelling support for either of these common explanations for the evolution of early life effects.

Keywords: developmental adaptive response; developmental plasticity; fitness; predictive adaptive response; silver spoon hypothesis.

Figure 1.

Difference between rainfall (top row) and dominance rank (bottom row) between development and adulthood. Top row: The distribution of rainfall deltas (i.e. the difference between the amount it rained during subjects’ first year of life and the amount it rained in the year of adulthood in which the outcome was measured (details in table 1)), expressed in mean mm per month. Zero on the x-axis indicates that it rained the same amount during development as it did during the year the fertility outcome was measured. Bottom row: The distribution of dominance rank deltas (i.e. the difference between subjects’ mothers’ proportional dominance rank when the subject was born, and the subjects’ proportional dominance rank in the period of adulthood in which the outcome was measured (details in table 2)). Zero on the x-axis represents a perfect match between rank at birth and adult rank when the outcome was measured.

Figure 2.

The relationship between dominance rank deltas and infant survival probability. We observed non-statistically significant support for the adaptive response (AR) hypothesis; females had a lower chance of successfully raising an infant to the average age at weaning when there were greater differences between their rank during development and their rank in adulthood than they did when these differences were smaller (unadjusted $p=0.081$, see table 4c; after multiple testing adjustment, $q=0.441$). The centre of the x axis (0) represents a developmental/adult environment delta of zero, meaning that the female held the same rank in adulthood as she did during development. The plot shows raw (i.e. unadjusted) data grouped into 50 bins, overlaid with a quadratic fit line. Total sample size = 933 live births, so each bin contains approximately 19 data points (2% of the sample).