animal kingdom

The Search for the First Multicellular Lifeform

Scientists are interested in the unicellular ancestors of plants, fungi and animals because they want to know if each transition into multicellularity was driven by similar evolutionary forces.

The Cheshire Cat in Alice’s Adventures in Wonderland is famous for its disappearing act: parts of its body vanish one by one until nothing remains but its ethereal grin. Scientists attempting to retrace the evolution of animals confront something equally curious. One might assume that going ever further back in evolutionary time, recently-evolved animal traits would drop away until a sort of ‘minimal animal’ remained. However, a growing body of data suggests that this minimal animal may not be an animal at all. Instead, sophisticated cellular processes once thought to be exclusive to animals are found across several unicellular eukaryotes: grins without (multicellular) cats!

So how do scientists reconstruct the deep history of multicellularity? The record for the oldest multicellular organism to be studied directly belongs to plants grown from 30,000-year-old seeds preserved in permafrost (Yashina et al., 2012). However, multicellular life is much older than this. Fossils of multicellular red algae have been dated to 1.6 billion years ago (Bengtson et al., 2017), and fossils of multicellular fungi date from about a billion years ago (Loron et al., 2019).

The oldest confidently-dated animal fossils are about half a billion years old (Bobrovskiy et al., 2018). Multicellularity arose independently in plants, fungi and animals (Brunet and King, 2017). Scientists are interested in the unicellular ancestors of these groups because they want to know if each transition into multicellularity was driven by similar evolutionary forces. Unfortunately, fossils reveal little about the cell biology of these primordial organisms.

Charles Darwin was well aware of this challenge. To reconstruct the evolutionary history of an organism, he wrote in On the Origin of Species, “we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible and we are forced in each case to look to … the collateral descendants from the same original parent-form.” That is, one must hope the traits of surviving organisms reveal those of their extinct ancestors.

Researchers now know that Darwin’s idea of “living fossils” was too simplistic. No organism remains entirely identical to its ancestor: genetic mutations constantly accumulate, driven by conflict, competition, and random chance. Nevertheless, one could hope to reconstruct the ancestor using a patchwork of different ancestral traits preserved across different surviving descendants.

A central theme of the emerging field of evolutionary cell biology is to study organisms that provide as much information as possible about the past. One way to do this is to develop new model organisms based on their position in the tree of life. As the evolution of animals is retraced, an ancestral unicellular species at the very threshold of multicellularity will eventually be reached. It is possible this species has no surviving descendants other than the animals themselves. To find more collateral descendants, one must push further back in time. The better life’s existing diversity is sampled, the more likely that a species will be found similar to the ancestors scientists want to reconstruct.

The billion-year-old clade known as Holozoa consists of animals and closely related unicellular species, including choanoflagellates, filastereans, and ichthyosporeans. Just a decade ago this was a sparsely sampled region of the eukaryotic tree. For example, the first choanoflagellate genome was only published in 2008 (Brunet and King, 2017). Today dozens of holozoan species have been cultured, sequenced, and studied, and they are a fertile hunting ground for interesting cell biology.

Codosiga sp. isolated from Siberian subsoil. This is a species of flagellate eukaryote of the choanoflagellate class and Codosiga genus. Photo: Daniel Stoupin/Wikimedia Commons, CC BY-SA 3.0

Importantly, the non-animal holozoans include species that can become transiently multicellular, at certain times or under certain conditions. Specifically, some choanoflagellates and ichthyosporeans have clonal multicellular life stages, while some filastereans form multicellular aggregates. But are these behaviours homologous to multicellularity in animals, and therefore representative of the ancestral state? Or are they examples of convergent evolution, driven by adaptations to similar environments?

One way to answer these questions is to resolve the molecular mechanisms that enable multicellular behaviour across holozoans. Suggestively, holozoan genomes encode transcription factors and cell adhesion genes known to be essential for animal multicellularity, but the roles of these genes had not been directly demonstrated (Grau-Bové et al., 2017; Richter et al., 2018). Now, two independent teams have reported the results of studies on certain animal-like behaviours in unicellular lineages that shed light on the evolution of animal multicellularity.

Non-animal species in the clade Holozoa exhibit coordinated contractions dependent on actomyosin complexes similar to those observed in modern animals. Caption and image: eLife 2019;8:e52805

In a paper in eLife, Iñaki Ruiz-Trillo and co-workers from Barcelona, Liverpool, Oslo, Shizuoka and Hiroshima – including Omaya Dudin and Andrej Ondracka as joint first authors – report how reproduction in an ichthyosporean called Sphaeroforma arctica involves a stage of growth that is reminiscent of the embryonic development of fruit flies. The nucleus of an initial single cell divides repeatedly to form a polarised epithelial layer, which then gives rise to multiple cells as its membrane undergoes coordinated invaginations (Dudin et al., 2019).

In a second paper in Science, Nicole King and co-workers from Berkeley and Amsterdam – including Thibaut Brunet, Ben Larson and Tess Linden as joint first authors – report the results of a study on a newly isolated choanoflagellate which they name Choanoeca flexa (Brunet et al., 2019). In bright light, this organism exists as a cup-shaped colony of cells, with their flagella pointing inwards. In the dark, however, the cup flips inside-out via a collective cellular contraction. This collective contraction is reminiscent of the contractions that generate curvature in developing animal tissues.

Both studies use imaging and pharmacological inhibition to demonstrate that these multicellular processes depend on the same molecular machinery: complexes of actin and myosin that can generate mechanical forces within cells. These results suggest that the last common ancestor of holozoans was an organism that was capable of transient multicellularity, with cells that could contract collectively. Among its descendants, only the animals evolved a permanently multicellular lifestyle, using the power of collective contraction to sculpt tissues and generate the “endless forms most beautiful” that so inspired Darwin.

This article was originally published by the journal eLife and has been republished here under a Creative Commons Attribution license.

Mukund Thattai is a member of the faculty of the National Centre for Biological Sciences, a part of the Tata Institute of Fundamental Research. His research focuses on the evolution of complex cells.

How Argentine Ants Built the Largest Known Cooperative Structure in Kingdom Animalia

This individually unremarkable insect has established populations in at least 15 countries on six continents.

The earliest record of the Argentine ant (Linepithema humile) dates to 1847, from the Atlantic island of Madeira, then a hub for Portugal’s trade with its colonies. This is strange for a species that’s native to the Panara river drainage system in South America, and is found in parts of Argentina, Brazil and Uruguay. Perhaps the Madeira record should have been the first call for attention to be paid to an invader caught in the throes of an endless, insatiable conquest.

The Argentine ant began to hitchhike on the backs of Spanish and Portuguese explorers in the 16th and 17th centuries, eventually stowing away on ships and other transportation systems in the mid-1800s and winding up in Europe and North America.

This individually unremarkable insect has since established populations in at least 15 countries on six continents. It is resourceful, adaptable and resilient to various shocks and stresses, and frequently displaces or wipes out native ant species, precipitating ecological disruption wherever it has gone.

The most interesting thing about the Argentine ant is how it has changed in the two centuries since the first populations arrived in Europe. Today, the foreign members of the species – introduced from outside a country to inside – differ from their native cousins in the biochemical particularities that govern how ant societies are formed.

Also read: There Is Nary a Pest as Hated as Mesquite in the Desert Flatland of Kutch

Ants are social organisms that form anonymous societies: unity doesn’t require individual recognition. Instead, ant species are characterised by a communal identity. Ants with similar genetic ancestry share chemical profiles and other cues, some of which also help identify what role each ant pays in a colony.

Most ant species are monogynous: a system of organisation in which one egg-laying queen builds a colony. A typical colony consists of the queen, her sterile female workers who forage and look after the larvae, and the drones: male ants whose sole purpose is to mate with the queen and die shortly afterwards.

During certain windows, the queen gives birth to reproductives: winged males and females that depart the nest in a swam to mate in other nests or to form entirely new colonies. This results in a feature called multi-coloniality, and each colony has a specific chemical profile shaped by the presence of cuticular hydrocarbons, usually defined by the queen.

Once an ant encounters a foreign chemical profile, it becomes aggressive. This helps maintain the colony’s integrity but results in both interspecific (between ant species) and intraspecific (within an ant species) aggression. In this organisational system, territorial boundaries are strictly demarcated but are limited in size.

However, the Argentine ant is polygynous: existing in colonies with multiple egg-laying queens. The mating season begins with a strange ritual in which worker ants kill a large fraction of existing queens.

After this, the reproductives begin mating, but instead of swarming out, female alates mate within the nests. As a result, nests can grow quickly in size thanks to the presence of so many queens giving birth. After a certain threshold, a queen may choose to embark on foot from a nest with a host of selected workers to establish a new colony, in a process called budding. The new nest retains the original nest’s chemical profile, resulting in the formation of a polydomous society, with multiple nests linked to one another across a geographical area. Queens and workers can move freely between them.

Unlike multi-colonial societies, a polydomous society exhibits a high degree of cooperation between different nests and a decrease in intraspecific aggression. Such behaviour is called unicolonial behaviour.

Unicoloniality lends itself to the formation of large networks of nests and colonies. However, colony size in the Argentine ant’s native range is limited by the emergence of genetically different colonies followed by intraspecific aggression. Even when unicoloniality supports the growth of super-colonies, these colonies remain much smaller than those formed by Argentine ants in non-native areas. Scientists have argued that this could be because of the presence of other, highly aggressive ant species, many of which are also successful invasive species in other parts of the world.

Argentine ants in foreign lands, on the other hand, have been known to form mind-bogglingly big colonies in the US, Europe, Australia, Northern Africa and Japan.

According to scientists, there are two reasons for this. Unlike in their native range, the Argentine ant doesn’t have to compete with other aggressive ant species for resources and can smoothly expand their colonies. Second, each super-colony across the world has its own founding population: a group of genetically similar ants that arrived from South America and founded a colony. Scientists believe they passed this genetic similarity on to their offspring, fostering the so-called founder effect.

For a long time, experts thought a super-colony in southern Europe, nearly 6,000 km long along the Mediterranean coast, was the world’s largest. In 2009, researchers from the University of Tokyo announced the discovery of a new record-holder. They had been able to link the genetic profiles of super-colonies in California and on the Japanese west coast together with the Mediterranean colony. When they brought individuals from these colonies close to each other, they cooperated instead of fighting, implying that three colonies in three continents were in fact one mega-colony spanning the globe – the largest known cooperative structure in the animal kingdom.

In effect, the Argentine ant isn’t an invasive species but an invasive community, comprising many billions of cooperative ants.

Also read: India Knows Its Invasive Species Problem But This Is Why Nobody Can Deal With it Properly

This is an evolutionary contradiction. Once a colony becomes so big that different parts of it are effectively in different environments, the ants living there typically evolve differently and show signs of aggression when brought together. How then do ants from opposite ends of the planet still work together? We don’t yet know.

New research has presented some clues. Scientists now believe unicoloniality has an inherent attribute triggered when Argentine ants are introduced to a non-native range with few threats. This attribute erases certain alleles that regulate recognition in ants, rendering the ants less discriminatory than they might be at home, and more willing to cooperate with any ant from the same species that is genetically similar.

This said, the Argentine ant has been facing stiffer competition of late. Territorial fights against the red imported fire ant (Solenopsis invicta), the tawny crazy ant (Nylanderia fulva) and the Asian needle ant (Pachycondyla chinensis) have weakened its holds in the US. The African big-headed ant (Pheidole megacephala) has prevented it from gaining ground in South Africa. In San Diego, a super-colony split in two, precipitating a battlefront many miles long in which 15 million ants died in six months of 2004.

Together with human efforts to hold such invasive species at bay and preserve endangered ecosystems, it seems the Sun may finally have started to set on the Argentine ant empire.

Kaustav Sood is a recent graduate from the history department at Ashoka University, Sonepat, and is currently interning with the National Skill Development Corporation. This article has been adapted from a research paper the author wrote while at Ashoka University.

Review: A Book on Robotics That’s Really About How Evolution Does It Better

David Hu’s is a world of constant wonder that never becomes overwhelming. Once you’re done, you’ll likely be scouring the internet for videos of ant rafts or swarm robots, and browsing through Hu’s studies on his website.

The title of David Hu’s new book, ‘How to Walk on Water and Climb Up Walls’, doesn’t immediately inspire interest. Journeying through 250 pages reading the author preach the gospel of a modern robotics revolution is a tedious proposition. Atlas, Boston Dynamics’ flagship bot, performing backflips like Simone Biles is enough proof that the revolution is in the offing, one way or another. We don’t need warnings of the obvious.

However, if you had somehow navigated to Hu’s research website before starting the book, you might actually pick it up for a read.

Hu specialises in fluid mechanics, drawing inspiration from a more organic source than his peers: the animal kingdom. He has made a name for himself uncovering the physical principles underlying the movements and functions of many animals. In 2018, for example, he co-presented a talk at a conference about how wombats excrete cube-shaped poop.

Combining theories rooted in classical physics with ingenious experimental setups, Hu’s team at the Georgia Institute of Technology, Atlanta, has studied worms, ants and snakes. However, the wackiness at play belies the rigour of science involved, even overshadowing the importance of studying the animal world from a mechanistic viewpoint.

Conscientiousness in Animals, Explaining Tidy Birds and Neat Bees

Our understanding of animal personality was stalled for years by both the fear of anthropomorphism among animal scientists, and a lack of consensus on how to describe it.

House-proud: An Anna’s Hummingbird on its nest. Credit: Mick Thompson/Flickr

Human personality theory has long revolved around what we know as the ‘Big Five’ – five dimensions of personality that cover a large swathe of how humans behave across time and contexts. These dimensions are conscientiousness (tendencies to be orderly and rule-abiding), agreeableness (easy to get along with), extraversion (outgoing), neuroticism (tendencies to be anxious, depressed or hostile), and openness to new experiences (creative and artistic inclinations). It’s the consistency in our behaviour in different situations that often teases apart why we aren’t all alike.

Just like physical traits, personality traits meet Charles Darwin’s criteria for evolution. First of all, personality traits show variability since the very concept of personality implies that we are all different in specific ways. Second, personality traits are not just influenced by the environment, they are all highly heritable. And finally, in many cases, certain traits make some individuals more likely to reproduce and pass on their genes than others, demonstrating clear fitness benefits.

Because human personality evolved, we should expect to find traces of it in other species. But our understanding of animal personality was stalled for years by both the fear of anthropomorphism among animal scientists, and a lack of consensus on how to describe it. Animal personality is sometimes referred to as ‘temperament’, ‘coping styles’, or ‘behavioural syndromes’ (which always struck me as sounding like more of an illness than a way of being). Often, animals are described simply in terms of their levels of boldness and aggressiveness. More recently, however, scientists have started using the Big Five as a framework for the examination of animal personality.

Conscientiousness as a personality dimension has multiple facets, and typically describes people who plan ahead, who are organised and reliable, hard-working, self-disciplined, and thorough. It can be difficult to see in other animals: right now, I’m looking at my cats lounging in a sunspot on the couch. They’ve been there all day. They might be reliable, but I’d hardly call them industrious. Perhaps the previous research findings are correct: conscientiousness involves traits that are too complex to attribute to animals, or just can’t be found.

But are we just being constrained by our human biases? Are the questions we are asking and the methods we are using really applicable to other species?

Most measures of human personality depend on self-report, where humans rate how well statements describe them, such as: ‘I seldom feel blue’, ‘I have a vivid imagination’, or ‘I often forget to put things back in their proper place’. From these responses, you get a separate score on each of the five personality dimensions.

And most measures of animal personality are also based on reports, not by the animals, of course, but by their owners or caretakers. By relying on human assessment, are we just trying to determine how animals are like humans, rather than what might define ‘personality’ in their own species-specific world?

Wondering if the lack of evidence for conscientiousness in the animal kingdom was due to such potential biases, my colleague, the psychologist Frank Sulloway at the University of California, Berkeley, and I tried a different approach, the results of which were recently published in the journal Psychological Bulletin. We first looked at all the descriptive terms that are commonly used to measure conscientiousness in surveys given to humans. Using this list of 103 terms, we thoroughly searched the literature to see when these terms were used to describe animal behaviour.

There are hundreds of examples of conscientiousness behaviours in the animal kingdom, from tidy fish building nests that attract potential mates, to mice who varied in impulse control or their ability to delay rewards. Some insects were methodical in their selection of a location to lay eggs, while others were not; some guide dogs were easily distracted, while others remained focused. Some guppies were ‘reckless’ in their selection of a mate, and cows showed an increase in heart level when they were successful at learning a new task.

Conscientiousness in animals doesn’t necessarily look like it does in humans. Bees might not make beds, but they vary in their cleanliness and orderliness. Bees who are better undertakers, removing dead bodies from their hives, experience less disease, and produce more and healthier offspring. Spiders who construct tidier webs catch more prey, and lazy birds are tolerated because, in times of desperation, they can provide backup babysitting for their relatives.

When subjected to statistical analyses, our data pointed to two types of conscientiousness in animals – one type related to orderly, industrious behaviour that was more likely to be reported in birds, and, to an extent, insects. The other type described more competent, achievement-striving behaviour, more commonly reported in primates and other mammals. Some species showed evidence for both types of conscientiousness, demonstrating that, just as in humans, conscientiousness in animals could be multifaceted.

However, we didn’t find strong evidence in animals for some aspects of human conscientiousness, such as virtue, traditionalism and self-discipline. These traits often involved a sense of moral obligation, which might just be too difficult to measure in animals, since they cannot communicate their motivations to us.

Although we found ample evidence for conscientiousness in other animals, our research was still limited by the fact that our starting point was lexically based, and still dependent on an anthropocentric launching pad. Perhaps we found more evidence for orderly behaviour in birds and bees because that is what scientists expect to find, whereas they might search for more cognitively complex behaviours in animals that we consider more similar to us.

What would be an even better approach is an overhaul of how we look at personality in all animals (including humans). We don’t have to throw the baby out with the bathwater, but the self-report questionnaire is limited in its ability to measure the true range of behaviour across the other 8.7 million animal species with whom we share the planet.

Instead, the psychologist Jana Uher at the University of Greenwich in London has proposed exploring animal personality using a behavioural approach. This means first determining what problems an individual species has evolved to solve, in regards to contexts such as predation, foraging and social interactions. Once those problems have been defined, behaviours, traits and their variability can be assessed in accordance with these problems.

It should not surprise us to find a continuum of all personality traits across the animal kingdom. But what should have surprised us much earlier was the denial of conscientiousness in non-human animals. As much as we like to think we are special and unique, to quote Darwin, the difference between us and other animals, ‘great as it is, certainly is one of degree and not of kind’.

This article was originally published at Aeon and has been republished under Creative Commons.

Whales and Dolphins Could Hold Clues to What Makes Humans So Advanced

Complex behaviour such as regional accents and cultural food preferences in whales and dolphins seems to be linked to brain size.

Michel Humpback Whales. Credit: Flickr, CC BY-NC-ND

Humans are like no other species. We have constructed stratified states, colonised nearly every habitat on Earth and we’re now looking to move to other planets. In fact, we are so advanced that some of our innovations – such as fossil fuel technologies, intensive agriculture and weapons of mass destruction – may ultimately lead to our downfall.

Even our closest relatives, the primates, lack traits such as developed language, cumulative culture, music, symbolism and religion. Yet scientists still haven’t come to a consensus on why, when and how humans evolved these traits. But, luckily, there are non-human animals that have evolved societies and culture to some extent. Our latest study, published in Nature Evolution & Ecology, investigates what cetaceans (whales and dolphins) can teach us about human evolution.

The reason it is so difficult to trace the origins of human traits is that social behaviour does not fossilise. It is therefore very hard to understand when and why cultural behaviour first arose in the human lineage. Material culture such as art, burial items, technologically sophisticated weapons and pottery is very rare in the archaeological record.

Previous research in primates has shown that a large primate brain is associated with larger social groups, cultural and behavioural richness, and learning ability. A larger brain is also tied to energy-rich diets, long life spans, extended juvenile periods and large bodies. But researchers trying to uncover whether each of these different traits are causes or consequences of large brains find themselves at odds with each other – often arguing at cross purposes.

One prevailing explanation is the social brain hypothesis, which argues that our minds and consequently our brains have evolved to solve the problems associated with living in an information rich, challenging and dynamic social environment. This comes with challenges such as competing for and allocating food and resources, coordinating behaviour, resolving conflicts and using information and innovations generated by others in the group.

Primates with large brains tend to be highly social animals. Credit: Wikipedia

However, despite the abundance of evidence for a link between brain size and social skills, the arguments rumble on about the role of social living in cognitive evolution. Alternative theories suggest that primate brains have evolved in response to the complexity of forest environments – either in terms of searching for fruit or visually navigating a three dimensional world.

Under the sea

But it’s not just primates that live in rich social worlds. Insects, birds, elephants, horses and cetaceans do, too.

The latter are especially interesting as, not only do we know that they do interesting things, some live in multi-generational societies and they also have the largest brains in the animal kingdom. In addition, they do not eat fruit, nor do they live in forests. For that reason, we decided to evaluate the evidence for the social or cultural brain in cetaceans.

Another advantage with cetaceans is that research groups around the world have spent decades documenting and uncovering their social worlds. These include signature whistles, which appear to identify individual animals, cooperative hunting, complex songs and vocalisations, social play and social learning. We compiled all this information into a database and evaluated whether a species’ cultural richness is associated with its brain size and the kind of society they live in.

We found that species with larger brains live in more structured societies and have more cultural and learned behaviours. The group of species with the largest relative brain size are the large, whale-like dolphins. These include the false killer whale and pilot whale.

To illustrate the two ends of the spectrum, killer whales have cultural food preferences – where some populations prefer fish and other seals. They also hunt cooperatively and have matriarchs leading the group. Sperm whales have actual dialects, which means that different populations have distinct vocalisations. In contrast, some of the large baleen whales, which have smaller brains, eat krill rather than fish or other mammals, live fairly solitary lives and only come together for breeding seasons and at rich food sources.

The lives of beaked whales are still a big mystery. Credit: Ted Cheeseman/wikipedia, CC BY-SA

We still have much to learn about these amazing creatures. Some of the species were not included in our analysis because we know so little about them. For example, there is a whole group of beaked whales with very large brains. However, because they dive and forage in deep water, sightings are rare and we know almost nothing about their behaviour and social relationships.

Nevertheless, this study certainly supports the idea that the richness of a species’ social world is predicted by their brain size. The fact that we’ve found it in an independent group so different from primates makes it all the more important.

Susanne Shultz is a University Research Fellow in University of Manchester.

This article was originally published on The Conversation. Read the original article.

How, or Why, Cobras Evolved Tissue-Destroying Venom

En route to answering this question, a new study shows that some components of snake venom can be used for defensive purposes also.

Black-necked spitting cobras not only have bright body banding but also red warning colours. Credit: Randy Ciuros

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Cobras are an extraordinary family of snakes. Even though they have enough venom to knock down a herd of oxen, they’ve developed defensive displays in a league of their own. They unfurl the elongate ribs of their necks to form hoods. Some like spectacled cobras startle predators with eye-like markings. Others can shoot venom with varying degrees of accuracy from considerable distance at their assailants’ eyes.

For a long time, scientists thought spitting cobras’ venom have more toxins that induce pain and destroy tissue than the venom of non-spitting cobras. These are the cytotoxins or cardiotoxins, a class of proteins. When the venom of spitters hits their antagonists’ eyes, it has an immediate reaction. Therefore, these cobras need to have more cytotoxic venom, the thinking went.

Rinkhals, closely related to cobras, advertise their cytotoxic venom with bold body-banding. Credit: Giuseppe Mazza

Other cobras possess cytotoxins to digest their preys’ innards even as their predators begin the cumbersome process of swallowing them whole. A similar process causes human victims of cobras to suffer skin and tissue damage, often permanently debilitating them. Despite the damage they cause, these toxins are not as potent as neurotoxins that attack the nerves.

Why do snakes that already have deadly neurotoxic venom evolve less lethal cytotoxins? A team of 26 scientists from five countries sought the answers in a new study.

“Evolution has one innovation come on to the scene at a time,” Bryan Grieg Fry, the lead scientist of the study, told The Wire. “Spitting as an innovation would not evolve in the absence of something worth delivering.”

By tracing the ancestry of cobras and using statistical models, the researchers discovered that cytotoxins evolved after cobras developed hoods. Despite the name, king cobras are not close relatives of cobras. They also independently evolved to spread their hoods and sit upright, facing their antagonists. Puffing up to look large is a common defence mechanism in the animal world. Soon afterward, the venom of cobras and king cobras developed cytotoxins, say the researchers.

In cobras, the tissue destroyers are tiny peptides called 3-finger toxins. King cobras developed L-amino acid oxidase to perform the same job.

By opting for different lifestyles, a few African species like the water (Naja annulata), tree (Naja goldii), and burrowing cobras (Naja multifasciata) opted out of this defence strategy and lost their ability to hood. Consequently, the cytotoxicity of their venom dropped.

The Malayan population of king cobras, with a bright orange throat, has the highest cytotoxicity of king cobras tested in the study. Credit: Kevin Messenger

What if the hooding bluff fails to deter? The most recent ancestor of cobras and rinkhals was probably a drab snake that could hood. Like the Egyptian cobra (Naja haje), its hood may not have had any markings and its venom was possibly moderately cytotoxic. When confronted, the ancestor perhaps behaved like its modern day lookalike – fleeing after striking nervously.

Three clans of closely related snakes – African cobras, Asian cobras, and rinkhals – developed a new weapon as plan B. They fine-tuned their venom delivery kit so they could shoot a fine jet of venom from a distance at the eyes of their assailants. The opening in the fangs of these snakes became narrow and migrated from the tip to squarely face the front, the better to target the eyes. Their venom became less viscous for greater reach. In addition to using their venom to inject and subdue prey, spitting cobras also used it in defence.

“Defensive venoms are characterised not by lethality but by pain,” says Fry, University of Queensland, Australia. “Predatory venoms however are selected for their potency. So these are mutually exclusive strategies. Thus predatory venoms are not selected as defensive plan Bs.”

Supplementing their already potent arsenal with non-fatal peptides allowed these snakes to stand their ground. After all, no animal wants to call the snakes’ bluff and suffer the searing pain caused by tissue-destroying venom in their eyes.

While many African cobras aim accurately, Asian cobras, likely being more recent innovators of this technique, are not as proficient. Some like the Chinese cobras (Naja atra) and monocled cobras (Naja kaouthia) spit only on rare occasions. Unlike Sumatran spitting cobras (Naja sumatrana) for instance, they didn’t develop special adaptations for spitting.

Many others didn’t go the spitting route or lost it later. Instead they developed startling eye-like markings on the back of their broad and round hoods like our spectacled and monocled cobras (Naja naja and Naja kaouthia).

The researchers tested the potency of the cytotoxins against healthy and cancerous cell lines. “We wanted to focus on the toxins that were indiscriminate killers,” says Fry. “So we looked for congruence between the two cell types as the guide for the truly potent cytotoxic activity.”

Red-spitting cobras dispense with markings in favour of a scarlet coloration. Credit: Wikimedia Commons

Indochinese cobras (Naja siamensis) and snouted cobras (Naja annulifera) developed startling black and white bands that serve the same purpose as the warning hood marks. Similarly, the more scarlet the snake, like red spitting cobras (Naja pallida), the more startling to predators. All these species that possess warning markings, bands, and colours have high cytotoxicity. Among king cobras, the Malaysian population that has a bright orange throat has more tissue-destroying toxins than other populations.

Across the board, whether they spat or not, Asian cobra venoms are highly cytotoxic. In contrast, drab, patternless African cobras that didn’t spit have lower cytotoxicity than ones with warning markings, bands, or colours.

All of this raises the question: Why don’t king cobras spit venom? After all, they already possess tissue-destroying factors in their venom. Unlike cobras, the cytotoxic elements of king cobra venom are large and globular that cannot easily attack the exposed surface of the eyes. So they never developed this weapon.

This study, perhaps the first, shows that some components of venom can be used for defensive purposes.

“I think it’s an excellent example of bringing multiple methods together to reveal a fascinating evolutionary story,” Rick Shine, a professor at the University of Sydney, Australia, told The Wire. “One of the aspects that fascinates me is the notion of ‘honesty in advertising.’ You might think that warning colours would help any snake to discourage predators, so those colours might evolve in fairly harmless species as well as deadly ones. But in fact this paper shows that a warning colour is generally a reliable indication that the snake does, indeed, possess a venom potent enough to cause major problems for any predator silly enough to attack it.”

However, cobras, rinkhals and king cobras aren’t the only venomous snakes to have cytotoxins. Many viper species that don’t spread hoods or spit venom possess them, too.

Fry says his team is conducting “a followup line of research with other snakes with significant warning displays, e.g. rattlesnakes, which have non-rattling ancestors.”

While cytotoxins cause a lot of human misery, they can also help us. They attack cancer cells with as much virulence as they do normal tissue. In the future, they may become the source of frontline drugs to treat cancer. “Anything that kills cells is a good thing in the search for new cancer medicines,” says Fry. “So our next step is to purify individual components out of the venoms and see if by fluke one happens to be more specific for cancer cells than healthy cells.”

The study was published is the journal Toxins on March 15, 2017.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.