Cover image: weaver-birds construct their nests from dried grasses, in colonies that can span several trees. © John King
Welcome to the ninth instalment of 100 Key Concepts. Last time we looked at how organisms move around. All organisms need to move to survive, and a key part of survival is breeding, along with the many varieties of family life that come with it.
69 MATE ATTRACTION
Organisms’ reproductive strategies can be broken into two (broad and overlapping) categories: asexual organisms can create offspring on their own, as clones of themselves, while sexual organisms need a partner to procreate. Mate-locating strategies all serve two purposes: first, to advertise a potential mate’s fitness, and second, its species. Hybrid offspring of two species are often infertile, and may struggle to survive, so avoiding cross-species mating is a top priority. Nature has come up with a plethora of ways for two organisms to find each other: some, like corals and grasses, let wind and ocean currents carry their gametes (reproductive cells, e.g. sperm and eggs) together without moving. Male birds may sing elaborate songs or perform dance-like rituals. The female emperor moth takes a sit-and-wait approach: having spent the night foraging, she finds a secluded location to roost, and begins to emit powerful pheromones. Day-flying males pick up these pheromones with their huge, feathery antennae, crossing the dangerous daytime landscape while the female stays safely hidden.
70 MATE COMPETITION
Finding a potential mate is just the first hurdle; many organisms then need to compete with other prospective wooers to win their beau’s affections. Across the tree of life, competition for these mating opportunities has taken a variety of forms, allowing potential mates to prove their fitness. One very obvious example of this plays out on the islands of Scotland every autumn, when red deer enter their rutting season. Having shed last year’s antlers and grown new ones over the summer, stags become increasingly aggressive and territorial as the days shorten, and testosterone floods their bloodstream. Stags compete with each other for possession of a herd of female deer, by roaring, sizing each other up, and if all else fails, fighting with their fresh antlers. The males which win these competitions ensure near-exclusive mating rights with the females in the herd, while those which lose might have to wait another year for their chance. This evolutionary strategy ensures that the healthiest males make the greatest contribution to the gene pool, while genes from weaklings are eradicated.
In some species, males have to compete for resources, such as nest sites of food, in order to win a mate. However, this is not always necessary, and in these cases, lekking is a common outcome. A lek is a gathering of animals (usually male, but with some exceptions) displaying in order to attract a mate. This allows males to attract mates without wasting energy on violent competition, and females can assess all their potential mates without having to move across a dangerous landscape. In spring, on the dunes and sandy heaths of western Europe, male natterjack toads gather in pools where they begin calling at night. Attracted by the males’ loud calls, females make their way to the pools where they lay the eggs, after which the males fertilise them. However, only big, old males make the effort of calling. Smaller, younger males lurk around the edges of a pool like satellites, intercepting an arriving female and clinging to her back. When she lays her eggs, he fertilises them before a calling male gets the chance. However, the callers still claim most of the mating opportunities.
72 THE HANDICAP PRINCIPLE
In nature, a handicap is a trait or feature which imposes a cost on the organism carrying it, reducing the organisms’ chances of survival. If the organism survives even with the handicap, it must indicate a fitter mate. This makes it a reliable signal of fitness, which cannot be faked or falsified: for instance, the volume of a stag’s roar depends on its size, so stags normally back down from competition with louder males, as they will always be bigger and stronger. The most famous example of this hypothesis is the tail of the peafowl. Like many other species in the pheasant family, the males (peacocks) and females (peahens) display sexual dimorphism – they’re visibly different. Not only are cocks much more colourful, they also have a long train of feathers resting on top of their real, much shorter tail, which the female lacks. These long feathers make the peacock heavier and less manoeuvrable, coupled with an already-poor flight ability. In their native environment in South Asia, this makes peacocks easy prey for tigers. Males with longer trains balance off increased attractiveness against decreased escape ability, so those which survive must be verifiably fitter than those which don’t.
73 KIN SELECTION
Mating allows individuals to pass their genes on to the next generation. But closely-related individuals – such as siblings – share most of their genes, so can pass them on by proxy when their relatives reproduce. Kin selection occurs when organisms increase the survival chances of their relatives, and their offspring, often at a cost to their own reproductive opportunities. This is the evolutionary basis for many of the most complex social systems in nature, such as elephant herds. In Asian and African species, herds are composed of females and immature individuals, and led by a matriarch – often the oldest female, whose life experience keeps the herd safe. Herds are families, and females usually stay in the same one their entire life, so all the members are closely related. For this reason, elephants don’t just look after their own offspring: the whole herd carries the responsibility of looking after juveniles, dramatically increasing a young elephant’s chances of survival. This means that an individual elephant can ensure copies of her own genes make it into the next generation, even if she herself doesn’t breed for several more years.
74 KIN RECOGNITION
In order to boost your relative’s chances of survival, you first need to be able to tell who your relatives are. This also helps to avoid inbreeding, which can produce unhealthy offspring as bad genes build up in the population. Long-tailed tits suffer very high rates of nest failure, but they have a long breeding season. When a bird’s first nest fails, it will often take up residence in the nest of a close relative, and help to raise those offspring instead of breeding again that year. Long-tailed tits can tell which birds are family, and which are strangers, based on the calls they make to keep in contact with one another. These calls are learned in the nest, allowing birds to locate kin even after years of separation.
75 PARENTAL CARE
Many organisms don’t bother looking after their children, producing large numbers of offspring to make up for the resulting high mortality rate. However, some animals can invest a great deal of parental care in their offspring to increase their chances of survival. The fewer offspring that are produced, the more important parental care becomes, as the probability of juveniles making it to adulthood by chance decreases. Orangutans have one of the longest childhoods in the natural world: they spend the first seven years of their life solely with their mother, and then another eight slowly building their independence. By taking so long to grow up, a female orangutan is unlikely to have more than four or five offspring in her lifetime, so each one is incredibly precious. Such long periods of childhood ensure that the young orangutan has time to learn all the tricks of surviving in the rainforest, keeping the species going.
76 PARENT-OFFSPRING CONFLICT
As any human parent will tell you, parenting isn’t smooth sailing. Often, parents and their offspring have different ‘ideas’ of what is in their best interests. For instance, offspring would like to be fed as much as possible, so they can quickly grow into strong adults. However, parents have to balance this with their own survival needs, and the interests of potential future offspring. This determines how much parental care offspring receive. For example, many eagle species lay two eggs each year, but only fledge a single offspring. Usually, one egg hatches a few days after the first, resulting in a size difference between the two chicks. The larger offspring outcompetes its sibling for food, and the weaker bird dies before it can leave the nest. It might seem cruel, but it’s insurance against producing a single, weak offspring.
77 BROOD PARASITISM
Some animals circumvent the problem of parent-offspring conflict, while still ensuring care for their offspring, by exploiting other animals. This allows the parent and the offspring to each maximise their chances of survival without impacting the other’s, and is often shown in birds. Brood parasitism can be intraspecific, between individuals of the same species. For instance, the goldeneye duck often lays its eggs in another female’s nest, leaving one female with the responsibility of raising a flotilla of ducklings while the real parents look after themselves. It can also be interspecific, as in the famous case of the cuckoo. Arriving in Europe from Africa every summer, the common cuckoo follows the calls of reed warblers until it locates their nest. The mother waits until the nest is unattended – her hawk-like silhouette is useful for scaring away the parents – before she swoops in. In a matter of seconds, she will lay an egg in the nest, taking one of the host’s eggs out in turn as a tasty snack, before vanishing completely – adult cuckoos may be back in their wintering grounds by July. Meanwhile, the unsuspecting warblers incubate the eggs until they hatch. The cuckoo’s egg is laid partially developed, so the chick hatches before its adoptive nestmates. It immediately follows its mothers example, pushing other eggs and chicks out of the nest until it’s the only one left. Monopolising the warblers’ attention, exploiting their instinctive attachment to anything inside the nest, the cuckoo chick rapidly outgrows its home, fledging three weeks later and flying alone to Africa. The reed warblers will get another chance next year, but the cuckoos will return as well.
It seems that families are messy in all species. But why did I choose the species I used in the examples here? Are some species just better than others? Well, you might find the answer in the next instalment, when we’ll be looking at the different ways that species can be considered ‘important’ from ecological and human perspectives.