Reappraising the exteriorization of the mammalian testes through evolutionary physiology

Abstract

A number of theories have been proposed to explain the exteriorization of the testicles in most mammalian species. None of these provide a consistent account for the wide variety of testicular locations found across the animal kingdom. It is proposed that testicular location is the result of coordinate action of testicular tissue ecologies to sustain preferential states of homeostatic equipoise throughout evolutionary development in response to the advent of endothermy.

Keywords: balanced ecological hypothesis; endothermy; exteriorization of the testes; gubernaculum; scrotum.

Figure 1.

The evolution of endothermy. Romer’s ‘Greenhouse Effect’ dried up bodies of water, forcing vertebrates on to land. During that period, the parathyroid Hormone-related Protein Receptor (PTHrPR) duplicated. PTHrP signaling appeared in the anterior pituitary, enhancing ACTH production; PTHrP signaling also appeared in the adrenal cortex, enhancing corticoid production. That stress-induce cascade increased adrenaline production, increasing production of lung surfactant in the alveoli, alleviating the constraint of hypoxia; it also increased the constraint of hypoxia; it also increased the production of free fatty acids from fat pads, increasing body temperature.

Figure 2.

Testicular descent. The testes emerge from the common gonadal tissue and the gubernaculum determines the level to which they descend. The production of testosterone determines the length of the gubernaculum, mediated by Insulin-like Hormone (IGF)-3 and heat shock protein A (HSPA)2. Renal location in the diagram is indicated in red and final testicular location in orange.