"Circle of life" of the species Testudo graeca in Israel: Effect of temperatures on sex determination, and yearly activity patterns, habitats preference and home range utilization

Mai Bernheim

Reptiles are one of the more susceptible groups to suffer from biodiversity loss as a result of human-induced changes in the environment, such as global climate change. There is solid evidence for the influence of global warming on species biology such as range-boundary changes and phenological shifts. Among reptiles there is a particular interest in the effects of climate change on species with temperature-dependent sex determination (TSD), where sex ratio is influenced by the temperature experienced by the eggs during the middle third of the incubation period. Therefore, finding reptiles’ habitat preferences and how temperature affect hatchlings’ sex may provide vital information for managing their populations under climate change scenarios.

Land tortoises are likely to be the most vulnerable to temperature changes, given their low dispersal capabilities, their occurrence in fragmented habitats and that most species are already threatened. The spur-thighed tortoise, Testudo graeca, is a terrestrial TSD tortoise with a circum-Mediterranean distribution. Courtship and mating season begin in March-April, but nesting occurs later between April and June, followed by 3-month incubation period. There is no data from natural environments in Israel on female’s nest habitats. Moreover, little data exists on adults’ seasonal activity patterns, home range and their micro-habitat preferences.

The objectives of this study were:

to identify tortoises’ yearly activity pattern, home ranges, preferred micro-habitats and biotic conditions affecting the distribution of tortoises in Ramat Hanadiv;
learn the effect of incubation temperature on sex determination of T. graeca in Israel, and the effect of nest location and temperature regimes on hatching success;
and finally, to identify behavioral patterns of males and females in relation to righting behavior as a tool for sex identification.

The research was conducted in two locations: Oranim academic college in Kiryat Tivon, where T. graeca eggs incubation experiments and hatchlings’ behavioral experiment were performed; Ramat Hanadiv Park, where I surveyed and marked 181 adults’ individuals and radio tagged 18 tortoises.
MaxEnt models were performed for predicting distribution of individuals of T. graeca in the study area.

The results show that between January and April both sexes exhibit maximum activity including courtship and copulations. Males precede females in decline of activity starting in May and lasting until October, while females decrease activity a month later. This study describes for the first time a long-term summer-autumn activity hiatus of T. graeca in Israel. Models for predicting their distribution revealed that they prefer heterogeneous habitats that enable access to open and closed patches. Both sexes share same size of home ranges. There is an overlap among home ranges, and the tortoises do not show territoriality. Tortoises had a non-random occurrence on dark rendzina soil, where they were mostly found in shrub margins showing thermoregulatory activity, sleeping, resting, and mating.

This micro-habitat probably provides them with relatively stable micro-climatic conditions. Sex determination of T. graeca in Israel follows the TSD mechanism, where incubation under constant temperatures above the pivotal point (a 1:1 sex ratio) during the thermo-sensitive period leads predominantly to females’ development and below this temperature predominantly to males.

In natural nests temperatures fluctuate and can rise significantly above the pivotal temperature without affecting the embryos’ survival. It appears that females carefully select the specific nest location to ensure adequate incubation conditions. The effect of temperature fluctuations on sex determination in T. graeca is not known yet, mainly due to a lack of a reliable sex determination method to study hatchlings in the field. However, this study revealed the potential of using a behavioral method to distinguish between the sexes. While this study did not directly deal with the hazards of global warming on T. graeca in Israel, it shows the sensitivity of the sex determining mechanism to incubation temperatures, which should raise concern if global temperatures continue to rise.

Additional links

MSc. Thesis, Mai Bernheim, Haifa University

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