Pregnancy sickness, a suite of "symptoms" that frequently co-occur during pregnancy, may be an adaptation providing behavioral prophylaxis against infection. Maternal immunosupression, necessary for tolerance of the fetus, results in gestational vulnerability to pathogens. Throughout the period of maximal vulnerability, dietary behavior is significantly altered via changes in nausea susceptibility and olfaction and the development of marked aversions and cravings. Of food types, meat is both the most likely to carry pathogens and the principal target of gestational aversions and pregnancy taboos. Because meat was prominent in ancestral human diets but hygienic procedures that effectively eliminate the risk of meat-borne infection are recent, such pathogens likely constituted a source of selective pressure on pregnant females throughout human history. Both the relatively low protein and energy demands of the first trimester and the existense of nonmeat alternatives would have allowed for the evolution of time-limited gestational meat-avoidance mechanisms.Complementing these mechanisms, gestational cravings target substances that may influence immune functioning and affect the availability of iron in the gastro-intestinal tract, thereby limiting the proliferation of iron-dependent pathogens. Clinical and ethnographic findings are examined in light of these proposals, and directions for future research are outlined.
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