Thursday, 16 February 2012

New Miniature Chameleons from Madagascar.

The Miniature Chameleons form a subgroup within the genus Brookesia (Leaf Chameleons), known as the Brokesia minima subgroup, after the first species discovered. These Chameleons tend to have extremely limited ranges, but are genetically quite distinct, implying the species diverged millions of years ago; this is quite surprising as the Malagasy climate has changed quite a lot during this time, which is not generally good news for species with limited ecological niches.

In a paper published in the journal PLoS One on 14 February 2012, a team led by Frank Glaw, of the Zoologische Staatssammlung München, carry out a review of the evolution and ecology of the Miniature Chameleons, and describe four new species within the group.

The first of these new species is Brookesia tristis, which reaches 24 mm in length and has lateral spines on the tail, a supranasal cone (horn above the nose), and a pelvic spine. B. tristis was found living in branches close to the ground (5-50 cm high) in a deciduous dry forest on a limestone mountain, the Montagne des Français. During the study a female was observed to lay two eggs, slightly under 6 mm in length, which hatched after 64 & 68 days, producing offspring 14 mm in length (more than half the adult length). The name tristris means doleful, or sorrowful, in reference to the threat of deforestation on Montagne des Français, despite its having recently been declared a nature reserve.

Male (A) and female (B) Brookesia tristis. From Glaw et al. (2012).

The second new species is Brookesia confidens, which reaches 23 mm, and lacks a supranasal cone, pelvic spines or lateral tail-spines. Found living on low branches (5-20 cm above the ground) in a deciduous dry forest in an area with numerous limestone outcrops, in the Ankarana National Park. The name confidens means confident, a reference to the good habitat protection in place in the Ankarana National Park. When alarmed B. confidens turns a dark colour, with a white stripe along its spine.

Male (A) and female (B) Brookesia confidens. From Glaw et al. (2012).

Brookesia micra measures under 20 mm in length, making it the smallest member of the group, and potentially (depending on how the measurements are taken) the smallest reptile and amniote (amniotes are tetrapod vertebrates that do not need to lay their eggs in water; reptiles birds and mammals, of which group the smallest members are small lizards). It has a short tail compared to other members of the group, which is an orange colour in adults. B. micra has a supranasal cone and a pelvic spine, but lacks lateral tail-spines. B. micra lives on the island of Nosy Hara off the north coast of Madagascar. The name micra means small.

Brookesia micra. (A) Adult male against a black ground in order to show the orange tail. (B) Juvenile on a finger tip. (C) Juvenile on a match-head. (E) Male in the wild. (F) Female in the wild. From Glaw et al. (2012).

Brookesia desperata is between 25 and 30 mm in length. It has three large tubercles on the side of the head, a supranasal cone, pelvic spines and well developed lateral tail spines. They were found living in the southern fringes of the Forêt d’Ambre Special Reserve, in branches 5-100 cm above the ground, and also living in banana plants where the forest had been cleared. A female was observed to lat two eggs. When alarmed the animals turn dark with a light line along the spine. The name desperata means desperate, a reference to the poor habitat protection in the Forêt d’Ambre Special Reserve.

Brookesia desperans. (A) Female with recently laid eggs; adult is showing stress colouration. (B) Close up showing (1) the pelvic spine and (2) the lateral tail spines. (G) Male. (H) Female. From Glaw et al. (2012).

The Miniature Chameleons are a curious group of animals. None of the known species are closely genetically related, suggesting the diverged millions of years ago. All species live in highly restricted habitats on an island we know to have undergone considerable climatic change in the last few million years. This would suggest that either the species are capable of surviving in a wider choice of habitats than they currently do, or that they have evolved to live in their current environments fairly recently. If the former is the case then an explanation as to why they currently inhabit such restricted ranges is needed. If the latter then it needs to be explained why none of the current species are closely related. We do not expect to see in nature a number of related species evolving to inhabit successive environments in parallel; rather some species should die out and others give rise to more than one daughter species - which would be closely related to one another.

Northern Madagascar, showing the distribution of six of the fifteen known species of Miniature Chameleons. From Glaw et al. (2012).

The Miniature Chameleons are also remarkable in their size; they are among the smallest amniotes, though some frogs and fish are smaller. The females lay very few, comparatively large eggs, no more than two in the smallest species. This suggests that it is the size of the egg, rather than the adult which is the limiting factor in amniote size; the eggs cannot get any smaller and remain viable, creating a restriction on how small the adults, who must lay the eggs, can get.

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