Featured Questions, Comments and Responses

Subject: EMCOM Queries
Question: Hello, I am preparing a paper on the colour changes in animals and would like some information on which colour changes are hormone dependent. Thanks in advance,
Antonin

Answer: Many vertebrates and invertebrates show colour changes associated with environmental factors or physiological/developmental events in their life-histories. These colour changes are a consequence of specialized cells called chromatophores that reside in the skin. Chromatophores are of various types: melanophores, iridophores, xanthophores, and erythrophores, depending largely upon the colour of pigment that they contain. The most common are the melanophores which are responsible for lightening and darkening of the skin as occurs in frogs or chameleons.

Two types of colour changes occur: morphological and physiological. In all vertebrates, there is the possibility of varying the amount of pigment in a chromatophore or varying the number of chromatophores in the skin; this leads to a morphological colour change, but it takes relatively long periods of time to achieve. Physiological colour changes involve the relatively rapid migration of pigment within the chromatophore, and among the vertebrates occurs only in fishes, amphibia, and reptiles. This occurs primarily within the melanophore where the pigment (melanin) is contained in discrete particles called melanosomes. When the pigment is aggregated in a tight ball, the skin is light. When the pigment is dispersed throughout the melanophore, the skin darkens.

Changes in the distribution of melanin within the melanophore is governed by both nervous and hormonal factors.. The pituitary gland secretes melanocyte stimulating hormone (MSH) and adrenocorticotrophic hormone (ACTH) which are the primary endocrine regulators of melanophores. MSH stimulates melanosome dispersion by binding to MSH receptors on melanophores to activate adenylate cyclase and, thus, causes a physiological darkening. MSH also stimulates activity of tyrosinase, a key enzyme in the pathway for melanin synthesis. It is via this route that MSH induces a slow morphological colour change. In most fishes, melanosome aggregation is under adrenergic neural control. The neurotransmitter norepinephrine (NE) promotes melanosome aggregation by action on alpha-adrenergic receptors.

I am not aware of the effects of environmental chemicals on physiological colour change. The most important factors appear to be the colour of the background and temperature.