SYNOPSIS. Integrating proximate and ultimate causes and effects simultaneously in the study of behavior is challenging and complex, but useful. This is equivalent to asking both “How?” (in the sense of proximate mechanisms) and “Why?” (in the sense of ultimate evolutionary payoffs) an organism operates in one way and not another. Sex differences in rattlesnake (i) size and growth and (ii) mating and reproductive strategies and tactics, provide a good theoretical and empirical context in which to attempt such integration. We employ interdisciplinary and multidisciplinary approaches in our behavioral and physiological work, but we mean something different by “integrative,” that amounts to the simultaneous study of both proximate and ultimate levels of causation and explanation. Though not always feasible, this approach represents an important goal to work towards because it attempts to represent faithfully the complexity inherent in living systems. To this end, we also employ a variety of modeling approaches, which entrain intuition, generate new hypotheses, and sharpen inference. Individual-based simulations, for example, offer promise for broad, integrative programs of biological research.