Long live the Tortoise
In my master's thesis, I studied the evolution of longevity of different vertebrate groups (amphibians, reptiles, birds, and mammals). Together with my colleagues, I published four papers (see below) examining how life history and ecological factors relate to mortality rates and how they affect longevity variation in more than 4,000 vertebrate species.
Long-lived reptilians: What causes reptiles to have longer lifespans?
Most recent studies of differences in longevity have focused on mammals and birds. My colleagues and I investigated how extrinsic mortality factors such as predation and environmental temperatures relate to differential longevity in lizards, snakes, turtles, and crocodiles. We found that larger reptiles that live on islands and in colder and more seasonal environments live longer.
Figure: The relationship between maximum longevity (log-transformed) and maximum body mass (log-transformed) of the major reptilian groups for the complete dataset (N = 1320): Lacertilia (green circles), Serpentes (black triangles), Testudines (blue diamonds) and Crocodilia (purple squares). Sphenodon punctatus is marked by an orange cross. *Only significant regression lines appear in the figure.
Does nocturnal activity prolong gecko lifespan?
Most gecko species are nocturnal, unlike other lizard families. Due to lower exposure to harmful UV radiation during the night, nocturnal species are expected to have lower intrinsic mortality rates because they accumulate less harmful mutations and metabolic waste compared to diurnal species. My colleagues (one of whom is my wife) and I compared diurnal and nocturnal geckos, as well as geckos with other lizards, to test whether nocturnal activity promotes a longer lifespan. We found that gecko activity time was not related to longevity. It is possible that mortality risk from extrinsic causes (e.g., predation) exerts much stronger selection pressure than intrinsic causes.
Figure: The relationship between maximum longevity (in years, log10 transformed), and body mass (in grams, log10 transformed), across
activity levels (diurnal: cyan triangles; nocturnal: black squares; cathemeral: red circles). Regression lines (purple) are for all species. (A) All
lizards, (B) Gekkotan species only
Living in cold and dark captivity: What drives the variation in amphibian lifespans?
Surprisingly, amphibians are the only tetrapod group where longevity differences have never been explored. Together with my supervisor, I studied the patterns and drivers of more than 500 amphibian species from all orders - anurans, caudatans, and caecilians. We found that large amphibians living in colder environments are likely to have slower growth and metabolism, reducing intrinsic drivers of mortality and extending their lifespans. Species that reduce extrinsic mortality pressures through chemical protection and nocturnal activity also have longer life spans.
Higher metabolism does not necessarily mean a shorter lifespan
It has long been assumed that animals with a slow metabolism live longer than those with a high metabolism. This notion is based on the assumption that animals with a high metabolic rate are more active, more threatened by predators, have a higher rate of potentially harmful somatic mutations, and produce more harmful metabolic byproducts such as free radicals. This trade-off between metabolism and life expectancy is commonly referred to as the theory of life rate. In our recent publication, my colleagues and I showed that the "rate-of-living" theory assumption, which has held for nearly a century, is not supported by the results of our large-scale study conducted on more than 4000 species of tetrapods. We found no relationship between animal metabolic rate and longevity, either when we examined all classes together or when we examined each class individually.
However, we did find that ambient temperature affects the lifespan of reptiles and amphibians, such that these species live longer in colder regions than their conspecifics in warmer environments. If rising ambient temperatures shorten the lifespan of these groups, ectothermic vertebrates may be at greater risk of extinction as temperatures rise at an unprecedented rate due to global warming.
However, we did find that ambient temperature affects the lifespan of reptiles and amphibians, such that these species live longer in colder regions than their conspecifics in warmer environments. If rising ambient temperatures shorten the lifespan of these groups, ectothermic vertebrates may be at greater risk of extinction as temperatures rise at an unprecedented rate due to global warming.
Figure: The relationship between longevity (y axis of all panels, log10 transformed) and (a–d) body mass (in g, log10 transformed) of amphibians (red circles), reptiles (black circles), birds (green triangles) and mammals (inverted blue triangles). (e–h) Mean annual temperature (regression lines only shown for amphibians and reptiles for which the relationship is significant), (i) basal metabolic rate (in ml O2/hr, log10 transformed), according to the colour codes depicted in the top plots and (j) field metabolic rate (kJ/day, log10 transformed)
Check out our paper in the media: Click Here
Brain Size Variation and Animal Lifespan
One of my side projects is to study the evolution and diversity of brain size in vertebrates. In my recent work, I have discovered how the brains of endothermic and ectothermic animals differ evolutionary, particularly in the way brain size is related to the energy required to produce them. Key finding: ectotherms (especially reptiles and sharks), which invest more in increasing the size of their brains, tend to live shorter lives, while the opposite is true for endotherms.
Figure: Top panels: the relationship between brain mass and body mass (both grams in log transformed) among (a) bony fish (Osteichthyes, black points, yellow dashed line) and (b) cartilaginous fish (Chondrichthyes, yellow points, black dashed line). Bottom panels: the relationship between relative brain size (extracted residuals from a brain mass in grams against body mass in grams log–log least-square linear regression) and maximum longevity (years in log transformed) among (c) bony fish and (d) cartilaginous fish