Year

2023

Degree Name

Bachelor of Conservation Biology (Honours)(Dean’s Scholar)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Phillip Byrne

Abstract

In the last four decades, amphibian declines have increased globally, with many species becoming threatened with extinction. These declines have been attributed to habitat loss, overharvesting and the introduction of invasive species, although the most prominent threat to amphibian biodiversity is the emergence of the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd). Population declines associated with this pathogen are well documented, however, not all amphibian species show declines when infected with Bd, with several species co-existing with the pathogen. This raises the question, what are the sublethal effects of Bd infection? A growing field of research has identified several sublethal impacts of Bd on anuran behaviour such as locomotion and reproduction, but few studies consider interrelationships between Bd and phenotypic traits and environmental factors that are also known to influence anuran behaviour. Furthermore, very few studies consider the sublethal effects of Bd on morphological attributes involved in signalling and communication. Using the Brown toadlet, Pseudophryne bibronii, as a study species, the aims of this thesis were twofold; to firstly investigate the sublethal effect of Bd infection on reproductive investment in calling, relative to other important phenotypic traits and environmental factors, and secondly to investigate the sublethal effect of Bd on colour expression, relative to phenotypic traits known to be associated with colour production. Field observations of P. bibronii were conducted over a period of two weeks, where males’ calls were recorded to measure temporal and spectral parameters of calling investment, and males were photographed to quantify colour parameters from aposematic shoulder patch colouration and a potentially testosterone-mediated throat colouration. Male phenotypic traits of snout-vent length and body condition, and nest site characteristics of soil pH, salinity, moisture and ambient temperature were also recorded. Although a low rate of Bd infection was observed in the study population (10.2%), infected males had more pulsatile calls than uninfected males. This suggests that when P. bibronii males iv become infected they increase reproductive investment. This supports either the terminal investment hypothesis, where diseased individuals increase reproductive investment to ensure fitness contributions, or the parasite manipulation hypothesis, where Bd could be manipulating host males to increase reproduction to facilitate pathogen transmission. In conjunction with Bd, several phenotypic and environmental factors were found to affect calling, with negative relationships observed between snout-vent length and call frequency, and between ambient temperature and call duration. These well-established relationships are suggested to be related to the properties of anuran vocal apparatus. Interestingly, novel relationships between soil pH and the proportion of advertisement calls, and between salinity and call rate were also found. Males increased investment in advertisement calls in lower pH soils, potentially due to favourable nesting conditions, and called at higher rates in soils with higher salinity, potentially indicating a terminal investment response. For P. bibronii colour expression, Bd was found to have no significant effect on either shoulder patch or throat colouration. However, a non-significant trend of Bd-infected males possessing darker throats, possibly associated with higher testosterone levels, than uninfected males was observed. This potentially supports the immunocompetence handicap hypothesis, which postulates that testosterone suppresses the immune system, thus increasing susceptibility to pathogens. Surprisingly, for the phenotypic traits, a negative relationship between snout-vent length and shoulder patch saturation was found, although this may be related to a differential age-dependent investment in aposematism. The present study advances our knowledge of the sublethal impacts of Bd on anurans, with preliminary trends found between Bd and colour expression warranting further research into this emerging field. The finding that males increase reproductive investment when infected with Bd, in conjunction with similar findings in other studies, has the potential to inform future conservation initiatives.

FoR codes (2020)

410401 Conservation and biodiversity

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.