Archive for October, 2009

Drought drives toads to mate with other species

By Ed Yong | October 31, 2009 12:00 pm

This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.

Spadefoot toads seek mates from other species under times of droughtWhen 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.

The south-western United States is home to two species of spadefoot toads with overlapping ranges – the Mexican spadefoot, Spea multiplicata and the Plains spadefoot, Spea bombifrons (more Kermit-like, according to Pfennig). Where both species mingle, they can breed and, as usual, the hybrid young are worse at spawning the next generation than their pure-blooded peers. Hybrid males are often sterile, and hybrid females lay fewer eggs.

Nonetheless, up to 40% of toads in certain areas can be hybrids and this intrigued Pfennig. She wanted to work out whether this was just incidental, or if some circumstances nudged the toads towards mating with individuals from a different species.

Their breeding grounds provided the answer; spadefoots lay eggs in temporary ponds and it’s often a race for tadpoles to turn into frogs before the water dries out. Pfennig noticed that hybrids were more common in shallower ponds that dry out quicker, and that’s because the two toad species develop at different rates.

On average, Mexican spadefoot tadpoles take less time to make the transition into frog-hood than Plains spadefoot ones, and hybrid tadpoles lie somewhere in the middle. This means that a Plains spadefoot female that’s faced with a short-lived pond might do better if she mates with a Mexican spadefoot male, for her young will be more likely to grow up in time.

Pfennig tested this idea by placing Plains spadefoot females in tanks simulating shallow and deep ponds and letting them choose between recorded calls from males of both species. In deep water, they favoured their own kind about 65% of the time, but in the shallower pools, they had no such preferences.

In contrast, Mexican spadefoot females also showed no willingness for breed with other species. Since their tadpoles develop quickly anyway, they gain nothing by courting Plains spadefoot males. Pfennig also found that only Plains spadefoot females that lived in the same areas as Mexican spadefoots had the ability to switch their mate preferences. In parts of the States where the two species are geographically segregated, females never made this choice.

A Plains spadefoot female’s health also affects which species she fancies. If she is fitter, she could provision her eggs with more nutrients and her tadpoles would grow faster. That would obviate her reliance on Mexican spadefoot males, even in shallower ponds.

Pfennig’s experiments confirmed her idea; the unhealthiest females were the most likely to switch their preferences, from mating with their own kind in deep ones to preferring the other species in shallow ones.

Biologists are used to viewing a female’s choice of partners solely in terms of the physical traits of males. But Pfennig’s results show that it isn’t just about which male has the flashiest colours, the most melodious song or the most impressive antlers. For females, mate choice is a much subtler affair, influenced by environment, personal health and probably many other factors that we have only begun to consider.

Reference: Pfennig. 2007. Facultative mate choice drives adaptive hybridization. Science 318: 965-7.

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Big-headed tiger snakes support long-neglected theory of genetic assimilation

By Ed Yong | October 30, 2009 9:34 am

Tiger snakes are a group of extremely venomous serpents found all over the southern half of Australia, and on many of its islands. Some were cut off from the mainland by rising sea levels more than 9,000 years ago, while others were inadvertently introduced by travelling humans and have been around for less than 30 years.

When the snakes first arrive on an island, they find prey that are generally larger than they’re used to on the mainland. That puts them under strong evolutionary pressure to have larger heads, in order to swallow larger meals. But by feeding snakes from different populations with prey of varying sizes, Fabien Aubret and Richard Shine have found that the more recent immigrants solve the need for larger heads in a very different way than the long-term residents. 

Young populations do it by being flexible. If growing tiger snakes from newly colonised islands are fed on large prey, their heads rapidly enlarge to cope with the sizeable morsels. This flexibility is an example of “phenotypic plasticity” and it doesn’t involve any genetic changes.

But Aubret and Shine found that older populations lack this flexibility – they have larger heads from birth and the size of the prey they eat doesn’t affect the way they grow. These adaptations are fixed in their genomes. In the heads of tiger snakes, Aubret and Shine have found evidence for a 67-year-old concept in evolution called “genetic assimilation“, which has very rarely been tested and is often neglected.

Tiger_snakes.jpg

Its name might conjure up images of science-fiction and DNA-stealing aliens, but genetic assimilation simply describes a means of adaptation. It was proposed in 1942 by Conrad Waddington, who suggested that species initially cope with fresh environments by being flexible – through plasticity. All species have a certain amount of variation built in to their developmental program, which they can exploit according to the challenges they face. In this case, the tiger snakes can grow larger heads if they encounter bigger meals.

But as populations face constant evolutionary pressures, natural selection eventually favours genes that produce the same results, the ones that plasticity once achieved. This is the crux of Waddington’s theory – in time, natural selection eliminates plasticity by fixing genes for the same traits. Such genes as said to be “canalised”.

Back in the 1950s, Waddington demonstrated this using fruit flies. He exposed developing flies to ether vapour and found that some developed a second thorax (the middle segment between the head and abdomen). By anyone’s standards, that’s a radical change, but one that was triggered by an unusual environment. Over time, Waddington selectively bred the double-thorax individuals and exposed each new generation to ether. After 20 rounds of this, he found that some flies developed a second thorax naturally, without being exposed to ether. The double-thorax trait, which was initially induced by the environment, eventually became governed by the fly’s own genes.

It was a neat idea, but finding other natural examples has been very tricky. Aubret and Shine thinks that genetic assimilation tends to happen over such short timescales (geologically speaking) that you can only really detect it under unusual circumstances. And the spread of tiger snakes across Australia certainly fits that bill.

Aubret and Shine’s experiments show that snakes from newly colonised areas had the greatest degree of plasticity when it comes to head size while those from the longest-colonised islands had the least. These differences become abundantly clear when you compare snakes from three populations.

Tiger snakes have only been on Trefoil Island for 30-40 years and the jaws of their hatchlings are still small. However, they’re also plastic – if they eat big meals, they’ll grow bigger. On Carnac Island, tiger snakes have been around for 90 years and there, the hatchlings have moderately sized jaws and a relatively high degree of plasticity. On Williams Island, the tiger snakes have been cut off from the mainland for 9,100 years and their jaws are not only large from birth but their growth has very little plasticity. 

The differences between the Trefoil and Carnac serpents are particularly interesting, because they suggest that the process of genetic assimilation can take place over a very short span of time, as others have predicted. It starts manifesting within just a few decades, even in animals like tiger snakes that only breed after their second or third birthday. This rapid pace could explain why it’s very difficult to observe this process in the wild.

Tigersnakes.jpg

Reference: Current Biology 10.1016/j.cub.2009.09.061

Images: Tiger snake by Ian Fieggan

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CATEGORIZED UNDER: Animals, Evolution, Reptiles, Snakes

Venomous shrews and lizards evolved toxic proteins in the same way

By Ed Yong | October 29, 2009 12:00 pm

The Northern short-tailed shrew is a small, energetic mammal that lives in central and eastern North America. The Mexican beaded lizard is a much larger reptile found in Mexico and Guatemala. These species are separated by a lot of a land and several million years of evolution, yet they share astonishing similarities. Not only are they both venomous, but the toxic proteins in their saliva have evolved in very similar ways from a common ancestor, converging on parallel lethal structures independently of one other. 

This discovery, from Yael Aminetzach at Harvard University, shows that adaptations are sometimes very predictable. Despite the many changes that could have shaped the course of venom proteins in lizards and shrews, they seem to have gone down a consistent and similar route.


Northern short-tailed shrew by Giles Gonthier; Mexican beaded lizard by PiccoloNamek

The northern short-tailed shrew is one of the few venomous mammals, but its poisonous bite is painful to humans and can kill smaller animals. The key to its venom is a protein called BLTX, whose job is to cut another protein in two. This chemical reaction frees a molecule called bradykinin, which widens blood vessels and lowers blood pressure. It’s a necessary job, but BLTX is so active that if floods the body with bradykinin – an overdose that leads to paralysis and death.

BLTX is a dark, hyperactive descendant of an ancestral protein called kallikrein-1, which does the same thing but in a much more restrained way. Aminetzach found that BLTX is a longer version of kallikrein and the extra amino acids it has gained have changed the structure of the protein’s ‘active site’.

The active site is the protein’s business end – it allows BLTX to latch onto the right targets and catalyse the relevant chemical reactions. It’s also the part of the protein that has changed the most from the harmless kallikrein model; amino acids around BLTX’s active site have changed about twice as much as the rest of the protein. As a result, the site is larger, more flexible and better at drawing in its target, and the protein as a whole has become hyperactive.

And amazingly, the Mexican beaded lizard has gone through similar changes. Its venom relies on a protein called GTX, which is also descended from kallikrein. Like BLTX, it too is a longer version of its ancestor, and while its extra amino acids have been shoved into different places, the results are the same. The changes have altered the protein’s active site so that it’s larger, more flexible and better at drawing in its target.

These changes are very specific to these toxic proteins. By studying 24 relatives of kallikrein, Aminetzach found that none of the non-toxic members of the family have any of the changes that BLTX and GTX share.

This study demonstrates that evolution doesn’t work with infinite possibilities. Often, there are only a few roads leading to the same destination. Through different amino acid changes, both BLTX and GTX have evolved similar structures and have turned into weapons. This predictability of venom evolution may be useful to us – for example, Aminetzach suggests that it could allow scientists to more easily identify toxins from others species, even distantly related ones.

Reference: Current Biology 10.1016/j.cub.2009.09.022

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CATEGORIZED UNDER: Animals, Evolution, Lizards, Mammals, Reptiles

Holy fellatio, Batman! Fruit bats use oral sex to prolong actual sex

By Ed Yong | October 27, 2009 7:56 pm

Many humans whinge about not getting oral sex often enough, but for most animals, it’s completely non-existent. In fact, we know of only animal apart from humans to regularly engage in fellatio – the short-nosed fruit bat (Cynopterus sphinx).

The bat’s sexual antics have only just been recorded by Min Tan of China’s Guangdong Entomological Institute (who are either branching out, or are confused about entomology). Tan captured 60 wild bats from a nearby park, housed them in pairs of the opposite sex and voyeuristically filmed their liaisons using a night-time camera. Twenty of the bats got busy, and their exploits were all caught on video.

Male bats create tents by biting leaves until they fall into shape. These provide shelter and double as harems, each housing several females who the male mates with. Fruit bat sex goes like this: the female approaches and sniffs the male, and both partners start to lick one another. The male makes approaches with his thumbs (like the Fonz) and mounts the female (like the Fonz). Sex itself is the typical rhythmic thrusting that we’re used to, and afterwards, the male licks his own penis for several seconds.

But Tan also found that female bat will often bend down to lick the shaft of her mate’s penis during sex itself. This behaviour happened on 70% of the videos, making it the only known example of regular fellatio in a non-human animal. It also prolonged the sexual encounter – males never withdrew their penises when they were being licked and, on average, the behaviour bought the couple an extra 100 seconds of sex over and above the usual 2 minutes. The licking itself only lasted for 20 seconds on average, so each second of it buys six extra seconds of penetration.  

NSFW – short-nosed fruit bats having sex. I will have you know that the music choice came with the video and has nothing to do with me.

Oral sex is rare in other animals. Bonobos do it (but really, what don’t they do?) but it’s more of a form of play among young males, and there’s one anecdotal instance of an orang-utan doing the same. Some animals, such as ring-tailed lemurs, lick each other’s genitals to judge whether they’re ready for mating, but there’s no evidence that they do so as an actual part of sex. As for other bats, it’s entirely possible that they too engage in oral sex. However, given their inaccessible roosts and nocturnal habits, we’re largely in their dark about their sex lives.

Nonetheless, Tan suggests a few possible reasons for the short-nosed fruit bat’s penchant for fellatio, aside from the anthropocentric conclusion of ‘pleasure-giving’. Bat penises contain erectile tissue much like our own. It gets stiffer if it’s stimulated, so females could use oral sex to prolong their encounters with males, by maintain their erections or lubricating it for easier entry.

While many of us might nod sagely at the need for longer sex, Tan suggests that for the bats, it could mean easier transport of sperm to the oviduct, or more secretions from the female that are conducive to fertilisation. It could also be a way of hogging a mate, keeping him away from rival females.

Alternatively, the antiseptic properties of saliva might help to strip the male’s penis of bacteria or fungi, and prevent the spread of sexually transmitted diseases. The fact that males lick their own penises after sex supports this idea.  

And finally, oral sex might help females to pick up chemical traces on her mate that might suggest if he’s a suitable mate. Obviously, they’d already be having sex, but female mammals often exert choice over their sexual partners after the fact, rejecting sperm from inferior males, or encouraging congress with superior ones to displace it. All of these explanations are just hypotheses for the moment, but they could all be tested in the future.

Reference: Tan, M., Jones, G., Zhu, G., Ye, J., Hong, T., Zhou, S., Zhang, S., & Zhang, L. (2009). Fellatio by Fruit Bats Prolongs Copulation Time PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007595

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Drinking blood makes vampire spider sexier

By Ed Yong | October 27, 2009 8:30 am

In East Africa lives a species of spider that drinks mammalian blood. But fear not – Evarcha culicivora is an indirect vampire – it sates its thirst by preying on female mosquitoes that have previously fed on blood themselves.

Even though its habitat is full of non-biting midges called “lake flies”, it can tell the difference between these insects and the blood-carrying mozzies it carries. Robert Jackson from the University of Canterbury discovered this behaviour a few years ago and one of his colleagues, Fiona Cross, has now found that the blood isn’t just a meal for the spiders, it’s an aphrodisiac too.

Photo of E.culicivora eating a mosquito, by R. Jackson.

Cross made spiders choose between two adults of the opposite sex, by wafting their smells down a tube on different days and seeing which drew the choosy spider’s attention for the longest time. The contenders had been fed on one of four diets: blood-fed female mosquitoes, sugar-fed female mosquitoes, male mosquitoes, or lake flies. 

She found that only a menu of blood-fed mosquitoes made spiders more attractive to the opposite sex, and both males and females shared this turn-on. If spiders were switched from a diet of lake flies to one of bloody mosquitoes, their scents became more attractive. Even a single meal of blood makes the spiders smell more attractive. Likewise, fasting, or moving from mozzies to lake flies even for just a day, curtails the sex appeal of an individual’s odour.

So for E.culicivora to maintain its sensuous scent, it needs to continuously feed on blood. In this way, spiders that smell of blood are probably those that are best at catching mosquitoes, and potential partners may be using the odours as a way of sussing out the quality of their mates. Of course, that’s just a hypothesis. Next, Cross plans to see if spiders on a blood diet actually mate more often, or produce more viable eggs and sperm.

The other alternative is that spiders are using the smell of blood to lure in potential mates, by tricking them into thinking that prey is near. But Cross thinks this is unlikely – spiders were only drawn to the smell of blood if it was given off by individuals of the opposite sex.

The importance of smell might come as a surprise, especially since E.culicivora is a jumping spider, a group that’s better known for their keen eyesight. But when it comes to mating, previous studies show that smell plays an equally important role in identifying a partner. If the smell was simply making them hungry, the gender of its source wouldn’t matter.

Perhaps the actual chemical lure is produced after blood is processed in the spider’s body. Perhaps it’s a combination of blood and a sex-specific chemical that piques a partner’s interest. The only real way to find out is to work out the precise chemicals that E.culicivora finds so appealing, and that’s next on Cross’s to-do list.

In the mean time, there are probably many other examples in nature of animals to rely on the same smells in courtship rituals as in other aspects of their lives. For examples, noctuid moths use sex pheromones that mimic smelly chemicals given off by plants, the same chemicals that they track to find somewhere to lay their eggs. And the European starling adds aromatic plants into its nest to attract females.

Reference: PNAS doi:10.1073/pnas.0904125106


A gallery of incredible spiders

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darwins_bark_spider_web
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evarcha
harpactea_sadistica
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philoponella
spider_cat
tarantula

How humans started a bacterial pandemic in chickens

By Ed Yong | October 26, 2009 6:13 pm

The prospect of infections spreading from animals to humans has become all too real with the onset of the current swine flu pandemic, and the threat of a bird flu still looming. But infections can jump the other way too. Decades before the world’s media were gripped with panic over bird flu, humans transferred a disease to chickens and it has since caused a poultry pandemic right under our noses. 

The infection in question is a familiar one – Staphylococcus aureus, a common human bacterium that’s behind everything from mild skin infections to life-threatening MRSA. It causes chicken diseases too, including septic arthritis and ‘bumblefoot‘. But in the 1970s, broiler chickens began developing a new type of S.aureus infection called ‘bacterial chrondronecrosis with osteomyelitis’ or, more simply, BCO. It’s a bone infection and it’s a major cause of lameness in broiler chickens.

This new disease had human origins. Bethan Lowder from the University of Edinburgh has shown that all of the bacteria behind BCO share a common ancestor, which jumped from humans to chickens in Poland, around 38 years ago. From that point on, the bacterium’s travel itinerary was set. Just as air travel has facilitated the spread of swine flu among humans, a global distribution network for chickens made it easy for S.aureus to spread all over the world aboard its new feathery hosts.

Lowder traced the common ancestry of S.aureus in chickens by analysing the genes of 57 samples. Of these, 48 came from healthy and diseased chickens across eight countries and four continents, and 9 were taken from different species of wild and domesticated birds. Amazingly, she found that two-thirds of all the broiler chicken samples came from a single strain of the bacterium called ST5.

ST5 infects humans all over the world and is one of the most successful strains of S.aureus to do so. But Lowder found that all of the chicken samples were more closely related to each other than they were to any of the human bacteria from the same strain. They all shared a common ancestor – a lineage of ST5 found only in Poland. Around 38 years ago, this pioneering bacterium made the leap from humans to chickens and its descendants have spread from Poland to countries as far as the US and Japan.

Since then, the ST5 strain has adapted to its new host. It has lost many of the genes it needs to cause disease in humans but it has picked up others that allow it to better infect chickens. A complete sequence of the bacterium’s genome reveals that since its human days, it has picked up five new genes from other bird sources, none of which are found in humans or other mammals. In fact, Lowder thinks that the ST5 strain may be particularly good at picking up mobile genes from other sources. That might explain why both human and chicken versions are so successful, and why the human one often picks up genes that allow it to shrug off powerful antibiotics.

It’s not clear how exactly these changes benefit the bacteria, but certainly, they’re much better at resisting a chicken’s immune system than their human predecessors. When faced with chicken heterophils – a type of white blood cell – the poultry strains were much more likely to survive than the human equivalents.

Lowder thinks that globalisation was the key to the new pandemic. In just the last fifty years, the broiler chicken industry has shifted from one dominated by small farms to a multi-billion dollar leviathan controlled by a small number of multinationals. These companies transport a relatively few breeding lines of chickens all over the world, and the low genetic diversity of these birds makes them vulnerable to infections as opportunistic as S.aureus.

She recommends that livestock are screened regularly so that emerging diseases can be picked up, and that stocks should often be cleansed of S.aureus, to nip potential new threats in the bud. Better regulations for international transport wouldn’t go amiss either – it’s no surprise that Australia, a country with stringent regulations on importing livestock, has no trace of the pandemic S.aureus strain.

Reference: PNAS: 10.1073/pnas.0909285106

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CATEGORIZED UNDER: Animals, Bacteria, Birds, Medicine & health

Mantis shrimp eyes outclass DVD players, inspire new technology

By Ed Yong | October 25, 2009 2:00 pm

The most incredible eyes in the animal world can be found under the sea, on the head of the mantis shrimps. Each eye can move independently and can focus on object with three different areas, giving the mantis shrimp “trinocular vision”. While we see in three colours, they see in twelve, and they can tune individual light-sensitive cells depending on local light levels. They can even see a special type of light – ‘circularly polarised light’ – that no other animal can.

But Nicholas Roberts from the University of Bristol has found a new twist to the mantis shrimp’s eye. It contains a technology that’s very similar to that found in CD and DVD players, but it completely outclasses our man-made efforts. If this biological design can be synthesised, it could form the basis of tomorrow’s multimedia players and hard drives.

Previous studies have found that mantis shrimps can detect polarised light – light that vibrates in a single plane as it travels. Think of attaching a piece of string to a wall and shaking it up and down, and you’ll get the idea. Last year, scientists discovered that they can also see circularly polarised light, which travels in the shape of a helix. To date, they are still the only animal that can see these spiralling beams of light.

Its secret lies at a microscopic level. Each eye is packed with light-sensitive cells called rhabdoms that are arranged in groups of eight. Seven sit in a cylinder and each has a tiny slit that polarised light can pass through if it’s vibrating in the right plane. The eighth cell sits on top and its slit is angled at 45 degrees to the seven below it. It’s this cell that converts circularly polarised light into its linear version. 

In technical terms, the eighth cell is a “quarter-wave plate”, because it rotates the plane in which light vibrates. Similar devices are also found in camera filters, CD players and DVD players but these man-made versions are far inferior to the mantis shrimp’s biological tech.

Synthetic wave plates only work well for one colour of light. If you change the wavelength slightly, they become ineffective, so designing a wave plate that works for many colours is exceptionally difficult. But the mantis shrimp has already done it. Its eyes work across the entire visible spectrum, from ultraviolet to infrared, achieving a level of performance that our technology can’t compete with.

What’s more, the same eighth cell not only manipulates circularly polarised light, but it can sense ultraviolet light too. It’s a detector and a converter – a two-for-one deal that nothing man-made shares.

Why the mantis shrimp needs such a sophisticated eye is unclear. It could help them to see their prey more clearly in water, which is rife with circularly polarised reflections. It needs good eyesight to be able to hit its prey accurately. Like a crustacean Thor, mantis shrimps shatter their victims with devastating hammer blows inflicted by the fastest arms on the planet. Their forearms, which end in clubs or spears, can travel through water at 10,000 times the acceleration of gravity and hit with the force of a rifle bullet.

Another option is that their super-eyes allow them to send and receive secret messages. A mantis shrimp’s shell reflects circularly polarised light, and males and females produce these reflections from different body parts. Their ability to see this type of light could give them a hidden channel of communication that only they can see, for use in courtship or combat.

Whatever the reason for it, Roberts thinks that the eye’s structure is “beautifully simple”. It’s all in the shapes of the cells, their size, and the amount of fat in their membranes. For all its outstanding performance, the eye’s abilities were probably easy to evolve, requiring only small tweaks to the basic blueprint of the light-detecting cells.

Now that we know about the microscopic structures behind the mantis shrimp’s amazing eye, Roberts is hopeful that engineers can mimic it using liquid crystals. “The cool thing is I think it’s actually something you could make and it would improve the workings of current technologies such as Blu-Ray, which uses multiple wavelengths of light, and of future data storage devices,” he said. It wouldn’t be the first time that crustaceans have inspired technology. A new type of X-ray telescope, for example, was based on the eye of the lobster

Reference: Nature Photonics DOI: 10.1038/NPHOTON.2009.189


The amazing ways in which animals see the world

heteronectes
bobtail-squid
box_jellyfish
cavefish
chiton
eye
eye_intro
mantis_shrimp
raccoon
spookfish
sunburst_beetle
zebrafish

EXTREME CLOSE-UP!!! Get something scanned under an electron microscope for free

By Ed Yong | October 24, 2009 12:44 pm

What would an extreme close-up of your sandwich filling look like? What about your hair? The cluster of dust in the corner of your living room? The grain of pollen stuck to your coat? Scientists, of course, have ways of finding out, using electron microscopes to look at the tiniest of objects in glorious detail.

Now you can do the same for free. A company called ASPEX, who bill themselves as “a leading producer of benchtop SEM (scanning electron microscopes”, is offering readers a chance to send in a sample of anything and see what it looks like in extreme close-up.

To take them up on the offer, download and fill in this form from the ASPEX website and send it (along with the sample you want scanned) to:

ASPEX Corporation

Free Sample Submissions

175 Sheffield Dr.

Delmont, PA 15626

It’ll take them about two weeks to complete the scan. Once they’re finished, they’ll notify you by email and post the images and the report on their website.

It sounds fun – if you take them up on the offer, post what you’ve sent to them here.

CATEGORIZED UNDER: Uncategorized

Fake cleaner fish dons multiple disguises

By Ed Yong | October 24, 2009 10:54 am

Guess which is which? (The top one is the real deal)

Nature is rife with charlatans. Hundreds of animals have evolved to look like other species in order to fool predators into thinking they’re more of a threat, or to sneak up on unsuspecting prey. In the Indo-Pacific lives a fish that does both and has the rare ability to switch between different disguises – the bluestriped fangblenny.

Common though it is, mimicry is usually restrictive and most fakers are stuck with one disguise. Until a few years ago, the only known animal that could switch between different acts was the amazing mimic octopus, which contorts its flexible body to look like seasnakes, lionfish, flounders and other poisonous underwater denizens.

In 2005, Isabelle Cote and Karen Cheney from the University of Queensland discovered that a small reef fish called the bluestriped fangblenny (Plagiotremus rhinorhynchos) is also a dynamic mimic.

Its model is the bluestreak cleaner wrasse Labroides dimidiatus, an industrious species that provides a cleaning service for other reef visitors by picking off parasites and mucus from hard-to-reach places. The fangblenny’s intentions are less welcome. Its resemblance to the helpful wrasse allows it to get close enough to mount quick attacks on larger fish, biting off scales and skin (see image below for why it got it’s name).

Why the fangblenny got its nameCote and Cheney found that fangblennies have two guises. In one, it has a black body and an electric blue stripe that mimics the wrasse, but in the other, it’s body is a very different brown, olive or orange with white or light-blue/green stripes. The fish can change from one to the other at will, and uses the non-mimicking colours to blend in with shoals of other fish.

Now, Cheney has provided further evidence for the opportunistic colour changes of this con artist. She captured 34 fangblennies of various colours and after 60 minutes alone, all the mimics had switched to non-mimic colours – it seems that there’s no point putting on a disguise if there’s no one around to see it.

When she added another fish, nothing happened unless it was a juvenile bluestreak cleaner wrasse. At that point, a third of the fangblennies swapped back to their black-and-blue coats. Cheney noticed that only the smaller individuals changed colours. She believes that as fangblennies grow larger, the rewards of looking like the smaller wrasse are reduced, so they don’t bother.

Her field experiments support this idea. On several swims, she noticed that the proportion of mimic to non-mimic fangblennies in the water was proportional to the number of juvenile cleaner wrasse around.The yellow fangblenny coat

A disguise may look right to us, but our colour vision is very different to that of most animals, including those whose reaction actually matters. To get a more objective view of the fangblenny’s disguise, Cheney analysed the light reflecting off its scales when it went through its different colour phases. Sure enough, its black-and-blue form reflected light in almost exactly the same way as a real cleaner wrasse would.

The fangblenny’s other colours also proved to be a match to other reef fish. The olive forms were most likely to be found among blue-green chromis, the brown forms mostly swam with the brown and white-coloured two-tone wrasse and the orange forms associated with orange Lyretail Anthias. In each of these cases, the pattern of light reflected off the fangblenny’s coat matched that of its preferred companion.

The bluestripe fangblenny’s many faces gives it great versatility. By matching the colours of a variety of different fish, it greatly expands the area of reef where it can safely hide from both predators and potential victims. Unlike the mimic octopus, it makes no effort to change its body shape and some of its models, like the chromis, are very different. But in a shoal, that hardly matters. A superficial resemblance to the surrounding throng may be advantage enough.

Reference: Cheney, Grutter & Marshall. 2007. Facultative mimicry: cues for colour change and colour accuracy in a coral reef fish. Proc Roy Soc B doi.10.1098/rspb.2007.0966

Images by K.Cheney and E.Schloeg.

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CATEGORIZED UNDER: Animal behaviour, Animals, Fish, Mimicry

Culture shapes the tools that chimps use to get honey

By Ed Yong | October 23, 2009 8:05 am

There is a deep hole in a tree trunk and within is a tasty dollop of sweet, nutritious honey. It’s a worthwhile prize for any animal skilled or clever enough to reach it, and chimpanzees certainly have both of these qualities. But the solutions they find aren’t always the same – they depend on cultural traditions.

Chimps from the Sonso community in Uganda are skilled at the use of sticks and unsurprisingly, they manufacture stick-based tools to reach the honey. Chimps from the Kanyawara community in a different part of Uganda have never been seen to use sticks in the wild. Instead, they bring their considerable leaf-based technology to the fore, using leaves a sponges to soak up the hidden honey.

This is hardly the first time that chimps have demonstrated cultural traditions. Chimps in different parts of Africa have their own peculiar styles of tool technology and these variations are some of the strongest pieces of evidence for the existence of animal culture. Captive chimps can also transmit traditions between each other, once seeded by scientists.

But some sceptics are unconvinced. Their riposte is that genetic or environmental differences could equally have shaped technological differences. Alternatively, faced with abstract problems in captivity, chimps could learn solutions through trial-and-error, rather than picking up answers from their peers. To discount these possibilities, Thibaud Gruber from the University of St Andrews wanted to see if different groups of wild chimps would solve new problems in different ways, even though they shared similar genes and environments.

He found two groups of participants in the Sonso and Kanyawara communities of Uganda. Both live in forests and both are genetically similar enough that you couldn’t tell which group an individual chimp belonged to based on its genes. And both groups like honey.

When the chimps weren’t around, Gruber drilled holes in fallen logs, filled them with liquid honey, and dotted honeycombs around the rim to alert passing chimps. For such chimps, it would have been an unusual sight – they often rob beehives but the holes they pilfer are on vertical trunks, and the honey is solid, waxy and easily reachable.

If the hole was shallow, the chimps from both communities could use their hands to get the honey. For deeper prizes that could only be reached with tools, their strategies strongly differed – some of the Sonso chimps sponged the honey up with leaves, while almost all of the Kanyawara chimps dipped into it with sticks. No Sonso chimp used sticks and no Kanyawara chimp used leaves.

Gruber thinks that it’s extremely unlikely that the chimps were using a trial-and-error method to extract the honey, for they solved the problem both quickly and accurately. Despite having similar environments, genes and tasks, the two communities had their own specific approaches to the task. Their divergent cultures are reflected not just in the tools they used, but their

Kanyawara chimps try to eat honey about twice a month, and they succeed on around half of their attempts. In Sonso, honey is a much rarer part of the chimp diet. At both places, bees attack invading chimps with equal ferocity, but the Kanyawara group have become persistent and learned to regularly revisit the same spot. The Sonso group only eat honey when the opportunity presents itself. It’s no surprise then that the Kanyawara chimps spent longer in their quest for the hidden honey than their Sonso peers.

Reference: Current Biology DOI: 10.1016/j.cub.2009.08.060

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MORE ABOUT: chimpanzee, honey, leaf, stick, tools
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