颶風使蜘蛛更兇猛

艾希禮·斯特里克蘭

In regions of the United States and Mexico that are prone to hurricanes， aggressive spiders are evolving to survive and ride out the storm.

When hurricanes rage along the Gulf of Mexico or charge up the East Coast， they can reshape an entire habitat in a short time. The winds destroy trees and spread debris for miles， putting new pressure on the creatures living in these environments. But how do creatures adapt when their habitats are so disturbed？ Researchers looked at Anelosimus studiosus spiders that live in storm-prone areas along the coast to see how they changed. Their study was published in the journal Nature Ecology and Evolution.

These weather systems are called “black swan events”： a surprising event with a major impact that is explained with the benefit of hindsight. They dont offer much lead time， so the researchers had to scramble to study the spider colonies before and after projected storms.

The team monitored Subtropical Storm Alberto and Hurricanes Florence and Michael during the 2018 hurricane season. They tried to anticipate the systems trajectories and study areas that included 240 female spider colonies， comparing them with areas where spider colonies were unaffected by such storms. The researchers returned to the sites hit by the storms 48 hours later. About 75% of the colonies survived the initial storm strikes.

“It is tremendously important to understand the environmental impacts of these ‘black swan weather events on evolution and natural selection，” said Jonathan Pruitt， lead study author， evolutionary biologist and Canada 150 Chair in McMaster Universitys Department of Psychology， Neuroscience & Behaviour. “As sea levels rise， the incidence of tropical storms will only increase. Now more than ever we need to contend with what the ecological and evolutionary impacts of these storms will be for non-human animals.”

The A. studiosis spider species has two main personality traits that are inherited： docile and aggressive. Aggressiveness can be determined based on their speed， attackers pursuing prey， cannibalizing male spiders as well as eggs and their vulnerability to the invasion of predatory spiders. On the one hand， aggressive colonies are better at gathering resources in times of scarcity. But when deprived of food or subjected to overheating， aggressive colonies can also suffer from infighting.

“Tropical cyclones likely impact both of these stressors by altering the numbers of flying prey and increasing sun exposure from a more open canopy layer，” Pruitt said. “Aggressiveness is passed down through generations in these colonies， from parent to daughter， and is a major factor in their survival and ability to reproduce.”

The researchers determined that after a storm passed， the colonies that aggressively pursued food and resources were able to produce more egg cases. The spiderlings also had a better chance of surviving into early winter. In areas that werent hit by storms， docile colonies thrived.

These findings were consistent even as the storms varied in duration， intensity and size. The researchers believe this shows that extreme events can shape animal behaviors. They would like to conduct more long-term studies to learn whether this is the case.

在美國和墨西哥經常遭受颶風襲擾的地區，兇猛好斗的蜘蛛變得對風暴更有抵御力，能安然度過災難。

颶風沿墨西哥灣肆虐或席卷美國東海岸時，短時間內可能就會把所到之處的生物棲息地弄得面目全非。強風摧毀樹木，殘枝碎屑散落數英里，給生活在這些地方的動植物造成新的生存壓力。那么，當生存環境遭到如此嚴重的破壞，各種生物是如何應變以繼續生存的呢？研究人員觀察了生活在沿海風暴頻發地區的櫛足蛛，探知它們的變化過程。該研究報告發表在了《自然·生態學與進化》雜志上。

這些天氣系統被稱為“黑天鵝事件”：突如其來，影響重大，人們往往后知后覺。它們來去匆匆，不會給研究提供很多時間，研究人員必須在預報的風暴發生前后抓緊研究蜘蛛群。

2018年颶風季，該研究團隊對亞熱帶風暴“艾伯托”、颶風“佛羅倫薩”和“邁克爾”進行了監測。研究人員盡力預測這些天氣系統的行進軌跡，在生活有240個雌蛛群的地區展開調研，把這些蛛群與沒受到風暴影響地區的蛛群進行對比。風暴過去48小時后，研究人員返回受災地區，發現約75%的蛛群抵抗住風暴最初的侵襲生存了下來。

該研究報告的主要作者、進化生物學家喬納森·普魯伊特是加拿大麥克馬斯特大學心理學、神經科學與行為學系獲得加拿大150“百人計劃”項目資助的首席專家。普魯伊特說：“這些‘黑天鵝天氣現象在環境方面對生物進化和自然選擇會產生諸多影響，搞清楚這些影響極為重要。隨著海平面上升，熱帶風暴只會更頻繁發生。我們現在比以往任何時候都需要去應對這些風暴對人類以外的動物造成的生態和進化影響。”

這種櫛足蛛在性情上主要有兩種遺傳特征：平和溫順和兇猛好斗。兇猛好斗可以從四個方面來判斷：行動迅速，進攻型捕食，吃雄蛛和卵，易受其他捕食性蜘蛛入侵。一方面，物資匱乏時，兇猛好斗型蛛群更善于積攢可用資源;但缺少食物或遭遇高溫時，它們也會發生內斗。

“熱帶氣旋可能改變飛行獵物的數量并利用更開闊的樹冠層增加陽光照射，以此影響那兩種壓力因素。”普魯伊特說，“兇猛好斗的特征在這些櫛足蛛群中會代代相傳，從父體或母體遺傳給雌性后代，這是它們得以生存繁衍的一個關鍵因素。”

研究人員發現，風暴結束后，攻擊型獲取食物和資源的蛛群能產下更多卵袋，它們的后代也更有機會活到入冬。而在沒有受到風暴襲擊的地區，繁衍旺盛的是平和溫順的蛛群。

即使風暴在持續時間、強度和規模上各不相同，但研究得到的結果一致。研究人員認為，這表明極端事件會對動物行為產生重要影響。他們希望開展更多的長期研究，弄清情況是否果真如此。