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DHLs--observations on feeding

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Posted by: 53kw at Tue Jun 17 14:59:47 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by 53kw ]  
   

While literature reports that horned lizards, especially the desert species, prey almost exclusively on ants, I am still not convinced they require some nutrient found only in ants, as that is not the way evolution works. It's commonplace for people, including scholars, to fall into the habit of imagining that evolution sees a need and fills it, but this is impossible. Of course, the way evolution really works is when an organism lucks into a mutation AND the mutation happens to have an immediate and substantively beneficial application. Neutral mutations are lost through dilution back into the general gene pool through matings with siblings who are just as successful as the mutated individuals, because the mutation does not confer any profound selective advantage that would enable the mutant to prosper to the extent it outstrips its non-mutant siblings. Negative mutations typically kill the individuals who express them, and that’s the end of that.

The ancestors of horned lizards lucked into a mutation that allowed them to detoxify ant venom, enabling them to exploit an abundant, ubiquitous food source, allowing them to inhabit relatively competition and predator free habitats like very dry deserts.

That said, I’m still reluctant to cut off wild-caught horned lizards from their native prey, and husbandry experience by horned lizard keepers suggests there may be more to ants in the diet than meets the eye. Despite my reservations regarding some magic nutrient found in ants, I’m not ready to risk the health of the animals on my suspicions.

Several weeks ago I located some Desert Horned Lizards in a local pet shop. I asked how long they had been there and what they were eating. I was told the lizards had been in the store for about four months and were eating vitamin dusted crickets with occasional mealworms. This does not seem like a diet likely to sustain health in horned lizards and yet the animals in the shop were bright-eyed, active and well fleshed. I am satisfied that the shop owners were accurate in their claims as to the length of time the lizards had been in the shop.

I asked if they would be willing to test some ideas for me regarding acceptability of local ants as prey items. Instead, they asked me to just take the lizards home and work on them there.

These Desert Horned Lizards are likely from southern Nevada. Sherbrooke reports that Desert Horned Lizards in that area prefer to feed on a harvester ant which forages alone. I offered the lizards some carpenter ants, which are very large, black ants, as individual ants and in small groups. I also offered some of the winged reproductive carpenter ants, which are larger still, nearly one-quarter inch long.

At first the lizards showed little interest in the ants, preferring crickets. After a few days, one lizard started eating the ants and little by little two others have followed suit. I find it interesting that they seemed at first to prefer ants which were injured or still sluggish from refrigeration. They even ate dead ants, including rarely eating ants which had been laying on the substrate for hours. I did not notice any preference for such ants, and I suspect the eating of long-dead ants is accidental—a consequence of routine tongue-touching to investigate anything on the ground and subsequent swallowing of the dead ant after it adhered to the lizard’s tongue. Ants which were injured or sluggish were deliberately targeted, however. The lizards also showed a strong interest in the reproductive ants, which as I said, are much larger than worker or soldier ants.

This size-based prey preference is intriguing. While the lizards will sit and wait for an ant to walk in front of them, or wait for an ant to be dropped from forceps to land in front of them, they will pursue reproductive ants and will also pursue crickets as vigorously as whiptail lizards pursue their prey. Perhaps an individual ant is not worth the energy invested in pursuit, but a larger meal like a cricket is. Additionally, because ants walk over literally every millimeter of soil, there is little need to chase them—one will be along soon enough. I had been thinking that wild caught horned lizards in particular might prefer ants since the lizards would have learned the habit of feeding on ants while wild. In that case, the lizards could be expected to show preference for prey the same size, color and activity pattern as an ant, and yet these lizards show a strong preference for larger prey which looks nothing like an ant. Given the choice between simultaneously offered crickets of different sizes, these horned lizards typically attack the largest available prey.

While these lizards did accept mealworms, two of them regurgitated stomach contents which included mealworms and I have stopped offering large mealworms. Very small mealworms and teneral mealworm pupae seem to be digested without problems. One of the lizards in particular, the only female—who appears gravid—eagerly feeds on small mealworms from a shallow dish, typically emptying the entire dish—about twenty small mealworms at a sitting. As I am trying to get the lizards to return to a more natural diet, small mealworms are offered only a few times weekly.

Examination of the lizards’ droppings shows that the composition of the droppings has changed slightly. When the lizards came home from the pet shop their droppings looked like the droppings of any insectivorous lizard—mostly cricket parts. Since their diet now includes so many ants their droppings more closely resemble the droppings of wild horned lizards—mostly ant parts.

This is all very preliminary and quite unscientific but interesting. I find that these Desert Horned Lizards are active and alert, and remain well fleshed on a mixed diet of ants and crickets. While these lizards do prey on large crickets, I prefer to offer smaller crickets just because I still cling to the notion they would be easier to digest. I must acknowledge that ants are probably one of the most difficult insects to digest (which is likely why wild horned lizard droppings contain so much undigested ant material), so perhaps my concern over the horned lizards’ digestive abilities is misplaced.

The enclosure is a design based on a zoo-originated discovery that venting the heated air from under the heat lamp is beneficial. The cage is four feet long, two feet deep and fourteen inches tall with a 65-watt spot bulb at one end. That same end has an axial fan rated at 55 cfm which extracts air from the cage through a screened vent under the fan. The far end of the cage has a vent to allow replacement of the vented air. This arrangement creates a steady breeze that refreshes the air in the cage several times each minute. Lights are two Lumichrome bulbs with a CRI of 98 and a temperature of 6700K, and a ZooMed 10.0 UVB bulb at the basking end. I find that the lizards close their eyes if the UV bulb is left on too long, so that bulb runs only a few hours at a time, a few days each week. Infrared thermometer readings show that temperatures at the basking site are about 105 F and surface temperatures of the lizards themselves are about 95-102. The lizards also get real sunlight by being placed into a sunning cage which is put outside on warm sunny days.

The particular points of this experience that most intrigue me are that these horned lizards show a strong preference for prey larger than ants, that they have well-developed pursuit behavior which they employ when attacking non-ant prey, that they will accept a species of ant which is not their native prey but is readily available locally, and that they showed a strong interest in injured and chilled ants to the extent that the availability of injured and chilled ants may have convinced the more reluctant lizards in this test to begin eating the available ants.

After many encounters with the particular species of carpenter ant in question, I wonder if this is a stinging species. I have had ants on me many times while disturbing the nests and have not been stung, though the ants do bite. There is another local species of native ant which is a mound-building harvester. This is a smaller species and may be suitable for smaller horned lizards such as hatchlings or Round-Tailed Horned Lizards.

I have kept and bred many reptiles including desert lizards but I can always benefit from suggestions of keepers who have more experience with horned lizards. If my female Desert Horned Lizard is gravid, does anyone have any tips to offer as to how to convince her to lay in a suitable nest site inside an indoor cage? Also, any thoughts on incubating the eggs would be most helpful.


   

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