尼泊爾埋葬蟲合作生殖群體內之繁殖分配

Abstract

繁殖分配係指生物合作生殖群體同性成員間子代數分配的情形，意謂合作生殖群體內的利益分配結果，是研究合作生殖行為演化的重要課題之一。生態、社會及個體條件等因子同時影響合作群體內個體的行為進而改變繁殖分配的結果。我們想了解在變動的環境中群內個體條件如何影響行為及繁殖分配。個體條件可能反映在其繁殖成本、打鬥能力及交配成本上，此成本影響個體繁殖投資、打鬥及交配行為的頻度，進而影響其子代數。我們以尼泊爾埋葬蟲作為研究對象進行野外試驗；尼泊爾埋葬蟲利用小型脊椎動物屍體繁殖，並能依據環境變化形成不同群大小的合作生殖群體。我們在合歡山沿海拔梯度設置繁殖裝置並設置氣溫記錄器，並隨機配對不同身體形質的埋葬蟲任其自由形成群體，同時錄影觀察其行為。雌蟲皆事先餵食可染卵殼之染色飼料，藉以辨認不同個體的產卵量並了解群體內繁殖分配結果。我們發現群體內雌性個體之交配行為為影響繁殖分配的關鍵。在尼泊爾埋葬蟲的合作生殖群體中交配次數變異度越小，繁殖分配也越趨平均。群內體型較大的雌性個體之交配次數越多，並能產下較多的卵。我們也發現交配次數較多的雌性個體可能較具攻擊性並且可能投資更多時間在其繁殖資源上。另外，氣溫變動影響雄蟲群大小，進而影響群內的交配行為頻度及變異度。我們的結果顯示群內個體的交配成本差異可能是造成繁殖分配均勻程度的關鍵；環境變動則會改變社會條件進而影響兩性間的交配行為。我們認為在研究雙親皆提供親代照護物種之合作生殖行為時，不僅止於關注同性個體間的差異，兩性衝突及可能存在的兩性性擇亦需納入考量。未來若能進一步釐清兩性在繁殖分配中扮演的角色，將使我們更加清楚社會群體演化的全貌。

Reproductive partitioning is how number of offspring is apportioned among the same sex group members in a cooperatively breeding group. It indicates how reproductive benefit is allocated within a group and is one of the key aspects in social evolution studies. Ecological and social factors and individual intrinsic condition could influence individual behaviors and further affect reproductive partitioning. We want to know how within group differences of individual intrinsic condition influence behaviors of group members and their reproductive partitioning. The individual intrinsic condition could show in cost of reproduction, fighting ability or cost of copulation, and, moreover, in behaviors of reproductive investment, aggression or copulation. These behaviors could further influence individual fecundity. We did our experiments with burying beetles, Nicrophorus nepalensis. Burying beetles take carcasses of small vertebrate as only breeding resource, and form cooperatively breeding groups on the carcass. The group size is varied in different environment. We did our experiments alone the elevation gradient in Hehuanshan and recorded the air temperature. We randomly grouped burying beetles of different body size and weight, and recorded the behavior videos. Burying beetles freely formed the cooperatively breeding group on the carcass we provided. In order to identify individual fecundity, female beetles were fed dyed beef to lay colored eggs. We found that female copulation is the key factor of fecundity and reproductive partitioning. Less the variance of copulation number, more even the reproductive partitioning. Larger females were more likely to copulate more, and could lay more eggs. We also found that females with more copulation showed more aggressiveness and invested more time on the carcass. Furthermore, air temperature was correlated to male group size. Male group size was positively correlated to total number and variance of female copulations. Our results show that within group differences of copulation cost influences reproductive partitioning, and environmental gradient changes social factors and then influence copulations. We believe when studying cooperative breeding of biparental care species, we should focus on not only the differences within the same sex but also the sexual conflict and possibly mutual sexual selection. If we know better about how reproductive partitioning is influenced by both sexes, the whole picture of the evolution of social group will be clearer.