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Population dynamics and age-dependent mortality processes in tropical reptiles

Understanding age-dependent mortality processes is a critical challenge for population biologists. Actuarial senescence appears to be a common process across the tree of life. Senescence patterns are highly variable in pluricellular organisms: senescence can be gradual or sharp and its onset may be early or delayed. By contrast, studies revealed that organisms may also not experience senescence at all while others display a “negative senescence”; i.e. a decrease of mortality rate with age. To date, studies on senescence have largely focused on human and other endotherm vertebrates, limiting our understanding of senescence in amniotes as a whole. By contrast, few have examined the diversity of senescence patterns in ectotherm vertebrates as reptiles. Here, we examined population dynamics and age-dependent mortality patterns in three tropical tortoises (Kinixys erosa, Kinixys homeana, Kinixys nogueyi) and snakes (Bitis gabonica, Bitis nasicornis, Causus maculatus). Our study revealed that tortoises of Kinixys genus had a higher survival and a lower recruitment than snakes of the genera Bitis and Causus, indicating that they have a slower life history. Furthermore, we showed that survival more slowly decreased with age in tortoises than in snakes. In addition, we highlighted contrasted patterns of age-dependent mortality in the three genera. In Kinixys, the relationship between mortality rate and age was positive and linear, suggesting gradual senescence over tortoise lifetime. By contrast, the relationship between mortality rate and age was negative and sharp in Bitis and Causus, possibly due to a “negative senescence” starting early in life. Our study highlighted various age-dependent mortality patterns in tropical reptiles. It also contributed to extend our knowledge of senescence in ectotherm vertebrates whose the demography is still poorly understood. In addition, while negative senescence has never been reported in endotherm vertebrates, our results showed that it can be common phenomenon in ectotherms.