The material property of leaf toughness is considered the crucial mechanical challenge facing folivorous primates. Mature leaves have higher recorded toughness values than young leaves on average, leading to many assumptions about the patterning of food breakdown that follow a tough/not-tough dichotomy. We tested three hypotheses about how leaves break down under repetitive loading cycles, predicting that mature leaves (i) experience more force during simulated occlusal loads, (ii) more effectively resist fragmentation into small pieces, and (iii) show a more gradual decline in resistance over consecutive cycles than young leaves. Under displacement control using a mechanical testing system, we subjected young and mature leaves to 20 cycles of axial loading using interlocking steel wedges, then collected and quantified the size of the leaf fragments. While we found that mature leaves experienced more overall force than young leaves (p < 0.001), they also shattered into smaller pieces (p = 0.004) and showed a steeper decline in their resistance to the cycles over the course of a test (p < 0.01). These results suggest that putatively ‘tougher’ foods (i.e. mature versus young leaves) do not necessarily resist fragmentation as commonly assumed. The current tough/not-tough paradigm of primate foods may not accurately reflect how leaves break down during masticatory behaviour.

叶片韧性的材料特性被认为是食叶灵长类动物面临的关键机械挑战。平均而言，成熟叶子的记录韧性值高于幼叶，这导致了许多关于食物分解模式的假设，这些假设遵循严格/不严格的二分法。我们测试了关于叶子在重复加载循环下如何分解的三个假设，预测成熟叶子（i）在模拟咬合负载期间承受更大的力，（ii）更有效地抵抗碎裂成小块，以及（iii）显示出更缓慢的下降连续周期的抗性高于幼叶。在使用机械测试系统的位移控制下，我们使用联锁钢楔对年轻和成熟的叶子进行 20 次轴向载荷循环，然后收集并量化叶子碎片的大小。虽然我们发现成熟的叶子比年轻的叶子经历了更多的整体力量（p < 0.001），它们也破碎成更小的碎片（p = 0.004），并且在测试过程中它们对周期的抵抗力下降得更厉害（p < 0.01）。这些结果表明，假定的“更坚硬”的食物（即成熟的叶子与年轻的叶子）并不一定能像通常假设的那样抵抗碎裂。目前灵长类食物的强硬/不强硬范式可能无法准确反映叶子在咀嚼行为期间是如何分解的。