Horned Lizards

A summer’s fascination in adaptation -- Horned lizards

Published Aug 20th 2008 in Wildlife

“I haven’t seen a horny toad in years,” people keep telling us. “They are endangered, aren’t they? Why has a local pet store sold them in recent years?” Other folks ask us, “They shoot blood from the eyes, right?” A visitor from Andrews asked if it was all right if his kids painted the horns on the head of the horny toads in their yard. “We are trying to figure out how many we have, and getting a head start on a Science Fair project by recording our observations of their behavior and the size of their range. ”

The Texas horny toad (shown on the right in the photo below) is on Texas’ endangered species list, but not the national list. We have two species on the Llano Estacado. We also have the round-tailed horny toad (shown on the left in the photo) in the rockier habitats. The Texas horny toad eats harvester ants (the large red ants) but the round-tailed eats honey-pot ants. The populations of both species in West Texas are doing fine, except in town where people poison the harvester ants and in yards where the imported fire ant is present.

Horny toads not only shoot blood, but their skin color changes, and they have a “third eye,” all because they are ectotherms (cold-blooded animals) and need physiological mechanisms for thermoregulation. When a horny toad wakes up in the morning from its slumber just underneath the soil surface, it first sticks its head out of the sand. Its eyes bulge as it constricts tiny muscles surrounding the major veins that leave the head of the lizard. Blood flow stops for a few seconds, then the blood is shunted to smaller veins for return to the body. It no longer passes next to the cooler arterial blood. Its body temperature climbs and it emerges from the soil. Once out of the soil it raises and tilts its body to bask its back in the rays of the sun, further warming its blood as they increase the capillary blood in their skin.

When it first emerges, the skin is darker than it will be when it is hot. Melanophores (pigment cells) move up and down in the skin’s surface depending on the temperature. The darkening occurs when a hormone is released from the brain into the blood, which signals the melanophores to move towards the skin surface.

When it is hot, they often seek the shade, or face the sun so its back is oriented away from the sun. they also pull their ribs backward to minimize the amount of sun striking their back. In the shade, they flatten themselves against the cooler ground, losing heat to the soil. Horny toads also have a black peritoneum (transparent in most animals) that shields the body cavity from visible and ultraviolet radiation that would damage their reproductive and other organs. When it becomes extremely hot, the lizard either goes underground, into dark shade, or, as in the case of the Texas Horny Toad, it will climb up into a bush well off the heated ground.

The “third eye” is the pineal gland, which can be seen through a translucent spot on the top of its head, midway between its eyes. It has an uncanny resemblance to vertebrate eyes, as it has a transparent outer covering, a lens for concentrating light rays, and a retina onto which the sun’s radiation is focused, but it lacks an iris or fluid filled chambers. It is not connected to the brain via the optic nerve, so it does not form a visual image. The pineal gland’s function is to monitor their total exposure to the sunlight. Too much exposure to the sun might alter their seasonal reproductive cycle, putting them out of phase with other nearby horny toads. Beyond this hypothetical function, scientists still consider the pineal gland a mysterious organ.

When a horny toad digs underground, they lower their head as they rapidly vibrate it. The scales on its lower jaws serve as a cutting edge. Its front legs are held against the body. Once the head and forebody is underground, the rest of the body begins to oscillate in waves until all is buried but the tip of its vibrating tail that soon disappears. It can burrow itself to a depth of two to three inches in less than a minute.

Horny toads have a wide array of defensive behaviors. They will raise high on all four legs and charge in a bluff, although they will sometimes bite. If captured, they dig their horns into their captor. They also inflate their bodies with air to not be swallowed. They can also hiss and continue to stand with their mouth open. They will also twitch their tail nervously, mimicking a rattlesnake. They can, in a single motion, flip around in midair to face a predator. They will also rise on one side, to present a spiky shield to a predator.

Children always know of their “death feigning” when the lizard is turned upside down. Some will turn over again on their own, and have been known to remain upside down for up to two hours. Their most oft-used defense is to freeze, remaining motionless. The Texas horny toad has a white stripe down its back, and if the lizard is under a dried glass clump, the white line (like a dead blade of grass) effectively draws the eye away from the shape of the body. The fringe of spikes along their bodies breaks up the shape of their shadow, so they become almost invisible, when pressed to the ground.

Less than four percent of Texas horny toads captured will shoot narrow streams of blood. The eyelids become swollen, then a fine stream of blood shoots out from the slightly parted edges of the closed eyelids, spurting as far as six feet, and can be directed either forwards or backwards. Each spurt from the ruptured capillary vessels in the sinuses lasts a second, and can be repeated several times. Doglike animals react with obvious distaste to the taste of blood, indicating some chemical component causes gustatory distress to canines. Round-tailed horny toads never squirt blood. I have only seen it happen three times, each time in late May or early June, which has led me and other observers to believe that it might be a defense by gravid females just before they lay their eggs.

On July 12th, Vicki Sybert of the Texas Parks and Wildlife Department trained local Master Naturalists how to take DNA samples for researchers at TCU, and over 15 horny toads have been tested and their home range marked by GPS. The past and present presidents of the Horned Lizard Conservation Societyspoke at Sibley on August 8th. Horny toads have often been the topic of conversation at the Sibley Nature Center these past weeks – and I am glad -- the adaptations of horny toads are amazing. Look for another story on horny toads soon!