species, also display this behavior. however, mating behavior that increases local relatedness with female age is prevalent in non-menopausal species, making it unlikely that it is the only factor that determines if menopause will evolve in a species. mother hypothesis the mother hypothesis suggests that menopause was selected for humans because of the extended development period of human offspring and high costs of reproduction so that mothers gain an advantage in reproductive fitness by redirecting their effort from new offspring with a low survival chance to existing children with a higher survival chance. grandmother hypothesis the grandmother hypothesis suggests that menopause was selected for humans because it promotes the survival of grandchildren. according to this hypothesis, post-reproductive women feed and care for children, adult nursing daughters, and grandchildren whose mothers have weaned them. human babies require large and steady supplies of glucose to feed the growing brain. in infants in the first year of life, the brain consumes 60% of all calories, so both babies and their mothers require a dependable food supply. some evidence suggests that hunters contribute less than half the total food budget of most hunter-gatherer societies, and often much less than half, so that foraging grandmothers can contribute substantially to the survival of grandchildren at times when mothers and fathers are unable to gather enough food for all of their children. in general, selection operates most powerfully during times of famine or other privation. so although grandmothers might not be necessary during good times, many grandchildren cannot survive without them during times of famine. post-reproductive female orcas tend to lead their pods, especially during years of food scarcity. furthermore, the increased mortality risk of an orca due to losing a grandmother is stronger in years of food scarcity analysis of historical data found that the length of a female's post-reproductive lifespan was reflected in the reproductive success of her offspring and the survival of her grandchildren. another study found comparative effects but only in the maternal grandmother—paternal grandmothers had a detrimental effect on infant mortality (probably due to paternity uncertainty). differing assistance strategies for maternal and paternal grandmothers have also been demonstrated. maternal grandmothers concentrate on offspring survival, whereas paternal grandmothers increase birth rates. some believe variations on the mother, or grandmother effect fail to explain longevity with continued spermatogenesis in males (oldest verified paternity is 94 years, 35 years beyond the oldest documented birth attributed to females). notably, the survival time past menopause is roughly the same as the maturation time for a human child. that a mother's presence could aid in the survival of a developing child, while an unidentified father's absence might not have affected survival, could explain the paternal fertility near the end of the father's lifespan. a man with no certainty of which children are his may merely attempt to father additional children, with support of existing children present but small. note the existence of partible paternity supporting this. some argue that the mother and grandmother hypotheses fail to explain the detrimental effects of losing ovarian follicular activity, such as osteoporosis, osteoarthritis, alzheimer's disease and coronary artery disease. the theories discussed above assume that evolution directly selected for menopause. another theory states that menopause is the byproduct of the evolutionary selection for follicular atresia, a factor that causes menopause. menopause results from having too few ovarian follicles to produce enough estrogen to maintain the ovarian-pituitary-hypothalamic loop, which results in the cessation of menses and the beginning of menopause. human females are born with approximately a million oocytes, and approximately 400 oocytes are lost to ovulation throughout life. reproductive conflict hypothesis in social vertebrates, the sharing of resources among the group places limits on how many offspring can be produced and supported by members of the group. this creates a situation in which each female must compete with others of the group to ensure they are the one that reproduces. the reproductive conflict hypothesis proposes that this female reproductive conflict favors the cessation of female reproductive potential in older age to avoid reproductive conflict, increasing the older female's fitness through inclusive benefits. female-biased dispersal or non-local mating leads to an increase in relatedness to the social group with female age. in the human case of female-biased dispersal, when a young female enters a new group, she is not related to any individual and she reproduces to produce an offspring with a relatedness of 0.5. an older female could also choose to reproduce, producing an offspring with a relatedness of 0.5, or she could refrain from reproducing and allow another pair to reproduce. because her relatedness to males in the group is high, there is a fair probability that the offspring will be her grandchild with a relatedness of 0.25. the younger female experiences no cost to her inclusive fitness from using the resources necessary to successfully rear offspring since she is not related to members of the group, but there is a cost for the older female. as a result, the younger female has the advantage in reproductive competition. although a female orca born into a social group is related to some members of the group, the whale case of non-local mating leads to similar outcomes because the younger female relatedness to the group as a whole is less than the relatedness of the older female. this behavior makes more likely the cessation of reproduction late in life to avoid reproductive conflict with younger females. research using both human and orca demographic data has been published that supports the role of reproductive conflict in the evolution of menopause. analysis of demographic data from pre-industrial finnish populations found significant reductions in offspring survivorship when mothers-in-laws and daughters-in-laws had overlapping births, supporting the idea that avoiding reproductive conflict is beneficial to offspring survivorship. humans, more so than other primates, rely on food sharing for survival, so the large survivorship reduction values could be caused by a straining of community resources. avoiding such straining is a possible explanation for why the reproductive overlap seen in humans is much lower than other primates. food sharing is also prevalent among another menopausal species, orcas. reproductive conflict has also been observed in orcas, with increased calf mortality seen when reproductive overlap between a younger and older generational female occurred. other animals menopause in the animal kingdom appears to be uncommon, but the presence of this phenomenon in different species has not been thoroughly researched. life histories show a varying degree of senescence; rapid senescing organisms (e.g., pacific salmon and annual plants) do not have a post-reproductive life-stage. gradual senescence is exhibited by all placental mammalian life histories. menopause has been observed in several species of nonhuman primates, including rhesus monkeys and chimpanzees. some research suggests that wild chimpanzees do not experience menopause, as their fertility declines are associated with declines in overall health. menopause also has been reported in a variety of other vertebrate species including elephants, short-finned pilot whales, orcas, narwhals, beluga whales, and the guppy. however, with the exception of the short-finned pilot whale, killer whale, narwhals, and beluga whales, such examples tend to be from captive individuals, and thus they are not necessarily representative of what happens in natural populations in the wild. dogs do not experience menopause; the canine estrus cycle simply becomes irregular and infrequent. although older female dogs are not considered good candidates for breeding, offspring have been produced by older animals. similar observations have been made in cats.