Evolutionary success is not about the number of one's children, but one's grandchildren: the children need to survive and pass on their genes.
David Attenborough, asked a few years ago by journalist Joanna Nikodemska about the animal he finds most interesting, answered after some consideration that he's most fascinated by a three-year-old human child, whose potential for development and adaptation are simply limitless.
The same journalist and I have been verifying this opinion for over eight years now – indeed, observing the development of a juvenile representative of the Homo sapiens species is a continuous, fascinating adventure.
More r or more K?
It's a fact that evolutionary success is determined not by the length of adult individuals' lives, but by the number of their offspring that carry their genes into subsequent generations. More precisely, it's not about the number of one's children, but one's grandchildren: the children need to survive and pass on their genes. Of course, in order to have children, one must beget them, or at least somehow initiate the development of the egg, as it happens in parthenogenetic species, where females don't bother with guys at all, or only rarely. But I've already written before about various original methods of completing that first stage, so let's focus on what happens later.
Ecology differentiates two strategies of reproduction: 'r-selection' and 'K-selection'. The symbols are taken from a complicated formula illustrating population dynamics developed in 1838, which systematized our thinking about animal success for the rest of the 19th century and for almost all of the 20th century. It was developed by Pierre François Verhulst (1804–1849), and its simplified version is as follows: dN/dt = rN (1 – N/K), where N is the population, r is its maximum growth rate, K is the carrying capacity of the local environment, and dN/dt is the rate of change of the population with time. According to this model, species that engage in r-selection produce as much offspring as quickly as possible, while K-selection involves an investment in quality rather than quantity. So we either have masses of children that we're not too worried about, hoping that things will work out and some of them will survive; or we have few, but we invest a lot in them and we try to make sure that they do as well as possible. Of course, as is often the case, in nature it's more of a continuum, where not only different species, but also different individuals from the same species, function somewhere between these extremes and we can only say that one is 'more r' or 'more K'.
For example, guppies – small fish from South and Central America, popular with both aquarists and evolutionary biologists – are very flexible in this regard. Researchers have been studying them for years in Trinidadian streams and it turns out that their strategies vary widely depending on the presence of predators, sometimes within the space of a few metres. In the upper reaches of the streams, where rocks make it impossible for bigger fish to get through, guppies have fewer, but larger and better-fed young, so they're 'more K', and their offspring grow up peacefully in calm waters. Below the rocks (sometimes literally one boulder is all it takes) they choose a strategy more closely aligned with r-selection – their offspring are smaller, but they're much more numerous, because in the face of the constant risk of being eaten it makes sense to have as many as possible. So, although science is currently leaving this classic model behind, speaking more often about the diversity of survival strategies, my opinion is that – with some reservations – these two letters make it easier for us to describe a complex reality.
Still, no matter how much offspring there is to be, they need to be brought into the world somehow. Here, there are fundamentally two methods. You can lay an egg with a yolk (the evolutionary equivalent of a packed lunch) from which after a while, with more or less assistance from the parents, your kids will hatch; or you can nourish the offspring within your own body and give birth to them ready-made. It's an easy guess that apart from oviparity and viviparity there's also a third option: ovoviviparity. It refers to embryos that develop in eggs that hatch while still in the mother's body, which the young leave later.
All the eggs in one basket?
Let's start ab ovo. The egg must be encased in something, so that it can protect the embryos at least a little from outside danger. Species that lay their eggs in water usually don't have to worry that they'll dry out, so for them a jelly-like membrane is usually enough; it means the contents of the egg stay where they should, instead of sloshing around. But if you live on land, you must – like many insects and arachnids, and all reptiles and birds, as well as mammals such as the platypus and the echidna – invest in something more watertight. The hard shell of a bird's egg also protects it from at least some predators. For example, the shell of an ostrich egg – incidentally, the largest single cell in the world – is so thick and strong that even lions have trouble breaking it.
Photo by Anna Sjöblom on Unsplash
Still, whatever the eggs are encased in, they all have a better chance of surviving if someone looks after them. We automatically associate incubating eggs with birds; indeed, they either take care of their clutch themselves or, like cuckoos, frame someone else into doing it. But other animals also provide many examples of parental dedication. Female octopuses spend the last weeks of their lives defending their eggs, tucked away in some underwater nook, oxygenating them and cleansing away algae and parasites. This work uses up all the time and energy they've got left after the enormous effort of producing and laying the eggs in a suitable place. When the young octopuses finally hatch, their mum is either already dead or about to die. Although this strategy seems to suit cephalopods, we owe our current position in the world to it – I suspect that if a mother octopus could pass her knowledge and experience to her offspring, Earth would be a very different place. As it is, despite their astonishing intelligence, each octopus must re-invent the wheel. Considering that their intelligence precedes ours by a few million years, I really think that if they could accumulate experience from generation to generation, I'd be writing this text for an eight-legged editor-in-chief, had she even been interested in the opinion of an organism as inferior as a human.
Although the sacrifice of the cephalopod mum is impressive, some invertebrates go further. Perhaps the most extreme form of parental devotion is matriphagy, or the consumption of the mother by her newly-hatched offspring. This phenomenon can be observed in some arachnid species: after laying the eggs, the female starts to dissolve the tissues of her body with digestive juices, so that when the adorable spider babies hatch, their mother is nothing more than an eight-legged chitin container filled with nutritious juice. The tots just need to bite through her skin and they can lap it up. Among insects, apart from the obvious examples of the Hymenoptera (i.e. ants, wasps and bees) and termites, earwigs provide another example of exemplary parental care. The Japanese species Anechura harmandi is the only insect known to science in which the mother also dies before the young hatch, to become their first meal. Even the common earwig is no stranger to motherly sacrifice. The females of these rather unpopular fearless vanquishers of aphids and silverfish frequently gather into groups to care for their clutches together, and then to feed their young and bravely defend them from predators.
The mixed method
Laying eggs has its obvious advantages. If they require no care, you can not only produce many, but also expect that they will spread around the world on their own. But carrying their offspring in their own bodies makes it easier for parents to provide suitable conditions for development. No wonder, then, that some animals (including many species of shark and the common European adder) have chosen the compromise of ovoviviparity during their evolution. In others – like in the viviparous lizard – one or the other method of reproduction dominates depending on environmental conditions. In Southern Europe these lizards, like most lizard species, lay eggs. But in cooler areas the females give birth to their young. Thanks to this flexible strategy, they can live in environments that are inaccessible to many other species, like high up in the mountains and the far north of Europe. It is the only reptile on our continent that also lives beyond the polar circle, although vipers – the northernmost of our snakes – reach almost as far north as that.
Another interesting issue is laying your eggs in someone else's body, although I'm not sure if that still counts as ovoviviparity. The most banal and drastic example are the many species of parasitoids – animals that exploit their host completely, living in it for a time, before killing it like the Alien from the famous science fiction film. Many wasps paralyse their victim (usually a caterpillar or a spider) and lay their eggs in that living larder; the larvae will later gradually bite their way out of it. But laying eggs into the body of one's own partner is even more interesting.
This is what happens in the Hippocampus, or the slowly moving fish known as seahorses. After their mating dance and successful consummation of the relationship, the female lays the fertilized eggs into a special pouch on the male's front. From then on, they will be in his care, so that one day he can give birth to hundreds of miniature seahorses, which he will still take care of after the birth.
But since early childhood, I have been fascinated by another organism. The common Suriname toad – a tailless amphibian (i.e. frog-adjacent) from the northern part of South America with the charming Latin name Pipa pipa – appeared in my life in the form of an illustration in an ancient animal atlas, and it immediately hopped onto the pedestal as one of my favourite species of all times. Just after the female lays the eggs, the male gathers them up and distributes them evenly on her sticky back. Her skin grows spongy, and the eggs sink into it and develop relatively safely; after a time, fully formed little frogs leave her back. It is undoubtedly one of the most interesting births in nature.
The strongest bond
If the young isn't separated from its mother's organism by the egg shell, she usually nourishes it via a placenta. This is, of course, the case in a substantial majority of mammals, but not exclusively. The placenta can also be found in some sharks and lizards, but true viviparity has evolved independently at least 150 times and occurs in many species of fish, amphibians, insects and arachnids. One of these unexpectedly caring parents is the infamous tsetse fly: the female flies around for nine months with a single, increasingly large larva in her abdomen, feeding it with a nutritious milky liquid. A more macabre version of feeding one's young can be observed in some Gymnophiona from the family of common caecilians. Their embryos have special teeth that allow them to feed on the epithelium of the mother's oviduct. After they're born, young common caecilians switch over to the female's outer epithelium and literally flay her, although fortunately she regenerates quickly.
After leaving the mother's body – one way or another – many young animals still need constant care. Because the physical connection is no longer there, persuading the parents to continue to provide food and shelter requires initiating a psychological bond. The parents must like their newly born or hatched children to keep taking care of them.
And so evolution has equipped young animals with a whole arsenal of signals that leave their carers helpless. In birds, it's frequently a lurid colouration of the inside of the beak and the area around it, visible when it is fully open. Adult birds find this irresistible and stuff food down the open, begging mouth, even if it doesn't belong to their children but, for example, to a fish taking advantage of the situation. It is due to our own primitive instincts that most of us also feel tenderness and an urgent need to take care of young animals (or ones that look young). What's more, the recipients of that care don't even need to be cute, pretty bunnies – I still remember how touched I was when, as a student, I discovered a wryneck nest in one of the nest boxes I was checking. The chicks of this woodpecker, with their thin, twisty necks and flat heads, look like mould-infested hallucinogenic mushrooms and they're certainly not pretty, but it works. Their relatively big eyes and squeaky sounds are all it takes. Of course, if the animal meets our criteria of beauty, the effect is even stronger. Cats blatantly exploit this – the charm of their small faces, large eyes and the meowing that emulates the voice of a human baby turns out to be so strong that even my geologist friend is unable to resist them. Although due to his profession he is used to communing with nature through the means of a hammer, he can't stop himself and constantly regales everyone with photos of his feline charges on social media.
There's no doubt, however, that in animals such as birds and mammals it's not only the case of a simple reflex. For some time now, researchers have been claiming more and more boldly that other animals also experience feelings and emotions, like fear, anger, boredom and love. And love for one's offspring is probably the easiest to observe. It is the simplest explanation for such dramatic examples as the behaviour of a killer whale called Tahlequah who, two years ago, carried the body of her dead child with her for 17 days. Parental love can also be the explanation – because there is no other – for more prosaic and happy examples of behaviour, such as the fact that I'm about to walk my daughter to school, even though I've spent all night writing this text.
Translated from the Polish by Marta Dziurosz.
The island rule hypothesizes that species shrink or supersize to fill insular niches not available to them on the mainland.
- Brookesia nana, the nano-chameleon, may be the smallest vertebrate ever discovered.
- The "island rule" states that when new species migrate to islands, they may shrink or grow as they evolve to fill new ecological niches.
- It remains unclear whether the island rule can explain the nano-chameleon or nature's other extreme miniaturizations.
The newly discovered nano-chameleon (Brookesia nana) is the latest contender for the title of the world's smallest reptile and amniote vertebrate. Found in a mountainous region in northern Madagascar, the males of this diminutive species sport a body size of 13.5 mm, meaning one could comfortably stand on the end of your finger.
Its wee challenger is the Jaragua dwarf gecko (Sphaerodactylus ariasae). These pocket-change-sized geckos—the genus is often pictured snogging the minted portraits of past presidents—come in at 16 mm from nose to tail. They were discovered in 2001 on Isla Beata, a small, forested Caribbean island just south of the Dominican Republic.
The title of the world's smallest, however, is difficult to award thanks to sexual size dimorphism. As Dr. Mark Scherz, herpetologist and evolutionary biologist, pointed out on his blog, nano-chameleon females are significantly larger than their male counterparts or Jaragua dwarf gecko females. "As a result, whether or not the new species is considered the smallest amniote in the world depends on whether we define that based on the male or female body size, or the midpoint of the two. It turns out this is quite a common problem in other species with size dimorphism as well, such as frogs," Scherz writes.
Beyond their shrimpy stature, these and other miniaturized species have another thing in common: They live on islands. That fact may explain why evolution has pushed them to shrink in a world full of giant competition.
Bigger isn't always better
The New Zealand little spotted kiwi evolved to be small to fill an ecological niche. Before the arrival of humans, its island ecosystem contained no land mammals to prey on these flightless birds.
Credit: Wikimedia Commons
Because of their geographic isolation, islands can have powerful effects on the evolution of their residential species. The massive Komodo dragon prowls its namesake island. The Barbados threadsnake is thin enough to slither through a straw. And the fossil record recounts a history of unusually sized and bedecked creatures who established homes far from the mainland, such as the Hoplitomeryx of the Mikrotia fauna.
One hypothesis for evolution's insular experimentation is "the island rule." The rule states that after establishing themselves on an island, smaller species will tend to evolve into oversized versions of their mainland ancestors. Meanwhile, larger species will tend to evolve into smaller variations. These processes are known as insular gigantism and insular dwarfism, respectively. They do this to fill the ecological niches available to them, which often differ from those they filled on the mainland.
The rule was first formulated by evolutionary biologist Leigh Van Valen and based on a 1964 study by mammologist J. Bristol Foster—which is why it is also known as Foster's rule. Since then, many observational studies have corroborated the island rule, and there is even evidence to suggest that new species introduced to islands will, for a time, evolve more rapidly to fill available niches.
A flock of migrant birds, for example, may find an island's lack of mammalian and reptilian predators opens the ground-living niche once forbidden to them. Such birds would then be free to grow larger, forage below the canopies, and lose the ability of flight.
This appears to be the origin story for New Zealand's flightless birds including the giant moa, which, at six-feet tall, is the tallest bird on record. This megafauna enjoyed all the benefits of being large and in charge: fewer predators, wider ranges, access to more and varied foods, and the ability to better survive trying times. The species enjoyed island life until roughly 600 years ago, when humans arrived on the scene and hunted them to extinction.
Conversely, large species may find island living restrictive as there's less room or food when compared to their mainland nurseries. Because of this, evolution may select for smaller body sizes as such bodies require less energy, and therefore fewer resources, to survive and reproduce.
This is the theory behind the miniaturization of the Channel Islands pygmy mammoths. As the story goes, in the search for food, a herd of Columbian mammoths embarked on a journey to the super island Santaroasae. Over time, the island was cut off from the mainland. Food became scarce, and smaller mammoths had an easier time surviving and reproducing, thus passing on their Shrinky-Dink genes. Thanks to a lack of oversized predators, such evolution proved fruitful, and in less than 20,000 years, the giant Columbian mammoths evolved into a new species—the (relatively) pint-sized, 6.5-foot-tall pygmy mammoths.
To be clear, the island rule doesn't state that any species that washes ashore must go either Lilliputian or Brobdingnag. It only states that if an ecological niche becomes available and improves survival and reproductive success, then such a change is likely.
Thanks to that island living?
Such constrained growth may be the cause of the Jaragua dwarf gecko's bantam evolution. The gecko eats tiny insects and may be filling a niche that's unavailable on the North American continent with its many, many insectivores. In fact, the island rule may explain why islands are so rich with endemic species—particularly the Caribbean, which is considered a biodiversity hotspot.
Of course, scientific rules are only provisional, and scientists are prepared to revise or completely disregard a hypothesis should new evidence appear. In a field as new as biogeography, the question of whether the island rule is truly a "rule" remains an open and hotly debated question.
One systematic review found empirical support for the island rule to be low, while another analysis argued the rule is simply a recognition of "a few clade-specific patterns." The latter's authors conclude that "[i]nstead of a rule, size evolution on islands is likely to be governed by the biotic and abiotic characteristics of different islands, the biology of the species in question and contingency."
That brings us back to the newly discovered nano-chameleon. While it seems to follow the island rule—Madagascar being an island known for its rich biodiversity—there is a wrinkle. The species' closest relative lives right next door. Brookesia karchei is near twice the size of the nano-chameleon but ranges in the same mountains on mainland Madagascar.
If the nano-chameleon evolved to fill an ecological niche, why didn't those same environmental pressures miniaturize the karchei chameleon? If not the island rule, what did lead to the nano-chameleon's smaller size? As is often the case in science, further evidence may one day answer these questions.
University of Tokyo scientists observe predicted quantum biochemical effects on cells.
We know at this point that there are species that can navigate using the Earth's magnetic field. Birds use this ability in their long-distance migrations, and the list of such species keeps getting longer, now including mole rats, turtles, lobsters, and even dogs. But exactly how they can do this remains unclear.
Scientists have for the first time observed changes in magnetism prompting a biomechanical reaction in cells. And if that's not cool enough, the cells involved in the research were human cells, lending support to theories that we ourselves may have what it takes to get around using the planet's magnetic field.
The research is published in PNAS.
The phenomenon observed by scientists from the University of Tokyo matched the predictions of a theory put forward in 1975 by Klaus Schulten of the Max Planck Institute. Schulten proposed the mechanism through which even a very weak magnetic field—such as our planet's—could influence chemical reactions in their cells, allowing birds to perceive magnetic lines and navigate as they seem to do.
Shulten's idea had to do with radical pairs. A radical is a molecule with an odd number of electrons. When two such electrons belonging to different molecules become entangled, they form a radical pair. Since there's no physical connection between the electrons, their short-lived relationship belongs in the realm of quantum mechanics.
Brief as their association is, it's long enough to affect their molecules' chemical reactions. The entangled electrons can either spin exactly in sync with each other, or exactly opposite each other. In the former case, chemical reactions are slow. In the latter case, they're faster.
Researchers Jonathan Woodward and Noboru Ikeya in their lab
Credit: © Xu Tao, CC BY-SA
Cryptochromes and flavins
Previous research has revealed that certain animal cells contain cryptochromes, proteins that are sensitive to magnetic fields. There is a subset of these called "flavins," molecules that glow, or autofluoresce, when exposed to blue light. The researchers worked with human HeLa cells (human cervical cancer cells), because they're rich in flavins. That makes them of special interest because it appears that geomagnetic navigation is light-sensitive.
When hit with blue light, flavins either glow or produce radical pairs — what happens is a balancing act in which the slower the spin of the pairs, the fewer molecules are unoccupied and available to fluoresce.
HeLa cells (left), showing fluorescence caused by blue light (center), closeup of fluorescence (right)
For the experiment, the HeLa cells were irradiated with blue light for about 40 seconds, causing them to fluoresce. The researchers' expectations were that this fluorescent light resulted in the generation of radical pairs.
Since magnetism can affect the spin of electrons, every four seconds the scientists swept a magnet over the cells. They observed that their fluorescence dimmed by about 3.5 percen each time they did this, as shown in the image at the beginning of this article.
Their interpretation is that the presence of the magnet caused the electrons in the radical pairs to align, slowing down chemical reactions in the cell so that there were fewer molecules available for producing fluorescence.
The short version: The magnet caused a quantum change in the radical pairs that suppressed the flavin's ability to fluoresce.
"The joyous thing about this research is to see that the relationship between the spins of two individual electrons can have a major effect on biology."
He notes, "We've not modified or added anything to these cells. We think we have extremely strong evidence that we've observed a purely quantum mechanical process affecting chemical activity at the cellular level."
One of the world's most isolated island groups has just been made one of the world's largest ocean reserves.
- The small island group of Tristan da Cunha has created one of the world's largest ocean sanctuaries.
- Neither fishing nor extractive activities will be allowed in the area, which is three times the size of the United Kingdom.
- Animals protected by this zone include penguins, sharks, and many seabirds.
Tristan da where now?
Tristan da Cunha is a British Overseas Territory consisting of an archipelago in the south Atlantic. The titular island is the largest in the group at about 100 square kilometers. Those hoping to visit will have to get there by a week-long boat ride from Cape Town. The island's government gleefully notes that it takes longer to get there than it takes astronauts to get to the Moon.
The marine protection zone will cover 627,247 square kilometers (over 242,000 miles) of the ocean around the islands. It will be the "gold standard" in ocean conservation, with neither fishing nor other extractive activities allowed, often referred to as "no-take." It will be the largest no-take zone in the Atlantic, and the fourth largest anywhere in the world.
The zone includes small areas just off the inhabited islands in which sustainable fishing will be allowed, but these areas are a small fraction of the no-take area's size. Given the historical reliance of the island's economy on the sea, this consideration is quite understandable.
These protected areas join many others covered by the United Kingdoms' Blue Belt Programme of marine protection, which aims to preserve 30 percent of the world's oceans by 2030.
In a press release issued by the government of Tristan da Cunha, the chief executive of the Royal Society for the Protection of Birds, Beccy Speight, left no doubt as to the environmental significance of this protection zone:
"This is a story two decades in the making, starting with the RSPB and Government of Tristan da Cunha commencing a conservation partnership, and culminating in the creation of this globally important protected area," Speight said. "The waters that surround this remote UK Overseas Territory are some of the richest in the world. Tens of millions of seabirds soar above the waves, penguins and seals cram onto the beaches, threatened sharks breed offshore and mysterious whales feed in the deep-water canyons. From today, we can say all of this is protected."
Speight added that in 2020, the need for these protections is greater than ever. "While Tristan da Cunha may be far away in distance it is still close to our hearts and protecting it is still the UK's responsibility. Closer to home, the crisis facing nature is also huge. So huge that our wellbeing, our economic future, and our very survival depend on the choices we make now about the natural world." Speight also used the statement to issue a call to action. "We need politicians to emulate the leadership of this small community to help us build the world we all want to live in. We hope today's fantastic announcement is the first of many more that help revive our world."
For the less romantic, there are also human-centered reasons why we ought to protect the oceans. A recent study suggests that keeping fishing boats out of a mere five percent of the ocean can raise catches everywhere else by a 20 percent. As it turns out, protecting the planet we live on provides benefits.
Most important of all, what animals are protected by this?
The now protected fish that inhabit the waters are a vital food source for many kinds of animals, all of which will benefit from not having to share their food supply with humans.
The vast area is home to many species of whales, sharks, and seals. Endangered species of albatross also drop by. Many birds that live on the islands and cannot be found elsewhere, such as the Wilkins bunting and the Inaccessible rail, also stand to benefit from the new protections.
Most adorable of all, the endangered northern rockhopper penguins make a home on one of the archipelago islands. With luck, they may not be endangered much longer.
Crows have their own version of the human cerebral cortex.
It's no surprise that corvids — the "crow family" of birds that also includes ravens, jays, magpies, and nutcrackers — are smart. They use tools, recognize faces, leave gifts for people they like, and there's even a video on Facebook showing a crow nudging a stubborn little hedgehog out of traffic. Corvids will also drop rocks into water to push floating food their way.
What is perhaps surprising is what the authors of a new study published last week in the journal Science have found: Crows are capable of thinking about their own thoughts as they work out problems. This is a level of self-awareness previously believed to signify the kind of higher intelligence that only humans and possibly a few other mammals possess. A crow knows what a crow knows, and if this brings the word sentience to your mind, you may be right.
It's long been assumed that higher intellectual functioning is strictly the product of a layered cerebral cortex. But bird brains are different. The authors of the study found crows' unlayered but neuron-dense pallium may play a similar role for the avians. Supporting this possibility, another study published last week in Science finds that the neuroanatomy of pigeons and barn owls may also support higher intelligence.
"It has been a good week for bird brains!" crow expert John Marzluff of the University of Washington tells Stat. (He was not involved in either study.)
Corvids are known to be as mentally capable as monkeys and great apes. However, bird neurons are so much smaller that their palliums actually contain more of them than would be found in an equivalent-sized primate cortex. This may constitute a clue regarding their expansive mental capabilities.
In any event, there appears to be a general correspondence between the number of neurons an animal has in its pallium and its intelligence, says Suzana Herculano-Houzel in her commentary on both new studies for Science. Humans, she says, sit "satisfyingly" atop this comparative chart, having even more neurons there than elephants, despite our much smaller body size. It's estimated that crow brains have about 1.5 billion neurons.
Fun with Ozzie and Glenn
Ozzie and Glenn not pictured
The kind of higher intelligence crows exhibited in the new research is similar to the way we solve problems. We catalog relevant knowledge and then explore different combinations of what we know to arrive at an action or solution.
The researchers, led by neurobiologist Andreas Nieder of the University of Tübingen in Germany, trained two carrion crows (Corvus corone), Ozzie and Glenn.
The crows were trained to watch for a flash — which didn't always appear — and then peck at a red or blue target to register whether or not a flash of light was seen. Ozzie and Glenn were also taught to understand a changing "rule key" that specified whether red or blue signified the presence of a flash with the other color signifying that no flash occurred.
In each round of a test, after a flash did or didn't appear, the crows were presented a rule key describing the current meaning of the red and blue targets, after which they pecked their response.
This sequence prevented the crows from simply rehearsing their response on auto-pilot, so to speak. In each test, they had to take the entire process from the top, seeing a flash or no flash, and then figuring out which target to peck.
As all this occurred, the researchers monitored their neuronal activity. When Ozzie or Glenn saw a flash, sensory neurons fired and then stopped as the bird worked out which target to peck. When there was no flash, no firing of the sensory neurons was observed before the crow paused to figure out the correct target.
Nieder's interpretation of this sequence is that Ozzie or Glenn had to see or not see a flash, deliberately note that there had or hadn't been a flash — exhibiting self-awareness of what had just been experienced — and then, in a few moments, connect that recollection to their knowledge of the current rule key before pecking the correct target.
During those few moments after the sensory neuron activity had died down, Nieder reported activity among a large population of neurons as the crows put the pieces together preparing to report what they'd seen. Among the busy areas in the crows' brains during this phase of the sequence was, not surprisingly, the pallium.
Overall, the study may eliminate the layered cerebral cortex as a requirement for higher intelligence. As we learn more about the intelligence of crows, we can at least say with some certainty that it would be wise to avoid angering one.