Immune and behavioural response of polymorphic urban lizards to recurring predation stress and immune challenges

Abstract

Introduction: Even though urban environment might hurt some species, others succeeded in adapting to them and successfully reproduce. However, this does not mean that those species don’t suffer from some of the characteristics typical of those anthropized environments, such as higher frequencies of interaction with humans. Those interactions can be perceived as predation attempts by lizards. Therefore, they can trigger the hormonal response that is part of the flight mechanisms. More precisely corticosterone is a hormone known to play an important part in the response to stressful events. Although its effects are beneficial in the short-term, corticosterone also has a negative effect on the immune function at long-term. Another hormone that has an immunosuppressive effect is testosterone. In some species that exhibit a colour polymorphism, such as the side-blotched lizard Uta stansburiana, it has been established that testosterone is a key component in the establishment of alternative strategies of reproduction. On the contrary, in other species, such as the common wall lizard Podarcis muralis, the characteristics that differentiate the alternative morphs are still mostly unknown. With the goal of learning more about the possible alternative strategies of reproduction in this species, as well as about the effects of urbanisation it might endure, we studied four of its populations. Material and methods: We captures white and orange common wall lizards at four sites, two being natural and two being urbanised. We then first evaluated the level of initial boldness of those animals by by simulating predator attacks and by observing the frequency with which each individual was visible outside of its refuge after a period of ten minutes. After that, we made blood smears in order to evaluate the parasite load, as well as the H:L ratio, a reliable indicator of chronic stress. We obtained this latter parameter by calculating the ratio between heterophiles and lymphocytes. Then, we submitted half of the animals to a treatment of simulated predator attacks that were repeated seven times a day during one week. After that treatment was completed, we evaluated a second time the levels of boldness, the parasite load, and the H:L ratio. Finally, we induced an immune reaction by injecting a solution of lipopolysaccharides, and repeated the evaluation of boldness a third time. Results: The animals that came from urban sites did not exhibit signs of chronic stress, but suffered higher parasite loads, a possible effect of higher population densities. Urban animals sowed higher levels of boldness, which could stem from a habituation to higher frequencies of interaction with humans. After the period of treatment, urban lizards showed a lesser increase in their H:L ratio compared to lizards from natural sites. This is consistent with a habituation effect. Surprisingly, animals that belonged to the control group saw their parasite load increase while animals that belonged to the manipulated group did not. The protective effect of the simulated attacks of predator might be due to the fact that it could have impeded lizards from the manipulated groups to reach their optimal body temperature. This might have disrupted the reproduction of blood parasites. Finally, after being immunologically challenged, the lizards from the manipulated group opted for an induced fever response while the lizards from the control group choose to induce a hypothermia response, possibly because of a weakened condition due to a higher parasite load. Regarding possible differences between morphs, we could only detect a weak tendency from orange lizards to develop higher parasite loads after the treatment. Therefore, our study tend to show that white and orange morphs of P. muralis to not differ in terms of behaviours or immunes capacities linked to higher levels of testosterone.