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In life, there are trade-offs between energy allocation for reproduction and lifespan. With a finite amount of metabolic energy available, where it is allocated makes a difference.
Scientists have observed on a species level that longer-lived animals typically produce fewer offspring. But does this tradeoff manifest at an individual animal level? A paper published this week in Nature presents evidence that it does.
Researchers from the University of Otago–Ōtākou Whakaihu Waka in New Zealand and several other universities analyzed data on 117 species of captive mammals housed at zoos and aquariums and conducted a meta-analysis on studies of several other vertebrate species.. Selecting species for which there were records for at least 30 individuals of each sex, they examined survival vis-à-vis reproductive output. On average, across species, sterilization increased animal life expectancy by almost 18 percent.
Variability cropped up with some animal care and demographic details, such as timing of the treatment, care environment, and sex of the individual. For example, the effects were distinct across males and females. In males, castration—i.e., removing the testicles—improved life expectancy, especially if it was done before puberty. But vasectomy—closing the tubes that move sperm out of storage in the testicles and into action—did not. So, extended lifespans in males apparently resulted from eliminating the sex hormones made by the testicles, which include testosterone and several others.
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“These hormones may interact with pathways that regulate the biology of aging, particularly during early-life development, since early-life castration has the strongest effects on lifespan,” explained co-author Mike Garratt, a reproductive biologist at the University of Otago, in a statement.
Castrated males also tend to engage in less aggressive behavior surrounding competition for mates, and may reap the benefits of fewer risky fights with other males.
In contrast, the lifespans of female mammals tended to increase with various forms of contraception, which Garratt attributed to the benefits that “arise from reducing the substantial energetic and physiological costs of pregnancy, lactation, and caring for offspring.” Female animals who were treated with hormonal contraceptives appeared to have more robust immune systems, buffering them from diseases, likely thanks to the energy saved by not having offspring.
Even foregoing reproduction in females that had previously reared offspring extended their lifespans, supporting arguments for “the evolutionary benefits of menopause, where reduced later-life reproductive investment contributes to improved longevity,” the study authors wrote.
There’s a caveat for females, however. When ovaries are removed in permanent surgical sterilization, the elimination of ovarian hormones makes females frail. Although they may live longer, that longevity is compromised by poor health.
The authors also considered data from human studies that retrospectively tested whether castration in historical populations was associated with improved survival. Those results, though mixed, revealed a similar average lifespan increase in castrated human males—about 18 percent.
As each animal traverses their own timeline, reproduction either happens or does not, and lives play out accordingly. But on a biological level, the tradeoffs involved seem to be coming into focus. 
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Lead image: Reto Buehler / Shutterstock
