The Evolution of Prolonged Post-Reproductive Lifespan in a Non-Human Mammal

了解女性在生命结束前停止生殖的原因是生物、医学和社会科学的一个关键目标。例如，在传统的人类社会中，女性通常在38岁时生下最后一个孩子，但可能会再活20年左右。这种现象并不局限于人类，在哺乳动物、鸟类和鱼类的许多物种中，雌性动物的寿命可能远远超过她们最后一次出生的时间。这是为什么？有三种可能的原因被提出：i)它可能仅仅是雌性长时间生活的副产品；ii)它可能通过提高后代和/或孙辈的存活率而使生育后的雌性受益，或者iii)老年雌性在争夺支持怀孕和产奶所需的食物时可能会输给年轻的雌性。在人类中，似乎是II和III的结合推动了更年期的进化。然而，目前几乎不知道是什么力量塑造了生活在紧密家庭群体中的非人类动物的生殖后寿命。在这个项目中，我们将首次测试目前关于非人类动物生殖后寿命的进化理论。我们的研究将集中在生活在北美海岸外的两个虎鲸种群。虎鲸是所有非人类动物中繁殖后寿命最长的；雌性虎鲸在30-40岁时停止繁殖，但可以活到90多岁。我们将使用过去30年收集的数据，在此期间，已记录了600多头鲸鱼。我们将使用出生和死亡的信息来研究社会因素如何影响生育率和存活率。我们将特别提出以下问题：(1)生殖后雌性如何从生殖后寿命中受益？(2)老年雌性如何支持其后代/孙代？(3)当与年轻雌性竞争繁殖所需的食物时，老年雌性是否会失去？(4)观察到的好处(问题1)和生殖竞争的后果(问题3)能否解释虎鲸繁殖后寿命较长的进化？我们将使用记录两个人口出生和死亡的长期数据来回答问题1和3。我们将使用类似于保险公司用来计算预期寿命的分析技术，来决定向人们收取人寿保险的保费。在我们的分析中，我们将研究繁殖后雌性的存在和行为如何影响生存。我们将通过使用视频和摄影记录来研究母亲和她们的后代/孙子之间的社会互动来回答问题2。我们将测试这种关系对生存的重要性。最后，我们将通过建立种群的模拟模型来解决问题4。我们将利用我们对鲸鱼的观察来设置模型中的参数[例如，生殖后雌性增加其后代存活率的数量]。然后，该模型将模拟进化，使我们能够检查我们在种群中看到的影响是否足以推动虎鲸繁殖后长寿命的进化。这项研究计划承诺推进我们对自然选择如何塑造生活在紧密家庭群体中的物种的生活史进化的理解。我们的工作将为非人类动物更年期进化的当前进化理论提供第一次检验，这项工作的结果将为人类生活史的进化提供信息比较。更广泛地说，我们的工作将促进我们对社会物种衰老过程的理解，以及个人社会关系和预期寿命之间的相互作用。

Understanding why females stop reproduction prior to the end of their lives is a key objective in the biological, medical and social sciences. In traditional human societies for example, women typically have their last child at 38 but may live for a further 20 years or so. This phenomenon is by no means restricted to humans and across many species of mammals, birds and fish, females may have a lifespan that extends far beyond their last birth. Why is this? Three possible reasons have been suggested: i) It could simply be a byproduct of females living for a long time; ii) it may benefit post-reproductive females by increasing the survival of their offspring and/or grand offspring or iii) old females may lose out to young females when competing for the food needed to support pregnancy and producing milk. In humans it seems that a combination of ii and iii have driven the evolution of menopause. Currently however, almost nothing is known about the forces that have shaped the post-reproductive lifespan in non-human animals that live in close-knit family groups. In this project we will test for the first time the current evolutionary theory for the post-reproductive lifespan in a non-human animal.Our study will focus on two populations of killer whales Orcinus orca that live off the coast of North America. Killer whales have the longest post-reproductive lifespan of all non-human animals; females stop reproducing in their 30s-40s but can survive into their 90s. We will use data collected over the last three decades during which time more than 600 whales have been recorded. We will use information about births and deaths to examine how social factors shape fertility and survival. In particular we will ask the following questions: (1) How do post-reproductive females benefit from a post-reproductive lifespan? (2) In what ways do older females provide support to their offspring / grand offspring? (3) Do older females lose out when competing with younger females for the food needed to reproduce? (4) Can the observed benefits (question 1) and the consequences of reproductive competition (question 3) explain the evolution of the long post-reproductive lifespan in killer whales? We will address questions 1 and 3 by using the long term data documenting births and deaths in both populations. We will use analysis techniques similar to those used by insurance companies to calculate life expectancy when deciding what premiums to charge people on their life insurance. In our analysis we will examine how survival is affected by the presence and behavior of post-reproductive females. We will address question 2 by using video and photographic records to examine social interactions between mothers and their offspring / grand offspring. We will test how important this relationship is for survival. Finally we will address question 4 by building a simulation model of the populations. We will use our observations from the whales to set the parameters in the model [e.g. the amount by which post-reproductive females increase the survival of their offspring]. The model will then simulate evolution, allowing us to examine if the effects we are seeing in the populations are sufficient to have driven the evolution of the long post-reproductive lifespan in killer whales. This programme of research promises to advance our understanding of how natural selection has shaped life history evolution in species that live in close-knit family groups. Our work will provide the first test of the current evolutionary theory for the evolution of menopause in non-human animals and the outputs of this work will provide an informative comparison for the evolution of human life history. More generally, our work will advance our understanding of the ageing process in social species and the interplay between an individual's social relationships and its life expectancy.