Guppy Study Suggests Menopause Not Shaped by Nature
Thursday, January 5 2006
Jan. 5, 2006 MOSCOW, Idaho – A study of guppies from Trinidad may offer some intriguing insights into the biology of human menopause, a University of Idaho biologist says. The study undermines the popular idea that nature favors long-lived grandmothers because they enhance survival of closely-related kin, said Donna Holmes, a UI research professor who specializes in aging. She joined evolutionary biologist David Reznick of the University of California at Riverside and statistician Michael Bryant of the California Institute of the Arts in Valencia to co-author a paper appearing in the January issue of Public Library of Science: Biology. Their paper drew on new results of Reznick’s extensive studies of guppies from Trinidad’s mountain streams, some with predators and others without. In earlier research, Reznick and colleagues generally have found that guppies that evolved in streams with a lot of predators had litters sooner, produced more offspring in each litter and had lived shorter lives in nature. Guppies that evolved in the absence of predators, on the other hand, matured later, produced fewer babies, but on average lived longer and aged more slowly in the wild. This finding is consistent with evolutionary theory, which predicts that in streams with predators, natural selection will favor guppies that mature quickly and produce young, thus passing along their genes for these traits before becoming prey to larger fish. In the protected environment of the laboratory, however, guppy females from high predation environments live longer than those from low predation environments. In their paper, the researchers evaluated this finding in greater detail, dividing the lifespan into three non-overlapping segments: birth to age at first reproduction, age at first reproduction to age at last reproduction, and age at last reproduction to age at death, Reznick said. When maintained in the laboratory most of the guppies, which bear live young, lived beyond the end of their normal capacity to produce litters and exhibited a post-reproductive period similar to that of menopause in mammals, Reznick said. The laboratory study also showed that the longer captive life spans of guppies from high predation localities is attributable solely to their longer reproductive life spans, the component of the life history that contributes directly to guppies’ evolutionary fitness, he added. Females from both groups tended to live about the same length of time after they stopped reproducing. "It was basically the luck of the draw whether the females lived for a short time or a long time after they became infertile,” Holmes said. “Unlike the fertile portion of their life spans, their post-reproductive life spans didn’t correlate at all with the natural environment in which they’d evolved originally.” Guppies can offer insights into human health and aging, Holmes said. As a gerontologist, she looks at statistical shifts in human life spans over the last century primarily as a reflection of improvements in health care, food and water safety. In the early 1900s, the average lifespan of a woman in the U.S. was approximately half that enjoyed by women today, many of whom now reach their 70s and 80s. This idea conflicts with the popular “grandmother hypothesis,” which says natural selection favors the midlife loss of childbearing so that older women can contribute instead to the survival of their daughters, grandchildren and other kin. While healthy older women certainly make contributions to their families in many human societies, the randomness of guppy females’ survival after they stop reproducing argues against a direct role of natural selection in shaping post-reproductive life spans, Holmes said. Her reading of the genes may be unpopular, she concedes. “The grandmother hypothesis has gotten a lot of attention because women like it so much,” Holmes said. “I like the idea, too, that grandmothers contribute to the quality of life of their kin, but that doesn’t mean it’s a result of natural selection.” The human health implications that emerge, Holmes said, are potentially provocative. “If women’s post-reproductive life spans were not shaped by natural selection, that may help to explain some of the troublesome symptoms many women experience around the time of menopause, like hot flashes, that often require some sort of intervention.” Many menopausal symptoms, as well as osteoporosis, are linked to declines in estrogen production during menopause. Although she does not advocate a particular hormone therapy, she believes women should not rule out medical support. “The idea that we have to tough menopause out because it’s ‘natural’ seems just as oppressive to me as being pushed into treatments for menopause because we’re somehow defective. “To me, understanding aging in a variety of vertebrate species is important for understanding how to treat diseases of aging and extend healthy human life spans,” Holmes said. Contacts: Donna Holmes, University of Idaho associate professor of biology, electric@uidaho.edu; or Bill Loftus, UI science writer, (208) 885-7694, bloftus@uidaho.edu -30- BL-1/5/2006-SCI
About the University of Idaho
The University of Idaho helps students to succeed and become leaders. Its land-grant mission furthers innovative scholarly and creative research to grow Idaho's economy and serve a statewide community. From its main campus in Moscow, Idaho, to 70 research and academic locations statewide, U-Idaho emphasizes real-world application as part of its student experience. U-Idaho combines the strength of a large university with the intimacy of small learning communities. It is home to the Vandals. For information, visit www.uidaho.edu.