Jan 01 2000 - Physiology Web Site

Melatonin and Uric Acid in Amniote Evolution : Their Selective Physiological Roles

J. Anim. Morphol. Physiol., Vol 47, Nos 1 & 2, pp 1 to 24, 2000

By J.C. George
Department of Zoology , University of Guelph, Guelph, Ontario, Canada N1G 2W1
Fax: (519) 767-1656; email: jgeorge@uoguelph.ca

Dedicated to the memories of the old masters of Indian Zoology:
P.R. Awati, K.N..Bahl., Caleekal John George, A.P. Mathew, A.B. Misra, H.K. Mookerjee, D.M. Munshi, N.V.Murti, Vishwa Nath, S.S.Prabhu,M.R. Rajasekareasetty, S.G.M. Ramanujam, S.P. Raychowdhury, and R.V. Seshaiya,
who had touched many lives.

ABSTRACT

In the colonization of dry land, the amniotes had to cope with scarcity of water, the threat of desiccation and also of hyperthermia caused by high environmental temperatures. On the other hand, the dynamic new environment offered them food and oxygen in abundance. Among the various adaptational aquisitions for survival on land, melatonin emerged as a thermoregulatory hormone protecting them from hypothermia. As a hypothermic hormone it also prevented indirectly excessive loss of bodywater. As a mechanism for conservation of water uricotely evolved in reptiles and birds.Uric acid also appears to have a hypothermic action. Some reptiles, however retained the urinary bladder and with greater postrenal reabsorption of water and electrolytes, became facltative uricoteles. Uricotely became well established in birds and ureotely in mammals. The high water solubility and low toxicity of urea and with a kidney that has a high capacity for reabsorption of water, enabled ureotely to become of selective value in mammals. It is suggested theta, while melatonin functions essentially as a thermoregulatory hormone in amniotes and uric acid excretion as a mechanism for water conservation in the sauropsids, both melatonin and uric acid originated during the course of evolution as antioxidants to protect organisms from oxidative damage from free radicals. Perhaps, all other functions of these two substances are fortuitous in the process of natural selection.

***