Real Zombie-Making Parasites Among Us

The mummified cat and the rat in the crypt of Christ Church in Dublin.
Photo by Adrian Grycuk at Wikimedia Commons.

The Happening, M. Night Shyamalan’s worst panned movie of all time, is a science fiction thriller about people going into a mysterious trance and committing suicide as a result of other mind-hacking species. One of the leading criticisms raised against this movie is the ridiculousness of the premise. One species can’t cause another to willingly commit suicide! …Or can they?

Toxoplasma gondii (we’ll call it T. gondii) is a protozoan parasite that has developed just such mind-hacking abilities! As far as we can tell, T. gondii only reproduces in the digestive tract of cat species, where it lays fertile eggs that are pooped out into the environment. From there, T. gondii eggs can contaminate any number of things that are consumed by other animals, such as rodents, birds, or even humans. When cats eat prey animals that are infected with T. gondii, another generation of parasites is now positioned to reproduce and the cycle continues.

However, prey animals can be pretty good at avoiding cats, in part by avoiding the smell of cats. This is a problem for the reproductive plans of T. gondii. The tiny protozoan has responded to this problem with remarkable biological sophistication: They alter the behavior of their rodent hosts so that the infected rodents find the smell of cat urine so irresistible that they run straight towards their predators! Now, researchers have found that T. gondii-infected rats don’t only like the smell of cat urine, but they even prefer the smell of wild cat urine over the smell of urine of weaker domesticated cats.

A rat checks out odor-soaked papers
in a Y-shaped apparatus. Image from
Kaushik, et al. (2014) in
Integrative and Comparative Biology.

Maya Kaushik, Sarah Knowles and Joanne Webster at the School of Public Heath at the Imperial College of London compared the responses of rats that were either infected with T. gondii or not to urine produced by domestic cats or wild cats. To do this, they put infected or uninfected rats into a Y-shaped apparatus. For each trial, tissue paper soaked in domestic cat urine or wild cat (cheetah or puma) urine was placed in two of the three arms and nothing was placed in the third arm. The researchers then measured how much time the rats spent in each of the three arms and how much they moved.

As expected, the T. gondii-infected rats avoided the cat-urine-soaked arms less than the uninfected rats did. Furthermore, when presented with a choice between arms with wild cat urine versus domestic cat urine, the infected rats (but not the uninfected rats) preferred the smell of the predatory wild cats over the domestic cats! Infected rats also moved more slowly around the wild cat urine compared to domestic cat urine, as if just begging any wild cats that may be around to eat them. It appears that T. gondii have developed a mechanism to turn rats into mindless zombies that practically run into the mouths of the nearest, most vicious cat they can find.

These mind-hacked rat-zombies may not be the only victims of T. gondii. People (particularly those that change their kitties’ litter boxes) can also become infected with the parasite. Some estimates suggest that nearly one-third of all people are already infected! Furthermore, people that test positive for T. gondii infection find the smell of cat urine more attractive than people who test negative! Although we are not likely to run to be eaten by our house-bound kitties, we may be more likely to change the litter box (or get more cats and become a crazy cat lady). So it looks like many of us are mind-hacked zombies too!

Want to know more? Check this out:

Kaushik, M., Knowles, S., & Webster, J. (2014). What Makes a Feline Fatal in Toxoplasma gondii's Fatal Feline Attraction? Infected Rats Choose Wild Cats Integrative and Comparative Biology, 54 (2), 118-128 DOI: 10.1093/icb/icu060

Caught in My Web: Creepy Animals, Animal Disguises, Pet-Love After Death, and Ebola

For this Halloween edition of Caught in My Web, we check out what the web has to say about creepy animal stories.


1. The CDC addresses questions about whether pets can get ebola

2. Julie Hecht, the author of Dog Spies at Scientific American blogs explores the love between dogs and their owners from beyond the grave

3. BuzzFeed presents 8 surprisingly creepy animals.

4. Dr. Doolittle at ScienceBlogs talks about the eyeless Mexican cavefish

5. Sara Mynott at Saltwater Science, a blog at Nature’s Scitable blog network, explains flounder disguises (they’re not just for Halloween)

Is That Lizard Possessed!? (A Guest Post)

By Tawny Liebe

Image from Chuck Heston on Flickr.

A creature straight from the depths of hell… or at least as close as you can get on this planet. The Texas horned lizard or “horny toad” is found in the deserts of the southwest United States and has an unusual adaptation to deter predation exclusively from a few species of canines. When threatened by coyotes, foxes, and dogs, the horned lizard squirts blood from its eyes to hit targets up to three feet away! A total of six species of horned lizard have been proven to respond this way to canine attack, while none have responded this way to other predators, such as the grasshopper mouse or the roadrunner. So what is the deal? How do they do this and how does it work as predator defense?


Veins have one-way valves
to prevent backflow.
Drawing by Tawny Liebe.

Before we get to that, there are a few things you need to know. First of all, the circulatory system includes a network of arteries and veins. Arteries carry blood full of oxygen to body tissues while veins carry blood that lacks oxygen from the rest of the body back to the heart. This means that for the blood, in someone’s foot for instance, to get all the way back to the heart through the veins, the blood must work against gravity. When you (or this lizard or many other species) move, blood is propelled from one chamber in the vein to the next until it reaches the heart. The blood is prevented from flowing back into the previous chamber by one-way valves, causing the blood to pool.

In order to squirt blood from their eyes, horned lizards manipulate the network of veins in their head so that they build up pressure, like a volcano getting ready to blow. By constricting a pair of throat muscles unique to reptiles, they effectively close their jugular veins and increase the blood pressure in their head. This is thought to be enhanced by another pair of muscles between the jugular veins and the eyes that help increase the blood pressure in the head even further, causing the blood to move into the sinuses of the eyes. The pressure continues to build until the blood breaks through the wall of the eye socket into the eyelids where it is forced into the tear duct and erupts like a Mentos in a Coke bottle all over whatever is chomping at the poor little lizard.

As if these lizards couldn’t get any more amazing, recent studies have shown that it is actually a compound in their blood that canines don’t like. Scientists have also found that this chemical is in their circulating blood, not just the blood that is squirted from their eye, rejecting an earlier hypothesis that the chemical is picked up in the tear duct. To top off all of this awesomeness, the chemical may be acquired through its main food source- harvester ants, which are venomous. These ants aren’t actually a requirement of the horned lizard’s diet and yet they are specialized to eat them. The horned lizard’s blood plasma binds to the venom, which neutralizes its toxicity and the resulting compound may be what deters these canines.

Now that we know how horned lizards are capable of this type of defense and how they most likely make their blood so undesirable, what is it about this chemical that is so appalling to these predators? It appears that the target area of the blood is the mouth since the horned lizard only squirts the blood when the canine begins to bite down on its head. This suggests that it may be the taste of the blood that prevents the horned lizard from becoming that coyote’s tasty snack.

The horned lizard’s ability to squirt blood at canines to prevent their untimely death is truly amazing and complex and there is still much to learn about it. Their ability makes me wonder how many other cool anti-predator adaptations there are out there in the animal and even the plant kingdom! Below is a video that will allow you to appreciate the full effect of this awesome defense strategy, enjoy!

References:

Heath, J.E. 1966. Venous shunts in the cephalic sinuses of horned lizards. Physiological Zoology 39(1): 30-35.Middendorf, G.A. and Sherbrooke, W.C. 1992. Canid elicitation of blood-squirting in a horned lizard (Phrynosoma cornutum). Copeia 1992(2): 519-527.

Middendorf, G.A. and Sherbrooke, W.C. 1992. Canid elicitation of blood-squirting in a horned lizard (Phrynosoma cornutum). Copeia 1992(2): 519-527.

Middendorf, G.A. et. al. 2001. Comparison of blood squirted from the circumorbital sinus and systemic blood in a horned lizard, Phyrnosoma cornutum. The Southwestern Naturalist 46(3): 384-387.

Middendorf, G.A. and Sherbrooke, W.C. 2004. Responses of kit foxes (Vulpes macrotis) to antipredator blood-squirting and blood of Texas horned lizards (Phrynosoma cornutum). Copeia 2004(3): 652-658.

Sherbrooke, W.C. 1992. Chiricahua Mountains Research Symposium. Horny “toad” tales from the Chiricahua mountains as, told by a biologist. Southwest Parks and Monuments Association, Tuscon, AZ. 78-80.

The Biology of Nagging

A female pied flycatcher can't feed herself sufficiently
while she incubates her eggs and newly-hatched
chicks. Photo by Alejandro Cantarero.

I have been blessed with the fortune of not just having two healthy and happy babies, but being able to spend much of the spring and summer nurturing them and watching them develop and grow. But it has not been all roses: their smiles beam through the fog of my sleep deprivation and exhaustion. Their tears are met with my own. Our clothes are stained in a rainbow of bodily fluids. Now I am back at work trying to remember how my life used to be and how to meet my obligations to countless people, all while trying to keep up with the ever-changing needs of my daughters. Luckily, I am not alone. The girls have a rotating schedule with their grandparents one day, with their dad another, at daycare for a few days, and with me on weekends and evenings. But as the expectations on me grow heavier, I find myself pushing my husband harder to do more with the girls and around the house. Now it seems like every time I open my mouth, I am accused of “being a nag” and “for no reason” no less. The truth is, nagging has a deep biologically-based reason and may even be critical to species survival.

We are not the only species that nags, although in other species these vocalizations are often called “begging signals”. Begging signals are commonly heard in bird species in which the female does most or all of the egg and chick incubation. Because these females cannot sufficiently feed themselves while ensuring the survival of their brood, their male partners need to spend extra time foraging for the females in addition to foraging for the chicks and themselves. There is an inherent conflict between how mated males would prefer to spend their time (feeding themselves, maintaining their dominance status, and flirting with females) and how their female partners want them to spend their time (providing as much as possible for the family). The male response to this conflict is often to see how little he can get away with contributing while he sneaks off to spend his time as he wishes. The female response is to produce loud, juvenile vocalizations and gestures until he brings food to the nest. Is this really necessary though? Maybe she is just being manipulative to try to get him to do more than he really needs to.

Alejandro Cantarero, Jimena López-Arrabé, Antonio Palma, and Juan Moreno from the National Museum of Natural Sciences in Madrid, Spain and Alberto Redondo from the University of Córdoba in Spain set out to test whether the begging signals made by female pied flycatchers were an honest signal of need or were just obnoxious melodrama. They predicted that if the females’ begging calls were an honest reflection of how much help they needed, then begging calls would increase as energy needs increase.

The researchers studied 71 pied flycatcher nests in a forest in central Spain. Behaviors were observed by nest-mounted video cameras five days after the females finished laying their clutches of eggs. Two days later, each female was caught and measured, fitted with an identifying leg band, and had her wings clipped. About half of these females had their primary feathers clipped at the base to impair their ability to fly (these females are called the “handicapped” group). The other half had their primary feathers clipped at the tip so that their ability to fly would not be affected (these are the “control” females). When the females were released, they all returned to their nests. Their behaviors were measured again three days later.

Females in the handicapped group lost weight and begged significantly more after their wings were clipped, whereas the control females did not. This suggests that females are adjusting their begging rates to accurately reflect their needs. Furthermore, male partners of the handicapped females fed their partners more after the wing-clipping, whereas the male partners of the control females did not. This shows that the males are responding to either their partners’ increased begging or increased need or both. Revealingly, the amount that females begged was positively correlated with the amount that the males fed them, even when the researchers statistically controlled for whether their wings were clipped or not. This means that males were feeding females more because they begged more (and not simply because they needed more, which was also true).

This nagging female gets exactly what she needs.
Video by Alejandro Cantarero.

So, at least among pied flycatchers, females don’t “nag for no reason”, but because they genuinely need the help to keep their bodies and families healthy and safe. And males respond to the calls for help by increasing their contributions. But the graph that shows that males feed females more because they beg more also reveals that males would not help enough if females did not beg enough. Nagging is an adaptive strategy that females must engage in to meet the needs of the family.

Is someone nagging you too much? If we are like our pied flycatcher friends, than if you meet that person’s needs, the nagging should stop.

Want to know more? Check this out:

Cantarero, A., López-Arrabé, J., Palma, A., Redondo, A., & Moreno, J. (2014). Males respond to female begging signals of need: a handicapping experiment in the pied flycatcher, Ficedula hypoleuca Animal Behaviour, 94, 167-173 DOI: 10.1016/j.anbehav.2014.05.002