Although it has been recently recognized that plants that are able to form cage architectures may be more likely to survive in environments with a high density of mammalian herbivores, how these cages form has received little attention.
Dr. Tristan Charles-Dominique of Xishuangbanna Tropical Botanical Garden (XTBG) and his cooperators conducted a study to test the effect of plant structure on the bite rate of large herbivores. They also investigated what developmental traits contributed to ‘caginess’.
The researchers estimating goat (Capra hircus, a generalist mammalian herbivore) bite rate on 11 savanna trees of contrasting architectures. For each tree species, they described the architecture and indexed the caginess by considering the number of axis categories that are woody and/or spiny.
They determined the effect of caginess on goat bite rate by comparing bite rate between whole trees and isolated branches. They then evaluated how the caginess of trees slowed down herbivores when feeding on the inner leaves in tree crowns.
They observed that the bite rate of goats on inner leaves of the cagiest trees was so severely reduced that they could not satisfy their daily nutritional requirements. The cage effect mostly affected herbivore feeding time in situations where opportunities to forage only on the periphery of trees was limited (e.g. high depletion level due to few palatable trees being available, or high competition levels).
Among the three components involved in building a cage (i.e. spinescence, stem woodiness and number of axis categories contributing to the cage), spinescence was probably the most important factor contributing to slowing down the bite rate.
Their study provides strong experimental support to the hypothesis that cagy architecture may help some plant species to survive in a landscape with high herbivory pressure.