Many animals are solitary – they just want to be alone. Primitive asexual creatures, such as a budding hydra, need no one else to make a success of their lives. However, most animals engage in some kind of relationship with another member of their species that is of mutual benefit to both parties. This may be as simple as pairing up with a mate, or it may be part of a more complex society where members of the same species cooperate to a lesser or greater extent.
For example, a piece of coral is actually a colony of thousands of individual animals called polyps. The polyps grow side by side, but feed and reproduce as individuals. However, at the microscopic level, members of the colony also work together to fend off encroachment from neighbouring corals. To understand animal societies and other animal relationships we have to weigh up their various benefits and disadvantages. For example, living in a group increases competition for food and mates but it also boosts safety and defence.
A symbiotic relationship is one in which members of two different species have evolved to live closely together for mutual gain. Flowering plants and honeybees exist in a symbiosis, for example: neither could survive without the other, and the partnership has come about by an extreme form of coevolution. Some symbioses are an even closer union. Giant clams and corals harbour photosynthetic bacteria called Zooxanthellae in their tissue. These so-called endosymbionts provide their hosts with sugar in return for a safe and stable place to live.
There are two modes of symbiosis. The examples above are mutualistic, with both species gaining from the relationship. But there are some symbiotic pairings where only one of the partners gains. The other one neither gains nor loses. This situation, known as commensalism, is less common than mutualism, but examples include cattle egrets (opposite) that follow herds of cattle or other large herbivores to prey on the ground insects that are disturbed by the herd’s hooves.
Coevolution can also create relationships between two unrelated species that benefit one partner and damage the other. This is parasitism – the beneficiary in the partnership is the parasite, and the loser is the host. Parasites can live inside or outside the host. Fleas (opposite) are ectoparasites, living on the skins of their furry hosts, while a tapeworm is an endoparasite, living in the gut of its host. Other parasitic worms enter the body proper and set up home in the blood and organs.
The life cycles of parasites are often convoluted, with parasites transitioning through several hosts, or ‘vectors’. For example, mosquitoes are the vectors for the malaria parasite, while river snails carry schistosomes, tiny worms that cause the tropical disease bilharzia. The most successful parasites do little harm to their hosts – killing a host means they must find a new place to live. However, some organisms called parasitoids do kill their hosts – eventually. Such animals (often tiny wasps) lays eggs on or inside the host, and the young slowly eat it alive.
A predator is an organism that kills and eats other organisms. There are a few examples of plants and fungi as predators (the Venus fly trap is famous for trapping and digesting insects, while some fungi snare microscopic worms in the soil). However, the great majority of predators are animals, and the organisms they target are their prey.
The common usage of these terms suggests that predators are fierce meat-eaters, frequently large and powerful beasts that are able to overpower their weaker prey. However, an assassin bug stalking a cricket or a ladybird larva grabbing an aphid has all the drama and violence of predator–prey relationships among larger animals. And there is also a dynamic relationship between the populations of any predator and their prey. When the prey population is large, predator numbers grow thanks to the surfeit of food. But with so many predators around, prey numbers begin to drop – and predators starve. With fewer predators, the prey population rises once more, and the cycle begins again.
Herds go by many names: flocks, shoals, even gaggles. All these animal groupings are loose, leaderless affiliations. The members are essentially living solitary lives, acting to maximize their own success, but this goal is best served by staying close to others of their species. It can look like members of a herd are all working together but this is an effect of them all behaving in the same way.
The most obvious reason to live in a herd is safety in numbers. Predators will attack individuals on the edge of the herd, and so those animals that find themselves on the periphery are always moving towards the centre, keeping the herd together. Each herd member remains on the lookout for danger, and moves to safety when it is alarmed. As a result, the whole herd is soon alerted to the threat and moves away en masse. Herding behaviour suits animals that live in habitats where food is widely distributed, such as grazers on a grassland. Animals that exploit small concentrated supplies of food, such as fruit-eaters, would be at a disadvantage if they moved in such large numbers.
Polygyny is a mating system where one male mates with several females. By contrast the female mates with a single male, perhaps because she becomes unreceptive to further courtships, but more often because her male mate keeps guard over her and his other mates. This creates a ‘harem’, where a single male controls the reproduction of a group of females and keeps other males away.
Many group-living animals, such as deer, hippos and cattle, use a polygynous mating system, as do many fish. A common factor is that these animals live in habitats with distributed food supplies. This makes it impossible for one animal to take control of a meaningful feeding territory. There is no point in a stag defending a parcel of land to control food resources, since his rivals will find plenty of food elsewhere. So to maximize success, a stag takes control of reproductive resources instead. This strategy leads to distinct sexual differences as males become fighting machines able to defend a harem.
Polyandry is the opposite of polygyny. A single female has several male mates, who are exclusively hers – they do not mate with other females during that breeding season. Polyandry is rarer in nature than polygyny, but it does exist. An extreme form is seen in anglerfish: the seafloor predators that lure prey with a glowing lantern are all female, since males never develop beyond their juvenile form. Instead, they bite into the body of a larger female, eventually becoming linked to her blood supply. Each adult female will have several such mates embedded in her skin, shedding sperm whenever she lays eggs. Other examples of polyandry are more conventional: spiders, reptiles and several birds, including the emu, all follow this strategy. Some females store sperm in order to produce a single clutch of young with multiple fathers. This means that male mates are all willing to share paternity and help the female look after all the young, since they do not know which ones may be theirs. In polygyny, males devote all their time and energy to mating. In polyandry, the female must shoulder a similar burden but also produce the young as well.
If we strip away the moral connotations of this word when applied to human behaviours, we find that many animal societies employ ‘promiscuous mating’ – including many of our closest relatives. Simply put, a promiscuous mating system is one in which both males and females mate with multiple partners – and do not form pair bonds.
This kind of sexual strategy is most common in so-called fusion–fission societies, as seen in many monkey and ape species, and in dolphins. Members of such societies generally cooperate to find food supplies, defend against dangers, and protect the young, irrespective of parentage. There is frequent mixing between groups, where two or more groups will meet and merge for a while and then split again into smaller units with a different set of members from the original ones. Such groups may have a leader – or a dominant leadership group of older individuals – but there are no barriers to members moving between groups.
The way animals breed is not always set in stone. Some species vary their mating system – or sexual strategy – to suit ecological conditions. Lions are a good example, famous for being the only cats that live in social groups. The most familiar grouping is the pride, in which one male controls a harem. This system works on the grasslands of Africa where lions must cooperate to catch fleet-footed herbivores. But in locations where food is easier to come by, lions form monogamous pairs, with one male and one female working together to raise their cubs. When lions lived in Europe, for example, they adopted this strategy.
Monogamy is the de facto system for many other animals. Forming long-lasting pairs reduces the need for males to expend energy competing for mates, allowing them to devote more effort to ensuring their offspring survive. Nevertheless, monogamous animals may mate elsewhere when they can – a female cheater gains by adding new genes to her offspring, while a male gains through another male raising his young.
Human sexuality has a social component beyond the realms of biology, bound up in ideas of sex (male or female), gender identity (masculine or feminine), and taboos and gender roles that vary from culture to culture. Is homosexuality caused by inheriting a gene? There is some evidence that both of a pair of identical twins being gay is a more likely outcome than just one, but this is thought to be an epigenetic, rather than genetic, effect (see here) – one proposed cause points to hormone levels in the uterus during pregnancy.
In human culture there remains a strong link between sexuality and personal identity, with individuals often required to declare as heterosexual, homosexual or bisexual. Do such distinctions in the natural world beyond humans? In a biological context sexuality can be treated simply as a set of behaviours. Homosexual behaviour is sometimes seen in many different animals, but is seldom the dominant mode during courtships: in species such as the bonobo, it provides a means of mediating complex social relationships.
For the majority of species, the ratio of male to female is roughly 1:1. Individual mothers may produce families that are all female or all male, but the chance of them producing a male or female offspring is always 50/50 each time. The reason for this equality is summed up in Fisher’s Principle, attributed to Ronald Fisher in 1930. If females outnumbered males, there would be an obvious advantage in producing males, since with a larger number of mates they would be able to sire more offspring. These offspring would also have a tendency to produce male offspring – pushing the sex ratio back towards 1:1. If it overshoots, a similar mechanism for the female sex would act to bring it back to a stable equilibrium. However, there are situations where the sex ratio remains skewed. For example, fig wasps spend their larval stage inside figs. The adults that emerge are mostly female, but there are a few males that mate with their sisters before they fly off to lay eggs in the next fig, thus removing the need for a balance of the sexes and maximizing reproductive output.
The rather obtuse phrase ‘r/K selection’ refers to the two main strategies for producing offspring. An r-selective strategy focuses an animal’s resources on the rate of reproduction (the r stands for rate) while a K-selective one focuses on maintaining the animal population at full capacity (the K stands for Kapazitätsgrenze, or ‘capacity limit’). A pinnacle of the r-selective species is the oceanic sunfish, the largest bony fish on Earth: a female produces 300 million eggs every year, by far the largest number of any vertebrate. Just a handful of these eggs will reach adulthood, but the fish is playing the numbers. If it can produce a few tens of millions of young more than its neighbour, it should have more success – and its babies will be able to take advantage of whatever openings in the ecosystem may arise. At the other extreme is the archetypal K-selective species, the orangutan. A baby stays with its mother for seven years, learning all it needs to survive in the forest. Only then will the mother have another child, thus limiting her lifetime fertility to an average of just two young.
The oceanic sunfish and the orangutan use very different strategies to achieve reproductive success.
The mechanisms that determine an animal’s sex are not universal. Most higher animals, such as birds and mammals, use a genetic system, but other methods are at play elsewhere in the animal kingdom. Mammals – including humans – use the sex chromosomes X and Y. A female has XX, while a male has XY. Females are homogametic, meaning a female gamete always carries an X chromosome. The male is heterogametic in that half his sperm carry an X and the other half a Y. Birds also use a genetic system, involving the ZW genes. In this case, however, the males are homogametic, with ZZ, while females are heterogametic with ZW. Many insects use a similar system where females have two sex chromosomes, and males have just one. The sex of crocodiles, turtles and some other reptiles depends on the temperature of the nest. In turtles, eggs that are at a lower temperature tend to be male, with the rest being female. In crocodilians, eggs that are in the mid-range of temperatures are male, while hot and cold ones become female. This system is prone to wide fluctuations year on year.
Ants, termites and honeybees are all examples of eusocial animals. They live in colonies where the offspring of a single queen act as workers that build a nest, collect food and raise more of their mother’s offspring. This kind of social unit thrives in arid areas where individuals would struggle without cooperation. A termite queen (opposite) is a giant egg-producer, many times bigger than the workers, with a king who lives alongside her: both male and female offspring that are kept infertile by pheromones she releases. Occasionally, fertile winged offspring are sent out to reproduce and start a new colony. In contrast ant, bee and wasp workers are all female. They and their mother, the queen, are diploid, while unfertilized haploid eggs develop into males, or drones, that leave the nest along with virgin queens to mate and start new colonies. The genetics are complex, but the system means that the sisterhood of ant workers are more closely related than normal sisters. This in turn ensures the workers will devote themselves to helping their mother to produce more siblings.
Competition for mates – generally between males – can be fierce and often deadly. However, once mating is over the competition does not end. Sperm are the product of meiosis and as such they have a different genetic makeup to their creator, and different genes from each other. As a result, each sperm is in direct competition with its neighbours. A leading hypothesis as to why crossing over evolved (see here) is to reduce the genetic differences between sex cells and so lessen this competitive streak. If it was left unfettered, natural selection would make sperm literally attack each other. Instead, evolution has resulted in numerous adaptations where the sperm from different males compete. The simplest one is mate guarding. A male harlequin toad stays attached to his mate for 19 days to stop rivals mating. Some males secrete a plug that blocks up their mate’s genitals. Meanwhile, in promiscuous species a male’s penis may scrape out earlier sperm deposits before leaving its own, after which it all comes down to a trial of speed and stamina as sperm race to the eggs.
One advantages of a K-selective strategy, where parents invest time and energy in raising a few young rather than simply producing offspring in vast quantities, is the ability to teach the young. Many behaviours, from hunting techniques to social interactions, are learned in childhood and passed down from generation to generation. The learned aspect of an animal’s behaviour has a cultural dimension because groups of the same species living in different parts of the world behave in different ways. A good example is killer whales: these ‘wolves of the sea’ hunt in packs, or pods, in all corners of the ocean. Yet each pod has a hunting style that suits where they live. Some target shoals of fish, others stalk whales, while others snatch seals – all using learned and well-practised cooperative hunting techniques. A fish-eating killer whale moving to a whale-catching group would struggle to fit in with the culture. Animal culture evolves and radiates in a way that mirrors natural selection, with novel behaviours taking root in one group before moving to another.
