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

了解为什么女性在生命结束前停止生殖是生物，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。这种现象绝不仅限于人类，在许多哺乳动物、鸟类和鱼类物种中，雌性的寿命可能远远超过它们的最后一次出生。为什么会这样呢？有三个可能的原因：i）它可能只是雌性生活很长时间的副产品; ii）它可能通过增加后代和/或后代的存活率而使生殖后的雌性受益; iii）老年雌性在竞争支持怀孕和产奶所需的食物时可能会输给年轻雌性。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，几乎没有什么是已知的力量，塑造了非人类动物的生殖后寿命生活在紧密的家庭群体。在这个项目中，我们将第一次测试目前的进化理论在非人类动物的生殖后寿命。我们的研究将集中在两个种群的虎鲸Orcinus虎鲸，生活在北美海岸。虎鲸是所有非人类动物中生殖后寿命最长的;雌性虎鲸在30 - 40岁时停止生殖，但可以存活到90多岁。我们将使用在过去三十年中收集的数据，在此期间记录了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生存率。特别是，我们将提出以下问题：（1）如何后生殖女性受益于后生殖寿命？(2)老年女性以何种方式为后代/祖子女提供支持？(3)年长的雌性在与年轻的雌性争夺繁殖所需的食物时会失败吗？(4)观察到的好处（问题1）和生殖竞争的后果（问题3）能否解释虎鲸生殖后寿命长的进化？我们将通过使用记录两个人群出生和死亡的长期数据来解决问题1和3。在决定向人们的人寿保险收取多少保费时，我们将使用类似于保险公司计算预期寿命所使用的分析技术。在我们的分析中，我们将研究如何生存的影响，生殖后的女性的存在和行为。我们将通过使用视频和照片记录来研究母亲与其后代/孙子之间的社会互动来解决问题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。我们将使用我们对鲸鱼的观察来设置模型中的参数[例如，繁殖后的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驱动虎鲸生殖后漫长寿命的进化。这项研究计划有望促进我们对自然选择如何塑造生活在紧密联系的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的进化提供当前进化理论的第一次测试，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，以及个人社会关系与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.