Peters S E, Nishikawa K C
Department of Biology, The University of North Carolina, Charlotte, North Carolina, USA.
J Morphol. 1999 Nov;242(2):107-24. doi: 10.1002/(SICI)1097-4687(199911)242:2<107::AID-JMOR4>3.0.CO;2-V.
Previous studies show that anurans feed in at least three different ways. Basal frogs have a broad tongue that shortens during protraction and emerges only a short distance from the mouth. Some frogs have long, narrow tongues that elongate dramatically due primarily to inertia from mouth opening, which is transferred to the tongue. A few species have a hydrostatic mechanism that produces tongue elongation during protraction. This functional diversity occurs among frogs that share the same two pairs of tongue muscles. Our study compares the isometric contractile properties of these tongue muscles among three frog species that represent each feeding mechanism. Nerves to the paired protractors and retractors were stimulated electrically in each species to record the force properties, contraction speeds, and fatigabilites of these muscles. Few differences were found in the isometric contractile properties of tongue muscles, and the greatest differences were found in the retractors, not the protractors. We propose that the unique arrangement of the tongue muscles in frogs results in a retractor that may also be coactivated with the protractor in order to produce normal tongue protraction. Inertial effects from body, head, and jaw movements, along with clear differences that we found in passive resistance of the tongues to elongation, may explain much of the behavioral variation in tongue use among species.
先前的研究表明，无尾目动物至少有三种不同的进食方式。基干蛙类有宽阔的舌头，在伸展时会缩短，仅从口中伸出很短的距离。一些蛙类有长而窄的舌头，主要由于张口时的惯性使其大幅伸长，这种惯性会传递到舌头上。少数物种有一种液压机制，在伸展时产生舌头伸长。这种功能多样性出现在拥有相同两对舌肌的蛙类中。我们的研究比较了代表每种进食机制的三种蛙类的这些舌肌的等长收缩特性。在每个物种中，对成对的伸展肌和收缩肌的神经进行电刺激，以记录这些肌肉的力量特性、收缩速度和疲劳性。在舌肌的等长收缩特性方面几乎没有发现差异，最大的差异出现在收缩肌，而非伸展肌。我们认为，蛙类舌肌的独特排列导致收缩肌可能也会与伸展肌共同激活，以便产生正常的舌头伸展。身体、头部和下颚运动产生的惯性效应，以及我们在舌头被动抗伸长方面发现的明显差异，可能解释了物种间舌头使用行为的大部分变化。
