Are mothers battling embryos or pathogens

In a recent Opinion article in TREE [1xGenetic conflicts in human pregnancy. Haig, D. Q. Rev. Biol. 1993; 68: 495–532Crossref | PubMedSee all References[1], Forbes champions Haig's [2.xGenetic conflicts in human pregnancy. Haig, D. Q. Rev. Biol. 1993; 68: 495–532Crossref | PubMedSee all References, 3.xAltercation of generations: genetic conflicts of pregnancy. Haig, D. Am. J. Reprod. Immunol. 1996; 35: 226–232Crossref | PubMedSee all References] proposal that the maternal–fetal conflict causes nausea and vomiting during pregnancy (NVP), and criticizes the hypothesis that NVP serves a prophylactic function [4.xMorning sickness: a mechanism for protecting mother and embryo. Flaxman, S.M. and Sherman, P.W. Q. Rev. Biol. 2000; 75: 113–148Crossref | PubMedSee all References, 5.xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References]. However, there are problems that plague Forbes’ position.Addressing the proximate cause of NVP, Forbes cites the link between female fetuses, associated with high levels of human chorionic gonadotropin (hCG), and NVP. However, the value of male and female offspring differs as a function of maternal rank [6xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References[6]; therefore, if NVP is due to genetic conflict, the association between fetal sex and NVP should be conditional rather than invariant.For the mother, the value of a given fetus is a function of her remaining reproductive potential [7.xParent–offspring conflict. Trivers, R.L. Am. Zool. 1974; 14: 247–262See all References, 8.xThe evolutionary biology of spontaneous abortion in humans. Forbes, L.S. Trends Ecol. Evol. 1997; 12: 446–450Abstract | Full Text PDF | PubMedSee all References]. If NVP is the result of maternal–fetal conflict, severity should therefore be inversely related to maternal age, yet little or no correlation occurs [9xEarly pregnancy nausea and vomiting in a high-altitude Andean population. Weigel, M.M. et al. Int. J. Gynaecol. Obstet. 2000; 69: 9–21Abstract | Full Text | Full Text PDF | PubMed | Scopus (10)See all References[9]. A mother's willingness to invest is influenced by her caloric reserves, whereas the fetus prioritizes investment over maternal maintenance [10xNatural selection and birthweight. Blurton Jones, N. Ann. Human Biol. 1978; 5: 487–489Crossref | PubMed | Scopus (19)See all References[10]. Maternal–fetal conflict should therefore be greater in lightweight than in heavyweight mothers, yet, as Forbes notes, the latter are more prone to NVP.Forbes concedes that the patterning of gestational dietary alterations suggests an adaptation, yet he argues that this is independent of NVP. However, as in nongestational aversions [11xTaste aversion learning: a contemporary perspective. Bernstein, I.L. Nutrition. 1999; 15: 229–234Abstract | Full Text | Full Text PDF | PubMed | Scopus (76)See all References[11], the development of gestational aversions is tied to nausea and vomiting [4.xMorning sickness: a mechanism for protecting mother and embryo. Flaxman, S.M. and Sherman, P.W. Q. Rev. Biol. 2000; 75: 113–148Crossref | PubMedSee all References, 12.xFood aversions and cravings during pregnancy: a temporal association with nausea and vomiting. Dye, L. et al. Int. J. Obesity. 1998; 22: S130See all References]. To assert that aversions are produced by a mechanism that does not involve nausea and vomiting is to posit an evolutionarily novel and hitherto undocumented adaptation.The prophylaxis hypothesis accounts for the coordination of NVP with immunosuppression and the association between aversions and the risk of infection [4.xMorning sickness: a mechanism for protecting mother and embryo. Flaxman, S.M. and Sherman, P.W. Q. Rev. Biol. 2000; 75: 113–148Crossref | PubMedSee all References, 5.xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References]. Challenges to this hypothesis include the lack of a clear association between NVP and progesterone, the keystone of immunosuppression [5xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References[5]. Moreover, criticisms of the genetic conflict hypothesis also weaken the prophylaxis hypothesis, as they are compatible: Because the mother pays a cost (immunovulnerability) to provide a benefit (freedom from attack by the maternal immune system) to the fetus, the level of immunosuppression, and hence of NVP, can be contested [5xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References[5].Forbes’ proposal to introduce hCG into rodents fails to test these hypotheses, because manipulation of a postulated proximate mechanism sheds no light on ultimate cause. However, if homologs of this mechanism exist in nonhuman primates, then, because maternal–fetal conflict varies with the degree of promiscuity [2.xGenetic conflicts in human pregnancy. Haig, D. Q. Rev. Biol. 1993; 68: 495–532Crossref | PubMedSee all References, 3.xAltercation of generations: genetic conflicts of pregnancy. Haig, D. Am. J. Reprod. Immunol. 1996; 35: 226–232Crossref | PubMedSee all References], these mechanisms should vary across mating systems. Conversely, if NVP serves a prophylactic function: (a) behavioral analogs of pregnancy sickness should occur in meat-eating omnivores but not in carnivores or non-meat-eating species; and (b) across women, the severity of NVP should correspond with the degree of immunosuppression [5xReproductive immunosuppression and diet: an evolutionary perspective on pregnancy sickness and meat consumption. Fessler, D.M.T. Curr. Anthropol. 2002; 43: 48–61See all References[5].