多任務處理如何影響最佳窩卵數在變動環境中的演化

Abstract

儘管窩卵數 (一次繁殖嘗試所產的子代個數) 在大自然中的適應已有相當多的討論，一般認為生物 (尤其是昆蟲與鳥類研究) 應該在較長的季節中減少窩卵數以換取更多繁殖嘗試的觀點並非完全正確。實際觀察中，窩卵數常常隨緯度升高、繁殖季減短而增加；然而，窩卵數卻隨海拔升高、繁殖季減短而減少，此現象被稱為產卵趨勢悖論。對於這相反的變化趨勢，多數假說 (如conservative bet-hedging) 沒有辦法提供合理解釋。在此研究中，我們提出多工任務處理假說以處理不可預期的變動環境中，繁殖季長短與窩卵數的關係。藉由個體為本模式 (individual-based model) 與解析模式 (analytical model) ，我們讓小窩卵數的策略因為比較高的切換任務 (繁殖與回復) 頻率而成為比較多工任務的策略；大窩卵數策略則反之，成為較不多工的策略。我們的模式顯示小窩卵數策略在短繁殖季條件下表現優於其他策略，因為繁殖所需的儲備能量與回復時間較短，可以在嚴格時間限制下，減少潛在浪費的能量。另一方面，我們的模式也顯示代表分擔風險 (bet-hedging) 的小窩卵數策略不會隨繁殖失敗率的平均或變異數改變而有不同的相對表現，即趨勢沒有發生變化。因此，節省時間應是短季節中小窩卵數策略的主要優勢。當我們同時考慮多工處理、季節性、與窩卵數依賴性捕食三樣假說時，產卵趨勢悖論就能被完整解釋。最終，我們的模式提供一個新的方法與觀點來了解變動環境中的生活史演化，特別是那些先前被認為是世代內分擔風險的案例更可被重新檢視。

Although the adaptive nature of clutch size variation has been studied for decades, the common belief that organisms, especially insects and birds, should decrease their clutch size (i.e. number of offspring per single reproductive attempt) to try more nesting attempts with an increase in breeding season length does not always hold true. Empirically, clutch size is often found to decrease with breeding season length along latitudinal gradients, but it has an opposite pattern along elevational gradient (fecundity gradient paradox). Most hypotheses to explain variation in clutch size with breeding season length (e.g. conservative bet-hedging) fail to explain these biogeographic patterns with latitude and elevation. Here, we propose a multitasking hypothesis for the evolution of clutch size in relation to the breeding season length in fluctuating environments. Using both individual-based and analytical models, we showed that laying smaller clutches represents a multitasking strategy because such a strategy has a higher switching rate between breeding and foraging than a large clutch size strategy. Our model demonstrates that the small clutch size strategy is favored in shorter breeding seasons because less time and energy are wasted under severe time constraints for multiple breeding attempts within a season. Our model also showed that bet-hedging strategy is not favored by nature selection under high mean and variance of predation risk even in longer breeding season. Thus, saving time is likely the primary benefit favoring the evolution of small clutches sizes in shorter breeding season in birds. The fecundity gradient paradox is solved when multitasking, seasonality, and clutch size-dependent predation are considered at the same time. Ultimately, our models provide a new approach for understanding life history evolution under fluctuating environments, especially those cases that have previously been considered to be within-generation bet-hedging strategies.