Variation in actuarial senescence does not reflect life span variation across mammals

Area of Science:

• Gerontology
• Evolutionary Biology
• Mammalogy

Background:

• Actuarial senescence, the increase in mortality with age, is often conflated with overall lifespan.
• This conflation obscures the roles of age-dependent and age-independent factors in shaping lifespan variation.
• Understanding these distinct processes is crucial for accurately studying aging and longevity.

Purpose of the Study:

• To disentangle the contributions of actuarial senescence and other factors to lifespan variation across mammal species.
• To analyze age-specific mortality patterns using a large dataset of individual mammal lifespans.
• To quantify the relative importance of different life stages and mortality parameters in driving lifespan differences.

Main Methods:

• Utilized the Species360 database containing individual lifespan records for 96 mammal species.
• Analyzed age-specific mortality patterns and calculated the sensitivity and elasticity of lifespan to key mortality parameters.
• Quantified between-species variance in juvenile duration, juvenile mortality, prime-age adult mortality, age at senescence onset, and senescence rate.

Main Results:

• Actuarial senescence accounted for only 35%-50% of lifespan variation across species, varying by body mass.
• The relative contribution of senescence intensity to lifespan variation increased with species size, while prime-age adult mortality's contribution decreased.
• Lifespan variation is driven by a complex interplay of factors, not solely by the rate or onset of aging.

Conclusions:

• Lifespan duration is an inadequate metric for quantifying the strength of actuarial senescence across species.
• A significant and variable portion of lifespan variation in mammals is independent of actuarial senescence.
• Future research should consider multiple age-specific mortality parameters to fully understand lifespan evolution and variation.