Why Does Salt Melt Snails (And Not Us)?

If you’ve ever seen a kid put salt on a snail (something you would never do, right?), you know they whither like the Wicked Witch of the West in a hot tub. But why is something as commonplace as table-salt so deadly to these little critters? And if salt is so lethal, why do some people pay spas beaucoup bucks to apply a salt scrub all over their bodies?

The Wicked Witch of the West melts after Dorothy dumps a bucket of water on her. Illustration by William Wallace Denslow from the original The Wonderful Wizard of Oz, by L. Frank Baum, 1900. Image available at Wikimedia Commons.

The cells that animals are made up of are essentially bags of salty water (with a little bit of specialized biological machinery). The outside of these bags, called cell membranes, are not waterproof but they are somewhat solute-proof (Solutes are all the dissolved stuff in the cell, including salts). That is, water can freely pass through the membranes, but salts (and other solutes) either can’t pass through or do so very slowly.

When solutes are in water, they like to be evenly spread out. This is pretty easy if you just have a glass of water with salt in it. But let’s say you put a membrane in this glass that allows water, but not salts, to pass through it. Now if you put saltier water on one side of the membrane and less salty water on the other side, what do you think would happen? If the salt can’t cross to the less salty side, then the water will cross to the more salty side.

If a membrane holds more salt (pink dots) on one side than the other, the water will
move to the side with more salt so that the salt and water can be evenly spaced out.

This principle isn’t just true for saltwater in a glass, it is also true for the contents of animal cells in an environment that is more or less salty than the cells are. For animals that live in freshwater, their cells generally have more solutes than the water around them. These animals and their cells are hyperosmotic to their environment, meaning water is constantly entering the cells and the water pressure is higher inside the cell pressing outwards. Salts slowly leak out of these cells, which helps to prevent them from exploding. But this process causes a problem for these animals: Left unchecked, these animals would gain too much water and lose too many salts, leaving them too diluted to function properly. They have to work to push out water and take in salts. Freshwater animals are okay with this, because this is what their species are used to.

If a cell is hyperosmotic to its environment, the water will move into it and salts will
leak out so that the salt and water can be evenly spaced out. Eventually, the cell
won’t function properly unless it can reabsorb some of these salts.

But what would happen if you take an animal that is used to a freshwater environment and suddenly expose it to high levels of salt? Water would rush out of the cells to mix with the salt on the outside! That is exactly what happens when you put salt on a freshwater snail: The salt mixes with the water in the mucous layer on the animal and almost all the water inside the animal’s body comes rushing out to mix with the resulting salty paste. The snail’s cells haven’t had to bring water in quickly before (remember, they are used to pushing water out), so the snail’s body just isn’t prepared to prevent the rapid loss of body water.

A freshwater ramshorn snail has nightmares about kids with salt-shakers.
Image by Alan R Walker at Wikimedia.

But then why don’t we shrivel up if you put salt on us? Unlike freshwater snails, which have bodies adapted to life in water, we are terrestrial animals and we have bodies adapted to life out in the air. In the heat, we run the risk of losing too much water by evaporation. To prevent this water loss, our skin is more waterproof than the skin of a snail. Your more waterproof skin can protect you from quickly losing all of your body water when the lady at the spa rubs salt paste all over your body for a “salt treatment”. So enjoy it – I promise you won’t melt.

Why Reptiles Won't Wear Fur

Have you ever seen a furry lizard? A fuzzy snake? A wooly turtle? Me neither. That's because a reptile in a permanent fur coat would whither like Superman with a pocket full of kryptonite. But why? Other animals are so content in their soft, luxurious layers... Why can't reptiles be?

"I wouldn't be caught dead in that fur coat you're wearing". Photo by Naypong at freedigitalphotos.net.

Animals exchange heat with their environments in four major ways: conduction, convection, radiation and evaporation:

• Conduction is when heat moves from a hotter area to a colder area across a still surface. If you stand barefoot on a cold sidewalk, the heat in your feet is going to transfer to the cooler surface of the sidewalk by conduction and you will get cooler (which is nice in the hot summer, but uncomfortable when the weather starts to get chilly). Conduction can happen when the body is in contact with a solid (like a sidewalk), a liquid (like a bath), or a gas (like the air around you).
• Convection is essentially conduction with movement, and this movement makes the transfer of heat even faster. If you are standing inside and it is 70ºF in the building, you will likely be fairly comfortable. But if you are outside on a windy 70º day, even though the environment is the same temperature, you will get colder faster.
• We are all familiar with the warming effects of the sun's radiation, but in reality, all objects give off electromagnetic radiation. Radiation within the visible spectrum we perceive as colored light, but most radiation is outside our visible range.
• Evaporation happens when water (like sweat or moist breath) converts from a liquid state to a gaseous state, taking heat away from the body. Animals are always in contact with something (like surfaces, air, or water), so conduction is always occurring.

The speed at which an animal's body heats or cools depends on the temperature difference between the animal's body and its environment. That is, in a very cold environment, an animal will cool quickly and in a very hot environment, an animal will heat up quickly, whereas in an environment that is close to the animal's body temperature, the animal will heat or cool very slowly. To put this in mathematical terms, let's call the animal's body temperature Tb and the environmental temperature Te. The bigger (Tb-Te), the faster the animal will cool. And the bigger (Te-Tb), the faster the animal will heat up. This difference between Tb and Te (in either direction) is called the driving force of heat exchange.

Imagine this circle is an animal's body,
Tb is the animal's body temperature and
Te is the environmental temperature.
The bigger (Tb-Te), the faster the
animal will lose heat and cool down.

This works the other way around, too.
The bigger (Te-Tb), the faster the
animal will heat up.

 

What happens if you put fur on that animal? Now you can imagine this animal as having two separate layers, a body (with the temperature Tb) and an insulation layer (with the temperature Ti). Now for heat to be exchanged, it has to be conducted twice, once between the environment and the insulation, and again between the insulation and the animal's body. Ti is always going to be some intermediate temperature between Tb and Te and so the driving force of heat exchange will be much lower and the animal will heat up or cool down much more slowly. The thicker this insulation layer, the more stable Ti becomes and heat exchange happens even more slowly. Also, because insulation prevents movement at the body's surface, insulation layers eliminate any heat exchange at the body's surface (but not the surface of the insulation layer) by convection. (By the way, this logic also holds true if the animal has feathers or blubber or even a winter coat).


This inside circle represents an animal's body and the outside circle shows its insulation
layer. Tb is the animal's body temperature, Te is the environmental temperature and Ti is
the insulation temperature. Ti is always between Tb and Te, so the driving force of
heat exchange is reduced and the animal's body temperature does not change quickly
at all, even if the environmental temperature is extreme.

Most animals that have fur are mammals, as are most animals with blubber layers (like seals and whales) and animals that wear coats (like people and Paris Hilton purse dogs) and most animals with feathers are birds. What do these insulated mammals and birds have in common? They are endotherms. They generate most of their own body heat. This means that by slowing the exchange of heat between the animal's body and environment, the animal is provided with more time to generate heat and the insulation then helps to preserve this heat.

But reptiles (as well as amphibians and fish) are ectotherms. They get almost all of their heat from their environments. They maintain their body temperatures behaviorally, by choosing what environment to hang out in and what position to put their body in. If they are cold, they go bask in the sun to absorb radiation heat or lay on a warmed rock to absorb conducted heat. If they are hot, they lay on a cool rock in the shade to lose heat by conduction or soak in a cool stream to lose heat by convection. To maintain a relatively constant body temperature, they are constantly moving between warm and cool areas to adjust their body temperature one direction or another.

Many ectotherms rely on their ability to adjust their body temperatures quickly, and this ability depends on creating large driving forces of heat exchange. If an ectothermic reptile were to have an insulation layer, like fur, it would reduce its ability to adjust its body temperature by conduction and convection. It would lose its heat slowly and not be able to replace it fast enough. In the end, it would become too cold. It may seem paradoxical, but a lizard in a fur coat would likely die of cold-related physical issues (if not embarrassment).

Interestingly enough, just because lizards don't have fur doesn't mean they couldn't have hair. In fact, some of them do have hair, but not how you may think. Hair, fur, feathers, and scales are all made up in large part by keratin proteins. Many gecko species are well known for their wide, sticky toes that help them climb smooth, vertical surfaces (like walls). Their secret? Ultra-thin keratin hairs growing out of the geckos' feet provide a chemical adhesive force to keep the animal secured to the wall surface. So reptiles may not have a need for fur, but some of them have an innovative use for hair.

Want to know more about hairy geckos?

Autumn K, Liang YA, Hsieh ST, Zesch W, Chan WP, Kenny TW, Fearing R, & Full RJ (2000). Adhesive force of a single gecko foot-hair. Nature, 405 (6787), 681-5 PMID: 10864324

What Do Animals Think of Their Dead?

You’re running around, going about your day, and suddenly you see a dead guy lying in the sidewalk. What do you feel? Sad? Scared? Do you look around to see if you might be in danger too? Would you feel any differently if the dead body on the sidewalk were that of a squirrel, and not a human? Do animals share these same emotional and thought processes when they come across their own dead?

Teresa Iglesias, Richard McElreath and Gail Patricelli at the University of California at Davis pondered this philosophical question themselves. Then they set off to scientifically test it. 

A western scrub-jay collecting peanuts from a windowsill.
Photo by Ingrid Taylar at Wikimedia.

Teresa, Richard and Gail had noticed that when a live western scrub-jay encounters a dead western scrub-jay, it hops from perch to perch while calling loudly, a response the researchers called a “cacophonous reaction”. This boisterous response usually attracts other scrub-jays, which either join in with their own cacophonous reaction or just sit quietly observing. Is this truly a response to seeing their own dead?

The researchers put bird feeders baited with peanuts in backyards all over Davis, California (with the permission of the backyard-owners, of course). Once they find a feeder, western scrub-jays take the peanuts one at a time and fly off to cache them away before returning for another peanut. While the scrub-jays were away caching a peanut, the researchers put a collection of painted wood pieces on the ground, arranged to vaguely look like a dead scrub-jay. Then they snuck away to watch if the scrub-jays responded when they returned. Several days later, they came back to the same feeders, waited until the scrub-jay was away caching a peanut, and then placed an actual scrub-jay carcass and feathers (usually found somewhere in the area). Then they snuck away again to watch if the scrub-jays responded any differently when they returned.

Watch the behavior of western scrub-jays before and after
the placement of a dead scrub-jay. The “after” response starts
about one minute into the video. Video by Teresa Iglesias.

And in a nutshell, they did. When the scrub-jays returned to find a dead scrub-jay, they called like crazy and hopped around in a full-blown cacophonous reaction. In most cases, this reaction attracted other scrub-jays who joined in the lively response. Additionally, when the dead scrub-jay was present, they took 90% fewer peanuts. None of this ever happened in response to a pile of painted wood. When a scrub-jay returned to find painted wood, it went about its day, calling at normal rates and collecting peanuts as usual. One jay was so unconcerned by the painted wood, it even cached peanuts under it!

A western scrub-jay thinks the painted wood makes
a good peanut-hideaway. Video by Teresa Iglesias.

This convinced the researchers that the scrub-jays were not simply responding to something new near the feeder, but were instead responding to dead bodies. But does it matter whether the body is a conspecific (the same species) or a heterospecific (different species)? And what do these group responses mean? Are they gathering in mourning? Or is their response a way of hollering, “Look out! Something out there is killing us!”?

To find out, the researchers did the same thing they had done before, but this time, they placed either a scrub-jay carcass or a mounted great horned owl (a scrub-jay predator). Interestingly, the scrub-jays responded with the same cacophonous reactions and avoided the peanuts in both cases. However, the scrub-jays called for longer and defensively swooped at the mounted owl, something they didn’t do to the scrub-jay carcass. To check if this heightened response to the owl mount was due to its lifelike position, they repeated the study, comparing scrub-jay responses to a scrub-jay carcass or a mounted scrub-jay. Although the dead-looking carcass always elicited cacophonous aggregations, mounted scrub-jays only elicited cacophonous aggregations a third of the time. But when jays did respond to the scrub-jay mounts, they often swooped at it as if it were a competitor, something they never did to a scrub-jay carcass.

What does this all mean? Western scrub-jays respond to conspecific (scrub-jay) carcasses not just because their appearance is surprising, but because they may represent some kind of risk. They seem to recognize that the carcass is not a living threat, because they don’t swoop at it like they do to both owl and scrub-jay mounts. But they do produce an alarm response, much as they do when a predator is present. So their responses to dead scrub-jays are not so much “funerals” in the way that people mourn and reflect on their dead, but rather a way to announce a risk of getting hurt or killed.

Are western scrub-jays uniquely aware of the risk a dead conspecific may represent? Maybe not. Although this was the first comprehensive study of this phenomenon, similar behavioral responses to dead conspecifics have been observed in ravens, crows and magpies, all members of the corvid family of birds, like scrub-jays. But rats and even bees have also been observed to avoid dead conspecifics. Many animals may be more cognizant of death than we give them credit for.

Want to know more? Check this out:

Iglesias, T.L., McElreath, R., & Patricelli, G.L. (2012). Western scrub-jay funerals: cacophonous aggregations in response to dead conspecifics Animal Behaviour DOI: 10.1016/j.anbehav.2012.08.007

Mmm… The Scent of a Stud

Your smell can say a lot about you… How often you bathe, for example. But in many species, smells can communicate much more… What else might they be saying? And how do you ask them?

What secrets do we hide
when we put on deodorant
and perfume? Image by
freedigitalphotos.net.

Field crickets are one of many species that use pheromones, compounds released by an animal that affect the physiology and/or behavior of others of the same species. Female field crickets can recognize individual males by the pheromones they produce… That is pretty specific information! If females can smell who a male is, what else can she tell about him based on his pheromones?

Male field crickets fight for and defend both females and shelters. Furthermore, females are very picky about what males they mate with and tend to go for males who are better fighters. Raine Kortet and Ann Hedrick at the University of California at Davis asked female field crickets whether they could smell the difference between winners and losers.

Raine and Ann took pairs of male crickets that were the same age and size and placed each one on a separate piece of filter paper in a petri dish for 24 hours. This process infuses each filter paper with that particular crickets’ pheromones. Then they put each of the two pheromone-infused filter papers, plus a third clean filter paper, into an arena. They placed a female in the arena and timed how long she spent on each of the three filter papers. Then they repeated the whole process again with another 58 pairs of males.

Next, Raine and Ann put the size-matched pairs of males together in the same arena and allowed them to compete. Cricket fights generally involve wrestling and biting and then one of the crickets will retreat and avoid his dominant competitor. At this point, they were assigned the ranks of “dominant” (winner) and “subordinate” (loser).

This drawing by Edward Julius Detmold from the 1921 book
Fabre's Book of Insects depicts a dominant cricket defending his
shelter while a subordinate cricket retreats. Image from Wikimedia.

Females spent way more time on the pheromone-infused filter papers than on the clean filter papers. But even more fascinating, the females spent more time on the filter papers infused with the dominant male smell than the papers that smelled like losers. Remember, this was before the males even competed. So now female crickets can predict the future?! …Yeah, kinda. Females can smell which males will win.

But maybe this isn’t as mysterious as it looks at first glance. Pheromones are chemical compounds created by the body – the very same body that wins or loses fights. Bodies that are not in good shape may not be able to produce high-quality pheromones. Another possibility is that the same hormones that influence dominant behavior and fighting ability may also influence pheromones. Or maybe males that are more energetic simply move around more and deposit more scent on the paper. In any case, by picking up the scent of the dominant male, females may be able to choose a mate that is a good fighter, in good physical health, and who may pass these traits on to her offspring.

When you think about it that way, smells can contain a lot of information… So be careful what signals you’re putting out there.

Want to know more? Check this out:

Kortet, R., & Hedrick, A. (2005). The scent of dominance: female field crickets use odour to predict the outcome of male competition Behavioral Ecology and Sociobiology, 59, 77-83 DOI: 10.1007/s00265-005-0011-1