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.

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.

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.

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.

Adult male cuckoo

Revolutions in the History of Science and the History of Life: The Influence of Kuhn on Gould and Eldredge’s theory of punctuated equilibria

I wrote this essay at the start of last summer at the end of my Lower Sixth year as part of an extension project for A-Level Philosophy inspired by finding Turner’s Paleontology: A Philosophical Introduction and being given The Structure of Scientific Revolutions at an impressionable age. I have now decided to dig it out and publish it unedited since then, retaining my youthful zeal, naivety and poor essay titling skills.

Thomas Kuhn was one of the most influential 20th century philosopher of science known primarily for his idea that science advances by a series of revolutions. During a paradigm shift, the fundamental theoretical foundation of a science is overturned and a new generally accepted theoretical foundation, or paradigm, is established. Kuhn’s analysis of the way that scientists work, detailed in his essay The Structure of Scientific Revolutions, has influenced the way that scientists in general have thought about the way in which they do science. But the intellectual framework of Kuhn’s theory has influenced scientist further, even in the extent of influencing heavily the formulation of a scientific theory.

Stephen Jay Gould and Niles Eldredge’s theory of punctuated equilibria states that it is best to take a literal reading of the fossil record, as it actually shows long periods of stasis where species stay the same, punctuated by the rapid appearance of new species. The theory is commonly described as “Marxist”, often as an insult, despite that Marx, and other Hegelians, believed that everything in history is leading to a single goal or end-point; a telos. Gould and Eldredge stressed the contingency of evolutionary history, and hence evolution does not occur towards a telos. Hence, the lack of an end goal that Gould and Eldredge posited for evolutionary history distinguishes his broad theory of evolutionary history from Marx’s theory of human history. However, we will see that the structure of the theory of punctuated equilibria is much more similar to the structure of Kuhn’s theory of scientific revolutions, and indeed, that punctuated equilibria could not have been formed as a theory without the intellectual framework Gould and Eldredge borrowed from Kuhn and other philosophers of science. Though many scientists do not consider detailed philosophical study to be important, I will argue that punctuated equilibria is a good example of where a scientific theory could not have been formulated without a study of philosophy and the adoption of the sort of thinking common amongst philosophers of science.

Kuhn suggested that the history of an established, mature science is characterised by long periods of normal science, under which almost all scientists work with the same paradigm. Paradigms are suitably opened ended so that scientists work to “patch up” the science by asking unanswered questions, but seldom question the theoretical foundation of the science. Occasionally, the paradigm strains under the increasing weight of anomalies that scientists find in the data, so the data cannot be seen to be compatible with the current paradigm. Then a scientific revolution may occur; where an often younger, outsider scientists proposes a new theory for the interpretation of the data that explains enough of the anomalies to challenge the current paradigm. If enough scientist pledge allegiance to this new theory, it will become a paradigm. This shift from one paradigm to another is not wholly rational; it is due to a wide range of personal, sociological, psychological and professional considerations as well as the strength of the evidence. The new paradigm becomes incorporated into normal science, and a period of stability occurs as scientists labour under the same, new paradigm. Scientists must use background theories to decide where and how to collect data and the ways in which they analyse data. But this data obtained are interpreted (or possibly actually seen) in line with the particular paradigm that the scientist is working under, so data is theory-laden. Furthermore, Kuhn argued that science is not advancing to a goal of an immutably truth, clearly set by nature. Rather science evolves through revolutions, but to no particular goal, in a similar way that living organisms evolve, without a goal or telos.

Therefore, the broad frame work of Kuhn’s theory is characterised by: long periods of stasis, where the paradigm remains the same, punctuated by rapid periods of revolution, where a new paradigm emerges, but does not move the scientific field towards the goal of absolute truth. Hence, we can trace the intellectual trail from Kuhn’s theory to Gould and Eldredge’s punctuated equilibria; which is similarly characterised by long periods of stasis, where species remain the same, punctuated by rapid periods of revolution, when new species appear, but does not move the evolution of life towards an absolute goal of evolutionary fitness.

In order to understand Gould and Eldredge’s theory, it is important to understand what it was formulated to counter. Darwin wrote that evolution occurred gradually, primarily by natural selection, involving the gradual evolution of one form or species into another, a process known as phyletic gradualism. Darwin himself said that this view of evolution did not reflect what is shown in the fossil record, as there is no evidence of enormous numbers of intermediate varieties, so the fossil record cannot be interpreted as revealing “any such finely graduated organic chain”1 of species splitting and evolving into different species. But Darwin conceded that “the geological record is extremely imperfect and this fact will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps.” So the phyletic gradualist would see the fossil record as showing gradual evolution littered with gaps, and in many of these gaps fall transitional forms.

However, in 1972, Stephen J Gould and Niles Eldredge presented their theory of punctuated equilibria2, though it is a different way of seeing the fossil record rather than a true theory. They posited that the fossil record in actuality shows an ancestral species in an older layer of rock and many descendent species in the next youngest layer of rock with no intermediary form between the ancestor and the descendants. Phyletic gradualists would see this as a “gap” in the deposition of sediment, due for example to a lake drying up, and therefore a thin layer of rock represents a long period of time in which speciation was occurring gradually. Hence, the phyletic gradualist reassures themselves again that speciation is gradual and the fossil record is incomplete. Gould and Eldredge bemoaned the confines of the phyletic gradualist picture: “We have all heard the traditional response so often that it has become extremely imprinted as a catechism that brooks no analysis: the fossil record is extremely imperfect. […] renders the picture of phyletic gradualism virtually unfalsifiable.”

Therefore, they said, what grounds do we have for not taking a literal reading of the fossil record? Maybe the apparent “jump” between ancestor and descendants is not an artifice due to geological particularities, but represents something actually occurring in evolution. The literal reading invites the proposition that, often, species do not differentiate gradually. Species could differentiate by a subpopulation breaking off from the main population, the small numbers of the isolated subpopulation on the periphery of the ancestor’s range experience a different environment, so the lineage splits into two new descendent species rapidly. The descendants then reinvaded the ancestor’s geographical range, a process known as allopatric speciation. When species become established, they do not change for long periods, until the lineage becomes punctuated by another speciation event. This happens in too short a time period and in a different geographical range to most of the rest of the population, therefore the fossils of the ancestor and the transitional forms are extremely unlikely to be found directly about each other in the same stratum until the new descendent species reinvades. So, most of the time, no “insensibly graded fossil series.”2 is captured in the fossil record. The ancestor, transitional forms, descendants series occurs at a faster tempo than Darwin suggests, and these events are interspersed with periods of stasis.

Though Darwin did not posit that evolution had a telos, he did formulate his theory using the standard frame-work of Victorian, ideas of gradual historical progress and Hegelian teleology. Indeed, it is very likely that Darwin could not have come up with punctuated equilibria however hard he tried, given the intellectual environment he worked in. Gould and Eldredge are indebted to Kuhn for their frame-work for punctuated equilibria. But furthermore, their presentation of their theory was done so in a way that acknowledges Kuhnian ideas of the theory-ladenness of science, remarkably modest behaviour.

Their formulation of punctuated equilibria is based on idea of the theory-ladennes of evidence used by Kuhn; that your background theories, or the paradigm you are working under, might lead you to interpret (or even actually see) the data in a way different from another scientist working under a different paradigm. Hence a phyletic gradualist and a punctuated equilibria adherent may each interpret the same fossil sequence differently from the other; the former would interpret there to be gaps in the formation of rock, giving the appearance of an interrupted gradual evolutionary process but the latter would interpret there to be no actual gaps in the rock formation, but rather the “gaps” show suggests a period of rapid evolution in a different geographical area. Gould and Eldredge write, in a remarkably Kuhnian style, in their original paper, that “the idea of punctuated equilibria is just as much a preconceived picture as that of phyletic gradualism. We readily admit our bias towards it and urge readers, in the ensuing discussion, to remember that our interpretations are as coloured by our preconceptions as are the claims of the champions of phyletic gradualism by theirs.”2 Hence, Gould and Eldredge stress punctuated equilibria as just another way of seeing, and they are as biased towards it as Kuhnians as Darwin was to gradualism as a Victorian.

In their original paper, Gould and Eldredge went so far as to say that “the data of palaeontology cannot decide which picture [phyletic gradualism or punctuated equilibria] is more adequate”. Though later claiming that punctuated equilibria could be verified, here the theory-ladennes of evidence is suggested to an extreme degree, as they suggest that our interpretations are not merely “coloured by our preconceptions” but wholly fogged, implying disturbingly that we cannot know anything inductively, as everything is hidden implicitly in our theories, or paradigms.

This extreme approach did, however, not bring many adherents to punctuated equilibria, hence their later concession that punctuated equilibria could survive tests against the data. With this verificationist approach, Gould and Eldredge have won over the vast majority of younger scientists to varying extents, and hence launched the palaeobiological revolution.

In conclusion, the work of Kuhn and other philosophers in the philosophy of science heavily influenced the formulation and presentation of the scientific theory of punctuated equilibria, itself a considerable contribution to modern evolutionary thought. Therefore, I stress the importance of scientists learning about philosophical ideas, if only for the sake of scientific innovation. Broadening the intellectual horizons can only bring the possibility of scientists applying theoretical frame-works in novel ways to allow radical reinterpretations of current theories. If scientists learn to think like philosophers, at least some of the time, it can bring about innovative interpretations of nature, which is vital for the continued relevance, usefulness and intellectual robustness of science.


  1. Darwin, C. On the Origin of the Species (1859)
  2. Gould, S. J. , Eldredge, N. Punctuated Equilibria: An Alternative to Phyletic Gradualism (1972)
  3. Turner, D., Paleontology: A Philosophical Introduction (2011)

Of Whales and Women: The Importance of Nature in Culture and Culture in Nature

J2 or “Granny”, an orca estimated to be 80-105 years old, who has been post-reproductive for over 40 years.

A recent documentary broadcast on Radio 4 presented by Victoria Gill no doubt sparked a recent editorial in The Guardian on the topic of the increased post-fertility lifespan, the menopause, in orcas. The documentary followed Darren Croft of the University of Exeter and Daniel Franks of the University of York and other studying the Southern Resident orca clan, which boasts a number of older female orcas who have survived well after their reproductive years have ending, including J2 or “Granny” who had her last calf in the 1960s and is still swimming at somewhere between 80-100+ years old today. This is similar to the menopause seen in women today, and in only a single other mammal species, the short-finned pilot whale. The phylogenic oddity of the menopause, appearing not in our close relatives the chimpanzee but in animals with very different evolutionary histories and habitats to us is enough alone to spark a inquiring scientist to investigate how the menopause evolved.

Furthermore, under an earlier and more narrow definition of evolutionary fitness, the menopause has been seen to be evolutionarily inexplicable. An understanding of the reproductive success of an individual as increasing the frequency of their alleles in subsequent generations means that suddenly stopping reproducing seems the exact opposite of a trait that evolved by natural selection. But Croft and Franks argue in this programme that the orca menopause did evolve by natural selection in part due to the post-reproductive females taking care of their adult sons they already have rather than pushing out as many kids as possible. This therefore increasing the number of their sons’ children surviving to breed and so on, so her genes increase in frequency in the population. Rather than increasing her own personal reproductive fitness, the post-reproductive orca uses her sons to increase the frequency of her genes in the population. This account is not particularly revelatory, it is an application of W.D. Hamilton’s ideas of inclusive fitness applied to a particular case.

However, in The Guardian editorial, the author argues that research into the evolution of any trait in any non-human animal is irrelevant to human society and attempts to infer “what constitutes a well-ordered society”, in this case the value of older women in a society, from facts about how a trait has evolved is dangerous. But such a wholesale dismissal of the cultural importance of an understanding of the evolutionary basis does not accomodate the view of biology and nature having a dialectic materialist relationship, the phenomenon of gene-culture coevolution which has been proposed to occur in both humans and several whale species notably including the orca. In the scientists studying animal culture, culture is considered to be behavioural practices transmitted through a population through social learning and not genetic inheritance, this is the lowest common denominator definition of culture and not as intricate as human culture, but significant none the less. By the gene-culture coevolution model, as Hal Whitehead and Luke Rendell describe in The Cultural Lives of Whales and Dolphins, the two streams of information in a cultural species, the genetic and the cultural, can interact. Whilst, as E.O. Wilson termed it, “genes hold culture on a leash” as our culture cannot reach beyond the limits of our biological limits, the favouring a behaviour by a culture, such as adult milk drinking in humans, can lead to the natural selection of genes allowing the most successful use of a cultural trait, which is why the frequency of genes for lactose tolerance is highest in pastoral populations.

Using an understanding of gene-culture coevolution, we can try to understand the evolutionary origins of our cultural practices, or most likely how the biological capacities to develop such cultural practices arose. Indeed, in the orcas the menopause may be one of the key elements in allowing the orcas to transcend the realm of purely genetic inheritance and learn socially and so develop their own culture.

As Whitehead and Rendell discuss, the menopause may have evolved as a means of preserving cultural knowledge. The older females are saved by the menopause from the risks of increasingly infrequent pregnancy at the age of 40 or older and so can live into their 80s and older. These grandmothers play an important role in helping raise children, especially in systems which Sarah Blaffer Hrdy describes as cooperative breeding, where the child is reared by a large extended family of parents, aunts and uncles, siblings, grandparents etc., the proverbial village it takes to raise a child, which she believes is likely the social organisation of early humans.Additionally, these older members of the (human) group will have amassed a great knowledge of the environment over time, which is very useful if a group is blighted by famines every 60 or so years and have to turn to alternative food sources, the knowledge of their edibility is preserved in the mind of the oldest grandmothers. Thus, cultural transmission of information may rely on these elders to preserve information, as in non-literate societies there is no way of preserving cultural information like DNA preserves genetic information, and applied to the early humans as to orcas. We look to orcas to give us clues as to how the human menopause evolved in part because of one of the key philosophical drivers of Darwin’s work, the principle of the consilience of inductions, as termed by William Whewell. By this, the power of a theory increase the more domains of empirical evidence it can explain. This is way Darwin bolstered this theory of evolution by natural selection using examples drawn from biogeography, embryology, behavioural instinct and the fossil record, and why modern menopause researchers use the evidence from orcas to increase the explanatory power of their theories. Crucially, the vast differences between women and whales, not least of all the lack of medical care received by the 80+ orcas, means that the menopause cannot be a pathological state in the orcas as their extended lifespan relative to the males cannot be explained by longevity alone. And explaining how the menopause evolved and therefore shows its advantages in a past environment, rather than dismiss as a pathological aberration of living longer, like Alzheimer’s disease, as some would argue, cannot be a dangerous thing.

I sense that I may be accused of spinning another “very interesting” evolutionary story, which has no relevance to human values and society now. But this society and values are supported by biological and social structures which have evolved, and upon which we have built our complex ethical and social systems. But we did not imagine these systems out of thin air a few thousand years ago, they have a long a gradual pre-history. We eventually had an explosion of social complexity in humans when our culture began to “rachet”, to become increasingly more complex as we built upon the knowledge of others, we stood on the shoulders of giants. It would be intellectually dishonest to refuse to try to encompass politically charged phenomena under an evolutionary frame-work if we call ourselves modern biologists, especially one so counter-intuitive as the menopause, in humans and in orcas. Indeed, not giving an accurate account of the origins of the menopause, gives power to those who would like to dismiss the menopause as pathological and showing womens’ inherent inferiority. Understanding the events of your body as having been selected for in evolutionary time for the advantages it brought your genes can only help understand and learn to value how you experience life. As Croft mentions in the radio programme, many menopausal women draw strength from learning of the menopasual orcas, as it shows “the important of older females in society really valued that story [and] empowered them to think what might be their role in society”. Revealing the evolutionary importance of a trait, thorugh comparison with those who also share this trait across the animal kingdom, can lead us to consider whether our social prejudices are the only way to be, and whether we could reassess our values in light of this.

I believe some of the confusion on these issues are draw in part from the different meanings of the word “value”. The evolutionary biologists uses the concept of evolutionary value as how a trait performs in a cost-benefit analysis in terms of the survival of the organisms. In contrast, value is used most commonly to refer to ethical values, which I will argue are mostly socially determined, though by the laws of causation these must have some sort of basis within the limits of biological possibility. Therefore the science vs. humanities debate comes to its apotheosis. Critics may claim that biologists equate is with ought, that what had the greatest evolutionary value in the human past is what we should value ethically today. I disagree with this position, but doing so does not mean that we should ignore the role of biology in being the basis from which culture springs, but something which does not tether all its ties to biology, as it can reach down and change it biological basis. Life is dialectic; never anything but subtle and dynamic.