Four Different Species Use the Same Odor to Exploit Each Other

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Chemical warfare. Hijacked communications. Stowaways. Eavesdropping. Sounds like the makings of a spy movie, but it’s just another day in nature. 

Plants have got to be fighters in this big bad world. When an animal that wants to eat them comes along to start chowing down, they can’t run or hide. They’ve got to fight back. Some of them have structural defenses like thorns or spines or nettles that jab at herbivores’ mouths. Some have waxy exteriors that make them too slippery for insects to land on, or produce resins and saps that create a sticky trap for bugs.

Others wage chemical warfare, sometimes in a roundabout way. Methyl salicylate (MeSA), also known as wintergreen oil, is produced by some plants when they’re damaged by herbivorous insects. The chemical doesn’t harm the insects directly, but acts like a mayday signal that attracts predatory bugs that come and eat the herbivores. In one case that University of Florida scientists have recently described, it’s also at the center of a web of species trying to exploit each other.

When citrus trees are damaged and release MeSA, it not only attracts helpful bugs, but also a jumping plant louse known as the Asian citrus psyllid (Diaphorina citri, top). The psyllids follow the chemical’s odor because a tree that’s already damaged is a good place to find food, places to lay eggs (which the psyllids can only do in new citrus shoots) and other bugs to mate with. 

Sometimes, though, there’s no meal to be had, because the tree has been infected by a bacterium called Candidatus Liberibacter asiaticus (Las). The psyllids are the bacterium’s primary vector for spreading from tree to tree. When Las infects a tree, it lowers the tree’s nutritional quality and also hijacks its odor production, forcing it to release MeSA as bait. When the psyllids show up and find there’s no less-than-ideal food, they move on in search of another tree—but not before the bacteria cling to them and hitch a ride to their next victim.

A group of entomologists in Florida, led by Xavier Martini, recently found that the trees, lice, and bacteria aren’t using these chemical signals in private, and that there’s another creature eavesdropping on them. The same odor that the lice use to find food, and that the bacteria exploit to lure the lice, also attracts a wasp called Tamarixia radiata. These parasites feed on the bodily fluids of citrus psyllids and also lay their eggs on the undersides of the immature insects. When the eggs hatch, the larvae attach to the psyllid and feed on their hemolymph (kind of the arthropod version of blood) until the host dies. Then, they crawl inside the mummified psyllids, where they’ll develop into adults and eventually break their way back out through the thorax or head. 

Martini and his team discovered that the wasps “eavesdrop” on the chemical cues produced by citrus trees to find their hosts, and were more attracted to the MeSA from Las-infected trees than non-infected trees. At trees where the bacteria was present, the wasps also parasitized five times as many psyllids as they did at the regular trees. 

That the wasps are attracted to the same chemical as the psyllids potentially complicates things for the Las bacteria. Martini found that the wasps sometimes arrive at an infected tree before the psyllids show up, which might drive them away or result in them being killed or parasitized before they can give the bacteria a lift to another tree. Then again, when the wasps show up after psyllids they might actually help the bacteria by causing the psyllids to disperse far and wide, which is exactly what the bacteria need to spread. 

A tree’s “cry for help” not only attracts more danger, but can also be hijacked by bacteria to help them spread infection, while an eavesdropping parasite uses the same odor to find a host that its children turn into a macabre nursery. Ain’t nature grand? 

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13 Facts About Opossums
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Opossums, which include the roughly 100 species in the order Didelphimorphia, are some of the most misunderstood animals in the Americas. They’re often thought of as dimwitted, dirty creatures whose most impressive trick is acting like roadkill. The truth is just the opposite: Opossums are smarter, cleaner, and more beneficial to humans than many of their woodland neighbors. Read on for more opossum facts.

1. OPOSSUMS AND POSSUMS AREN’T THE SAME ANIMAL.

In North America, opossum and possum describe the same thing, but in Australia the word possum refers to a completely different animal. Among the most well known of their respective types are the Virginia opossum and the brushtail possum. Both are small to medium sized, omnivorous marsupials, but the similarities end there. The possum looks like a cute cross between a squirrel and a chinchilla and it belongs to a different order than the North American mammal that shares (most of) its name. Despite the potential for confusion, possum is accepted as the shortened version of opossum in this part of the world (and if you see the word possum in this list, you can assume it’s referring to the animal from the Americas).

2. THEY’RE THE ONLY MARSUPIALS FOUND NORTH OF MEXICO.

Marsupials—mammals that carry and nurse their young in pouches—are absent from much of the world, and in Canada and the United States opossums are the sole representatives of the group. Like other marsupials, mother possums give birth to tiny, underdeveloped offspring (called joeys) that immediately crawl into a pouch where they live and nurse during their first months of life. Only once they’ve grown big and strong enough do they venture out, transitioning between their mother’s back and the warmth of the pouch until they mature into adults.

3. THEY CAN’T CHOOSE WHEN THEY PLAY DEAD.

Possum playing dead.
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Perhaps the most famous characteristic of the opossum is its tendency to play dead in front of predators. When the animal experiences intense fear in the face of danger, it seizes up and flops to the ground where it can remain for hours staring blankly ahead and sticking out its tongue. It’s an impressive defensive mechanism, but its effectiveness can’t be chalked up to the possum’s acting skills. Possums have no control over when they play dead or for how long they do it: The comatose-like state is an involuntary reaction triggered by stress.

4. AN OFFENSIVE ODOR SELLS THE PERFORMANCE.

A picture of a possum playing dead doesn’t really do it justice. To get the full experience, you need to be standing over to it to smell the putrid odor it emits when pretending to be a corpse. The smelly substance it secretes from its anus is just one more reason for foxes and bobcats to look for their dinner elsewhere.

5. THEY SLOW THE SPREAD OF LYME DISEASE.

Even if possums aren’t the cutest creatures in the forest, they should be a welcome addition to your backyard. Unlike other mammals that carry ticks, and therefore spread Lyme Disease, possums gobble up 90 percent of the ticks that attach to them. According to the National Wildlife Federation, a single possum consumes 5000 of the parasites per tick season. That means the more possums that are in your area, the fewer ticks you’ll encounter.

6. THEIR MEMORIES ARE SURPRISINGLY SHARP.

Possum looking up at table.
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Opossums have impressive memories—at least when it comes to food. Researchers found that possums are better at remembering which runway led to a tasty treat than rats, cats, dogs, and pigs. They can also recall the smell of toxic substances up to a year after trying them.

7. THEY’RE IMMUNE TO MOST SNAKE VENOM.

While most animals look at a snake and see danger, a possum sees its next meal. The animals are immune to the venom of nearly every type of snake found in their native range, the one exception being the coral snake. Possums take advantage of this adaptation by chowing down on snakes on a regular basis.

Researchers have been trying to harvest possums’ antivenom powers for decades. A few years ago, a team of scientists made progress on this front when they recreated a peptide found in possums and and found that mice given the peptide and rattlesnake venom were successfully protected from the venom’s harmful effects.

8. THEY ALMOST NEVER GET RABIES.

While possums aren’t totally immune to rabies (a few cases have been documented), finding a specimen with the disease is extremely unlikely. Marsupials like possums have a lower body temperature than the placental mammals that dominate North America—in other words, their bodies don’t provide a suitable environment for the virus.

9. THEIR TAIL ACTS AS A FIFTH APPENDAGE.

Baby opossum hanging from a tree branch by its tail.
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Opossums are one of a handful of animals with prehensile tails. These appendages are sometimes used as an extra arm: They can carry grass and leaves for building nests or grip the sides of trees to provide extra stability while climbing. Baby possums can even use their tails to hang from branches upside down as they’re often depicted doing in cartoons. But it’s a myth that possums sleep this way: Their tails are only strong enough to hold them for a short amount of time.

10. THEY’RE CONSTANTLY SELF-GROOMING.

Thanks to their whole acting-and-smelling-like-a-corpse routine, opossums aren’t known as the most sanitary animals in nature. But they take cleanliness seriously: The Washington Department of Fish and Wildlife writes that possums, like housecats, use their tongue and paws to groom themselves frequently and thoroughly. Possums largely lack sweat glands, and this behavior is believed to help them cool down. It also has the added effect of rendering them odorless (when they’re not secreting stinky predator-repellant, that is).

11. THEIR EYES AREN’T TOTALLY BLACK.

Close-up on opossum's face.
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One of the opossum’s most recognizable features is its pair of opaque eyes. Opossum eyes do have whites and irises, but because their pupils are so large, their eyes appear completely black from a distance. The exaggerated pupil dilation is thought to help the nocturnal animals see after the sun goes down.

12. THEY’RE SOCIAL CREATURES.

It was long assumed that opossums like to keep to themselves, but a study published in the journal Biology Letters suggests they have a social side. Researchers at the Federal University of Pernambuco in Recife, Brazil observed some possums in captivity sharing dens even if they weren’t mates. In one case, 13 white-eared opossums of various age groups were cohabiting the same space. The scientists suspect that male and female possums living in the wild may even build nests together as a way to trigger the female’s reproductive hormones.

13. THEIR REPRODUCTIVE SYSTEMS ARE COMPLICATED.

The way it gives birth and raises its young isn’t the only thing that’s interesting about the opossum's reproductive life. Females have two vaginal tracts and two uteri, and males in turn have a forked or bifurcated penis. This is fairly typical for marsupials, but when European colonizers first landed in North America centuries ago, they didn’t know what to make of the confusing genitalia. One explanation they came up with was that male opossums impregnated females through the nose.

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Hero Crayfish Cheats Death By Removing Its Own Claw to Escape Pot of Boiling Water
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There remains a perpetual debate over the ethical consequences of taking a crustacean and boiling it alive. In early 2018, Switzerland actually made it illegal to give living lobsters a scalding hot bath. (Instead, chefs are expected to stun them electronically before submersion.) Scientists can’t reach a conclusion over whether decapods feel pain—or if we can even define what that means for them.

While humans argue, some clawed sacrifices are taking action. A crayfish filmed by a Facebook user in China is making the internet rounds and being hailed as a hero after taking dramatic measures to escape a boiling pot of water.

In the footage, the crayfish appears to be unable to extricate its left appendage from a bubbling vat of doom. Rather than succumb, the crayfish uses its right claw to sever its compromised claw and scurry off. At 11 seconds, it’s the best summary of a Saw film possible.

“Juike,” the user who originally posted the video to the Weibo social media site, says he has taken the crafty invertebrate home and put him in an aquarium as a pet. The tiny survivalist may even regrow his lost limb, as crawfish are able to do, although it might not reach its former size.

Crayfish are in inherent danger of being turned into soup in China, where specialty restaurants devoted to their preparation are popping up. Some observers believe their popularity is due to diners having to step away from phones and social media in order to use both hands to peel away at their shells.

[h/t BBC]

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