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The costs of reproduction in Takydromus viridipunctatus: the association among reproduction, ectoparasitism, autotomy and survival
cost and benefit
cost and benefit
Evolution of life history traits has fascinated biologists for decades. For a better understanding of this issue, the cost and benefit in survival and mating success are needed. Nevertheless, field studies are relatively rare due to the challenges of accessibility of real cost in the wild that leads ambiguous conclusions in literature. In this dissertation, I focused on two critical life history traits, reproduction and autotomy, of Takydromus viridipunctatus under ecological contexts that showed the cost and benefit of these traits in survival and mating success. These series of studies began at a monthly capture-mark-recapture (CMR) experiment at Jinshan Cape of northern Taiwan during 2006 to 2013. This 8-yeared study have accumulated over 20,000 pieces of CMR data from around 11,000 individuals with their morphological records. I first described their fundamental demography and the reproductive cycle in Chapter 1. My results showed annual patterns of reproductive traits in adults and a following emergence of juvenile indicating the breeding season of this species, starting in May and end in October. Males were slightly bigger than females in body weight, as well as had higher survival rate than female during the breeding season. The sex differences in morphology and survival consisted to the seasonal sexual dimorphism in this species. In Chapter 2, I examined the cost of reproduction and its association to ectoparasitism under natural condition. The tradeoff between reproduction and survival is a central theme in life-history theory, however, empirical studies that link the cost to survival in the wild are rare. By using lateral coloration in males, and biting scars and pregnancy situation in females as reproductive indexes, I tested the association between reproduction and ectoparasitism, then linked them to survival with CMR modeling. I found a positive association between reproductive indexes and ectoparasitism, as well as a substantially negative association between survival and ectoparasite load in both sexes. These results suggested that individuals with higher reproductive indexes had higher ectoparasitism and lower survival rate than that with lower indexes. The wild correlations from this long-termed and huge dataset clearly indicated a severe survival cost of reproduction and implied the extrinsic agent, ectoparasites, may playing a critical role in this trade-off. In Chapter 3, I examined the immunocompetence handicap hypothesis in males, the potential mechanism causing the association in Chapter 2. In Takyromus lizards, the lateral coloration are assumed as a sexual signal but no study address on it so far. Here, I tested this assumption, then linked this color to testosterone, mating success and the immunity cost of reproduction. I used wild correlation, hormone manipulation and female preference experiments to confirm the cause-effect relationships among lateral coloration, testosterone, immunity, and mating success. There was a positively association between testosterone level and the area of green color in wild males. The hormone manipulation indeed enhanced the area of green spots, locomotor performance and size of genital while reduces immunity simultaneously. However, males with testosterone treatment and green spots were preferred by female indicating the function of lateral coloration, as a sexual signal for females. This chapter showed a possible mechanism of the associations in males appeared in Chapter 2 and linked the green spots to mating success. Summarizing Chapter 2 and 3, I suggested that testosterone was the physiological factor to cause the benefit of mating success and the cost of immunity. Then extrinsic ectoparasite may played an ecological role that link this intrinsic tradeoff to survival under natural condition. In Chapter 4, I studied survival cost and benefit of autotomy and regeneration. Caudal autotomy has intrigued scientists over one century. Nevertheless, the lack of empirical study in wild remains this topic equivocal. I used the long-termed CMR data and a contemporary bird census to unravel the complicated associations among tail loss, predation pressure, and the survival cost and benefit of tail loss and regeneration. Autotomy records and abundance of 4 candidate bird predators were used to examine the association between tail loss and predation pressure. CMR modelling was performed to evaluate the survival cost of tail loss and the alleviation by regeneration. The results suggested that cattle egrets cause direct mortality while shrikes and kestrels influenced autotomy, showing the mismatch between autotomy rate and predation pressure. The survival cost of autotomy was severe, especially the 30% decline in males during the breeding season. Surprisingly, the survival revived after tail regrowth clearly indicating the survival benefit of regeneration. This chapter provided field evidences to resolve debates of autotomy and regeneration that increase our understanding about how this dramatic behaviour are maintained in lizards.
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