IPBRG Investigators: Spence Behmer

Collaborators: Martin Shapiro (FSU), Steve Simpson (Sydney, AUS)

Learning is found in most animals, including insects, and it likely plays a role in a wide of range of activities. Invertebrates, particularly insects, are useful organisms for studying learning because they have relatively simple nervous systems and can be reared and maintained in large numbers with relative ease. We are particularly interested in how learning impinges on feeding and have recently developed, in collaboration with Martin Shapiro, a protocol (based on a Y-maze) for measuring the development and speed of learning in grasshoppers (Orthoptera: Acrididae). With respect to learning, grasshoppers offer some unique advantages over other insects, such as honeybees. First, much is known about their nutritional physiology, which permits broader interpretation of learned behavior related to food acquisition. Second, they readily eat synthetic foods, which means questions can be asked about nutrieint-specific appetitive learning. Third, their hemi-metabolous development allows for testing within and between developmental stages. Finally, since they are diverse, opportunities exist to explore how natural-history traits (e.g. specialists v. generalists, solitary v. gregarious individuals) influence learning abilities.

Using the Y-maze and the grasshopper Schistocerca gregaria (shown above), three separate experiments were conducted to test discrimination abilities with respect to size of rewards (both absolute and nutrient content). Our results were in agreement with previous work showing that acridids are capable of associative learning, but were unique because they provided new insights into the nature of associative learning in acridids by controlling levels of experience with each option, recording several measures of preference (choice, latency and extinction), examining reversal learning and documenting the change in performance during the acquisition of learning.

Grasshoppers have also been used to explore the phenomenon of 'state-dependent learned valuation' in animals. Models of decision-making often assume that choices are based on fitness consequences that each choice yields. Fitness gains, in turn, depend on both the intrinsic properties of the options and the state of the subject at the time of the choice. However, recent studies in humans and other vertebrates have shown that preferences may reflect more closely the subject's state at the time of learning than at the time of choice. We have recently described a similar behavior in a grasshopper, which suggests 'state-dependent learned valuation' is likely to be widespread in animals (Pompilio et al. 2006). This finding contrasts with normative theories of choice in biology and economics, which rely on present rather than past benefit and psychological models of reinforcement learning that use absolute reward magnitude rather than state-dependent benefits.

Most recently we have used grasshoppers to study appetitive learning within and across developmental stages (Nadell and Behmer, in review). Specifically, we asked how long positive memories might persist in the absence of reinforcement.

Many interesting questions remain to be pursued, and we believe that grasshoppers provide a system that has great potential for understanding shared processes and phyletic differences of the learning phenomena not only within invertebrates, but also with vertebrates. We also believe that grasshoppers can shed new light on the evolution of learning and the biological constraints on learning in simple systems.

Key publications:

Pompilio, L, Kacelnik, A. and Behmer, S.T. (2006) State-dependent
     learned valuation drives choice in an invertebrate. Science 313, 1613-1615. [pdf]

Behmer, S.T., Belt, C.E and Shapiro, M.S. (2005) Variable rewards and discrimination
      ability in an insect herbivore: what and how does a hungry locust learn?
     Journal of Experimental Biology 208, 3463-3473. [pdf]

Behmer, S.T. (2004) Insect Learning. In: Encyclopaedia of Entomology (Ed. J. Capinera),
     pp. 1278-1283. Kluwer Academic Publishers, Dordrecht, The Netherlands. [pdf]

Behmer, S.T., Elias, D.O. and Bernays, E.A. (1999) Post-ingestive feedbacks and associative
      learning regulate the intake of unsuitable sterols in a generalist grasshopper.
     Journal of Experimental Biology 202, 739-748. [pdf]

Behmer, S.T. and Elias, D.O. (1999) Phytosterol structure as a basis of food aversion
     learning in the grasshopper Schistocerca americana (Orthoptera: Acrididae).
     Physiological Entomology 24, 18-27. [pdf]