Physiological and Behavioural Flexibility in African Striped Mice: Testosterone and Environmental Influences

Abstract

Flexibility – reversible phenotypic changes in physiology, morphology, and / or behaviour – has been extensively studied in the framework of alternative reproductive tactics (ARTs). A reproductive tactic is a phenotype that results from a strategy – a decision rule based on genetic program. The relative plasticity hypothesis states that environmentally-induced hormonal changes cause the development of one out several possible ARTs. However, causality between hormones and ARTs has been poorly studied. Further, it is important to determine which environmental factors influence hormonal changes in males of different ARTs to determine in how far these hormonal changes are flexible. In this thesis, I experimentally studied the role of testosterone in causing physiological, morphological, and behavioural differences between two alternative reproductive tactics in male African striped mice, Rhabdomys pumilio: 1) group-living helpers showing alloparental care, low testosterone levels, and high corticosterone levels. 2) solitary-living roamers showing no parental care, high testosterone levels, and low corticosterone levels. Finally, I tested the flexibility of prolactin secretion as prolactin may play a role in the regulation of ARTs. For this, I studied the role of photoperiod and food availability in regulating prolactin levels of paternal dominant breeding males. In chapter 1 I studied which factors correlate with alloparental care. For the first time I demonstrated that both male and female, juvenile and adult philopatrics show extensive helping behaviour (14.7 % of their time). Corticosterone levels correlated with alloparental care in an age- and sex-dependent manner. Natural variation of testosterone levels between helpers did not correlate with alloparental care, suggesting that changes in alloparental care after the tactic switch are not caused by testosterone. In chapter 2, I tested whether an experimental increase of testosterone in male group living helpers influenced alloparental care and dispersal-like behaviours. An experimental increase of testosterone in male group-living helpers did not reduce alloparental care nor aggressive behaviour, but increased boldness, decreased anxiety, and lowered basal corticosterone levels. In the field, exogenous testosterone did not cause dispersal, but it caused male group-living helpers to expand their home ranges (chapter 3). I suggested that an increase of testosterone facilitate dispersal by reducing stress reactivity (low corticosterone levels and low anxiety). Exogenous testosterone also induced sexual maturation and spermatogenesis – testosterone-treated male group-living helpers became scrotal and showed larger testes and epididymis (chapter 3). Thus, I suggested that a quick increase of testosterone levels is an important event during the tactic switch to cause necessary physiological, morphological, and behavioural changes in group-living helpers to disperse and become solitary-living roamers. In chapter 4 I showed that free-ranging paternal dominant breeding males showed higher prolactin levels when they were breeding compared to the non-breeding season, independently whether this was during spring (increase of photoperiod) or during summer, i.e. normal non-breeding season (decrease of photoperiod). Food availability correlated with prolactin levels, suggesting that cues related to food availability may regulate prolactin levels. As paternal dominant breeding males have higher reproductive success than males following an alternative reproductive tactic (i.e. group-living helpers and solitary-living roamers), flexibility in prolactin secretion seems adaptive. My thesis demonstrated that testosterone plays an important role in physiological, morphological and behavioural differences between males of different ARTs in African striped mice. I also showed that the role of environmental factors (e.g. food availability) is crucial in hormonal flexibility (prolactin levels). Thus, studies from the African striped mouse suggest a complex relationship between hormonal and environmental factors in the regulation of ARTs. For future studies, I discussed how to integrate environmental factors in the behavioural endocrinology approach to study proximate mechanisms of ARTs.