Petra Frýdlová

Ontogeny of Sexual Size Dimorphism in Monitor Lizards: Males Grow for a Longer Period, but not at a Faster Rate

Monitor lizards belong to the largest and the most sexually dimorphic lizards in terms of size, making this group an ideal model for studies analyzing ontogenetic causes of sexual dimorphism. Understanding of these ontogenetic factors is essential to the current discussion concerning patterns of sexual dimorphism in animals. We examined the ontogenetic trajectories of body weight and snout-vent length to analyze the emergence of sexual size dimorphism. Experimental animals were 22 males and 13 females of mangrove-dwelling monitors (Varanus indicus) hatched at the Prague Zoo. They were regularly weighed and measured up to the age of 33–40 months, and subsequently sexed by ultrasonographic imaging. The logistic growth equation was used to describe and analyze the observed growth patterns. Our results confirm considerable sexual size dimorphism in the mangrove monitor. The mean asymptotic body weight of males was nearly three times higher than that of females. As the body size of male and female hatchlings is almost equal, and the growth rate parameter (K) of the logistic growth equation as well as the absolute growth rate up to the age of 12 months do not differ between the sexes, size differences between fully grown males and females should be attributed to timing of the postnatal growth. Males continue to grow several months after they reach the age when the growth of females is already reduced. Therefore, the sexual size dimorphism emerges and sharply increases at this period.

Is body shape of mangrove-dwelling monitor lizards (Varanus indicus; Varanidae) sexually dimorphic?

In monitor lizards, males are typically larger than conspecific females, but body shape is usually quite similar in both sexes. This not only represents a puzzle worthy of evolutionary explanation, but also makes field sex determination of monitor lizards difficult. We asked whether subtle differences in body shape follow the same pattern as in other sexually dimorphic lizard taxa and thus can be explained by the same selective forces. We tested the hypotheses that (1) females have a longer abdomen due to fecundity selection and (2) males possess bigger heads due to intrasexual selection. We also hypothesised that (3) male monitors show a wider chests and longer upper fore-limbs to win male-male wrestling matches. We monitored ontogeny in 35 mangrove-dwelling monitors (Varanus indicus). Seventeen body measurements were taken every three months up to the age of 24-34 months. Sex was determined by an ultrasonographic imaging. We employed multiple approaches to remove the effect of size and used both confirmation and exploratory statistics. The results revealed that sexual differences in body shape were small and emerged after maturity. Females have a relatively longer abdomen while males wider chest and longer upper fore-limbs. Thus, the differences in body shape between male and female varanid lizards may be attributed to both fecundity and sexual selection.

Morphological characteristics of blood cells in monitor lizards: is erythrocyte size linked to actual body size?

Blood cell morphology and count are not uniform across species. Recently, between-species comparisons revealed that the size of red blood cells is associated with body size in some lizard taxa, and this finding was interpreted in the context of the metabolic theory. In the present study, we examined the numbers and the size of blood cells in 2 species of monitor lizards, the mangrove-dwelling monitor (Varanus indicus) and the savannah monitor (V. exanthematicus), and we compared these traits in individuals of different body size. The results revealed that during the course of ontogeny, the size of red blood cells increases with body mass. Because the mass-specific metabolic rate decreases with body size and the cell volume-to-surface ratio decreases with the cell size, changes in the erythrocyte size might be the result of oxygen transport adjustment.

Development of behavioural profile in the Northern common boa (Boa imperator): Repeatable independent traits or personality?

Recent studies of animal personality have focused on its proximate causation and ecological and evolutionary significance in particular, but the question of its development was largely overlooked. The attributes of personality are defined as between-individual differences in behaviour, which are consistent over time (differential consistency) and contexts (contextual generality) and both can be affected by development. We assessed several candidates for personality variables measured in various tests with different contexts over several life-stages (juveniles, older juveniles, subadults and adults) in the Northern common boa. Variables describing foraging/feeding decision and some of the defensive behaviours expressed as individual average values are highly repeatable and consistent. We found two main personality axes—one associated with foraging/feeding and the speed of decision, the other reflecting agonistic behaviour. Intensity of behaviour in the feeding context changes during development, but the level of agonistic behaviour remains the same. The juveniles and adults have a similar personality structure, but there is a period of structural change of behaviour during the second year of life (subadults). These results require a new theoretical model to explain the selection pressures resulting in this developmental pattern of personality. We also studied the proximate factors and their relationship to behavioural characteristics. Physiological parameters (heart and breath rate stress response) measured in adults clustered with variables concerning the agonistic behavioural profile, while no relationship between the juvenile/adult body size and personality concerning feeding/foraging and the agonistic behavioural profile was found. Our study suggests that it is important for studies of personality development to focus on both the structural and differential consistency, because even though behaviour is differentially consistent, the structure can change.

Strong support for Rensch’s rule in an American clade of lizards (Teiidae and Gymnophtalmidae) and a paradox of the largest tejus

Rensch’s rule predicts an allometric relationship between male and female body size stating that the sexual size dimorphism (SSD) increases with body size in male-larger taxa and decreases in female-larger taxa in groups of related species. It means that the relationship between the male and female body size is hyperallometric, i.e., the allometric exponent of this relationship exceeds the unity. We explored the relationship between the male and female body size in a New World clade of lizards consisting of sister families Teiidae and Gymnophthalmidae, which exhibit a great variation in both their adult body sizes and SSD. All our estimates of the reduced major axis regression slopes ranged from 1.067 to 1.229 and clearly followed a pattern consistent with the Rensch’s rule. Despite a clear general trend, giant species from the subfamily Tupinambinae show paradoxically only poor SSD. The cases of extreme male-larger SSD were found in species of moderate body size belonging to the genera Ameiva and Cnemidophorus. The abovementioned deviations from the hyperallometric relationship between the male and female body size are surprising and require further examination.

A comparative study of growth: different body weight trajectories in three species of the genus Eublepharis and their hybrids

An extensive research effort is devoted to the evolution of life-histories and processes underlying the variation in adult body weight; however, in this regard, some animal taxa remain neglected. Here we report rates and timing of growth recorded in two wild-derived populations of a model lizard species, Eublepharis macularius (M, W), other two related species, i.e., E. angramainyu (A) and E. sp. (D), and their between-species hybrids. We detected clear differences among the examined species/populations, which can be interpreted in the terms of “fast – slow” continuum of life-history strategies. The mean asymptotic body size was the highest in A and further decreased in the following order: M, W, and D. In contrast, the growth rate showed an opposite pattern. Counter-intuitively, the largest species exhibited the slowest growth rates. The final body size was determined mainly by the inflexion point. This parameter reflecting the duration of exponential growth increased with mean asymptotic body size and easily overcompensated the effect of decreasing growth rates in larger species. Compared to the parental species, the F1 and backcross hybrids exhibited intermediate values of growth parameters. Thus, except for the case of the F2 hybrid of MxA, we failed to detect deleterious effects of hybridization in these animals with temperature sex determination.

Patterns of growth in monitor lizards (Varanidae) as revealed by computed tomography of femoral growth plates

Growth plate cartilage (GPC) is responsible for the growth of long bones due to endochondral ossification, which is the main mechanism of longitudinal skeletal growth in tetrapods. Degradation of GPC is a sign of determinate growth as it arrests the growth irreversibly. By contrast, indeterminate growth requires the persistence of GPC throughout the entire life. Monitor lizards (Varanidae) were previously reported to exhibit a dual type of growth, determinate as well as indeterminate. To reinvestigate this highly unexpected finding, we examined 13 species of varanids and their close relatives (Shinisauridae and Helodermatidae). In order to visualize GPC on the proximal part of the femur, we employed micro-radiography and micro-computed tomography. In large-bodied species, an extended capability of longitudinal growth was demonstrated; GPC was preserved for most of their lives. On the other hand, GPC senescence with complete disappearance of cartilage was found in adults of small-bodied varanids. These results confirm previous finding and, together with the absence of GPC in the outgroup species, challenge the universality of indeterminate growth in squamates. Moreover, we observed disappearance of GPC in an extremely old Varanus indicus, implying that GPC degradation is not entirely absent but only delayed to senescence in this large-bodied species. These findings raise the intriguing possibility that it is the timing, rather than other underlying mechanisms, what sets apart determinate from indeterminate growth. We therefore suggest that this dual type of growth represents an extreme case of heterochrony and is a consequence of strong sexual selection pressure to large-bodied varanids.

PLATFORM WITH CAMERA SYSTEM FOR MEASUREMENT OF COMPENSATORY MOVEMENTS OF SMALL ANIMALS

The article introduces systems and methods of a controllable rotational platform used for measuring compensatory movement of small animals. The system, based on a camera subsystem, is located on a mechanical platform powered by a set of three actuators. The subsystems and methods allow to measure angles of the platform’s orientation in space and body segment angles in both anatomical and Earth’s coordinate systems. The methods of video processing, selection of measurement parameters and detection of anatomical angles are thoroughly described in this article. The study also deals with the software designed in MatLab®, which controls the platform, records and processes videos, and obtains angles for the movement analysis. The system was tested for measuring a head rotation of a small reptile/amphibian and monitored reflective markers on the creature’s body by the camera system. This method has never been described before. The new subsystems of the platform and methods for monitoring animal’s head compensatory movements can be used in studies of the neural system and its evolution.

Offenders tend to be heavier: experimental encounters in mangrove-dwelling monitor lizards (Varanus indicus)

The evolution of male-biased sexual size dimorphism is often explained by sexual selection providing competitive advantage to the larger males. The aggressive interactions are often dangerous and energy consuming; thus, it is advantageous to reduce the risks by adjusting behavior to correspond with body size as a predictor of fighting success. Organization of contests into distinct phases with the initial displays preceding the real combat allows individuals to assess the body size and strength of the rival. We staged interactions between mangrove-dwelling monitor lizards (Varanus indicus) to uncover the initialization of aggression and factors determining the course of an encounter. The analyses revealed the importance of both absolute and relative body size of encountering males. The attack rate increases with the body weight of the lizard and offenders initializing a contact phase of the fight tend to be the heavier male of the dyad. Regardless of the final outcome of the combat, the results show that only short visual contact provides sufficient information about the body size of the opponent. This enables combatants to determine whether to initiate the fight or not. This finding together with the ethological details of contests provides the first evidence for the ability of mutual assessment in varanids.