The use of odors in burrow selection by the whiptail lizard Aspidoscelis laredoensis B (Squamata: Teiidae).
Whiptail lizards, genus Aspidoscelis (formerly Cnemidophorus, Reeder et al. 2002), family Teiidae, are ground-dwelling lizards found throughout Texas (Dixon 2000). When active, they employ a "wide-foraging" search tactic (Huey & Pianka 1981) of wandering over a large area, inserting their noses and flicking their tongues into small holes and crevices and under objects to locate their arthropod prey (Milstead 1957; Scudday & Dixon 1973). At the end of their daily activity period, whiptail lizards retire to burrows in the ground that they either dig themselves or appropriate from other small animals (Kennedy 1968; Leuck 1982; Walker et al. 1986). When a whiptail lizard locates a burrow, it inserts its nose into the burrow entrance and flicks its tongue a few times before entering (Rybiski & Paulissen 1995). Sometimes, however, after investigating a burrow entrance, a whiptail lizard will reject the burrow and move off to find another (Rybiski & Paulissen 1995). One reason a whiptail lizard might do this is that the burrow is already occupied. Anecdotal observations of several species of whiptail lizards suggest they are reluctant to enter an occupied burrow or rarely share burrows with other lizards (Fitch 1958; Hardy 1962; Kennedy 1968; Paulissen 2001). Behavioral studies on captive whiptail lizards suggest that many species do not share burrows readily (Leuck 1982; Paulissen 2002), though Aspidoscelis tesselata and A. neotesselata (formerly "triploid Cnemidophorus tesselatus", Walker et al. 1997) are exceptions to this general rule (Leuck 1982). Therefore it is reasonable to conclude that the investigative behaviors a whiptail performs prior to entering or rejecting a burrow are to ascertain if the burrow is already occupied. Presumably, if a burrow is occupied, the investigating lizard will detect the occupant and move on to another burrow.
Whiptail lizards have an excellent sense of smell that they use to locate prey and mates (Simon 1983; Cooper 1997), so it is reasonable to hypothesize that the sense of smell is important in helping a whiptail determine if a burrow is occupied or not. However, if a burrow was recently occupied by a lizard but is empty at the moment a whiptail lizard investigates it, the odors left by the recent occupant will still be present in the burrow. If odors in a burrow are the only sensory cue that a whiptail lizard uses to determine if a burrow is occupied, then a whiptail investigating an empty but recently occupied burrow may incorrectly 'conclude' that the burrow is occupied and reject it. If on the other hand a whiptail lizard uses sensory cues in addition to (or instead of) odors, then the investigating whiptail lizard should correctly determine that the recently occupied burrow is empty and choose it for its own. Subjecting whiptail lizards to a simple choice test, in which individual lizards are made to choose between a clean (no odor) burrow and a recently occupied burrow with lizard odors, should be able to distinguish between these alternatives.
This report presents the results of such a test conducted on individuals of one of the species of the all-female, parthenogenetic Aspidoscelis laredoensis (Laredo striped whiptail) complex. The A. laredoensis complex consists of two species: A. laredoensis A (=LAR-A), the form originally described by McKinney et al. (1973), and A. laredoensis B (=LAR-B) which was discovered in 1984 (Walker 1987) but has not been given a formal Linnaean name (see Paulissen & Walker 1998 for a review of nomenclature). This study focuses on A. laredoensis B. In Texas, it is found in the counties bordering the Rio Grande from Del Rio to just west of Brownsville, plus a few outlying areas (Abuhteba et al. 2001); it commonly co-occurs with the bisexual Texas spotted whiptail, A. gularis (Paulissen et al. 1992). It is a small species, snout-to-vent length 65-75 mm, ground-dwelling, and feeds on small arthropods (especially termites, Paulissen et al. 1988). Individuals of A. laredoensis B investigate burrows by inserting the head into an entrance and flicking the tongue before either entering or rejecting the burrow. Furthermore, laboratory studies showed that they rarely entered occupied burrows (Paulissen 2002). Therefore, this species is ideal to test the hypothesis that whiptail lizards use odors alone as the cue to determine if a burrow is occupied.
MATERIALS AND METHODS
Housing of captive lizards. -- Individuals of A. laredoensis B and both sexes of A. gularis were collected from various sites in Cameron and Hidalgo counties in Texas in June 1990, 1991, and 1992. The lizards collected in 1990 were transported to a lab room at Slippery Rock University where they were housed individually in 10-gallon terraria (50 by 26 by 31 cm) provided with a sand substrate, a 10 cm by 10 cm flat piece of cardboard under which a lizard could dig a "burrow", a food dish kept supplied with mealworms, and a water dish that was checked frequently and refilled as needed. The terraria were kept in a room with overhead lights controlled by a timer to provide a photoperiod of 14:10 L:D every 24 hours. Each terrarium was equipped with a 60 Watt heat lamp that was switched on five hours per day to provide lizards the opportunity to bask. Temperature in the lab room was always between 25 and 30[degrees]C. Five A. laredoensis B females, two A. gularis males, and five A. gularis females were used in the first series of tests conducted from September 1990 through April 1991.
Lizards collected in 1991 and 1992 were transported to a lab room at McNeese State University and again were housed individually in 10-gallon terraria. The only difference was that the room used at McNeese was cooler than the one used at Slippery Rock, so additional heat was provided by 250 watt infrared lamps hung from the shelves holding the terraria. These infrared lamps were switched on for five hours a day and warmed the room to 30-33[degrees]C during those hours. Eight A. laredoensis B females, eight A. gularis males and two A. gularis females were used in the second series of tests conducted July 1992 through April 1993. Two of the A. laredoensis B used in this round of the study were collected in June 1991 and held in captivity until used in 1992; all other lizards used in this segment of the study were collected in June 1992.
Experimental design. -- All tests were conducted using a 91 by 45 by 54 cm high glass terrarium provided with a substrate of about 3 cm of clean sand. Initially the terrarium was divided into 45 by 45 by 54 cm halves by a 50 cm tall piece of cardboard buried into the sand to prevent lizards from burrowing from one half to the other. Each half was provided with a water dish, a food dish containing mealworms, and a 10 by 10 cm piece of cardboard under which a lizard could dig a "burrow" to use as a retreat. An overhead light set to the same photoperiod as the lizard heat lamps was suspended 40 cm above the sand over each half of the divided terrarium to provide equal light and heat to the two halves. A lizard was placed in one half of the divided terrarium for a 48 hour conditioning period; the other half was left empty except for the food and water dishes (sides were chosen at random by coin flip). During the conditioning period, the lizard established a burrow under the cardboard retreat in its half of the terrarium leaving its odors in the burrow; this burrow is hereafter known as the "lizard-odor burrow". The cardboard retreat in the unoccupied half is hereafter known as the "clean (no odor) burrow". After the conditioning period was over, the conditioning lizard, the food and water dishes, and the cardboard partition were removed and a single A. laredoensis B was introduced. This test lizard was free to move from one half of the terrarium to the other and eventually chose to retreat for the night under either the lizard-odor burrow or the clean burrow. Two-tailed binomial tests were used to evaluate if A. laredoensis B showed a significant avoidance of, or preference for, the lizard-odor burrow. Binomial tests were also run to check if A. laredoensis B showed a significant preference for either the right or the left half of the terrarium.
Three sets of trials were run on each of the 13 A. laredoensis B: (1) using an A. gularis male as the conditioning lizard; (2) using an A. gularis female as the conditioning lizard; and (3) using another A. laredoensis B as the conditioning lizard. The order in which the trials were conducted was varied among the 13 test lizards. Due to the accidental death of one A. laredoensis B, n = 12 for the sets of trials involving A. gularis.
A limited fourth set of trials was run on five of the eight A. laredoensis B tested at McNeese State University. An A. laredoensis B was placed in one half of the divided terrarium and allowed to establish a burrow during the 48 hour conditioning period. This lizard was temporarily removed, the partition, food and water dishes removed, and then the conditioning lizard was returned to the undivided terrarium as the test lizard to see if it would choose its own (lizard-odor) burrow or the clean burrow. Results were again evaluated using a binomial test.
RESULTS AND DISCUSSION
Analysis of all trials combined revealed that the A. laredoensis B test lizards chose the left side burrow 19 times and the right side burrow 18 times indicating the test lizards had no significant side preferences in this experiment (P of binomial test = 0.85). With respect to choice of burrow based on odor, A. laredoensis B exhibited neither a significant avoidance of, nor a significant preference for, the lizard-odor burrow as its overnight retreat (Table 1). Regardless of the identity of the lizard that conditioned the lizard-odor burrow, A. laredoensis B chose its burrow essentially at random. This suggests that odors of burrows alone are not a factor, or at least are not the only factor, that A. laredoensis B uses in choosing a burrow.
The five trials in which an A. laredoensis B was made to choose between its own burrow and a clean burrow showed that test lizards chose their burrow twice and the clean burrow three times (P of binomial test = 0.69). This suggests A. laredoensis B does not prefer to use a previously occupied burrow, a result that agrees with a lab study that showed A. laredoensis B more often chose another lizard's burrow than its own as an overnight retreat (Paulissen 2002).
The results of this study, the lab study of Paulissen (2002), and field observations present the following overview of burrow choice in A. laredoensis B. When a lizard is seeking an overnight retreat and encounters a burrow entrance, it investigates the burrow by inserting its head in the entrance and flicking the tongue to discover if the burrow is already occupied. If the burrow is occupied, the lizard detects the occupant and moves off to locate another burrow. However, if the burrow is empty, the lizard moves into the burrow and claims it as its own, even if the burrow was recently occupied by another lizard and still bears the odors of the previous occupant. Clearly burrow odors alone do not convince an A. laredoensis B that a burrow is occupied and do not dissuade it from using a burrow. Furthermore, individuals of A. laredoensis B show no attachment to the burrows they used previously, being just as likely (or perhaps even more likely) to choose a clean burrow or another lizard's burrow than the one they used the night before. This is not surprising for a lizard that is not territorial and which wanders over a wide area during its daily activity period, reducing the likelihood that it will be near the burrow it used previously. Since all other whiptail species are also non-territorial and wide foragers (Wright & Vitt 1993; Martins 1994), they may also be expected to show little preference for burrow choice (other than avoiding burrows that are already occupied) and little burrow site fidelity.
I wish to thank the following persons for assistance in caring for captive lizards: S. Wu, J. Aguillard, and G. Heagler. The lab space and equipment made available by the Department of Biology, Slippery Rock University, and by the Department of Biological and Environmental Sciences, McNeese State University, is also gratefully acknowledged. Lizards were collected under authority of scientific collecting permit SPR-0691-408 granted by the Texas Parks and Wildlife Department. Lizard maintenance and experimental protocols were approved by the Institutional Animal Care and Use Committees of Slippery Rock University and McNeese State University. This manuscript benefited from constructive criticism of T. S. Hibbs.
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Mark A. Paulissen
Department of Biological and Environmental Sciences
McNeese State University, Lake Charles, Louisiana 70609
Table 1. Number of trials in which individuals of Aspidoscelis laredoensis B chose the Lizard-Odor Burrow versus Clean Burrow in burrow choice tests. The Lizard-Odor Burrow was conditioned with the odors of the Conditioning Lizard for 48 hours before the test; the Clean Burrow had no lizard odors. The P value is the result of a two-tailed Binomial Test. Lizard-Odor Burrow Clean Burrow Conditioning Lizard Chosen Chosen P A. gularis male 7 5 0.58 A. gularis female 8 4 0.27 A. laredoensis B female 7 6 0.79
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|Author:||Paulissen, Mark A.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2006|
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