Tuesday, November 16, 2010

Dwarf Chameleons, Brookesia, Some Recent Research


A composite of a series of video images illustrating how the Malagasy dwarf
chameleon’s tail curls and contacts the substrate during locomotion.
The Malagasy Dwarf Chameleons (Brookesia sp) are unusual for several reasons. They are exceptionally small for members of the Chamaeleonidae, they are very terrestrial (most chameleons are very arboreal), they have very short tails (most chameleons have relatively long tails), and they have bodies that mimic dried leaves. Several recent papers report on some interesting aspects on these interesting lizards.

Boistel et al. (2010) examined the walking behavior of these lizards and found that while they have grasping feet used to hold on to narrow substrates, they also place the tail on the substrate when walking on broad substrates to improve their stability. Using three-dimensional synchrotron X-ray phase-contrast imaging, the authors demonstrate a set of unique specializations in these unusual lizards. 3D reconstructions of the vertebrae inside the tail show morphological specializations that explain this ability. The tail tip is relatively mobile because there are fewer and finer vertebrae with stronger ventral tendons. The tail is therefore able to bend and provide the chameleon with a fifth point of contact with the ground. The inner ear morphology also is involved in balance. It has a highly specialized structure so that the ear could be adapted to detecting weak accelerations and help to correct posture. The high-resolution images obtained at the ESRF (The European Synchrotron Radiation Facility) were crucial in order to visualize the tail tendons, this could not have been accomplished with conventional X-ray technology. Also, the detailed reconstruction of the inner ears enabled precise comparison of the subtle structures within each species’ ear. The authors point out that it will be interesting to compare Brookesia's adaptations with those of genus Rhampholeon, an independent lineage of small ground-dwelling chameleons to test whether a similar tail-assisted locomotor mechanism is present.

Randrianantoandro et al. (2010) examined the abundance of chameleons at eight locations in Madagascar and found that in deciduous forest in Menabe, western Madagascar. Brookesia brygooi was the most abundant species, with a population density estimated at  35 per hectare. Furcifer species were less common, with densities of  about 7.2 per hectare for F. labordi, 3.0 per hectare for  Furcifer sp. and 1.3 per hectare F. oustaleti. Abundance of B. brygooi varied with altitude and the authors did not detect any clear problems created by logging. However, a lack of information about the chameleons diurnal habitat use is needed to assess the tolerance of these lizards to forest degradation. However, Rakotondravony et al. (2010) report that common species in original forest (e.g., Brookesia) were completely lacking in fragments.

Andrews and Karsten (2010) examined molecular phylogenies of chameleons and found that oviparity (egg-laying) is ancestral. Viviparity (live birth) has apparently evolved at least twice in the chameleons, once in Bradypodion and again in members of the Trioceros bitaeniatus clade. Viviparous species tend to be medium-sized as a result of convergence from either small-sized ancestors or large-sized ancestors, respectively. Also viviparous species do not differ from oviparous species in clutch size, hatchling size, or the trade-off between clutch and hatchling size. The authors found that the basal chameleons (Brookesia, Rhampholeon and Rieppeleon) are small-sized and have developmental rates comparable with those of other lizards. However, the more derived chameleons (Calumma, Chamaeleo, Trioceros and Furcifer) are mostly large and all have relatively slow developmental rates. Some of the derived chameleon clades also exhibit developmental arrest (the embryo goes into stasis) and incubation periods may be extended to 6–10 months or more. Developmental arrest is apparently an adaptation to dry, highly seasonal climates where the time period favorable for oviposition and hatching is short. Long incubation periods thus ensure that hatching occurs when temperatures, rainfall, and food availability are appropriate for the young.

Literature
Andrews, R. M. and K. B. Karsten. 2010. Evolutionary innovations of squamate reproductive and developmental biology in the family Chamaeleonidae. Biological Journal of the Linnean Society, 100: 656–668. doi: 10.1111/j.1095-8312.2010.01442.x

Boistel, R., A. Herrel, G. Daghfous, P.-A. Libourel, E. Boller, P. Tafforeau, and V. Bels. 2010. Assisted walking in Malagasy dwarf chamaeleons. Biology Letters 2010 : rsbl.2010.0322v1-rsbl20100322.

Rakotondravony, D., A. Raselimanana, J. Ratsimbazaf, J. S. Sparks, L. Wilmé and J. U. Ganzhorn. 2010. Patterns of species change in anthropogenically disturbed forests of Madagascar. Biological Conservation 143:2351-2362.

Randrianantoandro, C., B. Razafimahatratra, M. Soazandry, J. Ratsimbazafy, R. K. B. Jenkins. 2010. Habitat use by chameleons in a deciduous forest in western Madagascar. Amphibia -Reptilia 31:27-35.


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