The survival tactics of horned lizards

In the mid-16th century, when Spain sent its first scientific explorers to Mexico, Francisco Hernandez reported seeing a lizard squirt blood from its eyes. Horned lizards — found primarily in Mexico and the American Southwest.

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Imagine a horned lizard — fat and brown with horns pointing from the back of its head — sitting in the sand. It sees a fox approaching. The lizard sits still, hoping the fox won’t be able to distinguish its brown skin from the sand. But the fox sees the lizard and pounces! Its teeth clamp down on the lizard’s spiny head. Then the lizard does what only a few other lizards can do — it squirts blood from its eyes. The blood tastes horrible — and the fox drops the lizard in disgust.

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Phrynosoma cornutum

Only eight species of horned lizards are known to squirt blood from their eyes. They use this defense only against canine attackers — such as foxes or coyotes. Like many other lizards, horned lizards have blood-filled sinus cavities. This blood heats up quickly on a cold morning and is then circulated to colder parts of a lizard’s body. Besides staying still, blending in, and squirting blood, horned lizards can puff up their wide bodies until they look like “spiny balloons.”

Horned lizards have been around at least fifteen million years. But today some are disappearing. Fire ants are edging out their preferred food — big, red “harvester” ants. And fire ants have given all ants a bad name. So people are using more pesticides to kill ants — and horned lizards are going hungry.

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Horned lizards belong to a family of New World reptiles called “Phrynosoma,” which means “toad bodied.” They’re often mistakenly called “horny toads.”

Horned lizards are slow, fat and they multiply quickly. They sport horns and spines along their backs. They eat mostly ants. Ants are mostly made of chitin. To get enough energy, a horned lizard has to eat them in large amounts — over 200 ants a day. And this is one reason why a horned lizard is so fat and slow — it has a huge stomach.

Humans and Horned Lizards

To natives of North America — such as the Anasazi, the Hopi, the Navajo, and the Zuni — the horned lizard is a symbol of strength. But now the horned lizard is struggling to survive.

Much of the decline is due to the destruction of the lizards’ natural habitats by humans. But another key culprit — especially in Texas — is the fire ant. This ant species was accidentally introduced from South America several decades ago. Fire ants out-compete and eradicate the lizard’s primary food source — big, red “harvester” ants. Yet fire ants — with their different nutritional content — aren’t a good substitute food for the lizards. What’s more, fire ants have given all ants a bad name — because they’re aggressive and have a worse bite than native ants. So in recent years, people have used more pesticides to kill all ants — and horned lizards are finding it harder to get a good meal. So the Texas horned lizard is no longer found in about a third of its geographic range.

Warm Blooded or Cold Blooded?

Controversy over whether or not lizards are endothermic and exothermic. Wendy Hodges: “[Lizards are called] exothermic, but in reality, they keep a high body temperature — they have very hot blood and they can regulate their temperature through behavior — through solar heat, and through their sinuses. So there is a movement to call them endothermic. The majority of lizards are like this…”


Wendy Hodges on blood squirting: “A lot of lizards have this — well-developed sinuses filled with blood. Their day to day life goes like this: wake up in the morning, laying on top of sand, they pop up their head … the head fills up with blood, the sun comes up and the blood warms up quickly…. One of the sinuses goes to the eyelid … they can do a significant amount … The defense mechanism is directed only at cannids. The blood is distasteful and causes the animal to drop the lizard. It seems like this has evolved … Sand lizards also heat up themselves with blood in the head’s sinuses.”

Wendy Hodges on adaptation and success: “Part of their success is associated with their diet. The ants are a very reliable food source. The ants are active all year because they store grain for the entire year, not just when it rains or when it is cooler. That may be why the horned lizards have been very successful in deserts … they don’t require outside water… they use mostly metabolic water. They might lick dew from plants in the night and early morning. And, like all lizards, they produce solid urine (uric acid), this is how they conserve water.”

From an interview with Wendy Hodges:

Q. Why have these lizards practically disappeared in Texas?

A. “It’s primarily human induced impact (in my opinion) that is responsible for the horned lizard decline in Texas — and that means habitat loss (mostly). In Texas, it’s the conversion of habitat to agricultural land and urban areas. In the 1920s and 1930s, agriculture didn’t have such a big impact on lizards. People used to work in cotton fields and they would find the horny toads. They did the work by hand. But after WWII, there was a shift, people relied more heavily on mechanized agriculture and pesticide use. So this new type of agricultural activity had a bigger impact on the lizards.

“Imported fire ants are a big problem too. We had a decline of horned lizards prior to this invasion, [but it’s mostly been since the fire ants have come along] … The fire ants are contributing to the loss and preventing them from returning to areas even after [bad] agricultural use has been given up and the land changed back [to viable habitat] …. More indirectly, the fire ants forage under ground, which disturbs bird and reptile nests and burrows. Horned lizards burrow into the ground and lay eggs in the ground as well. …

“Fire ants first started hitting Texas in the 1950s. It began in Mobile, AL in the 1920s and 1930s. There are a red and a black species of fire ant that were introduced. The black ants were introduced first. In the 1930s, the red ones were introduced — this one was even better at invading the southern U.S. than the earlier, black fire ants. Now the fire ant occurs from Florida to Central Texas. It’s in all the Gulf States, and maybe Tennessee and Kentucky too. … Horned lizards are pretty strong. They’ll hang around for a while and then they’ll be gone.”

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