What do insect studies tell us about human lifespan, aging and death?
“Lots,” says UC Davis entomologist James R. Carey, director of a federally funded program on aging and lifespan that has just received a $3.4 million grant renewal from the National Institute on Aging.
“One of the paradoxes of aging science is that whereas much is known about the nature of aging, little is known about the nature of lifespan,” said Carey, a professor in the UC Davis Department of Entomology who has researched lifespan and aging for nearly 30 years. “For example, why do mice live only a few years while humans are capable of living 80 or more years?”
The grant is a two-year extension of his ongoing program, Biodemographic Determinants of Lifespan, a National Institutes of Health/National Institute of Aging-funded program involving scientists from UC Davis, UC Santa Barbara, UC Berkeley, Stanford and seven other academic institutions in the United States, the United Kingdom, and Greece. The scientists study aging in nematodes, honey bees, fruit flies, red deer, soay sheep and humans, and develop mathematical models targeting the evolutionary ecology of aging and lifespan. The program has been funded since 2003.
“Dr. Carey has expanded the boundaries of entomology with his research,” said Michael Parrella, professor and chair of the UC Davis Department of Entomology. “ Just as we have learned a great deal about human genetics by studying Drosophila fruit flies, Jim is expanding our overall understanding of mortality and lifespans by using various insects as model systems. He is known worldwide as one of the pioneers of biodemography, an emerging field in the interzone between biology and demography. His research is innovative and unique, and is one of many research programs that makes the Department of Entomology so strong.”
Carey said the broad aim of the research “is to develop an evolutionary demography of lifespan. All of the findings will be directly or indirectly relevant to an understanding of human aging and lifespan.”
“I am always asked what the results of studies on lifespan and aging in insects tell us about aging in humans,” he said. “My response is that a better way to frame the question is: ‘What do insect studies on aging tell us about the basic principles?’ In a word: Lots!”
Carey listed nine principles of lifespan and aging, most of which were either derived from or inspired by the results of his insect studies:
1. There is no sharply defined upper age limit to lifespan in any species (including humans).
2. The risk of death does not increase with age in either the very young or the very old. Thus, strictly speaking, centenarians are not aging since their annual mortality risk for each of the next 15 years is a constant 50 percent (that is, the probability of reaching 110 from 100 is equivalent to obtaining 10 heads in a row in a series of coin tosses).
3. Virtually all social species (including humans) are long-lived because longevity and sociality are mutually reinforcing — sociality creates conditions that extend lifespan; the increased longevity creates conditions that foster the evolution of greater degrees of sociality.
4. Higher female longevity is not a general principle of nature. Rather, the general principle is that, because of differences in physiology and behavior, the mortality responses between males and females living in identical environments are different.
5. The period of disability in insects near the end of their lives is proportional to their life expectancy. If also true for humans, this observation has profound medical and economic implications since life expectancies (and possibly disability periods) for both men and women in the U.S. continue to increase.
6. In life table analysis, age and dose are interchangeable. Thus, cause of death in the elderly can be thought of as an overdose of time.
7. Lifespan and reproduction are inextricably linked. Studies with UC Davis colleagues in which donor ovaries from young mice were transplanted into older females significantly extended their old-age survival. The results have important implications for human health and longevity.
8. The nutritional strategy for maximizing longevity is fundamentally different from that for optimizing reproduction, the former requiring calorie restriction and the latter requiring gastronomic profusion.
9. Old age is only halfway to extreme age. Actuarially speaking, a 92-year-old woman is only ‘halfway’ to 100 since her mortality risk from birth to 92 is identical to her risk of death between 92 and 100. The same holds for men.
Carey teaches a class on longevity through the Department of Entomology and the Human and Community Development Department, drawing 200 students per class.
“Among other things, I teach students that quantity of life flows from quality of life, and that the quality derives from healthful living at three interconnected levels including:
Level 1. Healthful ingredients such as eating right, exercise, social connectedness, and avoidance of vices (for example, smoking; and alcohol and other drugs)
Level 2. Healthful strategies where the different ingredients are brought together into a coherent healthy lifestyle, that is, individually tailored. It is virtually impossible for any individual to do everything ‘healthy.’
Level 3. Successful aging which builds on the first two levels but adds a dimension involving a person’s nobility of purpose and strength of character. Just eating right and exercising is not enough. We need direction and a deeper sense of meaning in life.”
Carey said that scientific research on death as a process is almost non-existent. However, he points to a recently published editorial in Nature that stresses the need for a deeper understanding of death in the context of the bioethics of organ transplants. “With aging research, insects can be used as models for studies in this area as well,” Carey said. As a lead-in for his classroom lecture on death, Carey quotes the late science fiction writer Isaac Asimov: “…life is pleasant, death is peaceful; it’s the transition that troubles.”
Carey, who has researched lifespan and aging since joining the UC Davis faculty in 1980, has authored three books on the subject. He was the lead author of a seminal life table study on medflies that, he said, showed “mortality slowed at older ages and thus supported the idea that there is no ‘wall of death’ — that there is not a fixed limit to lifespan.” The study, published in the journal Science in 1992, involved more than a million medflies.
His Oct. 21 seminar is the first Webcast of his work. He is the former chair of the UC Systemwide Academic Senate University Committee on Research Policy which is spearheading the piloting of Webinars on three UC campuses, with long-term plans to expand to the seven other campuses.
Carey said the Biodemographic Determinants of Lifespan blends projects, aims, experiments and models. Some projects focus on the ecology and evolution of lifespan, others on the mechanisms of aging, and others on both: the function of lifespan and the mechanisms involved in aging.
In his current research, Carey is testing specific hypotheses concerning aging, lifespan and impairment of insects in the wild and, with UC Davis colleagues, developing statistical models to estimate survival and age structure.
His UC Davis collaborators include statisticians Hans Müller and Jane-Ling Wang. “They are among the best statistical modelers currently publishing in aging science,” Carey said.
The director of another project based in part at UC Davis is honey bee behavioral geneticist Robert Page Jr., whom Carey described as “as one of the top scientists involved in the biology of aging.” Page, former chair of the UC Davis Department of Entomology, and now founding director of the School of Life Sciences at Arizona State University, studies caste demography and aging in honey bees.