Why does it make sense that monkeys in the former areas
evolved long tails, while those
in the latter areas did not?

Primates are characterized by relatively late ages at first reproduction, long lives and low fertility. Together, these traits define a life-history of reduced reproductive effort. Understanding the optimal allocation of reproductive effort, and specifically reduced reproductive effort, has been one of the key problems motivating the development of life history theory. Because of their unusual constellation of life-history traits, primates play an important role in the continued development of life history theory. In this review, I present the evidence for the reduced reproductive effort life histories of primates and discuss the ways that such life-history tactics are understood in contemporary theory. Such tactics are particularly consistent with the predictions of stochastic demographic models, suggesting a key role for environmental variability in the evolution of primate life histories. The tendency for primates to specialize in high-quality, high-variability food items may make them particularly susceptible to environmental variability and explain their low reproductive-effort tactics. I discuss recent applications of life history theory to human evolution and emphasize the continuity between models used to explain peculiarities of human reproduction and senescence with the long, slow life histories of primates more generally.

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Introduction

Explaining the great diversity of forms and lifestyles is the central goal of evolutionary biology. Natural selection is the primary force behind adaptive diversification, and selection favors those that persist and increase over time. Evolutionary success is ultimately founded on two fundamental demographic processes: first, surviving to an age at which reproduction is possible (‘recruitment’), and second, reproducing successfully. Life-history theory seeks to explain the diversity of tactics through which different organisms achieve evolutionary persistence and increase, why the tempo and mode of reproduction can vary so much across taxa, and why life cycles vary from species to species. R. A. Fisher[1] defined the modern study of life-history theory saying that: “it would be instructive to know…what circumstances in the life-history and the environment would render profitable the diversion of a greater or lesser share of the available resources toward reproduction”. In an influential essay, Stephen Stearns [2] suggested that studies of life-history phenomena “naturally elicit a research viewpoint that combines the study of reproduction, growth, and genetics in an ecological setting to produce hypotheses concerning evolutionary changes.” As successful reproduction is immediately proximate to fitness, life-history theory lies at the heart of any understanding of adaptation in evolutionary biology.

then that particular organisms become extinct.