Life history and age-dependent mortality processes in tropical reptiles

Actuarial senescence appears to be a common process, and senescence patterns are highly variable across the tree of life. To date, studies on animal senescence have largely focused on model species, such as as fruit flies, humans and a few other endotherms. In contrast, our knowledge about ageing remains fragmentary in ectotherm vertebrates, such as reptiles. Here, we examined life history and age-dependent mortality patterns in three tropical tortoises (Kinixys erosa, Kinixys homeana and Kinixys nogueyi) and snakes (Bitis gabonica, Bitis nasicornis and Causus maculatus). Our study revealed that tortoises of the genus Kinixys had a higher survival and a lower recruitment than snakes of the genera Bitis and Causus, indicating a slower life history. Furthermore, we confirmed that survival decreased more slowly with age in tortoises than in snakes. In addition, we highlighted contrasting patterns of age-dependent mortality among the three genera. In Kinixys, the relationship between mortality rate and age was positive and linear, suggesting gradual senescence over tortoise lifetime. In contrast, the relationship between mortality rate and age was negative and sharp in Bitis and Causus, possibly owing to a ‘negative senescence’. Our study is one of the few to have examined and compared the demography and age-dependent mortality patterns of tropical reptiles. Among other things, our results suggest that although negative senescence has never been reported in endotherm vertebrates, it could be a common phenomenon in ectotherms.

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 ofKinixysgenus had a higher survival and a lower recruitment than snakes of the generaBitisandCausus, 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. InKinixys, 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 inBitisandCausus, 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.

Bioinspired Design

Bioinspired design involves the use of concepts observed in natural biological materials in engineering design. The hope is that the leveraging of biological materials in the engineering domain can lead to many technological innovations and novel products. This work presents the initial material characterization of kinixys erosa tortoise shell using a combination of x-ray diffraction, optical/scanning electron microscopy and micro-mechanical testing. The results were used in the analytical/computational modelling of shell structures. The potential implications or the results were then discussed to give fundamental understanding of deformation and stress responses of shell structures

Food niche overlap between sympatric potential competitors increases with habitat alteration at different trophic levels in rain-forest reptiles (omnivorous tortoises and carnivorous vipers)

Comparisons of sympatric reptile species were used to assess the variation in niche overlap for food between potential competitors at different trophic levels. Omnivorous tortoises and carnivorous vipers inhabiting the rain-forest region of West Africa were used as study models. Food niche overlap between species increased with habitat alteration in both the independent study systems: tortoises (Kinixys homeana and Kinixys erosa) and vipers (Bitis gabonica and Bitis nasicornis) showed lower values of Pianka's niche overlap index in the pristine habitat than in the altered habitat, and these differences in overlap values did not depend on chance after Monte Carlo simulations. There were higher inter-habitat food niche overlaps within-species than between-species. Permutation tests (assessed after 5000 iterations) revealed that, for both study systems, the P-values became significantly smaller with fewer resource states, thus showing the niche overlap between species really increases after habitat alteration. The observed increases in food niche overlap between species accomplished with rain-forest habitat degradation in turn may be predicted to have cumulative effects on reducing the level of forest biodiversity.