Human males are reproductively active throughout their life span but not the females. why???
Answers
Answer:
The relationships between reproductive aging, reproductive cessation, the emergence of extended, post-reproductive life, and social behavior in humans continue to be topics of both theoretical and empirical interest. Debate continues over the physiological processes underlying human reproductive aging (Downs and Wise, 2009; Perheentupa and Huhtaniemi, 2009; Ferrell et al., 2012), the uniqueness of human menopause (Packer et al., 1998; Herndon and Walker, 2010; Levitis et al., 2013), and the evolutionary forces that may have shaped late life and reproduction in humans (Johnstone and Cant, 2010; Kaplan et al., 2010; Hawkes et al., 2011; Mittledorf and Goodnight, 2012; Chu and Lee, 2013). However, from a comparative perspective, it is critical to recognize the existence of highly conserved mechanisms in these lifelong processes and to identify similarities as well as unique differences across vertebrates. Follicular depletion is widely considered to be the primary cause of the phenomenon of human menopause, as well as of cessation of ovarian function in other birds and mammals (Edson et al., 2009; Perheentupa and Huhtaniemi, 2009; Finch, 2013), but the degree to which this imposes a constraint on evolution is not clear. There is also disagreement over whether the trajectory of follicular depletion in humans displays evidence of significant acceleration prior to menopause (Richardson et al., 1987; Faddy and Gosden, 1995; Hansen et al., 2008; Coxworth and Hawkes, 2010;), as well as over recent evidence for follicular renewal throughout life (Johnson et al., 2004; Eggan et al., 2006; Faddy and Gosden, 2007; Begum et al., 2008; Kerr et al., 2012).
Other components of reproductive aging, including gonadal senescence in males and changes in hypothalamic-pituitary function in both sexes, are often neglected in these debates. There is also debate over the relative frequency of significant periods of post-reproductive survival in nature and its phylogenetic distribution (Cohen, 2004; Pollycove et al., 2011). Finally, there are theoretical debates regarding the evolutionary origin of the currently observed human pattern of long, regularly occurring post-reproductive life and its relationship to human social behavior, ranging from those who see it as a consequence of intergenerational conflict to those who see it as a consequence of intergenerational cooperation (Mace, 2000; Hawkes, 2003; Cant and Johnstone, 2008; Johnstone and Cant, 2010; Chu and Lee, 2013).
In this paper, we will attempt to bring some of these debates into comparative perspective, focusing particularly on vertebrates. We will first consider the mechanisms of reproductive aging and cessation in vertebrates and particularly in birds and mammals, then the phenomenological distribution of post-reproductive life in captivity and in the wild. In addition, we will consider the insights provided by laboratory or captive populations in order to compare and contrast conserved mechanisms as opposed to unique adaptations in some populations. Finally, we will return to questions of the evolutionary origins of human post-reproductive life in particular.