Reproductive endocrinology and stress physiology in Galápagos land iguanas

Abstract

Free-living animals must struggle against a variety of challenges and man-mediated alterations that can cause high stress conditions. Among the most hostile consequences of prolonged stress are disruption of behaviour and reproductive physiology and the alteration of population fitness. Understanding if and how threatened free-living populations reproduce and respond to changes is necessary for determining vulnerability and setting conservation priorities. Conolophus marthae (Critically Endangered, IUCN Red List) and Conolophus subcristatus (Vulnerable, IUCN Red List) are two land iguana species living in syntopy on Volcán Wolf (Isabela Island, Galápagos Archipelago, Ecuador). Little is known about the reproductive biology of these two species, and data available are based only on circumstantial observations. As these two congeneric species may compete for nesting sites, comprehending species-specific times of reproduction can be crucial to understand interspecific interactions. Thus, in the first part of the thesis, plasma levels of progesterone (P4) and 17b-estradiol (E2) were examined and explained through enzyme-linked immunosorbent assay (ELISA), a methodology never applied to iguana species before. Overall, the endocrinology allowed the description of a specific breeding season for Conolophus subcristatus on Volcán Wolf. Reproduction appeared still ongoing in June to finish in July. Unfortunately, for Conolophus marthae the description of a breeding season was not possible with the same precision. Reproduction in this species seemed hampered, determining attrition. This aspect of reproductive endocrinology was combined with the analysis of the stress physiology in relation to parasitic infection. Indeed, in the Galápagos Islands some populations of land iguanas are strongly impacted by ticks, ectoparasites vectors of the apicomplexan haemoparasite Hepatozoon (Hepatozoidae, Hemogregarines). In general, ecto- and endoparasites are important stressors for wild animals; they may affect individual health and fitness-related traits. These stressors may cause endocrine and immunity variations and the glucocorticoids hormones are key mediators of the stress response. In vertebrates, glucocorticoids, whose synthesis is regulated by the hypothalamic–pituitary–adrenal axis (HPA), are involved in the responses to a variety of stressors and are often used as a hormonal measure of stress. Parasites and glucocorticoid hormones interact and affect a multiplicity of processes, such as immune response and reproduction. However, the nature of the relationship between parasitic infection and levels of glucocorticoids and their possible covariation with haematological profiles has received relatively little attention in wild animals, even with equivocal results especially in reptiles. In the second part of the thesis, the relation between ecto- and endoparasites and the stress physiology of these two land iguana species were analysed by using: (i) leukocyte profiles as haematological markers and specifically the heterophils/lymphocytes ratio (H/L), commonly used as diagnostic tool for assessing stress in vertebrates (ii) endocrinological markers as baseline corticosterone plasma levels, the primary adrenal glucocorticoid hormone produced in response to stressful events in reptiles. For a comparative approach, a population of C. subcristatus from Bahia Urbina, an area where notoriously ecto-parasites and haemoparasites are marginally present, was used as a control for haematologic and hormonal comparisons. Overall, thanks to haematologic profiles, it was described how Hepatozoon caused an activation of immune system especially of phagocytic heterophils forms; nevertheless it was also clarified that both reproduction and parasites infection contributed to the observed leukocytes’ profiles. In this study, the physiological capability of iguanas to modulate the activity of the immune system in response to different stressors (reproduction and endoparasitic infection) emerged explicitly for the first time in reptiles. Moreover, the H/L ratio appeared to be a more persistent indicator of stress by parasites than the more sensitive corticosterone levels. It was suggested that baseline corticosterone and H/L ratio cannot be used interchangeably as indicators of stress by parasites, but together can provide a comprehensive picture about the stress status of iguana species in in-situ studies. In fact, the stress hormone levels rose exclusively as a response to a natural and life-threatening situation as reproduction. These results confirmed that the interaction between adrenal and gonadal systems could be important for reproductive efficiency, dispelling the assumption that adrenal activation is necessarily deleterious.