Cuckoo Logic: The Alien World of the Brood Parasite

The cuckoo is at the emotional heart of evolutionary biology. Whilst puzzles such as altruistic behaviour and the peacock’s tail can be solved by a tweaking of evolutionary theory and a realigning of our image of nature; the behaviour of the European cuckoo, Cuculus canorus, is different. The cuckoo’s behaviour cannot be dressed up with human ideals of the family and morality. In the figure of the brood parasite, we see the bones of natural selection lain bare, naked and unavoidable.

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A cuckoo egg mimicking the two reed warbler eggs in a reed warbler nest

The behaviour of the female cuckoo contradicts cultural ideas about what parenting, particularly mothering, is. There is no concept of parental love for the cuckoo; the mother will slip into the nest of the selected host, pluck out a host’s egg, quickly deposit her own egg and leave. She pays no part in nurturing the offspring; she is the proverbial bad mother. But the more paternal aspect of parenting is also absent in cuckoos. Most human cultures value parents passing down the knowledge and values they accumulate in addition to their genetic material to their children. Familial traditions have no meaning for the cuckoo, for it receives nothing from it’s parents except their gametes and the home range they return to in the breeding season.

The cuckoo hatchling comes of age in an environment very different from our bustling human world. They are fed by their hosts, each subspecies or gentes of cuckoo is host species- specific, and are unlikely to see any other cuckoos before they fledge. Despite this, by adulthood they must have an idea that only European cuckoos, not the host species, are sexual partners. The cuckoo chick must on some level know that the creature caring for them is not ‘their kind’, one of the first things they must do is understand themselves as a stranger.

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A cuckoo chick ejecting reed warbler eggs

When the cuckoo chick hatches, sightless in a nest full of host eggs, it behaves in a way chilling to see, a puppet governed by pure evolutionary logic. Like a murderous sleepwalker, the naked red cuckoo will hoist the hosts’ eggs (or hatchlings) into a hollow of its back, shuffle to the edge of the nest and discard the hosts’ progeny, to die outside the nest. This grisly scene is repeated until all rivals are gone. It is left lord of the nest, and can enjoy the food the hosts brings unhindered. We may recoil from the cuckoo chick and it’s “odious instinct”, horrified by a babe whose first action is sin. But this exemplifies the error of applying human morals to all animals indiscriminately. They were never innocent so cannot fall from grace – they just act out a strategy honed by their ancestors.

See a short, if sentimental, video of this here.

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Illustration by Laura Cooper of a cuckoo chick being fed by a reed warbler host

What baffles most about the cuckoo’s world is the behaviour of the host. Here “host” refers to the parasitic relationship between the cuckoo who exploits the resources of the unrelated pair who raise it, just as I may be the host exploited by Plasmodium in malaria. But the case of the cuckoo suggests a different meaning of “host”. The host pair appear to welcome the cuckoo chick; they are passive as it ejects their offspring and feed it even as it grows to monstrous sizes. The sight of a reed warbler contorting itself to ram an insect down the throat of a cuckoo twice its size suggests a relationship that has passed hospitality and become subservient.

This can lead us to see the host as either foolish or manipulated by the Machiavellian chick. Both of these descriptions are partially true. But the host is only temporarily foolish. In evolutionary time the species will become wise and the cuckoo will lose the evolutionary arms race and move onto a naive host species. The blackbird still has vestiges of its victory over the cuckoos, its chicks will eject strange eggs from the nest though cuckoos must have long given up parasitising this species.

The cuckoo chick does manipulate – it’s calls mimic a whole brood of the hosts’ chicks to trick the hosts to give it enough food to fill several host chick bellies. But as Nick Davies in the book Cuckoo: cheating by nature suggests, manipulation by the chick is not the sole reason for hosts’ care. Davies reports that reed warbler parents will accept odd chicks of many other species, it is not due to the skills of the cuckoo. It is instead due to the breeding patterns of the host itself.

The reed warblers have evolved great skill at recognizing cuckoos attempting to parasitise nests and rejecting cuckoo eggs. This is because at this stage there is still a significant chance they can have another brood before they migrate. But once a given pair has had a cuckoo hatch in the nest, it is too late to have another brood before they migrate, and only a 50% chance of making it to the next breeding season. Therefore, the hosts carry on as if they had their own offspring, as they can do nothing now to increase their reproductive success.

It is likely that it would benefit the host pair to reject cuckoo chicks and save the energy spent caring for them on preparing for the next breeding season. But this strategy cannot evolve. The low probability that a cuckoo-rejecting pair will ever breed means that the genes associated with this behaviour can’t pass to the next generation and so meet an evolutionary dead end.

Instead of the foolishness or manipulation, what is really seen when the reed warbler feeds the stranger in its nest is something that exists because of an absence of positive or negative selection pressures, as selection cannot touch it. The reed warbler responds to the cuckoo chick by replicating the behaviour it uses to feed its offspring as no counter-strike against the cuckoo can be evolved.

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Adult male cuckoo
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On Bats

The evening was warm and muggy, covered in a blanket of mites which had descended with the sun at 10pm. Sultry summer days can become tedious, stretched out and languorous like you are left in the heat. But nights – perhaps because of their frenzied shortness, perhaps because of the way night breezes can cut through the malaise of the day – become freer, wilder.

I sat on a garden table, legs crossed like a pixie on a toadstool, a small figure within the baroque ballet my garden becomes in the shadows. Things rustled in the undergrowth behind me, but I look up into the domed theatre of the sky. Little black shapes zip around the eaves of the row of terraced houses, like enchanted scraps of velvet ripped from the theatre curtains. Unaided, to me they fly silently. When I produce my bat detector their echolocation calls are lowered to within my hearing range, and I can hear them navigate. My bats are common pipistrelles whose echolocation calls are a little like popcorn popping, a little like a sputtering engine, a little like tutting. A little zip noise indicates a capture.

It does not feel like I am watching mammals. They look like shadow puppets, gliding along on sticks supported by the unseen hand of the puppeteer. They seem mechanical, eternal, indestructible.

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Whilst seeing these mammals soar fills me with wonder, I can see how someone less besotted with the Chiroptera could link them to forces of evil. They are elusive things flitting across the darkening sky, so like us and yet so strange. These uncanny mammals have been apt models for symbols of our worst natures – Bosch’s demons, the vampire. But even in a secular age, they are too often scape-goats for the transmission of most zoonotic diseases. Not unwarranted, but throughout most of the world domesticated dogs pose a greater relative threat. But as so many of us look our dogs in the eye and feel we understand them on an emotional level, we override fear and do not condemn this species. But for bats, it is easy to throw a fist skywards and wish a plague upon all 20 of their families, for we have few opportunities to understand them as a feeling, thinking being. This ignorance breeds fear breeds ignorance and the cycle perpetuates, which can be observed at this time of year by seeing that the scariest, spookiest bat themed Halloween decorations are the least anatomically correct. For me, whilst seeing them soar and reading about them inspires awe, coming close to a bat in the hand is what brings across their true nature as a feeling thing.

Noctule bat  (Nyctalus noctula)Captive

Though I had come close to bats whilst doing surveys of bats emerging from maternity roosts on the site of an airport with a local bat group, my first encounter with a bat in the hand came at a public talk by a member of a local bat group. The bat was a noctule female called Nicky who had been taken in when she was found grounded and kept as a long term captive by the group, to serve as an ambassador for her Order. There was nothing visibly wrong with her, she just did not fly, likely due to a brain injury. This in itself may had contributed to her placidity. She nestled quite placidly into the hoodie of her handler and remained calm whilst we peered at her. Bats look very different with their wings folded in at the elbow and wrist, their patagia sagging at their sides and sticking up their thumbs. Without the elegant width of the wingspan, their furry bodies make them look more like mice. Noctules are one of the largest UK bats, chunky things with distinct light brown fur. Their beady, intelligent looking eyes are either side of a blunt snout with prominent nostrils and a little cleft chin. Noctules echolocate at around 20kHz, which for a sharp eared young ‘un like me is just in the audible range, if I strain to hear. Out of Nicky comes a twitchy echoey sound like no other. It is not a sound made and heard passively. Instead it is a dynamic sound. It is received by the same animal that made it to determine what their environment is like based on how for example the beating of a moth’s wing alters this sound. The bat integrates this information to turn uses this information to change their environment, such as by eating that moth. Bats treat sound very differently to humans, for us it is principally communication, for them navigation.

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Whilst the concept of echolocation is understandable to humans, the “what it is like to” of echolocation (if you will, the qualia of echolocation) is at least very difficult to comprehend, if not unimaginable. To seamlessly process auditory information for navigational cues as we do with visual information is a system so alien to us it could be seen as a barrier to appreciating how these animals live. But given how differently even other people seem to experience the world, to say nothing of the heightened fragrances and dulled colours of the world of the family dog, to draw the line of incomprehensibility at bats is to arbitrarily severe a continuum of sensory experiences. Indeed, trying to understand the alien sensory world of the bat allows us to reconsider what aspects of our own sensory systems we took for granted as not necessarily the only, or the best, way for experiencing the world. From there, we can ultimately transcend these difference and appreciate what we share as mammals whose ancestors have fought, tricked and cooperated successfully through evolutionary time.

Fundamentally, what connects me to bats is how they always strike me as an animal with a particularly precarious existence. Yes, all animals do live on a knife edge, forever at risk of starvation or a failure to reproduce or both. But I believe that the remarkable achievements of the bats – flight, echolocation, their extraordinary lifespans – and the conditions upon these – hibernation, their enormous food requirements- means it feels like they must go to exceptional lengths to merely cheat death everyday. The need to hurl their furry bodies into the air every evening before winter and hibernation cuts them short gives them a manic desperate intensity. Their body temperature is considerably higher than other mammals, in order to power flight they live in a permanent state of “fever” which may explain why they frequently incubate zoonotic diseases. To do extraordinary things in life, they must go to extraordinary lengths to support themselves, but in the end merely resulting in what even the most slothful of corals achieves, reproduction and the increase in frequency of their alleles. But whilst it lasted, it was wonderful.

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When I was given my first bat to care for, to feed and water before release, I held this impossibly small, squeaking, wriggling thing under my gloved thumb. Their manic despair is more evident when they are grounded. I held such a tiny, delicate thing, merely a scrap of fur desperately burning up in their own metabolic furnaces. Driven by a basal need to get out, fly, catch insects as this is what a bat must do, injured bats are restless, they have too much living to get on with to stay in a box. For me, seeing such an elaborate creature as at its base a thing driven by survival is consoling.

We humans may have so much angst about our purpose and our desires, but the bat has had these questions solved for it already. The bat must perform marvelous feats to survive and reproduce, for survival is it’s sole aim and sole pleasure. The bat exemplified the art of survival, a thing we can all do with reacquainting ourselves with, whatever traumas we have faced.