On size and survival: progress and pitfalls in the allometry of life span

The hyperfunction theory: An emerging paradigm for the biology of aging

The process of senescence (aging) is predominantly determined by the action of wild-type genes. For most organisms, this does not reflect any adaptive function that senescence serves, but rather evolutionary effects of declining selection against genes with deleterious effects later in life. To understand aging requires an account of how evolutionary mechanisms give rise to pathogenic gene action and late-life disease, that integrates evolutionary (ultimate) and mechanistic (proximate) causes into a single explanation. A well-supported evolutionary explanation by G.C. Williams argues that senescence can evolve due to pleiotropic effects of alleles with antagonistic effects on fitness and late-life health (antagonistic pleiotropy, AP). What has remained unclear is how gene action gives rise to late-life disease pathophysiology. One ultimate-proximate account is T.B.L. Kirkwood's disposable soma theory. Based on the hypothesis that stochastic molecular damage causes senescence, this reasons that aging is coupled to reproductive fitness due to preferential investment of resources into reproduction, rather than somatic maintenance. An alternative and more recent ultimate-proximate theory argues that aging is largely caused by programmatic, developmental-type mechanisms. Here ideas about AP and programmatic aging are reviewed, particularly those of M.V. Blagosklonny (the hyperfunction theory) and J.P. de Magalhães (the developmental theory), and their capacity to make sense of diverse experimental findings is assessed.

Origins and evolution of extreme life span in Pacific Ocean rockfishes

Pacific Ocean rockfishes (genus Sebastes) exhibit extreme variation in life span, with some species being among the most long-lived extant vertebrates. We de novo assembled the genomes of 88 rockfish species and from these identified repeated signatures of positive selection in DNA repair pathways in long-lived taxa and 137 longevity-associated genes with direct effects on life span through insulin signaling and with pleiotropic effects through size and environmental adaptations. A genome-wide screen of structural variation reveals copy number expansions in the immune modulatory butyrophilin gene family in long-lived species. The evolution of different rockfish life histories is coupled to genetic diversity and reshapes the mutational spectrum driving segregating CpG→TpG variants in long-lived species. These analyses highlight the genetic innovations that underlie life history trait adaptations and, in turn, how they shape genomic diversity.

Cellular metabolism and IL-6 concentrations during stimulated inflammation in primary fibroblasts from small and large dog breeds as they age

The immune system undergoes marked changes during aging characterized by a state of chronic, low-grade inflammation termed 'inflammaging'. We explore this phenomenon in domestic dogs, which are the most morphologically and physiologically diverse group of mammals, with the widest range in body sizes for a single species. Additionally, smaller dogs tend to live significantly longer than larger dogs across all breeds. Body size is intricately linked to mass-specific metabolism and aging rates, which suggests that dogs are exemplary for studies in inflammaging. Dermal fibroblast cells play an important role in skin inflammation, making them a good model for inflammatory patterns across dog breed, body sizes and ages. Here, we examined aerobic and glycolytic cellular metabolism, and IL-6 concentrations in primary fibroblast cells isolated from small and large dog breeds, that were either recently born puppies or old dogs after death. We found no differences in cellular metabolism when isolated fibroblasts were treated with lipopolysaccharide (LPS) from Escherichia coli to stimulate an inflammatory phenotype. Unlike responses observed in mice and humans, there was a less drastic amplification of IL-6 concentration after LPS treatment in the geriatric population of dogs compared with recently born dogs. In young dogs, we also found evidence that untreated fibroblasts from large breeds had significantly lower IL-6 concentrations than observed for smaller breeds. This implies that the patterns of inflammaging in dogs may be distinct and different from other mammals commonly studied.

Differences in Reproductive Success in Young and Old Females of a Long-Lived Species

Long-lived species are particularly interesting for investigation of trade-offs that shape reproductive allocation and the effective contribution to the next generations. Life history theory predicts that these species will buffer environmental stochasticity via changes in the reproductive investment, while maintaining high adult survival rates. The spur-thighed tortoise was selected as a case study in order to investigate the relationship between the linked maternal characteristics (size and age) and related traits in their hatchlings. We tracked naturally emerging hatchlings from young and old females under semi-natural conditions to test variations in hatchling numbers, body mass, size and survival over two years. We used linear mixed-effect models to analyze variations in hatchling body mass and size, and a mark-release-recapture framework to model their survival. Our study illustrates that old females of long-lived species have greater offspring numbers, greater survival and smaller size when compared with those of young females. The interannual variability evidenced the reduced offspring number and survival in the lower autumn rainfall and spring mean temperature year. Our results highlight the role of maternal age and climatic conditions in the population dynamics and the need for long-term studies of reproduction traits for designating adequate conservation strategies.

Metabolic rate is negatively linked to adult survival but does not explain latitudinal differences in songbirds

Survival rates vary dramatically among species and predictably across latitudes, but causes of this variation are unclear. The rate-of-living hypothesis posits that physiological damage from metabolism causes species with faster metabolic rates to exhibit lower survival rates. However, whether increased survival commonly observed in tropical and south temperate latitudes is associated with slower metabolic rate remains unclear. We compared metabolic rates and annual survival rates that we measured across 46 species, and from literature data across 147 species of birds in northern, southern and tropical latitudes. High metabolic rates were associated with lower survival but survival varied substantially among latitudinal regions independent of metabolism. The inability of metabolic rate to explain latitudinal variation in survival suggests (1) species may evolve physiological mechanisms that mitigate physiological damage from cellular metabolism and (2) extrinsic rather than intrinsic sources of mortality are the primary causes of latitudinal differences in survival.

Adaptation to developmental diet influences the response to selection on age at reproduction in the fruit fly

Experimental evolution (EE) is a powerful tool for addressing how environmental factors influence life-history evolution. While in nature different selection pressures experienced across the lifespan shape life histories, EE studies typically apply selection pressures one at a time. Here, we assess the consequences of adaptation to three different developmental diets in combination with classical selection for early or late reproduction in the fruit fly Drosophila melanogaster. We find that the response to each selection pressure is similar to that observed when they are applied independently, but the overall magnitude of the response depends on the selection regime experienced in the other life stage. For example, adaptation to increased age at reproduction increased lifespan across all diets; however, the extent of the increase was dependent on the dietary selection regime. Similarly, adaptation to a lower calorie developmental diet led to faster development and decreased adult weight, but the magnitude of the response was dependent on the age-at-reproduction selection regime. Given that multiple selection pressures are prevalent in nature, our findings suggest that trade-offs should be considered not only among traits within an organism, but also among adaptive responses to different-sometimes conflicting-selection pressures, including across life stages.

Adaptive sequence convergence of the tumor suppressor ADAMTS9 between small-bodied mammals displaying exceptional longevity

Maximum lifespan varies by two orders of magnitude across mammals. How such divergent lifespans have evolved remains an open question, with ramifications that may potentially lead to therapies for age-related diseases in humans. Several species of microbats as well as the naked mole-rat live much longer than expected given their small sizes, show reduced susceptibility to neoplasia, and largely remain healthy and reproductively capable throughout the majority of their extended lifespans. The convergent evolution of extreme longevity in these two groups allows for the opportunity to identify potentially important aging related genes that have undergone adaptive sequence convergence in these long-lived, yet small-bodied species. Here, we have tested 4,628 genes for evidence of convergence between the microbats and naked mole-rat. We find a strong signal of adaptive sequence convergence in the gene A disintegrin-like and metalloprotease with thrombospondin type 1 motifs 9 (ADAMTS9). We also provide evidence that the shared substitutions were driven by selection. Intriguingly, ADAMTS9 is a known inhibitor of the mTor pathway and has been implicated in several aging related processes.

A comparative perspective on longevity: the effect of body size dominates over ecology in moths

Both physiologically and ecologically based explanations have been proposed to account for among-species differences in lifespan, but they remain poorly tested. Phylogenetically explicit comparative analyses are still scarce and those that exist are biased towards homoeothermic vertebrates. Insect studies can significantly contribute as lifespan can feasibly be measured in a high number of species, and the selective forces that have shaped it may differ largely between species and from those acting on larger animals. We recorded adult lifespan in 98 species of geometrid moths. Phylogenetic comparative analyses were applied to study variation in species-specific values of lifespan and to reveal its ecological and life-history correlates. Among-species and between-gender differences in lifespan were found to be notably limited; there was also no evidence of phylogenetic signal in this trait. Larger moth species were found to live longer, with this result supporting a physiological rather than ecological explanation of this relationship. Species-specific lifespan values could not be explained by traits such as reproductive season and larval diet breadth, strengthening the evidence for the dominance of physiological determinants of longevity over ecological ones.

Sex, long life and the evolutionary transition to cooperative breeding in birds

Long life is a typical feature of individuals living in cooperative societies. One explanation is that group living lowers mortality, which selects for longer life. Alternatively, long life may make the evolution of cooperation more likely by ensuring a long breeding tenure, making helping behaviour and queuing for breeding positions worthwhile. The benefit of queuing will, however, depend on whether individuals gain indirect fitness benefits while helping, which is determined by female promiscuity. Where promiscuity is high and therefore the indirect fitness benefits of helping are low, cooperation can still be favoured by an even longer life span. We present the results of comparative analyses designed to test the likelihood of a causal relationship between longevity and cooperative breeding by reconstructing ancestral states of cooperative breeding across birds, and by examining the effect of female promiscuity on the relationship between these two traits. We found that long life makes the evolution of cooperation more likely and that promiscuous cooperative species are exceptionally long lived. These results make sense of promiscuity in cooperative breeders and clarify the importance of life-history traits in the evolution of cooperative breeding, illustrating that cooperation can evolve via the combination of indirect and direct fitness benefits.

The Companion Dog as a Model for the Longevity Dividend

The companion dog is the most phenotypically diverse species on the planet. This enormous variability between breeds extends not only to morphology and behavior but also to longevity and the disorders that affect dogs. There are remarkable overlaps and similarities between the human and canine species. Dogs closely share our human environment, including its many risk factors, and the veterinary infrastructure to manage health in dogs is second only to the medical infrastructure for humans. Distinct breed-based health profiles, along with their well-developed health record system and high overlap with the human environment, make the companion dog an exceptional model to improve understanding of the physiological, social, and economic impacts of the longevity dividend (LD). In this review, we describe what is already known about age-specific patterns of morbidity and mortality in companion dogs, and then explore whether this existing evidence supports the LD. We also discuss some potential limitations to using dogs as models of aging, including the fact that many dogs are euthanized before they have lived out their natural life span. Overall, we conclude that the companion dog offers high potential as a model system that will enable deeper research into the LD than is otherwise possible.

The Mass-Longevity Triangle: Pareto Optimality and the Geometry of Life-History Trait Space

When organisms need to perform multiple tasks they face a fundamental tradeoff: no phenotype can be optimal at all tasks. This situation was recently analyzed using Pareto optimality, showing that tradeoffs between tasks lead to phenotypes distributed on low dimensional polygons in trait space. The vertices of these polygons are archetypes--phenotypes optimal at a single task. This theory was applied to examples from animal morphology and gene expression. Here we ask whether Pareto optimality theory can apply to life history traits, which include longevity, fecundity and mass. To comprehensively explore the geometry of life history trait space, we analyze a dataset of life history traits of 2105 endothermic species. We find that, to a first approximation, life history traits fall on a triangle in log-mass log-longevity space. The vertices of the triangle suggest three archetypal strategies, exemplified by bats, shrews and whales, with specialists near the vertices and generalists in the middle of the triangle. To a second approximation, the data lies in a tetrahedron, whose extra vertex above the mass-longevity triangle suggests a fourth strategy related to carnivory. Each animal species can thus be placed in a coordinate system according to its distance from the archetypes, which may be useful for genome-scale comparative studies of mammalian aging and other biological aspects. We further demonstrate that Pareto optimality can explain a range of previous studies which found animal and plant phenotypes which lie in triangles in trait space. This study demonstrates the applicability of multi-objective optimization principles to understand life history traits and to infer archetypal strategies that suggest why some mammalian species live much longer than others of similar mass.

Why is aging conserved and what can we do about it?

The field of aging research has progressed rapidly over the past few decades. Genetic modulators of aging rate that are conserved over a broad evolutionary distance have now been identified. Several physiological and environmental interventions have also been shown to influence the rate of aging in organisms ranging from yeast to mammals. Here we briefly review these conserved pathways and interventions and highlight some key unsolved challenges that remain. Although the molecular mechanisms by which these modifiers of aging act are only partially understood, interventions to slow aging are nearing clinical application, and it is likely that we will begin to reap the benefits of aging research prior to solving all of the mysteries that the biology of aging has to offer.

Old age is the single greatest risk factor for the leading causes of death in the developed world. Advances in aging research promise to alleviate the diseases of aging by targeting aging itself.

Evolution of lifespan

Present-day evolutionary theory, modern synthesis and evo-devo, appear to explain evolution. There remain however several points of contention. These include: biological time, direction, macroevolution verses microevolution, ageing and the extent of internal as opposed to external mediation. A new theoretical model for the control of biological time in vertebrates/bilaterians is introduced. Rather than biological time being controlled solely by a molecular cascade domino effect, it is suggested there is also an intracellular oscillatory clock. This clock (life's timekeeper) is synchronised across all cells in an organism and runs at a constant frequency throughout life. Slower frequencies extend lifespan, increase body/brain size and advance behaviour. They also create a time void which could aid additional evolutionary change. Faster frequencies shorten lifespan, reduce body/brain size and diminish behaviour. They are therefore less likely to mediate evolution in vertebrates/mammals. It is concluded that in vertebrates, especially mammals, there is a direction in evolution towards longer lifespan/advanced behaviour. Lifespan extension could equate with macroevolution and subsequent modifications with microevolution. As life's timekeeper controls the rate of ageing it constitutes a new genetic theory of ageing. Finally, as lifespan extension is internally mediated, this suggests a major role for internal mediation in evolution.

Ecology and mode-of-life explain lifespan variation in birds and mammals

Maximum lifespan in birds and mammals varies strongly with body mass such that large species tend to live longer than smaller species. However, many species live far longer than expected given their body mass. This may reflect interspecific variation in extrinsic mortality, as life-history theory predicts investment in long-term survival is under positive selection when extrinsic mortality is reduced. Here, we investigate how multiple ecological and mode-of-life traits that should reduce extrinsic mortality (including volancy (flight capability), activity period, foraging environment and fossoriality), simultaneously influence lifespan across endotherms. Using novel phylogenetic comparative analyses and to our knowledge, the most species analysed to date (n = 1368), we show that, over and above the effect of body mass, the most important factor enabling longer lifespan is the ability to fly. Within volant species, lifespan depended upon when (day, night, dusk or dawn), but not where (in the air, in trees or on the ground), species are active. However, the opposite was true for non-volant species, where lifespan correlated positively with both arboreality and fossoriality. Our results highlight that when studying the molecular basis behind cellular processes such as those underlying lifespan, it is important to consider the ecological selection pressures that shaped them over evolutionary time.

Life's timekeeper

Life's timekeeper is a 'free-running' intracellular oscillator synchronised across all cells. It runs throughout life splitting lifespan into equal length phases. During the maturational period it controls the overall rate of progression whereas in the post-maturational period it controls the overall rate of ageing. This includes the rate of senescence and hence time to death. As such life's timekeeper equates maturational and post-maturational time, hence explains the tight correlation between these time periods that has existed throughout mammalian evolution. Life's timekeeper is proposed to have played an important role in vertebrate evolution. A slower oscillatory frequency results in proportional life phase prolongation. This leads to increased body and brain size, together with extended lifespan. Higher brain centres, neocortex in mammals, are disproportionately enlarged. Hence behavioural capacity is increased. The extended post-maturational period ensures that there is enough time in order that the behavioural advantages can be fully manifest in the environment. A faster oscillatory frequency would result in proportional life phase reduction. This process however would lead to reduced behavioural capacity, and is hence unlikely to be positively selected. Therefore throughout evolution life's timekeeper has operated to extend lifespan. It has hence functioned to promote longevity as opposed to ageing.

Testing the oxidative stress hypothesis of aging in primate fibroblasts: is there a correlation between species longevity and cellular ROS production?

The present study was conducted to test predictions of the oxidative stress theory of aging assessing reactive oxygen species production and oxidative stress resistance in cultured fibroblasts from 13 primate species ranging in body size from 0.25 to 120 kg and in longevity from 20 to 90 years. We assessed both basal and stress-induced reactive oxygen species production in fibroblasts from five great apes (human, chimpanzee, bonobo, gorilla, and orangutan), four Old World monkeys (baboon, rhesus and crested black macaques, and patas monkey), three New World monkeys (common marmoset, red-bellied tamarin, and woolly monkey), and one lemur (ring-tailed lemur). Measurements of cellular MitoSox fluorescence, an indicator of mitochondrial superoxide (O2(·-)) generation, showed an inverse correlation between longevity and steady state or metabolic stress-induced mitochondrial O2(·-) production, but this correlation was lost when the effects of body mass were removed, and the data were analyzed using phylogenetically independent contrasts. Fibroblasts from longer-lived primate species also exhibited superior resistance to H(2)O(2)-induced apoptotic cell death than cells from shorter-living primates. After correction for body mass and lack of phylogenetic independence, this correlation, although still discernible, fell short of significance by regression analysis. Thus, increased longevity in this sample of primates is not causally associated with low cellular reactive oxygen species generation, but further studies are warranted to test the association between increased cellular resistance to oxidative stressor and primate longevity.

Stress resistance in the naked mole-rat: the bare essentials - a mini-review

BACKGROUND

Studies comparing similar-sized species with disparate longevity may elucidate novel mechanisms that abrogate aging and prolong good health. We focus on the longest living rodent, the naked mole-rat. This mouse-sized mammal lives ~8 times longer than do mice and, despite high levels of oxidative damage evident at a young age, it is not only very resistant to spontaneous neoplasia but also shows minimal decline in age-associated physiological traits.

OBJECTIVES

We assess the current status of stress resistance and longevity, focusing in particular on the molecular and cellular responses to cytotoxins and other stressors between the short-lived laboratory mouse and the naked mole-rat.

RESULTS

Like other experimental animal models of lifespan extension, naked mole-rat fibroblasts are extremely tolerant of a broad spectrum of cytotoxins including heat, heavy metals, DNA-damaging agents and xenobiotics, showing LD(50) values between 2- and 20-fold greater than those of fibroblasts of shorter-lived mice. Our new data reveal that naked mole-rat fibroblasts stop proliferating even at low doses of toxin whereas those mouse fibroblasts that survive treatment rapidly re-enter the cell cycle and may proliferate with DNA damage. Naked mole-rat fibroblasts also show significantly higher constitutive levels of both p53 and Nrf2 protein levels and activity, and this increases even further in response to toxins.

CONCLUSION

Enhanced cell signaling via p53 and Nrf2 protects cells against proliferating with damage, augments clearance of damaged proteins and organelles and facilitates the maintenance of both genomic and protein integrity. These pathways collectively regulate a myriad of mechanisms which may contribute to the attenuated aging profile and sustained healthspan of the naked mole-rat. Understanding how these are regulated may be also integral to sustaining positive human healthspan well into old age and may elucidate novel therapeutics for delaying the onset and progression of physiological declines that characterize the aging process.

Hibernation is associated with increased survival and the evolution of slow life histories among mammals

Survival probability is predicted to underlie the evolution of life histories along a slow-fast continuum. Hibernation allows a diverse range of small mammals to exhibit seasonal dormancy, which might increase survival and consequently be associated with relatively slow life histories. We used phylogenetically informed GLS models to test for an effect of hibernation on seasonal and annual survival, and on key attributes of life histories among mammals. Monthly survival was in most cases higher during hibernation compared with the active season, probably because inactivity minimizes predation. Hibernators also have approximately 15 per cent higher annual survival than similar sized non-hibernating species. As predicted, we found an effect of hibernation on the relationships between life history attributes and body mass: small hibernating mammals generally have longer maximum life spans (50% greater for a 50 g species), reproduce at slower rates, mature at older ages and have longer generation times compared with similar-sized non-hibernators. In accordance with evolutionary theories, however, hibernating species do not have longer life spans than non-hibernators with similar survival rates, nor do they have lower reproductive rates than non-hibernators with similar maximum life spans. Thus, our combined results suggest that (i) hibernation is associated with high rates of overwinter and annual survival, and (ii) an increase in survival in hibernating species is linked with the coevolution of traits indicative of relatively slow life histories.

[Biology of aging: theories, mechanisms, and perspectives]

Abstract The article reviews the major biological theories of aging, and discusses the most relevant mechanisms to explain the aging process. It begins with the evolutionary theories, explores the molecular-cellular mechanisms, and presents the perspective of the systemic theories. The complex etiology of aging is a challenge to the researchers. The knowledge on that phenomenon develops towards an integrative approach.

Maximum shell size, growth rate, and maturation age correlate with longevity in bivalve molluscs

Bivalve molluscs are newly discovered models of successful aging, and this invertebrate group includes Arctica islandica, with the longest metazoan life span. Despite an increasing biogerontological focus on bivalves, their life history traits in relation to maximum age are not as comprehensively understood as those in vertebrate model aging organisms. We explore the allometric scaling of longevity and the relationship between development schedules (time to maturity and growth rate) and longevity in the Bivalvia. Using a traditional nonphylogenetic approach and the phylogenetically independent contrasts method, the relationship among these life history parameters is analyzed. It is demonstrated that in bivalves, maximum shell size, development, and growth rates all associate with longevity. Our findings support the observations of life history patterns in mammals and fish. This is the first investigation into the relationship among longevity, size, and development schedules throughout this group, and the results strengthened by the control for phylogenetic independence.

A proposal in relation to a genetic control of lifespan in mammals

This article proposes that behavioural advancement during mammalian evolution had been in part mediated through extension of total developmental time. Such time extensions would have resulted in increased numbers of neuronal precursor cells, hence larger brains and a disproportionate increase in the neocortex. Larger neocortical areas enabled new connections to be formed during development and hence expansion of existing behavioural circuits. To have been positively selected such behavioural advances would have required enough postdevelopmental time to enable the behaviour to be fully manifest. It is therefore proposed that the success of mammalian evolution depended on initiating a genetic control of total postdevelopmental time. This could have been mediated through the redeployment of gene regulatory networks controlling total developmental time to additionally control total postdevelopmental time. The result would be that any extension of developmental time, leading to a behavioural advancement, would be accompanied by a proportional extension to postdevelopmental time. In effect it is proposed that mammalian lifespan as a whole is genetically controlled.

Life-history connections to rates of aging in terrestrial vertebrates

The actuarial senescence (i.e., the rate of increase in adult mortality with age) was related to body mass, development period, and age at sexual maturity across 124 taxonomic families of terrestrial vertebrates. Model selection based on Akaike's information criterion values adjusted for small size showed that the rate of aging decreases with increasing body mass, gestation period, age at maturity, and possession of flight. Among families of mammals, actuarial senescence was related to extrinsic mortality rate (standardized regression coefficient = 0.215), gestation period (-0.217), and age at maturity (-0.553). Although rate of aging in birds also was related to the embryo development period, birds grow several times more rapidly than mammals, and therefore, the connection between rate of early development and rate of aging is unclear. The strong vertebrate-wide relationship between rate of aging, or life span, and age at maturity can be explained by density-dependent feedback of adult survival rate on the recruitment of young individuals into the breeding population. Thus, age at maturity seems to reflect extrinsic mortality, which, in turn, influences selection on mechanisms that postpone physiological and actuarial senescence. Because rate of embryo development influences rate of aging independently of the age at maturity, in a statistical sense, the evolutionary diversification of development and aging seem to be connected in both birds and mammals; however, the linking mechanisms are not known.

Insights from comparative analyses of aging in birds and mammals

Many laboratory models used in aging research are inappropriate for understanding senescence in mammals, including humans, because of fundamental differences in life history, maintenance in artificial environments, and selection for early aging and high reproductive rate. Comparative studies of senescence in birds and mammals reveal a broad range in rates of aging among a variety of taxa with similar physiology and patterns of development. These comparisons suggest that senescence is a shared property of all vertebrates with determinate growth, that the rate of senescence has been modified by evolution in response to the potential life span allowed by extrinsic mortality factors, and that most variation among species in the rate of senescence is independent of commonly ascribed causes of aging, such as oxidative damage. Individuals of potentially long-lived species, particularly birds, appear to maintain high condition to near the end of life. Because most individuals in natural populations of such species die of aging-related causes, these populations likely harbor little genetic variation for mechanisms that could extend life further, or these mechanisms are very costly. This, and the apparent evolutionary conservatism in the rate of increase in mortality with age, suggests that variation in the rate of senescence reflects fundamental changes in organism structure, likely associated with the rate of development, rather than physiological or biochemical processes influenced by a few genes. Understanding these evolved differences between long-lived and short-lived organisms would seem to be an essential foundation for designing therapeutic interventions with respect to human aging and longevity.

Energetics and longevity in birds

The links between energy expenditure and ageing are different at different levels of enquiry. When studies have examined the relationships between different species within a given class the association is generally negative--animals with greater metabolism per gram of tissue live shorter lives. Within species, or between classes (e.g. between birds and mammals) the association is the opposite--animals with higher metabolic rates live longer. We have previously shown in mammals that the negative association between lifespan and metabolic rate is in fact an artefact of using resting rather than daily energy expenditure, and of failing to adequately take into account the confounding effects of body size and the lack of phylogenetic independence of species data. When these factors are accounted for, across species of mammals, the ones with higher metabolism also have the largest lifetime expenditures of energy-consistent with the inter-class and intra-specific data. A previous analysis in birds did not yield the same pattern, but this may have been due to a lack of sufficient power in the analysis. Here we present an analysis of a much enlarged data set (>300 species) for metabolic and longevity traits in birds. These data show very similar patterns to those in mammals. Larger individuals have longer lives and lower per-gram resting and daily energy expenditures, hence there is a strong negative relationship between longevity and mass-specific metabolism. This relationship disappears when the confounding effects of body mass and phylogeny are accounted for. Across species of birds, lifetime expenditure of energy per gram of tissue based on both daily and resting energy expenditure is positively related to metabolic intensity, mirroring these statistical relationships in mammals and synergizing with the positive associations of metabolism with lifespan within species and between vertebrate classes.

An analysis of the relationship between metabolism, developmental schedules, and longevity using phylogenetic independent contrasts

Comparative studies of aging are often difficult to interpret because of the different factors that tend to correlate with longevity. We used the AnAge database to study these factors, particularly metabolism and developmental schedules, previously associated with longevity in vertebrate species. Our results show that, after correcting for body mass and phylogeny, basal metabolic rate does not correlate with longevity in eutherians or birds, although it negatively correlates with marsupial longevity and time to maturity. We confirm the idea that age at maturity is typically proportional to adult life span, and show that mammals that live longer for their body size, such as bats and primates, also tend to have a longer developmental time for their body size. Lastly, postnatal growth rates were negatively correlated with adult life span in mammals but not in birds. Our work provides a detailed view of factors related to species longevity with implications for how comparative studies of aging are interpreted.

Large Brains and Lengthened Life History Periods in Odontocetes

Previous work on primates and birds suggests that large brains require longer periods of juvenile growth, leading to reproductive constraints due to delayed maturation. However, longevity is often extended in large-brained species, possibly compensating for delayed maturation. We examined the relationship between brain size and life history periods in cetaceans, a large-brained mammalian order that has been largely ignored. We looked at males and females of twenty-five species of Odontocetes, using independent contrasts and multiple regressions to disentangle possible phylogenetic effects and inter-correlations among life history traits. We corrected all variables for body size allometry and separated life span into adult and juvenile periods. For females and both sexes combined, gestation, time to sexual maturity, time as an adult and life span were all positively associated with residual brain size in simple regressions; in multiple regressions, maximum life span and time as an adult were the best predictors of brain size. Males showed few significant trends. Our results suggest that brain size has co-evolved with extended life history periods in Odontocetes, as it has in primates and birds, and that a lengthened adult period could have been an important component of encephalization in cetaceans.

Correlations between physiology and lifespan--two widely ignored problems with comparative studies

Comparative differences between species provide a powerful source of information that may inform our understanding of the aging process. However, two problems regularly attend such analyses. The co-variation of traits with body mass is frequently ignored, along with the lack of independence of the data due to a shared phylogenetic history. These problems undermine the use of simple correlations between various factors and maximum lifespan potential (MLSP) across different species as evidence that the factors in question have causal effects on aging. Both of these problems have been widely addressed by comparative biologists working in fields other than aging research, and statistical solutions to these issues are available. Using these statistical approaches, of making analyses of residual traits with the effects of body mass removed, and deriving phylogenetically independent contrasts, will allow analyses of the relationships between physiology and maximum lifespan potential to proceed unhindered by these difficulties, potentially leading to many useful insights into the aging process.

Justified and unjustified use of growth hormone

Growth hormone (GH) replacement therapy for children and adults with proven GH deficiency due to a pituitary disorder has become an accepted therapy with proven efficacy. GH is increasingly suggested, however, as a potential treatment for frailty, osteoporosis, morbid obesity, cardiac failure, and various catabolic conditions. However, the available placebo controlled studies have not reported many significant beneficial effects, and it might even be dangerous to use excessive GH dosages in conditions in which the body has just decided to decrease GH actions. GH can indeed induce changes in body composition that are considered to be advantageous to GH deficient and non-GH deficient subjects. In contrast to GH replacement therapy in GH deficient subjects, however, excessive GH action due to GH misuse seems to be ineffective in improving muscle power. Moreover, there are no available study data to indicate that the use of GH for non-GH deficient subjects should be advocated, especially as animal data suggest that lower GH levels are positively correlated with longevity.

Telomeres and Telomerase: A Modern Fountain of Youth?

Since ageing is a universal human feature, it is not surprising that, from the Babylonian epic of Gilgamesh to Ponce de Leon seeking the "Fountain of Youth," countless people have dreamed of finding a way to avoid ageing, to no avail. Yet the search continues. In this review, we present one of the latest candidates: the enzyme telomerase, capable of elongating the tips of chromosomes, the telomeres. Research into the causes of cellular ageing established the telomeres as the molecular clock that counts the number of times cells divide and triggers cellular senescence. Herein, we review arguments both in favor and against the use of telomerase as an anti-ageing therapy. The importance of the telomeres in cellular ageing, the low or non-existent levels of telomerase activity in human tissues, and the ability of telomerase to immortalize human cells suggest that telomerase can be used as an anti-ageing therapy. On the other hand, recent experiments in mice have raised doubts whether telomerase affects organismal ageing. Results from human cells expressing telomerase have also suggested telomerase may promote tumorigenesis. We conclude that, though telomerase may be used in regenerative medicine and to treat specific diseases, it is unlikely to become a source of anti-ageing therapies.

Hormone use and abuse: what is the difference between hormones as fountain of youth and doping in sports?

GH can induce changes in body composition that are considered to be advantageous to aging subjects especially. However, there are no results indicating that the use of GH during aging should be advocated, because of the lack of any proven efficacy for whatever parameter. Also, data indicate that calorie restriction can extend life spans by altering the rate of decline in reserve capacity as well as by reducing the cumulative exposure to GH. Moreover, animal data suggest that lower GH actions are positively correlated with longevity. The abuse of GH by sportsmen is based on the belief that it has potent anabolic effects, while it is difficult to detect the abuse. Again, this supposed efficacy cannot be supported by any scientific data.

Is height related to longevity?

Over the last 100 years, studies have provided mixed results on the mortality and health of tall and short people. However, during the last 30 years, several researchers have found a negative correlation between greater height and longevity based on relatively homogeneous deceased population samples. Findings based on millions of deaths suggest that shorter, smaller bodies have lower death rates and fewer diet-related chronic diseases, especially past middle age. Shorter people also appear to have longer average lifespans. The authors suggest that the differences in longevity between the sexes is due to their height differences because men average about 8.0% taller than women and have a 7.9% lower life expectancy at birth. Animal experiments also show that smaller animals within the same species generally live longer. The relation between height and health has become more important in recent years because rapid developments in genetic engineering will offer parents the opportunity to increase the heights of their children in the near future. The authors contend that we should not be swept along into a new world of increasingly taller generations without careful consideration of the impact of a worldwide population of taller and heavier people.

Effect of MPTP on Dopamine metabolism in Ames dwarf mice

Hypopituitary dwarf mice exhibit a heightened antioxidative capacity and live extensively longer than age-matched controls. Importantly, dwarf mice resist peripheral oxidative stress induced by paraquat, and behaviorally, they maintain cognitive function and locomotor activity at levels above those observed in old wild-type animals. We assessed monoaminergic neurotransmitters in nigrostriatal tract and cerebellum after the administration of the dopaminergic neurotoxin, MPTP. There was no significant change in mitochondrial monoamine oxidase (MAO)-B and total MAO activity in the substantia nigra and nucleus caudatus putamen of wild-type and dwarf mice. Coenzymes Q-9 and Q-10 were present in similar quantities, as were dopamine, norepinephrine, and serotonin levels in the cerebellum and nigrostriatal tract. MPTP set off tremor, hind limb abduction, and straub tail behavior and induced significant dopamine depletion in the striatum of both dwarf and normal mice. This study shows that the MAO activity and the coenzyme content of dwarf mice are similar to those of their wild-type controls and hence susceptible to MPTP-induced toxicity.

The evolution of mammalian aging

The incidence of aging is different between mammals and their closer ancestors (e.g. reptiles and amphibians). While all studied mammals express a well-defined aging phenotype, many amphibians and reptiles fail to show signs of aging. In addition, mammalian species show great similarities in their aging phenotype, suggesting that a common origin might be at work. The proposed hypothesis is that mammalian aging evolved together with the ancestry of modern mammals. In turn, this suggests that the fundamental cause of human aging is common to most, if not all, mammals and might be a unique phenomenon. Experimental procedures capable of testing these theories and how to map the causes of mammalian and thus, human aging, are predicted.

Growth negatively impacts the life span of mammals

A negative intraspecific relationship between growth and longevity was proposed in the early 20th century. Indeed, stunting the growth of rodents by restricting their food dramatically extended life span. Subsequently, however, the hypothesis that growth exacerbates aging rates fell into disfavor. Contributing to this was (a) the establishment of a positive relationship between body size and longevity interspecifically, (b) purported antiaging impacts of growth hormone, and (c) the fact that the longevity of even mature rodents that had completed growth was extended by dietary restriction. Furthermore, intraspecific analytical studies failed to provide any clear resolution. This article presents the first global analyses of maximal longevity versus maximum mature mass for laboratory rats and mice, based on a relatively comprehensive compilation of research across the 20th century. Peak body mass (which reflects juvenile growth rates) was negatively associated with longevity within both species. Proximal mechanisms for impacts of growth on longevity appear congruent with the free radical and immunological theories of aging.

Mitochondrial oxidant generation and oxidative damage in Ames dwarf and GH transgenic mice

Aging is associated with an accumulation of oxidative damage to proteins, lipids and DNA. Cellular mechanisms designed to prevent oxidative damage decline with aging and in diseases associated with aging. A long-lived mouse, the Ames dwarf, exhibits growth hormone deficiency and heightened antioxidative defenses. In contrast, animals that over express GH have suppressed antioxidative capacity and live half as long as wild type mice. In this study, we examined the generation of H2O2 from liver mitochondria of Ames dwarf and wild type mice and determined the level of oxidative damage to proteins, lipids and DNA in various tissues of these animals. Dwarf liver mitochondria (24 months) produced less H2O2 than normal liver in the presence of succinate (p<0.03) and ADP (p<0.003). Levels of oxidative DNA damage (8ÕHdG) were variable and dependent on tissue and age in dwarf and normal mice. Forty-seven percent fewer protein carbonyls were detected in 24-month old dwarf liver tissue compared to controls (p<0.04). Forty percent more (p<0.04) protein carbonyls were detected in liver tissue (3-month old) of GH transgenic mice compared to wild types while 12 month old brain tissue had 53% more protein carbonyls compared to controls (p<0.005). Levels of liver malonaldehyde (lipid peroxidation) were not different at 3 and 12 months of age but were greater in Ames dwarf mice at 24 months compared to normal mice. Previous studies indicate a strong negative correlation between plasma GH levels and antioxidative defense. Taken together, these studies show that altered GH-signaling may contribute to differences in the generation of reactive oxygen species, the ability to counter oxidative stress and life span.

Catalase expression in delayed and premature aging mouse models

The physiological decline that occurs with aging is thought to result, in part, from accumulation of oxidative damage produced by reactive oxygen species (ROS) generated during normal metabolism. Two genetic mouse models of aging, the Ames dwarf and growth hormone (GH) transgenic, suggest that hormone levels may play a role in antioxidative defense and aging. To explore this possibility, catalase (CAT), an enzyme involved in elimination of ROS, was evaluated in long-lived dwarf and short-lived transgenic mice. Catalase activity and/or protein was significantly elevated in livers from dwarf mice at 3, 6, 13-15, and 24 months of age when compared to age-matched wild type mice. In contrast, a 50 and 38% reduction (P<0.05) in CAT protein was observed in 3 and 10 to 12 month old GH transgenics respectively, when compared to wild type mice. Kidneys from old dwarf mice exhibited significantly increased CAT activity (22%), protein (16%) and mRNA expression (59%) compared to wild type mice. Conversely, kidneys from GH transgenic mice showed reductions in CAT activity. The results of this study suggest that hormonal status modulates antioxidative mechanisms and that CAT is important in overall defense capacity with respect to lifespan in both decelerated (dwarf) and accelerated (transgenic) mammalian models of aging.

When will the biology of aging become useful? Future landmarks in biomedical gerontology

Where should we look to find the causes of and cure for aging? This essay considers a number of scientific discoveries, not yet made, that might dramatically increase the proportion of biogerontologists doing useful work. I will consider, in turn, problems and prospects in the areas of comparative biology, mammalian and invertebrate genetics, biomarker research, caloric restriction, and clonal senescence and then conclude with a discussion of potential links between aging and late life disease.