birds do it. bees probably do it. no one's sure whether educated fleas do it. what they do is have same-sex relationships and, in a new review of published research on the subject, biologists have started to consider what it might mean for the evolution of the animals in question.
nathan bailey and marlene zuk, biologists at the university of california, riverside, found that same-sex relationships were a universal phenomenon in the animal kingdom, seen in everything from worms to frogs to birds. "it's clear that same-sex sexual behavior extends far beyond the well-known examples that dominate both the scientific and popular literature: for example bonobos, dolphins, penguins and fruit flies," said bailey.
penguins have been known to form long-term same-sex bonds where males will engage in sexual activity. toads generally don't discriminate between sexes while marine snails all start out male and, when they mate with another male, one of them helpfully changes sex. dolphins will often touch their genitals together or one male might even mount another and penetrate its blowhole. bonobos go the furthest in same-sex bonding with regular copulation among males.
but not all relationships should be considered the same. a male fruit fly, for example, may court other males because it lacks a gene that allows it to tell the difference between the sexes. "but that is very different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or female laysan albatross that can remain pair-bonded for life and cooperatively rear young," said bailey.
writing in the journal trends in ecology & evolution, the authors said that lots of previous studies had considered how same-sex relationships might have come about but very few studies had considered whether the relationships shape the course of evolution.
"same-sex behaviors – courtship, mounting or parenting – are traits that may have been shaped by natural selection, a basic mechanism of evolution that occurs over successive generations," bailey said. "but our review of studies also suggests that these same-sex behaviors might act as selective forces in and of themselves."
in other words same-sex relationships might shape evolution in subtle and important ways for many animals. when bilogists think about selective pressure in evolution, they tend to focus on environmental concerns such as weather, temperature, or geographic features in a particular locality. social circumstances can also have an impact and bailey argues that same-sex relationships could "radically change those social circumstances, for example by removing some individuals from the pool of animals available for mating."
in addition, the behaviour can lead to the evolution of defence mechanisms. "for example, male-male copulations in locusts can be costly for the mounted male, and this cost may in turn increase selection pressure for males' tendency to release a chemical called panacetylnitrile, which dissuades other males from mounting them," said bailey.
bailey and zuk are also researching the laysan albatross, a species in which females form same-sex pairs and rear young together. "same-sex behavior in this species may not be aberrant, but instead can arise as an alternative reproductive strategy," they said.
almost a third of laysan albatross couples are female-female pairs and they are more successful than unpaired females when it comes to rearing chicks.
"same-sex sexual behaviors are flexibly deployed in a variety of circumstances, for example as alternative reproductive tactics, as cooperative breeding strategies, as facilitators of social bonding or as mediators of intrasexual conflict. once this flexibility is established, it becomes in and of itself a selective force that can drive selection on other aspects of physiology, life history, social behaviour and even morphology," said bailey.
鸟这样，蜜蜂可能也这样，没有人能确定受过训练的跳蚤是否也这样，它们都存在同性关系。最近几名生物学家撰写的一份报告对该主题之前已发表的研究进行了回顾及评论，同时开始考虑：这种同性关系对动物物种本身进化究竟意味着什么。
美国加州大学河滨分校的生物学家内森·贝利（nathan bailey）和玛琳·祖克（marlene zuk）发现，同性关系在动物王国里是一种普遍现象，从蠕虫到青蛙到鸟类，都存在这种现象。贝利说：“显然，同性性行为所涉及的动物物种远远不只包括科学文献及大众文学中频繁曝光而众所周知的那些动物：比如倭黑猩猩、海豚、企鹅及果蝇。”
我们都知道，企鹅常常会建立长期的同性关系，其中雄性企鹅将涉足性活动。蟾蜍通常是雌雄平等、一视同仁，而海洋蜗牛生来就都是雄性，当他们与另一雄性交配时，其中一个便及时变性。海豚常常一起触磨生殖器，有时雄性海豚甚至会骑在另一头海豚身上，将生殖器插入其呼吸孔。倭黑猩猩在同性关系方面步子迈得更大，他们雄性之间经常交媾。
但并非所有的关系都应视为相同无异。比如，雄性果蝇可能会追求其他雄性果蝇，那是因为它缺少一个能分辨雌雄的基因。贝利说：“但是这完全不同于雄性的宽吻海豚，他们同性交往是为了促进群体关系，建立一个和谐社会；也完全不同于雌性的黑背信天翁，她们可以厮守终生，同性协力抚养后代。”
这份评论发表在《生态和进化趋势》杂志上，该评论的作者表示，以前的许多研究都关注于同性关系究竟是如何发生的这一问题上，但很少有研究关注于同性关系是否影响生物进化历程。”
“同性行为——求偶、上位交媾或养育后代——这些特性的形成可能都归因于自然选择，自然选择是一种代代延续的进化基本机制，”贝利说，“但是，我们对以往的研究进行回顾所做的评论也表明，这些同性行为本质上可能就是选择力。”
换句话说，对许多动物物种来说，同性关系可能以微妙而又重要的方式影响着它们的进化历程。当生物学家考虑进化中的选择压力时，往往把重点放在环境问题上，比如某特定地区的天气、温度、或者地理特征。社会环境也同样具有影响，贝利认为，同性关系可以“从根本上改变这些社会环境，比如相应减少了该动物物种中可供交配的对象。”
此外，同性行为也可能催生出防御机制的演化。贝利说：“举例来说，雄性蝉之间的交媾对上位的雄性蝉来说代价昂贵，会付出生命的代价，而这种代价可能反过来增加了选择压力，促使雄性蝉倾向于释放一种称为panacetylnitrile的化学物，这种化学物会阻止其他雄性蝉，避免他们上位交媾。”
贝利和祖克也在一直研究黑背信天翁，这种动物物种的雌性个体会建立同性配对，并一同抚养后代。他们说：“这一动物物种的同性行为或许并非异常，但可能是作为另一种替代性繁殖战略出现。”
黑背信天翁对偶中几乎三分之一是雌雌配对，和未配对的雌性相比，她们更能成功地抚养后代。
贝利说：“同性性行为在不同情况下相应起着不同的作用，比如它可以作为替代性生殖策略、合作型繁殖战略、社会亲密关系的促进手段、或者同性冲突的调解工具。这种适应性一经确定，它本质上就成为一个选择力，能够推动物种其他方面的选择，如生理、生活史、社会行为、甚至物种形态。”