PS 26-104 - Shiny beetles and levels of causation: a hands-on exercise exploring forms of explanation

Abstract: The concept of proximate versus ultimate causation is often central to ecology and evolutionary biology undergraduate courses. This concept can be difficult for students to grasp. While they may be able to identify level of causation when presented with explanatory statements, students often struggle with generating appropriately distinctive explanations themselves. I present here a laboratory exercise for a field-based ecology course designed to encourage students to reflect on proximate versus ultimate causation while generating data through a hands-on experiment. I also present analysis of student responses to a questionnaire designed to gauge improvement of understanding in the context of experimental results.

The exercise was built around recent publication of a study testing the effect of beetle iridescence on predation risk. I obtained a digital 3D model of a tiger beetle, and printed it in two halves to create silicon molds. Students in one group in an animal behavior course then used modeling clay to create life-size beetle models. Models (n = 202) were painted using iridescent and non-iridescent paint, coated with either a glossy or matte finish, and then glued to substrates in 3 different habitats. All models were collected three days later. Attacks on models by birds, hymenopterans and other insects were recorded by damage to the clay. Predation risk was affected by habitat but not by iridescence or sheen. Remaining students in the class then provided responses to the question, “Why are beetles iridescent?” at both proximate and ultimate levels, before and after discussing design and results of the experiment. Written responses were scored as to whether they accurately reflected proximate versus ultimate explanations and whether students tied their explanation specifically to beetle iridescence.

I observed a significant increase in student ability to posit proximate explanations after discussion of the experiment; prior to discussion, student proximate explanations were overly broad and unfocused on either proximate mechanism or beetle iridescence. After discussion, proximate explanations very clearly invoked mechanism and iridescence. There was no significant change in ability to pose ultimate explanations; even prior to discussion, students readily provided ultimate explanations that were focused on iridescence and beetle ecology. This exercise excited student interest in the application of 3D technology and the clear results of predator attacks being recorded simply and inexpensively. The exercise also provided an ability to highlight student understanding of proximate versus ultimate causation in an immersive, hands-on experience.