A measure for describing and comparing postreproductive life span as a population trait

Abstract

Summary 1. While-classical life-history theory does not predict post-reproductive lifespan (PRLS), it has been detected in a great number of taxa, leading to the view that it is a broadly conserved trait, and attempts to reconcile theory with these observations. We suggest an alternative: the apparently wide distribution of significant PRLS is an artifact of insufficient methods. 2. PRLS is traditionally measured in units of time between each individual’s last parturition and death, after excluding those individuals for whom this interval is short. A mean of this measure is then calculated as a population value. We show this traditional population measure (which we denote PrT) to be inconsistently calculated, inherently biased, strongly correlated with overall longevity, uninformative on the importance of PRLS in a population’s life-history, unable to use the most-commonly available form of relevant data and without a realistic null hypothesis. Using data altered to ensure that the null hypothesis is true, we find a false positive rate of 0.47 for PrT. 3. We propose an alternative population measure, using life-table methods. Post-reproductive Representation (PrR) is the proportion of adult years lived which are post-reproductive. We briefly derive PrR and discuss its properties. We employ a demographic simulation, based on the null hypothesis of simultaneous and proportional decline in survivorship and fecundity, to produce a null distribution for PrR based on the age-specific rates of a population. 4. In an example analysis, using data on 84 populations of human and non-human primates, we demonstrate the ability of PrR to represent the effects of artificial protection from mortality and of humanness on PRLS. PrR is found to be higher for all human populations under a wide range of conditions than for any non-human primate in our sample. A strong effect of artificial protection is found, but humans under the most-adverse conditions still achieve PrR of >0.3. 5. PrT should not be used as a population measure, and should be used as an individual measure only with great caution. The use of PrR as an intuitive, statistically valid and intercomparable population life-history measure is encouraged.

Reproductive cessation and post-reproductive lifespan in Asian elephants and pre-industrial humans

“…The PrR value of 0.13 calculated for Myanmar elephants is higher than those found in primates living in wild or semi-wild conditions ( Pan troglodytes 0.018; Papio hamadryas 0.084; Macaca fuscata 0.055) (Figure 5) and corresponds to values found in many zoo populations of primates [36]. The value, 0.51, calculated for Finnish women correspond with patterns found in other historical or hunter-gatherer populations [36], where PrR ranges from 0.3 to 0.47 (in modern populations <0.76).…”

“…Post-reproductive representation values and maximum longevities in Asian elephants and humans from this study, other PrR-values according to Levitis and Lackey 2011 [36]. Maximum longevities in Macaca fuscata from [44], Pan troglodytes from [45], Papio hamadryas from [46] and Globicephala macrorhyncus from [47].…”

Reproductive aging patterns in primates reveal that humans are distinct

“…The 90th percentiles of these two distributions are quite similar for the nonhuman primates, but they are quite different for the !Kung (Table S5). Finally, comparison of the postreproductive representation of adults (3,12) in the nonhuman primates and humans is also striking: among the nonhuman primate species, 1-6% of adult-female-years are lived by postreproductive individuals, whereas in the !Kung this metric reaches 42.5% (Table 1).…”

“…Recently, Levitis et al (3) examined reproductive senescence in humans and in three nonhuman primate species, and concluded that the postfertile lifespan is a distinct life phase that occurs in humans but may be absent in many (but not all) other animals. The authors address the problem of how to identify a true postfertile lifespan from the occasional occurrence of postfertile individuals in a population (12). This approach also highlights the fact that to place humans precisely on the comparative landscape, data from wild populations and from multiple species of primates and other mammals are required.…”

Human mortality improvement in evolutionary context

“…The hunter-gatherer groups included in the Siler fit are the Hadza, Ache Forest, Hiwi, !Kung, and Agta. The (37) and were also analyzed by Levitis and Lackey (38).…”