Adapting to Less: Revisiting the Island Syndrome Through MacArthur’s Blurry Glasses

What is the island syndrome, and why does it emerge repeatedly across taxa on small-to-medium islands, acrosslatitudes, and throughout geological time? The wide range of interpretations surrounding this phenomenon oftenobscures broader evolutionary patterns—like a forest hidden by its trees.This paper approaches the island syndrome from a bioeconomic perspective and proposes a classification of itsdefining traits. To guide this analysis, I draw comparisons with troglomorphism—the cave equivalent of theisland syndrome—which is better understood and offers a useful structural framework. Amid the complexitiesof island adaptation, the extinct Balearic caprine Myotragus, with its five-million-year evolutionary history andextreme insular traits—from metabolic to sensory, locomotor, digestive and life history adaptations—serves as avaluable model.Following a brief historical overview, I examine the ecological parallels between caves and islands and exploreenergy use from the perspective of metabolic economy. Because organ size and function strongly influenceresting metabolic rate, I investigate reductions in energetically costly structures—such as brain regions—alongside compensatory expansions in others. Changes in field metabolic rate and behavioural adaptations arealso considered.By tracing these bioeconomic shifts and momentarily adopting MacArthur’s “blurry glasses” to favour generalpatterns over fine detail, I propose a preliminary classification of island syndrome traits—organized into thecategories of reduction, enhancement, and time. This framework, inspired by studies of cave evolution,highlights the directional nature of adaptation under constraint and offers a new perspective on how energylimitedenvironments shape life.