Journal Abstracts: Iguana iguana Visual and Chemical Reception
Compiled by Melissa Kaplan, 2002
Green iguanas, like other reptiles and most (if not all) other animals, have many ways in which they receive input from their environment. The ones we tend to think of are the ones that we humans consciously make the most use of: sight, sound, smell, touch and taste. What we remain largely unconscious of is something that many researchers - and perfume designers - are well aware of: we also receive chemical input that is processed through different pathways than our more obvious senses.
What does this have to do with green iguanas and other animals? Humans tend to forget that animals rely more heavily on chemical input than we do As a result, how well they do - or don't do - in captivity may be affected by not only what they see and hear, but by the chemicals they are picking up with their tongue and vomeronasal (Jacobson's) organ. There is one more important aspect of their keen chemosensory system: they can detect our pheromones and changes in our own hormone levels. Normally, this is not a big deal. But when the animal happens to be a green iguana or other reptile who normally lives in a dynamic social group where status is achieved and challenged, or the animal is a male green iguana living with post-pubescent human females, then the keeper needs to be aware lof what kinds of behavioral changes - and challenges - may occur as a result of chemicals wafting about the human/reptile environment.
The following abstracts document some of the findings relating to green iguanas, the lizard species that is probably the most problematic in captivity due to the danger a male iguana poses to his human keepers during the iguana's breeding season.
of chemical and visual exposure to adults on growth, hormones, and behavior
of juvenile green iguanas.
For 1 year, groups of juvenile male green iguanas (Iguana iguana) were housed such that one group was visually exposed to an adult female, a second group was both visually and chemically exposed to an adult female, a third group was visually exposed to an adult male, and a fourth group was both visually and chemically exposed to an adult male. Juvenile males exposed to signals from adult males showed signs of chronic stress, including reduced growth rates, lower testosterone levels, higher corticosterone levels, and decreased frequencies of headbob display. Although visual exposure alone was sufficient to suppress growth, testosterone levels, and rates of display, the addition of chemical exposure strengthened some of these effects. Both chemical and visual exposure to an adult male were necessary to produce elevated corticosterone levels. In contrast, juvenile males exposed to visual and chemical signals from an adult female exhibited growth rates, hormone levels, and behavior patterns typical of juvenile males housed in the absence of adults of either sex. These results suggest that visual and chemical signals from dominants may serve to reinforce social relationships among males through their influence on the physiology and behavior of receivers.
and behavioral studies of femoral gland secretions in iguanid lizards.
Comparative studies on the chemistry and behavioral significance of femoral gland secretions in desert iguanas (Dipsosaurus dorsalis) and green iguanas (Iguana iguana) are reviewed. Field and laboratory studies suggest that femoral gland secretions function in conspecific recognition and range marking. In desert iguanas, secretions are of low volatility and may be detected initially using long-range ultraviolet visual cues. In contrast, green iguana secretions contain a diversity of volatile lipids and appear to be localized by chemoreception. Interspecific differences in femoral gland chemistry may reflect adaptations to the diverse climatic conditions of arid desert and tropical forest environments.
iguana: a model species for studying the ontogeny of behavior/hormone
Several features of the green iguana, Iguana iguana, make this species an excellent subject for biological research, especially with respect to collecting comparative data on behavior/hormone interactions. This species thrives on captivity, and behavioral interactions appear substantially similar to those observed in natural populations, given effective captive environments. Because hatchlings do not necessarily associate with adult conspecifics in wild populations, behavioral patterns exhibited by and among juveniles in either natural conditions or captivity are probably unlearned behaviors. The species is large enough to obtain blood samples at frequent intervals at an early age. Because the species is herbivorous the maturation process can be controlled by the caloric base made available to individual lizards.
productivity of lizard femoral glands: relationship to social dominance
and androgen levels.
Social and hormonal correlates of femoral gland productivity were investigated in groups of adult and juvenile green iguanas (Iguana iguana) over 12 months. Femoral gland productivity, pore size, and the percentage of lipids in the secretions were correlated with plasma testosterone (T) levels in dominant, although not in subordinate, adult males. Secretory activity peaked during the breeding months, at which time dominants produced more secretion than subordinates. Pore size in juvenile males was positively correlated with plasma T levels and frequency of headbob displays in the months corresponding to the adult breeding season. After 18 months of age, individuals that performed visual displays had significantly larger pores than individuals that did not display. These results indicate that femoral gland secretions could function in the ontogeny and maintenance of dominance relationships.
Note: I included the following abstracts both to provide information for anole keepers but also because researchers are finding a great deal of simularity amongst the iguanid lizards, which includes the frequently studied Anolis and Sceloporous lizards.
and anaerobic metabolism of paired male lizards (Anolis carolinensis).
The associations of physical activity, skin color, body mass difference and conspecific olfactory cues with short-term aerobic and anaerobic metabolism in paired male Anolis carolinesis were examined. Control measurements for mild laboratory manipulation (such as the movement of a metabolic chamber) yielded significant increases in the rate of oxygen consumption, but not in the lactate concentration of these animals; stronger manipulation increased both. A possible influence of conspecific olfactory cues on the metabolism of lizards introduced into empty chambers was undetected. Anolis that turned from green to brown with handling showed an increase in oxygen uptake, but an association between this color shift and total body lactate level was not detected. Elevated rates of oxygen use and glycolysis were found in pairs of males in the absence of physical activity. Lactate levels of the individuals of a pair were positively correlated with one another; the lactate concentrations of lizards placed into occupied metabolic chambers were correlated with the difference in body mass of the pair.
Central and endocrine
aspects of tongue-flicking and exploratory behavior in Anolis carolinensis.
Tongue-flicking in reptiles is frequently seen to increase in novel habitats and is presumed to involve the acquisition of chemical information. Like most behavioral patterns, however, tongue-flicking has both multiple causes and multiple functions, only some of which involve chemosensation. This paper describes units of exploratory behavior, including use of the tongue, in a microsmatic lizard, the green anole, Anolis carolinensis. This species employs its tongue in a manner suggestive of chemosensory exploration, but it possesses modest peripheral chemosensory organs and reduced central representation of chemosensory target cell groups. Further, specific elements of exploratory behavior are affected differently by treatments that involve altering levels of stress-sensitive hormones, androgen, and central catecholamines. Such differential responses suggest that exploratory behavior is an ensemble of individual units, some of which are selectively sensitive to specific physiological elements of the stress response and of elevated arousal.
delivery to the vomeronasal organs and functional domain of squamate chemoreception.
While evidence exists that many squamate behaviors are released by chemical stimuli, the specific sensory system that detects such stimuli and mediates subsequent behavior has been determined infrequently. Techniques and experimental approaches that have been used to determine the roles that the main olfactory and vomeronasal systems play in mediation of specific behaviors are discussed.
evolution of chemoreception in squamate reptiles: a phylogenetic approach.
Recent advances in the field of squamate reptile chemoreception have been paralleled by the growth and preeminence of cladistics in the field of systematics, but for the most part, workers in the former have failed to incorporate the conceptual and informational advances of the latter. In this paper, I attempt a preliminary rapprochement by combining the methods of phylogenetic systematics and current hypotheses of squamate relationships with an overview of squamate chemosensory biology. This purely phylogenetic approach leads to a number of falsifiable generalizations about the evolution of chemoreception in squamates: 1) Evolution of this system is conservative rather than plastic, reflecting to a large extent suprafamilial attributes rather than adaptation to local conditions; 2) Anguimorphs are highly chemosensory and teiids show convergence with this group; 3) Tongue-flicking, a bifurcated tongue tip, a vomeronasal (VNO) mushroom body, and a complete circular muscle system in the tongue are a correlated character complex associated with the attainment, in squamates, of a direct VNO-oral connection and the loss of a VNO-nasal connection; 4) There is little support for a visual-chemosensory dichotomy within Squamata; 5) Gekkotans are allied with Autarchoglossa, both phylogenetically and in terms of chemosensory biology; 6) Iguania are highly variable in chemosensory development; iguanids represent the primitive iguanian condition, while agamids and chamaeleonids have secondarily reduced or lost their chemosensory abilities; 7) Apparent contradictions in chemosensory behavior among iguanids probably represent intrafamilial divergence; 8) Ecological correlates within Iguanidae and other taxa might be spurious, resulting from historical factors unrelated to the adaptations in question; 9) The mechanical demands of lingual food prehension have constrained chemosensory evolution in Iguania; chemosensory evolution within Scleroglossa was permitted by the liberation of the tongue from this ancestral role.
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