Colour-changing chameleons evolved to stand out, not blend in

Chameleons aren’t exactly known for being showy. Indeed, they are so synonymous with blending in that we use the term ‘social chameleon’ to refer to people who are at home in any social setting. But new research suggests that this reputation needs a rethink. The chameleon’s ability to change colour evolved not to blend in, but to stand out.

Chameleons are a group of small lizards that are almost synonymous with camouflage. Common folklore has it that their vaunted ability to change their skin colour allows them to go undetected in a variety of environments.

Certainly, their default colours match their surroundings well. But Devi Stuart-Fox and Adnan Moussalli from South Africa have found that the changing hues they are best known for evolved for communication not disguise. They allow chameleons to make themselves incredibly but temporarily noticeable to mates and rivals, while remaining inconspicuous for the rest of the time.

Sex runs hot and cold – why does temperature control the gender of Jacky dragons?

Among Jacky dragons, females are both hot and cool, while males are merely luke-warm. For this small Australian lizard, sex is a question of temperature. If its eggs are incubated at low temperatures (23-26ºC) or high ones (30-33ºC), they all hatch as females; anywhere in the idle, and both sexes are born.

This strategy – known as ‘temperature-dependent sex determination (TSD) – seems unusual to us, with our neat gender-assigning X and Y chromosomes, but it’s a fairly common one for reptiles. Crocodiles are all-male at high temperatures and all-female at low ones, while turtles flip the rules around and produce more males in cooler climes. Now, a thirty-year old idea to explain this puzzling system has finally been confirmed.

Assigning gender based on temperature is not uncommon but it is nonetheless puzzling. Gender seems like an incredibly fundamental physical trait to leave to something as variable as the temperature of your surroundings. How has such a system evolved? What possible benefits could a species receive by switching control of from chromosomes to the environment?

Drought drives toads to mate with other species

When it comes to sex, it makes sense to stick to your own species. Even putting aside our own innate revulsion, inter-species liaisons are a bad idea because they mostly fail to produce any young. In the few instances they do, the hybrid progeny aren’t exactly racing ahead in the survival stakes and are often sterile (think mules).

But having poor unfit young is still better than having no young at all and if an animal’s options are limited, siring a generation of hybrids may be a last resort. Karin Pfennig from the University of North Carolina found that the plains spadefoot toad uses just this strategy in times of need.

Female toads breed just once a year, so it pays for them to make the right choice. According to Pfennig’s work, they take their health and their environment into account when choosing mates. If their bodies are weak and their surroundings are precarious, the benefits that another species’ genes can provide to their young are enough to outweigh the risks.

Ground squirrels use infrared signals to fool heat-seeking rattlesnakes

Rattlesnakes can track their prey using the infrared light given off by the warm bodies. But ground squirrels can use this super sense against the snakes. By pumping blood into their tails, they give off infrared signals that fool the snakes’ heat-seekers.

It seems like an uneven match. In one corner, the unassuming California ground squirrel (Spermophilus beechyi), 30cm in length. In the other, the northern Pacific rattlesnake (Crotalus oreganos), more than twice the length of the squirrel, and armed with hinged fangs that pack a lethal venom. But thanks to a cunning adaptation, the squirrel often gets an unexpected upper hand in this bout.

Squirrels vs snakes

Ground squirrels live in a series of burrows that keep them out of reach of most predators. Snakes, however, have exactly the right body plan for infiltrating long sinuous tunnels, and it’s not surprising that they are the squirrels’ major predators. It’s equally unsurprising that the squirrels have developed ways of defending themselves against snakes.

When the heat is on, male dragons become females

It seems almost fashionable now to blame everything on climate change, but the most unusual claim yet is that it could lead to sex-changing lizards.

For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.

In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual’s sex is determined by the temperature that their eggs were incubated at.

Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.

But Alexander Quinn and colleagues form the University of Canberra have found that an Australian lizard, the central bearded dragon (Pogona vitticeps) flouts this rule. It has become the first animal known to use two separate methods to determine the sex of individuals.

The dragon uses the Z/W system, where the males carry two Z chromosomes and the females have a Z and a W. But Quinn found that these genes are only the dominant influence on gender if eggs are incubated between 20 and 32 degrees Celsius.

At higher temperatures, males ignore their genetic heritage and become females instead. When Quinn incubated broods of eggs between 34 and 37 degrees Celsius, the hatchlings were almost invariably female. And as predicted, about half of these sisters were genetically male. For dragons at least, when the heat is on, the men turn into women

Quinn believes that the key to the manliness of boy dragons lies in a temperature-sensitive protein produced by the Z chromosome. The protein’s activity needs to surpass a certain threshold before a dragon can become male. For that, there need to be two copies of Z, and the temperature must be just right.

Reptiles that use temperature to assign gender must have fine-tuned their systems over time to cope with an ever-changing environment. But Quinn fears that the current pace of climate change may be too rapid for these animals to adapt to.

If temperatures rise far enough to bias an entire species over to a single gender, extinction would be all but inevitable. These warnings have been sounded before, and Quinn’s work suggests that they should be shouted a little bit louder.

More about animal sex and reproduction: 
Virgin birth by Komodo dragons
Butterflies evolve resistance to male-killing bacteria in record time 
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

More on the effects of climate change: 
Icebergs are hotspots for life
Climate change responsible for decline of Costa Rican amphibians and reptiles
Hope for corals – swapping algae improves tolerance to global warming
Corals survive acid oceans by switching to soft-bodied mode

 

 

Reference: Quinn, Georges, Sarre, Guarino, Ezaz & Graves. 2007. Temperature sex reversal implies sex gene dosage in a reptile. Science 316: 411.

Technorati Tags: bearded dragon, sex determination, temperature sex, temperature lizards, science

Climate change responsible for decline of Costa Rican amphibians and reptiles

Amphibians around the world are facing extinction from habitat loss and a killer fungus. Now, climate change joins their list of enemies. In Costa Rica, warmer and wetter days have led to a loss of rainforest leaf litter that has sent amphibian and reptile populations crashing.

Miners used to take canaries into unfamiliar shafts to act as early warning systems for the presence of poisons. Today, climate scientists have their own canaries – amphibians.

Amphibians – the frogs, toads and salamanders – are particularly susceptible to environmental changes because of their fondness for water, and their porous absorbing skins. They are usually the first to feel the impact of environmental changes.

And feel it they have. They are one of the most threatened groups of animals and one in three species currently faces extinction. The beautiful golden toad (right) was one of the first casualties and disappeared for good in 1989. Even though they are less glamorous than tigers, pandas or polar bears, amphibians are a top priority for conservationists.

The usual factors – introduced predators and vanishing habitats – are partially to blame, but many populations have plummeted in parts of the world untouched by pesky humans.

More recently, a large number of these deaths have been pinned on a fatal fungal disease called chytridiomycosis. Hapless individuals become infected when they swim in water used by diseased peers, and fungal spores attach to their skins. The disease had decimated amphibians across the Americans.

The extent of the damage may be even worse than we think. We have very little long-term data on the population sizes of many amphibian species, particularly in the tropics, where the greatest diversity exists. One of the few sites to buck the trend of ignorance is La Selva Biological Station in Costa Rica, which has been monitoring amphibian populations since the 1950s.

Steven Whitfield and colleagues from Florida International University used the La Selva data to analyse the populations of a species living among the leaf litter that covers the local rainforest floor. The team ran their census of about 30 species of amphibians, as well as many reptiles (lizards and snakes).

To their astonishment, the populations of these species had plummeted by 75% in 35 years. This massive decline is worrying for many reasons, the least of which is that La Selva sits within a protected area. Habitat destruction is non-existent here, so something else must be happening.

Nor is chytridiomycosis to blame. The fungus doesn’t tolerate high temperatures and only grows in temperate regions or mountainous ones. La Selva is neither. The killer fungus marks its presence with rapid falls in amphibian numbers within months, but these declines took place over decades.

And most tellingly of all, the reptiles suffered population losses as great as those of the amphibians. With their dry, scaly skins, reptiles lack the amphibian vulnerability to chemicals and chytridiomycosis. Something else is afoot.

Whitfield believes that climate change is the answer. Over the past 35 years, La Selva has experienced wetter and warmer days. Temperatures have gone up by one degree Celsius, which slows the growth of local trees, and reduces the volume of leaves that they shed. The number of dry days has halved, and with more rainfall, the leaves that do fall decay faster.

So these combined climate changes have conspired to reduce the levels of leaf litter in the forest, robbing amphibians and reptiles alike of their homes. Even in this protected area, habitat destruction is going on right under our feet.

The climate change idea explains another odd finding. Whitfield saw that amphibian and reptile numbers had not declined in nearby abandoned cacao plantations. That’s because cacao trees shed their leaves throughout the year and provide a continual supply of new leaf litter.

The picture for the world’s amphibians seemed bleak enough, but it seems that we have been ignoring a larger simmering danger in the face of the immediate threat of chytridiomycosis. It is telling that all but one of the disappearing species in this study are listed as ‘least concern’ by the World Conservation Union (IUCN). Whitfield’s study should be a call to action for conservationists.

Reference: Whitfield, Bell, Phillippi, Sasa, Bolanos, Chaves, Savage & Donnelly. 2007. Amphibian and reptile declines over 35 years at La Selva, Costa Rica. PNAS doi.0611256104.

Technorati Tags: reptiles, amphibians, amphibian extinction, Costa Rica, amphibian conservation, amphibian climate change, chytridiomycosis, science

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Salamander robot walks, swims and sheds light on evolutionary step from sea to land

A team of scientists have developed a robot salamander that walks and swims using an electronic spinal cord. The robot provides us with clues about how our ancestral animals made the evolutionary leap from the sea to the land.

Moving robots are becoming more and more advanced, from Honda’s astronaut-like Asimo to the dancing Robo Sapien, a perennial favourite of Christmas stockings.

But these advances are still fairly superficial. Most robots still move using pre-defined programmes and making a single robot switch between very different movements, such as walking or swimming, is very difficult. Each movement type would require significant programming effort.

But robotics engineers are now looking to nature for inspiration. Animals of course, are capable of a multitude of different styles of movement. They have been smoothly switching from swimming to walking for hundreds of millions of years, when our distant ancestors first invaded the land from the sea.

This ancient pioneer probably looked a fair bit like the salamanders of today’s rivers and ponds. On the land, modern salamanders walk by stepping forward with diagonally opposite pairs of legs, while its body sways about its hips and shoulders. In the water, they use a different tactic. Their limbs fold back and they swim by rapidly sending S-like waves down their bodies.

Both of these movements, as in all back-boned animals, are controlled by bundles of neurons called central pattern generators (CPGs). These bundles run down either side of the animal’s spine (its body CPG) and in each of its four limbs (its limb CPGs).

The CPGs produce rhythmic movements in muscles, by sending them carefully timed pulses of electrical signals. The brain is a casual bystander in this process, stepping in only to tell the CPGs to switch from a walking to a swimming rhythm, or vice versa.

By directly stimulating the brains of salamanders, Jean-Marie Calguen from the University of Bordeaux, managed to trigger the switch between walking and swimming gaits. With low levels of stimulation, the hapless animal made walking movements, and at higher levels, it tried to swim.

Calguen, along with Auke Ijspeert from the Ecole Polytechnique Federale de Lausanne, came up with a model for how this switch works and tested it by building a robot salamander. The metre-long and grandiosely named Salamandra Robotica was designed to mimic its biological counterpart.

Its movements are controlled by an ‘spinal cord’ that uses electronic CPGs to control its body and limbs. Like a real salamander, these are overseen by signals from the robot’s ‘brain’ – in this case, a wireless human-controlled laptop.

Ijspeert and Calguen used different CPG programs to control the robot’s body and limbs. When the robot receives any stimulation from its laptop brain, its body CPG produces the S-like body waves used by swimming salamanders.

At low levels of stimulation, the limb CPGs overpower the bodily ones, and the robot walks. But the limb CPGs cannot cope with higher levels of stimulation and switch off, leaving the bodily CPG free to start a swimming motion.

This model was a success. When they tested Salamandra Robotica on the shores of Lake Geneva, Ijspeert and Calguen found that their robot reproduced the same swimming and walking gaits seen in living salamanders, abruptly switching between the two depending on how much stimulation its CPGs received.

The model shows one way in which evolution could have modified an aquatic animal’s movements to a walking way of life. This was a key evolutionary step and provided the impetus for the spread of life from sea to land.

Salamandra’s success also shows that the studies of robotics and biology can successfully work together. Robots can be used to test biological ideas, while biology in turn can inspire successful solutions to engineering problems.

The robot salamander is part of a new era in robotics, where robot movements are controlled by artificial nervous systems.

 

Reference: Ijspeert, Crespi, Ryczko & Cabelguen. 2007. From swimming to walking with a salamander robot driven by a spinal cord model. Science 315: 1416-1419.

(Salamander photo by Marek Szczepanek. Photo of Salamandra Robotica from EPFL. Drawing of Acanthostega by Arthur Weasley.)

Technorati Tags: evolution, invasion of land, salamanders, salamander robot, robotics, artifiical nervous systems, walking, science

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The snake that eat toads to steal their poison

Many snakes are well-known for making their own poison. But the tiger keelback doesn’t bother – it appropriates its venom from the toads it preys on.

Many animals use poisonous secretions to protect themselves from predators. But poisons are complex chemicals and can take a lot of energy to make. Why invest in them, when you can steal someone else’s?

Poison thieves are well-known in the animal kingdom. Many species of brightly coloured poison arrow frogs acquire their poisons from beetles, while some sea slugs make a living by hunting for jellyfish, transporting their stinging cells into their own limbs.

Now, another species joins this guild of thieves – the tiger keelback snake, Rhabdophis tigrinis (image right, by Deborah Hutchinson).

The tiger keelback lives in Japan and uses its poisons for defence rather than attack. When threatened, it angles two glands on the back of its neck towards the predator. The fluid that oozes from these ‘nuchal glands’ contains chemicals called bufadienolides, that irritate airways and affect heart muscle.

But the glands themselves lack any of the secretory cells that you might expect in a poison-producing organ. So where does the snake get its poison from?

The answer lies in its diet – the snakes eat poisonous toads (from which bufadienolides get their name), and defend themselves with the weapons of their prey.

Deborah Hutchinson and colleagues from Old Dominion University, Virginia collected tiger keelbacks from islands across Japan. She found that snakes from islands with thriving toad populations had secreted nuchal fluid rich in bufadienolides, while those on toad-free islands completely lacked these defensive chemicals.

Hutchinson reared newly hatched snakes and found that they lack defensive poisons, but rapidly build up a supply if they are fed a diet of toads – a classic case of ‘you are what you eat’. In fact, the hatchlings had something of a predilection for toads and showed an instinctive preference for prey that will help to provide them with chemical defences in later life.

The researchers also found evidence that the snakes biochemically process some of the poisons they steal, making them even more toxic than they were in their toad donors.

The poisons make the snakes bold, and those that lack them are more likely to flee in the face of danger. But some hatchlings already had a head-start. Mothers with a full bufadienolide supply were capable of provisioning their stolen poisons to their young. The hatchlings were then free to hunt different varieties of toad to build up their repertoire.

Just like well-off human children benefit from their parents’ wealth, so do the offspring of envenomed mothers reap the benefits of her poisonous legacy.

Technorati Tags: snakes, snake venom, keelback snake, snake poison, snake toads, animal poisons, science

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Christmas special – Virgin birth by Komodo dragons

A virgin birth is a key part of the Christmas story. Now, scientists have found that two Komodo dragons in English zoos have done the same. This ability helps dragon populations to recover in the wild, but it may push the remaining few closer to extinction.

According to Christian lore, Mary gave birth to baby Jesus without ever having had sex with Joseph. A biologist might describe this with the unwieldy word ‘parthenogenesis’, the Greek version of the more familiar term ‘virgin birth’(‘parthenos’ means virgin, and ‘genesis’ means birth.)

The New Testament aside, shunning fertilisation and giving birth to young through parthenogenesis is rare among higher animals, occurring in only one in every thousand species. Nonetheless, in a few short weeks, eight more virgin births are expected in the English town of Chester. The mother is called Flora, and she is a komodo dragon.

Komodo dragons are an endangered species in their island homes of Indonesia. Fifty-two zoos around the world co-operate in a dedicated breeding programme that aim to boost the natural populations of these largest of lizards.

In Europe, only two female dragons, both living in England, are sexually mature. One of these, Flora, lives at Chester Zoo where she has laid a clutch of 25 eggs despite never having been kept with a male.

Three of Flora’s eggs tragically collapsed while they were being incubated, but this provided Phillip Watts and colleagues from the University of Liverpool to trace their origins. They analysed the genetic make-up of the lost eggs using genetic fingerprinting and found that their genomes matched those of their mothers.

Children born through sex have two copies of every gene, one inherited from their father and one from their mother. But in the case of Flora’s babies, all of their genes were identical, suggesting that they all came from Flora alone.

Watts found a similar situation in London Zoo, where a late female called Sungai had given birth to four healthy dragon-lings (see left, image courtesy of Ian Stephen/Nature), over two years after she lost contact with a male.

Scientists had suspected that the babies were the result of sperm that Sungai had stored during that time, but genetic tests confirmed that she was the sole parent.

This double-sighting of parthenogenesis in Komodo dragons suggests that this unusual strategy is not so unusual in these lizards. It could even be used to help populations weather hard times.

Komodo dragons have Z and W chromosomes, rather than our Xs and Ys and in their case, it is the ones with a matching pair who are males (ZZ or WW), and the ones with a dual set who are females (WZ). As a result, parthenogenetic dragons are always male and when populations dwindle, they can kick-start numbers by mating with their own mothers.

This strategy could cause large problems for conservationists. By causing all an individual’s gene pairs to be identical, parthenogenesis achieves what inbreeding usually takes generations to do.

In rare cases, it could help struggling populations to recover, but if dragon numbers become so small that parthenogenesis becomes the norm, reduced genetic diversity could push the species further towards extinction

Zoos need to take heed as well. Females are usually kept apart from males, who are transferred between zoos to act as reptilian studs. This reduces the risk of aggression on the part of the larger males, but it could lead to a excessive number of virgin births.

Clearly, the key to saving this magnificent animal is more research into how best to account for its new-found ability, and breed a healthy diverse population.

More about animal sex and reproduction: 
Butterflies evolve resistance to male-killing bacteria in record time 
When the heat is on, male dragons become females
Chimerism, or How a marmoset’s sperm is really his brother’s
Aphids get superpowers through sex

Reference:  Watts, Buley, Sanderson, Boardman, Ciofi & Gibson. 2006. Nature 444: 1021-1022.

Technorati Tags: komodo dragons, virgin birth, parthenogenesis, zoos, breeding programmes, science

Natural selection does a handbrake turn – quick evolution at work

Evolution works over centuries and millennia. But occasionally, scientists can stumble across situations where evolution happens quickly enough to be experimentally tested. Islands provide such opportunities as invading species can drastically change or reverse the evolutionary pressures on the locals.

Our lifespans of decades and, with luck, centuries seem like vast stretches of time to us. But to the forces of evolution, they are mere temporary blips. The common knowledge has it that evolution occurs over geological timescales – thousands and millions of years.

As such, evolutionary biology takes a lot of criticism for being a ‘descriptive science’, being less open than other fields to that fundamental aspect of science – experimentation. Those who study evolution must be content to observe snapshots of life, either present or entombed in rock, and make inferences from there.

But this is not always so. Occasionally, evolution happens at astonishingly fast rates, as epitomised by the case of the peppered moths.

Today, canny scientists are on the look-out for similarly speedy evolutionary events, that are more amenable to testing. Jonathan Losos and colleagues at Washington University, St Louis, have found one such example in a small Caribbean lizard.

The brown anole lives in the Bahamas and spends much of its time foraging on the ground. But occasionally, its island homes are invaded a larger predatory species, the curly-tailed lizard.

Losos’s earlier work had shown that these invasions caused the anole populations to head for the trees, abandoning their vulnerable land-based activities over a few generations. He spotted the signs of quick evolution at work and set about testing it.

Losos deliberately introduced curly-tailed lizards to islands containing brown anoles. A year later, and the percentage of brown anoles caught on the ground fell from about 40% to under 10% in a year, but not in other islands untouched by the curly-tails.

In the first six months, the anole populations on invaded islands shifted towards individuals with longer legs, who were better at outrunning the predators. But six months later, and the survivors were those with much shorter legs, which allowed them to hide from curly-tails in narrow and irregular tree-top spaces.

Within a single generation, Losos had shown that the evolutionary forces, or ‘selection pressures’, acting on the anoles went through very quick reversals.

As the lizards’ behaviour changed and they started to leave the ground, traits that had once been gifts became hindrances. Natural selection, it seems, is a fickle master.

Over more generations, the persisting threat of the curly-tailed lizards will drive the evolution of shorter and shorter legs in the anole population. The endpoint of this process can be seen on other islands, where some lizards species have evolved very short legs indeed and become ‘twig specialists’.

These rare sightings of ‘microevolution’ help to show us the essence of a process that takes several of our lifetimes. In doing so, they greatly enrich our appreciation of how life on earth became as rich as it is today.

Losos, Schoener, Langerhans & Spiller. 2006. Science 314: 1111.

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