Evolutionary functional morphology of the prey transport apparatus of elapid and Atractaspidid snakes: Functional integration and constraints.

摘要

The extent to which structure defines behavior was tested by examining form and function in the upper jaws of three different types of snakes. Specifically, swallowing (prey transport) was examined in snakes lacking a connection between palatine and pterygoid (two species of Atractaspis) and in elapid snakes with connected palatines and pterygoids but with complex (erector) or simple (dragger) palatines. It was tested if functional interactions among the characters within the functional units producing transport and envenomation result in constraints on the evolution of those functional units. Quantitative analysis of bone movements from video records of feeding behavior and manipulations of anaesthetized animals was combined with reassessment of palatal and related morphology by dissection of preserved specimens. Species within each group of elapids are united by shared characters that differ between groups but produce similar palatine behaviors during transport. In erectors, the palatine attaches to the hypochoanal cartilage, the retractor pterygoideus does not span the mobile palato-pterygoid joint, and palatine erection does not depend on the maxilla but occurs independently. In draggers, the palatine is free from the snout and the retractor pterygoideus spans the palato-pterygoid joint, which is ligamentously connected to the maxilla. Both groups of elapids share with most colubroids a core functional unit of plesiomorphic characters integrated to produce the pterygoid walk involving palatine erection. Structural variations comprise subordinate character sets operating within the thresholds of system function. In Atractaspis, structural modification of the palatomaxillary apparatus is associated with a novel unilateral envenomating behavior. The gap between palatine and pterygoid allows maxillary rotation while the palatine remains attached to a stiff snout adapted for a fossorial lifestyle. The long fang precludes pterygoid protraction during transport, resulting in loss of teeth and the pterygoid walk. The different structural arrangements of the upper jaw of elapids have little effect on the basic behavioral properties of the transport functional unit. In Atractaspis, because of participation of structures in both the prey transport and envenomation functional units, strong selection for envenomation resulted in a breakdown of the pterygoid walk prey transport functional unit.