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Examining the terminal investment hypothesis in humans and chimpanzees: Associations among maternal age, parity, and birth weight
Author(s) -
Fessler Daniel M.T.,
Navarrete C. David,
Hopkins William,
Izard M. Kay
Publication year - 2005
Publication title -
american journal of physical anthropology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.146
H-Index - 119
eISSN - 1096-8644
pISSN - 0002-9483
DOI - 10.1002/ajpa.20039
Subject(s) - parity (physics) , terminal (telecommunication) , psychology , demography , developmental psychology , biology , computer science , sociology , physics , telecommunications , particle physics
The terminal investment hypothesis (Williams [1966] Adaptation and Natural Selection; Princeton, NJ: Princeton University Press) holds that reproductive effort should increase over time in iteroparous species in which reproductive value declines with age. Attempts to model this hypothesis and test it in various species have produced mixed results. Clutton‐Brock ([ 1984] Am. Nat. 123: 212–229) argued that simply testing for changes in propagule size with age fails to recognize that the costs of producing offspring of a given size may increase over the lifespan, hence absence of a positive correlation does not defeat the hypothesis. However, this interpretation is weakened by evidence of sequential increases in propagule size independent of age, as such changes reveal a capacity to increase absolute investment over time. Humans and chimpanzees meet the preconditions of the terminal investment hypothesis. Surveying the obstetrics literature, we show that the majority of published studies indicate that parity has a positive effect on birth weight, but age has no effect. Analyzing 436 captive chimpanzee births, we document a positive influence of parity and a negative influence of age. We therefore conclude that, though it is yet to be replaced by a more compelling alternative, the terminal investment hypothesis is not supported in these two species, as absence of a positive effect of age on birth weight cannot be interpreted in a manner congruent with the hypothesis. Am J Phys Anthropol, 2004. © 2004 Wiley‐Liss, Inc.