How head shape and substrate particle size affect fossorial locomotion in lizards

Granular substrates ranging from silt to gravel cover much of the Earth’s
land area, providing an important habitat for fossorial animals. Many of
these animals use their heads to penetrate the substrate. Although there
is considerable variation in head shape, how head shape affects fossorial
locomotor performance in different granular substrates is poorly
understood. Here, head shape variation for 152 species of fossorial
lizards was quantified for head diameter, slope and pointiness of the
snout. The force needed to penetrate different substrates was measured
using 28 physical models spanning this evolved variation was constructed.
Ten substrates were considered, ranging in particle size from 0.025 to 4mm
in diameter and consisting of spherical or angular particles. Head shape
evolved in a weakly correlated manner, with snouts that were gently sloped
being blunter. There were also significant clade differences in head shape
among fossorial lizards. Experiments with physical models showed that as
head diameter increased, absolute penetration force increased but force
normalized by cross-sectional area decreased. Penetration force decreased
for snouts that tapered more gradually and were pointier. Larger and
angular particles required higher penetration forces, although
intermediate size spherical particles, consistent with coarse sand,
required the lowest force. Particle size and head diameter effect were
largest, indicating that fossorial burrowers should evolve narrow heads
and bodies, and select relatively fine particles. However, variation in
evolved head shapes and recorded penetration forces suggest that
kinematics of fossorial movement are likely an important factor in
explaining evolved diversity.

从粉砂到砾石的颗粒基质（granular substrates）覆盖了地球陆地的大部分区域，为穴居动物（fossorial animals）提供了重要的栖息生境。其中诸多物种依靠头部贯穿基质。尽管头部形态存在显著差异，但目前学界对不同颗粒基质中头部形态如何影响穴居运动性能的认知仍较为匮乏。本研究针对152种穴居蜥蜴的头部形态变异展开量化分析，测量指标包括头部直径、吻部斜度与尖锐程度。我们构建了涵盖该类群演化变异的28种物理模型，用以测量穿透不同基质所需的作用力。实验共设置10种基质，颗粒直径范围为0.025至4mm，颗粒形态涵盖球形与棱角状两类。头部形态的演化呈现弱相关性：吻部斜度平缓的个体，其头部更钝圆。穴居蜥蜴的头部形态在不同演化支（clade）间亦存在显著差异。物理模型实验结果显示，头部直径增大时，绝对穿透作用力随之上升，但经横截面积归一化后的相对作用力则下降。吻部渐变细且更尖锐的个体，其穿透作用力更低。棱角状颗粒与更大尺寸的颗粒需要更高的穿透作用力，而与粗砂特性一致的中等粒径球形颗粒所需穿透作用力最低。颗粒粒径与头部直径的影响最为显著，这表明穴居掘穴者应演化出窄小的头部与躯体，并偏好相对精细的颗粒基质。然而，已观测到的头部形态演化变异与穿透作用力数据提示，穴居运动的运动学（kinematics）特征或许是解释演化多样性的关键因素之一。