Sex differences in immunity: the role of sexual selection

动物并非事事完美。为了生存、繁殖并确保它们的基因在后代中得到代表，它们必须将时间和精力分配到寻找食物和对抗疾病等重要活动上。为了成功繁殖，两性都需要比竞争对手生育更多的后代。对于雄性来说，繁殖后代的关键是与尽可能多的雌性交配，为了实现这一目标，它必须胜过对手，并说服雌性与它交配。在整个动物王国里，雄性进化出了非凡的特征，比如令人印象深刻的鹿角和美丽的羽毛，以确保在这两项任务中都能成功，查尔斯·达尔文（Charles Darwin）创造了这一过程。总的来说，雌性不需要这样的技巧来最大限度地提高它们的后代数量。女性的卵子比男性的精子更大，制造成本也更高，因此可以制造的卵子更少。产卵的成本往往会使雌性在交配的频率和与谁交配方面做出选择。对于雌性来说，生存比与许多雄性交配更重要，因为寿命的延长为生育后代提供了充足的时间。因此，自然选择的特征，如强大的免疫系统，对女性降低死亡风险至关重要。细菌、病毒和其他动物的天敌分布广泛，对它们的宿主具有毁灭性。像黏液病这样的传染病经常使兔子数量大量减少，动物已经进化出强大的防御能力来对抗感染并摆脱感染。然而，免疫系统可能是昂贵的，因为它们是由能量和营养物质驱动的。由于这些动物的供应是有限的，他们必须仔细安排自己的活动。科学家们认为，对雄性来说，这意味着将资源用于吸引配偶，即使这意味着更快地死亡，而对雌性来说，这意味着拥有强大的免疫力，以确保生存。因此，我们预测，与女性相比，男性的免疫系统应该更弱，更容易患病。我们的研究将在果蝇身上验证这一观点，果蝇已经被允许在不同程度的性选择下进化。通过鼓励果蝇与许多个体交配，我们引起了雄果蝇之间激烈竞争的强烈性选择。在这种情况下，雄性应该付出很大的努力来获得交配和受精的机会。事实上，我们发现雄性向雌性唱歌的方式发生了变化（它们通过振动翅膀来做到这一点），这可能使它们更善于说服雌性与它们交配。我们预测，这些雄性将会有更少的能量和更少的营养来投资于免疫，使他们更容易感染。由于女性需要投资于提高存活率的特征，她们应该更能抵抗感染。相比之下，当我们只允许一夫一妻制的交配时，也就是说，一只雌性和一只雄性一生住在一起，性选择就不会发生。由于一夫一妻制的雄性不需要在交配竞争中竞争，他们应该有更多的能量和营养来投资他们的免疫系统，使他们在对抗疾病方面和一夫一妻制的雌性一样好。对昆虫的研究很重要。它们可以揭示很多其他动物的生物学，包括哺乳动物，因为昆虫和哺乳动物有一些共同的对抗疾病和感染的机制。这项引人入胜的研究将帮助我们理解生物学的重要方面，比如雄性和雌性何时以及多久患病，何时死亡，以及它们能养育多少幼崽。更重要的是，如果宿主的性别影响害虫和疾病的行为，它可能会影响感染的繁殖和传播速度，这将改变生活在我们环境中的宿主和天敌的数量，以及它们如何相互进化。这些变化甚至可能影响物种灭绝或新物种出现的机会，而这些因素决定了我们星球上动物生命的多样性。

Animals do not do all things perfectly. In order to survive, reproduce and ensure their genes are represented in future generations, they must divide their time and energy between vital activities such as finding food and fighting disease. To be successful in reproduction, both sexes need to produce more young than their competitors. For a male, the key to producing many young is to mate with as many females as possible and to achieve this he must out-compete rivals and persuade females to mate with him. Across the animal kingdom males have evolved extraordinary features such as impressive antlers and fabulous plumage to ensure success in both tasks, a process Charles Darwin coined sexual selection. On the whole, females do not need such tricks to maximise the number of young they produce. Females' eggs are larger and more expensive to manufacture than males' sperm, so fewer can be made. The expense of producing eggs tends to make females choosy about how often to mate and who to mate with. For females, survival is more important than mating with many males, as this increase in lifespan provides plenty of time to produce young. Naturally-selected features, such as a strong immune system, are therefore vital for females to reduce the risk of dying. Bacteria, viruses and other natural enemies of animals are widespread and can be devastating to their hosts. Infections like myxomotosis regularly decimate rabbit populations and animals have evolved powerful defences to fight infections and rid themselves of infestations. However, immune systems can be expensive as they are powered by energy and nutrients. As supplies of these are limited, animals they must prioritise their activities carefully. Scientists think that for males this means channelling resources into attracting mates, even if that means dying sooner, whilst for females this means having strong immunity to ensure survival. We therefore predict that males should have weaker immune systems and be more prone to disease compared to females. Our research will test this idea in fruitflies that have been allowed to evolve under different levels of sexual selection. By encouraging fruitflies to mate with many individuals we caused strong sexual selection with intense competition among males. Under these conditions, males should put a lot of effort into gaining opportunities to mate and fertilise eggs. Indeed, we found changes in the way males sing to females (they do this by vibrating their wings) which may make them better at persuading females to mate with them. We predict that these males will then have less energy and fewer nutrients left to invest in immunity, making them more likely to catch infections. As females need to invest in features that increase survival, they should be more resistant to catching infections. In contrast, when we allowed only monogamous mating, that is, one female housed with one male for their entire lives, sexual selection did not occur. As monogamous males do not have to compete in mating contests they should have more energy and nutrients to invest in their immune systems, making them just as good as monogamous females at fighting disease. Studies of insects are important. They can reveal a lot about the biology of other animals, including mammals, because insects share some of the devices used by mammals to combat disease and infection. This fascinating research will help us to understand important aspects of biology such as when and how often males and females get ill, when they die, and how many young they manage to raise. What's more, if the sex of a host influences how pests and diseases behave, it could affect how fast infections multiply and spread which will alter the number of hosts and natural enemies that live in our environment and also how they evolve along-side one another. These changes may even affect the chances of species going extinct or new ones arising, factors that determine the diversity of animal life on our planet.