Theoretically, zoology is dominated by the theory of evolution. Zoologists' nearly single-minded devotion to that theory makes zoology quite different than psychology. Specifically, zoologists tend to be much more likely to search for functional explanations of behavior than are most psychologists. For example, the question of why two sexes exist is one that has received considerable attention in the literature of theoretical biology. The two sexes evolved from a single sex, and then two sexes came to be the dominant pattern; there are many more sexually reproducing species than there are asexually reproducing ones.
Theoretical questions about why two sexes evolved and then succeeded so mightily are hotly debated. However, at least until only very recently, most social psychologists studying human sexual behavior would never have stopped to consider the question of why two sexes exist. Recently, some social psychologists have been framing their research questions in a more biological way, and they have produced some very interesting research.
Another difference between zoologists and psychologists is the stature of Charles Darwin. In psychology, there is no person with the equivalent intellectual stature that Darwin has in zoology. There are good reasons for that difference. Darwin's theory transformed biology and thus zoology nearly completely. Before Darwin, biology was mostly concerned with collecting and counting specimens, and there was no underlying theoretical foundation. After Darwin and evolutionary theory, the data in biology began to make theoretical sense. In other words, Darwin gave biology and zoology its marching orders, and they have been following those orders since 1859.