Female survival advantage is well documented in certain mammalian species, most notably humans (Austad, 2006
). This translates into a greater likelihood that women will live to 100 years, outnumbering men by approximately 4:1 (Terry et al., 2008
). Little progress, however, has been made to elucidate the mechanisms of sex differences in human longevity. Transplantation of mouse ovaries from young donors into ovariectomized female mice increased life expectancy proportional to age of the recipient (Cargill et al., 2003
). But the mouse, biogerontology’s most trusted mammalian workhorse, is not likely the most trustworthy mimic of the sex differences in human longevity, because female mice are typically outlived by their male counterparts (Turturro et al., 2002
; Austad, 2006
Whether ovary removal early in life can re-set longevity parameters in humans has not been evaluated, as few young women undergo ovariectomy. In contrast, a large percentage of pet dogs undergo elective ovariectomy at different ages throughout the life course, creating a research opportunity to study the dose–response between endogenous ovarian function and longevity. Here, by studying pet dogs living in the same environment as humans (Waters & Wildasin, 2006
), we test the hypothesis that lifetime ovary exposure is significantly associated with exceptional longevity.
A database was established to construct lifetime medical histories for a cohort of 119 oldest-old Rottweiler dogs living in North America. These pet dogs lived with their owners and females underwent elective ovariectomy at different ages throughout the life course. Information on medical history, age at death and cause of death was collected by questionnaire and telephone interviews with pet owners and veterinarians as previously reported (Cooley et al., 2003
). Rottweilers with exceptional longevity lived ≥ 13 years, i.e. more than 30% longer than average life expectancy for the breed (9.4 years). In each case, date of birth obtained from American Kennel Club registration records was used to validate exceptional longevity. Sex (female:male) and lifetime duration of ovary exposure in the oldest-old dogs were compared with information collected from another cohort of 186 Rottweilers in the same catchment area that had usual longevity (age at death 8.0–10.8 years).
Like women, female dogs were more likely than males to achieve exceptional longevity (OR, 95% CI = 2.0, 1.2–3.3; P= 0.006). However, removal of ovaries during the first 4 years of life (i.e. median age at ovariectomy) erased the female survival advantage over males (OR, 95% CI = 1.2, 0.7–2.2; P= 0.55). In females that retained their ovaries for more than 4 years, likelihood of exceptional longevity increased to more than three times that of males (OR, 95% CI = 3.2, 1.8–5.7; P< 0.0001).
To further define the dose–response relationship between ovaries and longevity, we focused our analysis on the 83 exceptional longevity females and 100 usual longevity females (). This enabled us to address the following question: Is the duration of ovary exposure during the first 8 years of life associated with an increased likelihood of achieving exceptional longevity? When females from the exceptional longevity and usual longevity cohorts were combined then subdivided into tertiles based upon ovary exposure during the first 8 years of life, dogs with the longest ovary exposure (6.1–8.0 years) were 3.2 times more likely to reach exceptional longevity than dogs with shortest exposure (P
= 0.002) (; Supporting Fig. S1
). In multivariate analysis, the association between increasing ovary exposure and exceptional longevity remained strong even after considering other factors that might influence longevity, such as height, body weight and whether mother achieved exceptional longevity ().
Characteristics of female Rottweilers in study population
Endogenous ovary exposure and likelihood of exceptional longevity in female Rottweiler dogs
Finally, we evaluated whether the survival advantage in females with intact ovaries could be explained by a protective effect of ovaries against a particular disease. In Rottweilers with usual longevity, the major cause of death and major death category were bone sarcoma and cancer-all types, accounting for 38% and 73% deaths, respectively. We found that, after excluding bone cancer deaths or all cancer deaths, the strong association between intact ovaries and exceptional longevity persisted. After excluding all cancer deaths, females who kept their ovaries during the first 7 years of life (i.e. highest tertile of ovary exposure) were more than nine times more likely to reach exceptional longevity than females with shortest ovary exposure (P= 0.001) ().
Our results show that in Rottweiler dogs, like in humans, there is a strong female sex advantage for reaching exceptional longevity. Importantly, the longevity advantage over males is abolished in females that undergo early or mid-life ovarian removal. To our knowledge, this is the first demonstration that lifetime ovary exposure is significantly associated with exceptional longevity in any mammalian species.
By studying dogs that underwent elective ovariectomy at different ages, we were able to probe the dose–response relationship between endogenous ovarian function and exceptional longevity. A possible longevity-promoting effect of ovaries in exceptionally long-lived dogs in this study is supported by data from another cohort of pet dogs that we have investigated (Supporting Fig. S2
). In a population of 237 female Rottweiler dogs who died at 1.3–12.9 years, females that had intact ovaries for the first 4.5 years of life had 37% lower mortality than females that underwent elective ovariectomy before 4.5 years, i.e., median age at ovariectomy (hazard ratio, 95% CI = 0.63, 0.49–0.82; P
< 0.0001 log-rank test). The research strategy advanced here by our group – using a national population of registered, pure-bred dogs of a single breed, carefully quantitating the duration of ovary exposure rather than binning dogs into a category of either ‘intact’ or ‘ovariectomized’– lessens the likelihood of misclassification bias that likely plagued previous dog studies (Bronson, 1982
; Michell, 1999
Recognizing that observed associations between exposures and outcomes may not necessarily be causal, we explored alternative, non-causal explanations for the association between ovaries and exceptional longevity. But, we found no evidence that factors which may influence a pet owner’s decision on age at ovariectomy – for example, earlier ovariectomy in dogs with substandard conformation or delayed ovariectomy to obtain more offspring in daughters of long-lived mothers – can adequately account for the strong association (Supporting Appendix S1
). Further, our results mirror the recent findings from more than 29 000 women in the Nurses’ Health Study (Parker et al., 2009
). In that study, women who had elective hysterectomy with ovary sparing had lower overall mortality than those who underwent hysterectomy with ovariectomy. Notably, the benefit of keeping ovaries experienced by women under 50 years was attributable to decreased cardiovascular and cancer mortality (Parker et al., 2009
). Taken together, the findings from dogs and women support the hypothesis that early life physiological influences, such as ovarian hormones, lay the foundation for adult health outcomes including longevity. Further testing of the ovary-longevity hypothesis should utilize experimental designs that capture the broad range of lifetime ovary exposure seen in the pet dog population so that the critical windows of ovary exposure can be better defined.
Specific mechanisms have been proposed by which ovaries might promote longevity, including estrogen-induced enhanced immune response (Austad, 2006
; Straub, 2007
) and protection against oxidative stress (Borras et al., 2003
). We could not attribute the ovary-associated longevity advantage in dogs to resistance against a particular disease. Instead, we observed a robust ovarian association with longevity that was independent of cause of death
, suggesting that a network of processes regulating the intrinsic rate of aging is under ovarian control. Future studies in dogs and women are warranted to define specific ovarian longevity factors and to identify ovary-sensitive biological processes that promote healthy longevity. Pet dogs should provide a tractable mammalian model to investigate the mechanisms of how ovaries orchestrate extended longevity in both species.