寄生蟲對台灣森鼠存活、繁殖表現及身體狀況的影響

Abstract

寄生蟲對宿主生理、行為與生活史的影響可產生複雜生態演化效應。除了對宿主生存與繁殖的直接影響，寄生蟲也可能改變宿主在生存與繁殖間的權衡，且這些影響皆可能隨宿主性別而異。本研究檢測腸胃道寄生蟲（helminths）對不同性別之台灣森鼠（Apodemus semotus）在生存、繁殖以及身體狀況上的影響。實驗方式為隨機給予一半宿主個體寄生蟲移除藥劑（ivermectin），並持續追蹤宿主的生存、繁殖及身體狀況達9個月（2011年8月至2012年5月）。我利用重複捕捉標放的記錄來量化宿主的生存，並藉由雌性陰道口開、乳頭呈哺乳狀態、懷孕，以及雄性睪丸下降等性徵量化宿主繁殖表現。宿主身體狀況則是利用體重改變量進行量化。結果顯示，宿主繁殖隨寄生蟲量增加而提升，且移除寄生蟲降低雌性宿主的生存。同時，我發現雌性宿主在當下繁殖與未來生存間有權衡的現象（trade-offs between current reproduction and future survival），顯示寄生蟲移除可能促使雌性宿主提升對當下繁殖的投資，導致未來生存機率的降低。寄生蟲移除對宿主身體狀況沒有影響。此外，我並沒有發現文獻中常提及的雄性宿主寄生蟲量較高的現象（male-biased parasitism）。這個研究提供了一個寄生蟲對不同性別宿主會有不同影響的實證，且幫助我們了解寄生蟲在宿主生活史權衡上可能扮演的複雜角色。Parasites can influence host physiology, behavior and life history, leading to complex ecological and evolutionary consequences. In addition to directly influencing host survival and reproduction, parasites can also alter the trade-offs between host survival and reproduction. For sexual host species, many parasitism effects are sex-specific. In this study, I tested sex differences in parasitism effects from intestinal helminths on the survival, reproduction and body condition of a rodent host, Apodemus semotus. I conducted a field experiment where I randomly assigned parasite removal treatment (ivermectin) to half of the rodents and tracked their survival, reproduction and body condition for 9 months from August 2011 to May 2012. I quantified host survival with capture-recapture data, their reproduction by physical signs of perforated vagina, lactation or pregnancy in females and descended testicles in males, and their body condition with body mass changes. My results showed that host reproduction generally increases with parasite loads, and parasite removal treatment reduced female A. semotus survival. In the meantime, A. semotus exhibit female-only trade-offs between current reproduction and future survival, suggesting that parasite removal might increase female hosts’ investment in current reproduction at the expense of their future survival. I did not find parasite removal effects on host body condition. Contrary to the male-biased parasitism commonly reported in mammalian hosts, I did not find sex-biased parasitism in A. semotus. This study provided an empirical case of sex-specific parasitism effects in a common rodent host, and highlighted the complex role of parasitism in host life-history trade-offs.