It’s become an annual tradition for Dean Williams – when the weather is hot, it’s time to travel to Karnes County to study horned frogs, also known as horned lizards.
The Texas Christian University biology professor makes a pilgrimage to South Texas in June, July and August to study the somewhat exotic creature that thrives locally.
“In Kenedy and Karnes City, we found horned lizards at much higher densities than in more natural (ranch areas),” Williams said. “We have been interested in why this is the case and so we have looked at their home ranges, genetics, vegetation patterns, diet, predation, and the temperature profiles in town.
“We found that many places in town still had a fair amount of native Texas vegetation and harvester ant mounds. There were also a number of places they could hide such as vegetation along alleyways, around trees, bushes, old sheds, and brush piles. Our early studies found that the horned lizards in town have very small home ranges (a town block on average) and that they are isolated from lizards outside of town - the reason for this may be roads - they rarely crossed roads and large ones like Highway 181 and Panna Maria in the Karnes City area appears to even keep different groups of lizards in town separated from each other.”
Abundant food supply
The food source for horned lizards – primarily ants and termites – is obviously a key to their survival.
“The number of harvester ant mounds in the towns were not enough to support all the lizards we found (it has been estimated that you need about six mounds per lizard) and so we started to study their diet by collecting their distinctive fecal pellets and identifying and counting all the insects they were eating (the insects hard exoskeletons stay intact through the horned lizard digestive system).
“We made an interesting discovery when we did this. We found that there is another food source, other than harvester ants that they can eat in South Texas – harvester termites. They eat thousands of these termites. These termites come from a group of termites that mainly occur in the tropics – there are several species that make it into South Texas, southern New Mexico and Arizona. These are not the termites that infest peoples houses or that build the little mud casings around vegetation (desert termites) – instead they come out of the ground when it is cool and damp, and eat vegetation on the surface. They move in columns much like ants, so horned lizards can just sit there and eat them as they go by.
“Termites are also a good source of protein and fats compared to ants. We also found out that they eat some harvester ants and a lot of smaller ants and sweat bees (which nest in the ground), and a few small beetles. One of the arguments for them eating harvester ants is that they are much larger and so they do not have to spend so much time eating out in the open (where they are exposed to predators) to meet their nutritional needs.
Horned toad enemies
“With the exception of feral cats– and (lawn mowers and weed whackers – there do not seem to be many of their natural predators in town so we conducted an experiment where we put out foam models of horned lizards in different parts of town and on a ranch in Dimmit County, Williams said. “The ones on the ranch were often attacked and had missing heads, limbs, peck marks while the ones in Kenedy and Karnes City were untouched.
“Unfortunately we cannot measure cat predation in town because the models do not move, but our experiment does suggest overall predation is lower in the towns than out at ranches and so maybe they can afford to spend more time in the open eating small things in town.”
The presence or absence of vegetation is another key factor in horned lizards’ ability to survive.
“During the course of our study they have disappeared completely from lots and alleyways where vegetation was cleaned up and hiding places like woodpiles were removed,” Williams said. “Horned lizards can not regulate their body temperature like we can (internally) and so they need sunny and shady areas to keep their temperatures in their preferred range. We are currently looking at horned lizard temperatures in town as well as temperatures in places they normally hang out by using temperature loggers in model horned lizards (open areas, bushes/vegetation, slightly buried).
“Horned lizards can handle hotter temperatures than most lizards and prefer to be at 97-100 Fahrenheit. Their upper limit before they die is about 118F degrees. We have found that in the summer there are about five hours during the day where open ground temperatures far exceed their upper limit and so we think their inability to regulate their temperature may be the main reason they disappear from lots with little vegetation like bushes – inside a bush or a thick grass patch it never gets above their critical limit and so provides a good refuge from summer heat.”
A tradition begins
It was an invitation to Karnes County which initially led Williams to the area.
“I was contacted by Ryan Darr, a wildlife biologist from Texas Parks and Wildlife in 2011 about horned lizards living in Kenedy and Karnes City,” Williams said. “I went down in 2012 and met Wade Phelps who ran the Kenedy horned toad club and then started the study in 2013. We have been down every year since then for a couple of weeks in June, a week in July, and a week in August.
“We monitor the same 15-20 sites every year – alleyways, school yards, abandoned lots. We map all ant mounds, catch, measure, genetically sample, mark, and release all horned lizards. We used to use injectable pit tags to mark them (small versions of what you use for dogs) but we have found that you can tell individuals apart by the spots on their bellies – which is much cheaper, and so now we take pictures of their bellies and keep a photo catalog to tell them apart.
“During a single year we also mark them with a dot of fingernail polish on their underside so we know we caught that one earlier – they will shed their skin once or twice a year so that usually comes off before the next year but their spots stay the same.”
What can people do to help protect and promote the local horned lizard population?
“Planting native vegetation is a big one – especially native grasses that provide food for harvester ants and harvester termites and also allow horned lizards to move around more easily,” Williams said. “They do not do well in lawn grass, like St. Augustine, and they need bushes/cactus, fence row vegetation, and vegetation around the base of trees – brush piles are also good. Leaving harvester ants alone is also beneficial. Horned lizards do best with a patchwork of open areas, low vegetation, and bunch grasses.”