Ecology and Species Comparisons of the Short-Horned Lizard (Phrynosoma douglassi) and the Desert Horned Lizard (Phrynosoma platyrhinos)

Brodie Hylton

Introduction

Horned lizards (Phrynosoma spp.) are a unique genus of lizards with specialized morphological, behavioral, and ecological adaptations that set them apart from other species of lizards. "For example, compared to other lizards, many horned lizards 1) are strongly dorso-ventrally flattened, 2) are generally armed with sharp spines, particularly around the occipital region, 3) have relatively short legs and are slow and awkward, 4) are noticeably reluctant to run when approached by a large animal, 5) are typically active over a long time span, 6) often have more variable body temperatures than other sympatric lizard species, 7) have a conspicuously specialized diet which consists largely of ants and/or, sometimes, beetles, 8) tend to have an exceptional large stomach capacity for their body size, 9) have a specialized dentition which facilitates ant-eating (myrmecophagy), 10) produce large numbers of relatively small eggs or young and 11) expend a considerable amount of energy on each clutch or litter of young" (Pianka and Parker, 1975). Here, in Oregon, we have two species of horned lizard. These are: the short-horned lizard (Phrynosoma douglassi) and the desert horned lizard (Phrynosoma platyrhinos). The distribution, diet, morphology, defenses to predation, thermoregulation, and reproduction of this genus differs greatly from most other species of lizards which therefore makes them an interesting topic of study.

Distribution

Phrynosoma are restricted to western and central North America from British Columbia and Alberta in the north to Guatemala in the south. Within this range there are 12 recognized species that are, for the most part, restricted to arid and semi-arid habitats such as desert, thorn scrub, and grassland (Pianka and Parker, 1975). The range of the desert horned lizard, P. platyrhinos, includes the Great Basin and the Mojave and Sonoran Deserts while the short-horned lizard, P. douglassi, is distributed from south-eastern British Columbia, to North Dakota, and south to New Mexico. In general P. douglassi inhabit regions of higher elevation and cooler average temperatures than P. platyrhinos. Although both species cover almost the same wide range, P. douglassi is more common in the foothill and mountain regions while P. platyrhinos is found more often on valley floors west of the Rocky Mountains. Both lizards require sufficiently friable soil in order to stay hidden and to avoid temperature pessima (Dumas, 1964). Dumas (1964) states that distribution of P. douglassi and P. platyrhinos in the northern intermountain areas are primarily due to temperature factors with predation playing a minor role. His research showed that interspecific competition between the species' was not a factor in maintaining the allopatry of the species-pair. The ranges of these two species do not often overlap.

The upper limit (both elevation and latitude) to both species of Phrynosoma is temperature. The body temperature at which normal activity takes place is the same for both lizards, however P. platyrhinos takes twice as long to "warm-up" because of its much larger body size (Dumas, 1964). In order for any lizard to feed normally it must reach an optimal body temperature for a certain amount of time. In mountainous regions P. douglassi is able to do this while P. platyrhinos cannot. More detailed coverage on this topic in included under thermoregulation.

Breeding characteristics also allow for P. douglassi to live at higher elevations. P. douglassi is ovoviviparous which is characteristic of reptiles living in cold or wet conditions. P. platyrhinos is oviparous and because of this would not be a very successful breeder in the harsher conditions in the range of P. douglassi. More detailed coverage of breeding of the two lizards can be found further, under reproduction.

According to Dumas (1964) the lower limit of P. douglassi is set in part by predation by the leopard lizard and the whiptail. Wherever these potential predators of P. douglassi are found, the short-horned lizard itself is scarce or, more often, absent. P. platyrhinos is apparently to much for the leopard lizard or the whiptail because of its larger spines and overall larger size. In field and laboratory experiments by Dumas (1964), adult P. douglassi were quickly eaten by leopard lizards and whiptails in areas where leopard lizards, whiptails, and desert horned lizards were abundant. Both leopard lizards and whiptails released P. platyrhinos individuals without permanent injury.

Diet

Although there are a few exceptions in some species of Phrynosoma, both P. douglassi and P. platyrhinos as well as most other species dine exclusively on ants. As stated earlier, Phrynosoma are very dorso-ventrally flattened. This allows them to be very close to the ground in order to easily consume ants as well as bury themselves in the sand or dirt. Horned lizards are lie-and-wait predators with fantastic camouflage. They are the only lizards in North America that are myrmecophagous and there are few others in the world. Phrynosoma are often found in the open, near ant nests. Besides ants, beetles may dominate the diet when they are abundant (Pianka and Parker, 1975). In studies done by Pianka and Parker (1975) P. platyrhinos individuals stomach contents were composed of 35-65% ants and 20-50% beetles. It does appear, however, that ants are the favored food of Phrynosoma. Whenever captive, Phrynosoma will rarely survive if not given a regular offering of live ants (Sherbrooke, 1995).

Morphology

The morphology of the horned lizards is the most noticeably unique characteristic of this genus. For the most part, the morphology of Phrynosoma is facilitated towards ant-eating, their main dietary source (Montanucci, 1987). Because of their diet, Phrynosoma possess a very large stomach. Presumably this is a result of the amount of ants eaten per day coupled with the much unusable chitin contained in ants (Pianka and Parker, 1975). Their short, stout bodies and short legs do not allow for rapid movement. Their dorso-ventrally flattened bodies and near perfect camouflage allow Phrynosoma to be easily buried and hidden in loose substrate. The body and head of Phrynosoma is covered with spines which probably serve as both protection and camouflage. The head of P. platyrhinos has much larger "horns" than P. douglassi. The horns are a means of protection against predation from other reptiles, and since leopard lizards, whiptails, and other predators of Phrynosoma are rare if not absent in the range of P. douglassi, long horns are not necessary for this species (Dumas, 1964).

Defenses to Predation

Although many of the defenses to predation have been touched on previously, there are some that have not been discussed or not been discussed with any detail. It has been noted that the main defense against predation for Phrynosoma is it's near perfect camouflage and subsequent reluctance to move (Pianka and Parker, 1975). If spotted by a possible land predator, such as another lizard or a snake, the horns of Phrynosoma often prove to be too much, except of course in the case of P. douglassi which has much smaller horns but very few land predators (Dumas, 1964). The only predators Phrynosoma really need to be wary of are birds (Pianka and Parker, 1975). Sometimes the camouflage isn't enough, and since Phrynosoma are reluctant to move and often out in the open they are an easy target from the air when spotted.

Like all lizards, Phrynosoma are able to lose their tails. As mentioned earlier, they have few land predators and because of this it is rare to find a horned lizard with a broken tail. In studies done by Pianka and Parker (1975) only about 5% of Phrynosoma had tails which had been broken.

The least used defense to predation of Phrynosoma is also one of the most interesting and unique characteristics to the genus. When threatened, presumably from the front, it will shoot blood from small openings just below the eyes. This characteristic is rarely seen by humans as mock attacks or rough handling rarely elicits blood-squirting (Mendez, 1992). It is now being considered, after a discovery made at an American Museum of Natural History Research Station in Portal, Arizona, that blood-squirting is a canine defense mechanism (Mendez, 1992). No matter what time of day or what temperature the lizard was, a dog was able to get Phrynosoma (no species mentioned) to consistently shoot blood (Mendez, 1992). Whether blood-squirting is indeed a specific canine anti-predatory device is still unclear (foxes and coyotes would be natural predators), but the fact remains that it is a defense mechanism, and quite an incredible one at that.

Thermoregulation

All species of Phrynosoma are able to be active over much larger time spans, and thus much wider temperature ranges than most other lizards. But, since P. platyrhinos and P. douglassi inhabit very different temperature ranges, thermoregulation differs greatly between the two species. The temperature at which normal activity takes place is about the same in both species, 26.5-40.5 degrees C (Dumas, 1964). However, P. platyrhinos takes twice as long to warm up as P. douglassi because of it's much larger body size (Dumas, 1964). Inversely, the small body size of P. douglassi enables it to warm up quickly in order to feed if temperatures are only briefly high enough for normal activity. P. douglassi is also darker in color than P. platyrhinos which would increase heat absorption. The lethal limit for both species is 41 degrees C, but because of the lower rate of heating of P. platyrhinos it is better able to survive in temperatures near the upper limit (Dumas, 1964).

Cooling in these two species also takes place at a different rate. P. douglassi cools much more slowly than P. platyrhinos, again, because of the smaller body size of P. douglassi (Dumas, 1964). A more compact body reduces surface area for radiation of heat (Dumas, 1964). The ability of P. douglassi to warm up more quickly and cool down more slowly could be a limiting factor to its distribution in the hotter regions inhabited by P. platyrhinos. The opposite pattern of temperature regulation and related distribution seems to be true for P. platyrhinos.

Reproduction

Reproductive tactics of horned lizards are, like many of their characteristics, somewhat unusual among lizards. They have a very high reproductive potential, expending large amounts of matter and energy on their clutch or litter (Pianka and Parker, 1975). Phrynosoma produce large numbers of eggs or offspring in order to compensate for a relatively high mortality of the young (Pianka and Parker, 1975). As adults however, survivorship is very high (Powell and Russell, 1991).

One of the most noticeable differences between P. platyrhinos and P. douglassi is the fact that the former is oviparous and the latter is ovoviviparous. When thinking about the distribution of the two species, it is understandable why the characteristics of each have been selected for in this particular way. Reptile eggs must be kept warm and relatively dry, and in the case of P. douglassi this would be very difficult if not impossible because of their distribution. Retaining eggs within the body and bearing live young is probably essential to the survival of this species because there is no incubation time necessary outside the body of the gravid female allowing the female to regulate the temperature of the unborn young herself in the warmer summer months. P. platyrhinos, on the other hand, inhabits warmer regions and breeds during the middle to end of July, a time that supports temperatures that effectively incubate her eggs (Pianka and Parker, 1975). Eggs are buried in warm, moist, loose substrate and incubate for about 20 days before the young emerge (Pianka and Parker, 1975). Since there is such a large amount of young born to each gravid female (anywhere from 2 to 16 in P. platyrhinos in particular in studies done by Pianka and Parker, 1975), the young are very small and helpless for the first stage of their life which would explain the high rate of mortality.

Many features of distribution, diet, morphology, defenses to predation, thermoregulation, and reproduction of P. platyrhinos and P. douglassi can be "interrelated and interpreted as a series of coadapted phenotypic traits, whose functions complement one another to make (these lizards) ecologically successful (Pianka and Parker, 1975)." Most importantly, the adaptations all facilitate efficient exploitation of ants as a food source setting horned lizards apart from other species of lizards. In other words, their niche is uncontested and therefore success as a species relies on defenses to predation rather than competitive advantages. Again, to summarize the strange and interesting characteristics of Phrynosoma discussed in this paper; "compared to many other lizard species, Phrynosoma 1) are strongly flattened dorso-ventrally, 2) are usually armed with sharp occipital spines, 3) have relatively short legs and are slow and awkward, 4) are noticeably reluctant to move when approached by a potential predator, 5) are often active over a long daily time span, 6) frequently have more variable body temperatures than other sympatric species of lizards, 7) have a conspicuously specialized diet consisting largely of ants, 8) possess an exceptionally large stomach for their body size, 9) have a specialized dentition that facilitates ant-eating, 10) produce large numbers of relatively small eggs or young and 11) expend a considerable amount of matter and energy on each clutch or litter (Pianka and Parker, 1975)."

References:

Dumas, P.C., 1964. Species-pair allopatry in the genera Rana and Phrynosoma. Ecology 45 (1): 178-181.

Pianka E.R. and W.S. Parker, 1975. Ecology of horned lizards: a review with special reference to Phrynosoma platyrhinos. Copeia 1975 (1): 141-162.

Sherbrooke, W.C., 1995. Collecting and feeding harvester ants to captive horned lizards. Herpetelogical Review 26 (1): 25-26.

Powell, G.L. and A.P. Russell, 1991. Partuition and clutch characteristics of short-horned lizards from Alberta. Canadian Journal of Zoology 69 (11): 2759-2764.

Mendez, R.A., 1992. Seeing red. Natural History 101 (10): 40.

Montanucci, R.R., 1987. A phylogenetic study of the horned lizards, genus Phrynosoma, based on skeletal and external morphology. Contributions in Science: Natural History Museum of Los Angeles County 18 Dec. (390).

Heath, J.E., 1965. Temperature regulation and diurnal activity in horned lizards. University of California Publications in Zoology. University of California Press 64 (3): 97-136.