Topic: Ecological adaptations in Moloch and Phrynosoma lizards

Lizards of the genera Phrynosoma and Moloch have been considered a classic example of convergent evolution J. J. Meyers & A. Herrel (2005) The Journal of Experimental Biology, vol. 208, p. 114

In the deserts of Australia and North America similar environmental pressures have independently shaped the adaptations of two unusual lizard species, providing us with one of the most radiant examples of convergent evolution. Moloch horridusThe ‘thorny devil’ (Moloch horridus) of Australia is ecologically equivalent to the distantly related ‘desert horned lizard’ (Phrynosoma cornutum) of North American arid regions such as those in parts of Texas. Moloch is an ‘agamid’ lizard, but Phrynosoma is an ‘iguanid’, so they represent two distinct groups within the diverse clade Iguania, and have been diverging independently in Australia and North America for approximately 150 million years. In spite of this wide evolutionary and geographical separation, both lizards have converged on similar solutions to the problems of food scarcity, water scarcity and a need for camouflage in their largely open desert habitat. The most notable aspects of evolutionary convergence between Moloch and Phrynosoma are their specialisations for eating ants (‘myrmecophagy’) and for collecting and transporting drinking water through modified structures within the skin. Some of the adaptations for myrmecophagy in these two desert lizards are also convergent with other ant-eating animals, most notably amongst mammals such as anteaters, pangolins and echidnas.

Phrynosoma cornutumMoloch horridus eats ants exclusively, whereas species of Phrynosoma show varied degrees of ant-eating in their dietary habits; the most morphologically specialised are dependent on a diet of ants and include the species P. cornutum, Phrynosoma solare, P. platyrhinos, P. modestum and P. mcalli. Adaptations to myrmecophagy in Moloch and Phrynosoma include cryptic coloration, a spiny body, relatively mobile/elongated neck, reduced teeth, slender mandible, a large stomach, and modifications of the tongue, mouth and jaw that enable fast prey capture and ingestion. These features are related to their strategy of catching a very large number of small prey (i.e. ants) and passing them directly to the back of the mouth for swallowing, with no (or only minimal) ‘prey processing’ – i.e. chewing, crushing or ‘mastication’ before ingestion. As a diet of ants also removes the need for sharp teeth and strong jaws for immobilising or chewing prey, Moloch and Phrynosoma lizards have evolved reduced teeth, reduced or absent ‘epipterygoid’ bones (those that form a brace against excessive chewing/swallowing forces) and a slender mandible with reduced area for jaw muscle attachment. Phrynosoma (head)These convergent anatomical features have interesting parallels in mammalian ant-eating taxa, which have either no teeth (e.g. anteaters, pangolins, echidnas) or reduced teeth (e.g. armadillos, sloths, numbats), and display very reduced jaw adductor muscles, similarly evolved in response to absence of mastication. Among further adaptations to myrmecophagy in Moloch and Phrynosoma, their cryptic colouration, spiny form and good neck mobility allow them to stand directly beside columns of ants, motionless save for head and tongue movements. Both lizards have relatively large stomachs for digestion of the large quantities of ants required daily for proper nutrition, but they have divergent approaches to achieving efficient prey capture; Moloch is capable of the most rapid, straight tongue protrusion seen in the Iguania, and Phrynosoma covers its prey in a thick mucous, enabling swallowing without any physical processing at all, whereas Moloch uses its palate to crush the ants before swallowing. Deployment of an elongated, rapidly protruding tongue is another character convergent with highly myrmecophagous mammals (e.g. anteaters Myrmecophaga and Tamandua, and pangolins of the genus Manis), Giant anteaterin which a long extensible tongue permits rapid feeding, penetration of ant/termite nests and capture of multiple prey on the large tongue surface.

Drinking adaptations that are shared between Moloch horridus and Phrynosoma species (e.g. P. cornutum and P. helioscopus) include a semi-tubular network of inter-scalar channels within the skin for capturing water, and use of rhythmic jaw/tongue movements to generate a pumping action, drawing water into the mouth. Water is initially drawn into and through inter-scalar channels by capillary action (enhanced by convoluted, sculptured surfaces lining the inter-scalar hinge ducts and channels), but when the network of tubular channels is full, repeated tongue and jaw movements create negative pressure in the network, keeping water flowing continuously in the direction of the mouth. Drinking in Moloch and Phrynosoma is further promoted by the secretion of water-absorbing (hygroscopic) mucous onto the lips, and various structural modifications (e.g. spines, tubercles and flaps) at or within the corner of the mouth.

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December 15, 2017

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(Topic created 10th October 2008) | Last modified: 22nd July 2010