David Crews

David Pafford Crews is the Ashbel Smith Professor of Zoology and Psychology at the University of Texas at Austin.

He has been a pioneer in several areas of reproductive biology, including evolution of sexual behavior and differentiation, neural and phenotypic plasticity, and the role of endocrine disruptors on brain and behavior.

Crews enrolled at the Munich campus of the University of Maryland in 1965, then transferred to the College Park campus in 1967.

He graduated with a B.A. (Psychology and Sociology majors) in 1969.

Following a summer as a research assistant at Walter Reed Army Institute of Research in the Department of Experimental Psychology he decided to pursue a degree in psychology.

Crews received a Ph.D. in Psychobiology as a National Institute of Mental Health Predoctoral Trainee at the Institute of Animal Behavior at Rutgers University in 1973 under the mentorship of Daniel S. Lehrman and Jay S. Rosenblatt.

He completed a National Science Foundation Postdoctoral Fellowship mentored by Paul Licht at the Department of Integrative Biology at the University of California, Berkeley until 1975.

He was then appointed as a lecturer in the Departments of Biology and Psychology at Harvard University.

He was promoted to assistant professor in 1976 and to associate professor in 1979.

While at Harvard he was also an associate at the Museum of Comparative Zoology.

In 1982, he joined the faculty of the Department of Zoology (now Integrative Biology) at the University of Texas at Austin, where he became the Ashbel Smith Professor of Zoology and Psychology in 1998.

Crews has argued that the primary function of sexual behavior, namely the stimulation and coordination of the reproductive physiologies of the interacting individuals (usually male and female), originated with the first unicellular organisms and hence predates the evolution of sexual recombination.

He has challenged the Organizational/Default doctrine of sex determination, extending it to sexual differentiation of the brain and arguing for its replacement with an Ancestral (female)/Derived (male) paradigm.

This concept has led to questions such as why might males be more like females than females are like males?

The utility of this concept is becoming apparent as we continue to gather evidence for gender differences in genetic and mental disorders.

He also has been a major player in the area of the evolution and diversity of steroid hormone receptors.

Crews discovered the important principle that sexual behavior, gamete production, and steroid hormone secretion could be dissociated, in his studies of the red-sided garter snake (T. s. parietalis).

These snakes are the northernmost reptile and hibernate for much of the year, responding to temperature both to emerge from winter dormancy and to engage in sexual behavior.

It was his work with this species that provided the first demonstration that the activation of sexual behavior could be independent of sex steroid hormones, depending instead upon increasing spring temperature.

This work also led to the first isolation, identification and synthesis of a new class of pheromones.

The desert grassland whiptail lizard (A. uniparens), presented an opportunity to study first-hand how the neuroendocrine substrates underlying sexual behavior can evolve.

In this instance, Crews used a parthenogenetic species derived from interbreeding of two sexual species.

Remarkably, although the descendant species consists only of females, reproducing by obligate parthenogenesis, individuals continue to display sexual behaviors that are typical of both females and males, alternating behaviors depending upon their individual hormone profiles across reproductive cycles.

Although it is not immediately apparent what benefit could come of females engaging in male-typical behaviors in a parthenogenetic species, Crews has shown that this behavior is important to stimulate reproduction of both individuals in these pairings.

By comparing the unisexual descendants with their sexual ancestors, Crews revealed how hormone-brain controlling mechanisms evolve.

This work led to the examination of how novel hormone-brain controlling mechanisms might respond to new challenges.

Of particular note is the revelation that the male-typical sexual behaviors which parthenogenetic females display turn out to be under the control of the postovulatory surge of progesterone rather than androgen, which the parthenogens do not produce.

This discovery in the parthenogenetic lizard led Crews to extend his work to genetically modified mice and rats, demonstrating that progesterone is not a “female-specific” hormone but plays a critical role in sexual behavior in males.

Indeed, Crews demonstrated that androgen and progesterone synergize in males to control copulatory behavior much as estrogen and progesterone synergize in females to facilitate sexual receptivity.

These discoveries have shed light on recent work in humans suggesting a clinical significance of progesterone in male sexual behavior.

Crews has been the leader in determining the physiological and molecular bases of temperature-dependent sex determination (TSD).

Sex determination is a case study in how evolution has produced different mechanisms for achieving the same end.

In many reptiles the sex of the offspring depends on the incubation temperature of the egg, not on genotype as in mammals.

One question concerns how the physical stimulus of temperature is transduced into a molecular and physiological stimulus to determine an individual's gonadal sex.

Crews demonstrated that incubation temperature acts on a group of genes homologous to those in mammals that affect gonadal differentiation.

This work helped overturn the classic tenet that males are the “organized” sex and females the “default” sex.

Today we recognize both sexes as organized and the question now becomes how the activation of a conserved network of genes leads to a binary response (ovary or testis).

He is a pioneer in the relatively new field (actually rebirth) of phenotypic plasticity, or the process by which the environment induces different phenotypes from a given genotype.

When considering that species without sex chromosomes possess all of the genes necessary to develop the phenotype of both sexes, it becomes apparent that the process of sex determination and sexual differentiation represents a form of phenotypic plasticity.

Using the leopard gecko (E. macularius) as the animal model system, Crews determined how the experience of temperature during a narrowly defined period of embryogenesis affects the total phenotype of the adult organism, accounting for much of the variation observed among individuals in morphology, growth, endocrinology, neural activity, and neuroanatomy.