Evolution of permanent estrus displays in hominids

Abstract

Accounts of the reality of “concealed ovulation" (i.e., concealed estrus) in humans do not stand up to tests supplied by the facts of morphology and physiology when these are viewed in a fully Darwinian perspective of descent. The origin and transformation of morphology and physiology occur together with behaviors. The latter cannot be decoupled from the somatic attributes of organisms. Although there is natural selection of organisms, this success is largely the result of selection for survival strategies, the latter being directly causal to differential reproduction. Reproductive strategies are thus constrained by feeding and locomotion. Historical and functional attributes of the quadrupedal ape ancestor with a temporary, visually oriented, estrus display (TED) channelled the transformation of the sexual signal system of the newly bipedal protohominid. Mechanical constraints of bipedality ushered in this system based on the attractivity of the ancestral vulva-focused estrus swellings, but in a permanent form. Permanent estrus displays (PED) of hominids are a continuation from a chimp-like ancestry of swollen and naked perineal signals. PED, realized when bipedality evolved, occurred through a phyletic automimickry (of the antecedent condition in the lineage) of the swollen labia of estrus by the fatty buttocks and by the spreading of hairless skin. A zoologically unique sexually dimorphic deposition of fat altered the buttocks and female body contours emphasizing the rear, and thus making permanent the signal paraphernalia of the ancestral estrus state. Further spread of such skin onto thighs and torso, and the development of pendulous breasts, in automimicry of the swollen perineal region of the ancestry, and the hominid buttocks, are the results of continuous sexual selection based on a successful somatic and behavioral female survival and reproductive strategy. In this example of inseparable sexual and natural selection, the continuous attractivity of PED females resulted in both feeding advantages obtained through the males, and probable anti-infanticidal strategy against males. This further fueled sexual selection of males as a result of both intermale and female-male selection. Male competition within a complex reconciliatory hierarchy of the group would have increasingly selected for phenotypes with a behavioral ability to deal with tensions created by the PED. Male hair reduction is the hormonally-mediated epiphenomenon of successful female traits. We explore explanations of human attributes in the framework of a historical-narrative explanation of hominid origins.