Plasma steroids and steroid-binding capacity in male semelparous dasyurid marsupials (Phascogale tapoatafa) that survive beyond the breeding season in captivity

Programmed ageing: decline of stem cell renewal, immunosenescence, and Alzheimer's disease

The characteristic maximum lifespan varies enormously across animal species from a few hours to hundreds of years. This argues that maximum lifespan, and the ageing process that itself dictates lifespan, are to a large extent genetically determined. Although controversial, this is supported by firm evidence that semelparous species display evolutionarily programmed ageing in response to reproductive and environmental cues. Parabiosis experiments reveal that ageing is orchestrated systemically through the circulation, accompanied by programmed changes in hormone levels across a lifetime. This implies that, like the circadian and circannual clocks, there is a master 'clock of age' (circavital clock) located in the limbic brain of mammals that modulates systemic changes in growth factor and hormone secretion over the lifespan, as well as systemic alterations in gene expression as revealed by genomic methylation analysis. Studies on accelerated ageing in mice, as well as human longevity genes, converge on evolutionarily conserved fibroblast growth factors (FGFs) and their receptors, including KLOTHO, as well as insulin-like growth factors (IGFs) and steroid hormones, as key players mediating the systemic effects of ageing. Age-related changes in these and multiple other factors are inferred to cause a progressive decline in tissue maintenance through failure of stem cell replenishment. This most severely affects the immune system, which requires constant renewal from bone marrow stem cells. Age-related immune decline increases risk of infection whereas lifespan can be extended in germfree animals. This and other evidence suggests that infection is the major cause of death in higher organisms. Immune decline is also associated with age-related diseases. Taking the example of Alzheimer's disease (AD), we assess the evidence that AD is caused by immunosenescence and infection. The signature protein of AD brain, Aβ, is now known to be an antimicrobial peptide, and Aβ deposits in AD brain may be a response to infection rather than a cause of disease. Because some cognitively normal elderly individuals show extensive neuropathology, we argue that the location of the pathology is crucial - specifically, lesions to limbic brain are likely to accentuate immunosenescence, and could thus underlie a vicious cycle of accelerated immune decline and microbial proliferation that culminates in AD. This general model may extend to other age-related diseases, and we propose a general paradigm of organismal senescence in which declining stem cell proliferation leads to programmed immunosenescence and mortality.

Resting disparity in quoll semelparity: examining the sex-linked behaviours of wild roaming northern quolls (Dasyurus hallucatus) during breeding season

Semelparity is a breeding strategy whereby an individual invests large amounts of resources into a single breeding season, leading to the death of the individual. Male northern quolls (Dasyurus hallucatus) are the largest known mammal to experience a post-breeding die-off; however, the cause of their death is unknown, dissimilar from causes in other semelparous dasyurids. To identify potential differences between male northern quolls that breed once, and females that can breed for up to four seasons, the behaviours, activity budgets, speeds and distances travelled were examined. Northern quolls were captured on Groote Eylandt off the coast of the Northern Territory, Australia, and were fitted with accelerometers. A machine learning algorithm (Self-organizing Map) was trained on more than 76 h of recorded footage of quoll behaviours and used to predict behaviours in 42 days of data from wild roaming quolls (7M : 6F). Male northern quolls were more active (male 1.27 g, s.d. = 0.41; female 1.18 g, s.d. = 0.36), spent more time walking (13.09% male: 8.93% female) and engaged in less lying/resting behaviour than female northern quolls (7.67% male: 23.65% female). Reduced resting behaviour among males could explain the post-breeding death as the deterioration in appearance reflects that reported for sleep-deprived rodents.

Highly variable lifespan in an annual reptile, Labord's chameleon (Furcifer labordi)

Among tetrapods, the current record holder for shortest lifespan is Labord’s chameleon, Furcifer labordi. These reptiles from the arid southwest of Madagascar have a reported lifespan of 4–5 months during the annual rainy season and spend the majority of their life (8–9 months) as a developing embryo. This semelparous, annual life history is unique among tetrapods, but only one population (Ranobe) in the southernmost distribution range has been studied. We therefore investigated the potential for environmentally-dependent variability in lifespan in a population in Kirindy Forest, which has a much longer warm rainy season. While no adults were found after March in Ranobe, the disappearance of adults was delayed by several months in Kirindy. Our data also revealed sex-biased mortality, suggesting that females have a longevity advantage. Furthermore, we found that, after an unusually long previous rainy season, one female was capable of surviving until a second breeding season. Keeping F. labordi in cages under ambient conditions demonstrated that also males can also survive until the next season of activity under these conditions. Our study therefore revealed considerable variability in the extreme life history of this tetrapod that is linked to variation in ecological factors.

Does the degree of endocrine dyscrasia post-reproduction dictate post-reproductive lifespan? Lessons from semelparous and iteroparous species

Post-reproductive lifespan varies greatly among species; human post-reproductive lifespan comprises ~30-50% of their total longevity, while semelparous salmon and dasyurid marsupials post-reproductive lifespan comprises <4% of their total longevity. To examine if the magnitude of hypothalamic-pituitary-gonadal (HPG) axis dyscrasia at the time of reproductive senescence determines post-reproductive lifespan, we examined the difference between pre- and post-reproductive (1) circulating sex hormones and (2) the ratio of sex steroids to gonadotropins (e.g., 17β-estradiol/follicle-stimulating hormone (FSH)), an index of the dysregulation of the HPG axis and the level of dyotic (death) signaling post-reproduction. Animals with a shorter post-reproductive lifespan (<4% total longevity) had a more marked decline in circulating sex steroids and corresponding elevation in gonadotropins compared to animals with a longer post-reproductive lifespan (30-60% total longevity). In semelparous female salmon of short post-reproductive lifespan (1%), these divergent changes in circulating hormone concentration post-reproduction equated to a 711-fold decrease in the ratio of 17β-estradiol/FSH between the reproductive and post-reproductive periods. In contrast, the decrease in the ratio of 17β-estradiol/FSH in iteroparous female mammals with long post-reproductive lifespan was significantly less (1.7-34-fold) post-reproduction. Likewise, in male semelparous salmon, the decrease in the ratio of testosterone/FSH (82-fold) was considerably larger than for iteroparous species (1.3-11-fold). These results suggest that (1) organisms with greater reproductive endocrine dyscrasia more rapidly undergo senescence and die, and (2) the contribution post-reproduction by non-gonadal (and perhaps gonadal) tissues to circulating sex hormones dictates post-reproductive tissue health and longevity. In this way, reproduction and longevity are coupled, with the degree of non-gonadal tissue hormone production dictating the rate of somatic tissue demise post-reproduction and the differences in post-reproductive lifespans between species.

Seasonal programming, not competition or testosterone, drives stress-axis changes in a partially-semelparous mammal

Animals must make tradeoffs between reproduction and longevity. This is particularly pronounced in male arctic ground squirrels (Urocitellus parryii), that compete aggressively for territories and mates during a three-week breeding season. Breeding males have high rates of severe wounding, high mortality rates, and high free cortisol levels, along with downstream consequences of chronic stress (weight loss, reduced immune function) that appear to contribute to their early death. The elevated cortisol levels are thought to be a result of the intense intrasexual competition. An alternative hypothesis, however, is that the hormonal change is a seasonal adaptation facilitating the tradeoff of immediate competitive advantage at the expense of long-term survival. We tested a two-part hypothesis: first, that elevated free cortisol during the breeding period is a seasonal change that will still occur in the absence of actual competition, and second, that testosterone maintains this increase. We measured plasma cortisol, corticosteroid-binding globulin, and fecal glucocorticoid metabolites in three groups: wild male ground squirrels, captive males prevented from fighting, and captive castrated males. There were no differences amongst these three groups in free and total plasma cortisol, fecal glucocorticoids, or downstream measures of chronic stress. This suggests that high free cortisol and its effects on breeding males are not a consequence of contest competition during the breeding season, but rather a generalized seasonal change. We found no evidence that testosterone plays a role in maintaining elevated free cortisol in arctic ground squirrel males.

A review of factors influencing the stress response in Australian marsupials

Many Australian marsupials are threatened species. In order to manage in situ and ex situ populations effectively, it is important to understand how marsupials respond to threats. Stress physiology (the study of the response of animals to challenging stimuli), a key approach in conservation physiology, can be used to characterize the physiological response of wildlife to threats. We reviewed the literature on the measurement of glucocorticoids (GCs), endocrine indicators of stress, in order to understand the stress response to conservation-relevant stressors in Australian marsupials and identified 29 studies. These studies employed a range of methods to measure GCs, with faecal glucocorticoid metabolite enzyme immunoassay being the most common method. The main stressors considered in studies of marsupials were capture and handling. To date, the benefits of stress physiology have yet to be harnessed fully in marsupial conservation. Despite a theoretical base dating back to the 1960s, GCs have only been used to understand how 21 of the 142 extant species of Australian marsupial respond to stressors. These studies include merely six of the 60 marsupial species of conservation concern (IUCN Near Threatened to Critically Endangered). Furthermore, the fitness consequences of stress for Australian marsupials are rarely examined. Individual and species differences in the physiological stress response also require further investigation, because significant species-specific variations in GC levels in response to stressors can shed light on why some individuals or species are more vulnerable to stress factors while others appear more resilient. This review summarizes trends, knowledge gaps and future research directions for stress physiology research in Australian marsupial conservation.

Fecal cortisol levels predict breeding but not survival of females in the short-lived rodent, Octodon degus

The cort-adaptation hypothesis indicates that an association between glucocorticoid (cort) levels and fitness may vary with the extent to which reproduction or breeding effort is a major determinant of cort levels. Support for a context dependent association between cort and fitness comes mostly from relatively long-lived, bird species. We tested the hypothesis that there are gender and context (life-history) specific cort-fitness relationships in degus, a short-lived and generally semelparous social rodent. In particular, we used demographical records on a natural population to estimate adult survival through seasons and years and linked that to records of baseline cort (based on fecal cortisol metabolites). We found no evidence for a direct relationship between baseline cort and adult survival across seasons, and this lack of association was recorded irrespective of sex and life history stage. Yet, cort levels during early lactation predicted the probability that females produce a second litter during the same breeding season, supporting a connection between baseline cort levels and breeding effort. Overall, the differential effects of cort on survival and breeding supported that the extent of cort-fitness relationships depends on the fitness component examined.

Senemorphism: a novel perspective on aging patterns and its implication for diet-related biology

Aging can be described as the accumulation of changes in organisms over time. Aging in organisms undergoing caloric restriction (CR) is widely considered as a slowed version of aging under ad libitum (AL) conditions. However, here we argue that aging under optimized CR is fundamentally different from aging under AL based on the following facts: (1) Comparing the two dietary groups, several age-related changes run in the opposite direction over time; (2) Switching from an AL to a CR diet clearly reverts (not only delays) several "normal" accumulated changes; (3) major causes of death are as different between both groups as they are between species. These observations support the idea that CR and AL initially modulate different metabolic and physiological programs, which exclusively over time generate two biologically different organisms. Such distinct diet-related senescence is analogous to the divergent aging processes and causes of death observed between castes of social insects, such as queens versus workers ("caste-related-senescence") and also between breeding versus non-breeding semelparous animals ("reproduction-related-senescence"). All these aging phenotypes are different not because they accumulate changes at a different rate, but because they accumulate different changes over time. Thus, the environment does not simply affect the individual aging rate through stochastic effects (e.g. U.V.) but also modulates the activation of a particular program/strategy that influences lifespan (e.g. caste, calorie intake). We refer to the environment-dependent aging patterns encoded by the genome as "senemorphism". Based on this idea we propose experimental schemes for aging, evolution and biomedical research.