The Seasonality Thermostat: Female Reproductive Synchrony and Male Behavior in Monkeys, Neanderthals, and Modern Humans

Reproductive synchrony or desynchrony of primate females influences number and fitness of males in mating
systems. Langur monkey populations provide a natural ex
periment for observing alternative female strategies of
confusing or concentrating paternity. Where females escape seasonal reproductive constraints, they desynchro
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nize fertility and show visible cues (menstruation), enabling single males to monopolize matings. This increases
female fitness by reducing food competition. Where langurs are seasonally constrained, females conceal fertility,
confusing paternity and reducing infanticide. These case studies illuminate how hominin females could increase
male numbers and investment. Fitness payoffs to male investors will be affected by degree of reproductive sea
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sonal constraint, and by females either concealing or confusing menstrual cues of imminent fertility. Among
ancestors of modern humans and Neanderthals these strategies diverged. Under pressure of encephalization,
modern human female ancestors, less seasonally constrained, pursued a strategy of cosmeticization of menstrual
signals. This Female Cosmetic Coalitions model accounts for the African Middle Stone Age record of pigment use.
Among Neanderthals, strategies alternated. Severe seasonality during glacial cycles tied Neanderthal males into
pair-bonds, suppressing cosmetic signaling. Only during interglacials when seasonality relaxed would Neander
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thal females require blood-red cosmetics. Our Seasonality Thermostat model explains why European ochre use
correlates with climate through the Middle to Late Pleistocene.