Countershading is a kind of pattern in animal coloration which means the dorsal hair or feather color is darker than the ventral hair or feather color. Most of us think it is a kind of crypsis for animals not to be found easily by others. But in fact, people are still debating about why countershading and we still have little understanding about this common phenomenon in many species. There are two aspects for the question of why countershading, the function of countershading on animals and the molecular mechanism of countershading development.
One main problem about crypsis theory is that why ventral hair or feather color is lighter? We will all agree with that when looking upward, it will be hard to find the fish because their ventral color is light and very close to the color of the sky, same thing about looking fish downward. But is that situation the same in some mammals? It seems no chance for others to look at a rodent, a felid or a Artiodactyla upward. If the hair colors of these animals are all dark in dorsal and in ventral, it seems to be a better crypsis. So, there are several alternative theories to explain the function of lighter ventral hair color, like illuminate the food under the body, protection from ultraviolet light; thermoregulation; and protection from abrasion. Other theory says the lighter color in ventral help the cubs finding the papilla easier which convinced me more.
While looking at birds, most of the birds have countershading and the main function of that may be crypsis because their situation is similar with fish rather than mammals. But another phenomenon confused me is the sexual dichromatism, which means the pattern of males and females are different.
Pic.1 Cardinal: left female, right male. (Photo from Google)
Pic.2 Penacook: left female, right male. (Photo from Google)
Pic.3 Mallard: left female, right male. (Photo from Google)
Pic.4 Red Jungle fowl (ancestor of all domestic chickens): left male, right female. (Photo from Google)
In these birds above, we can find sexual dichromatism and countershading only happened in females not in males. Here is a problem; sexual dichromatism is more common in birds rather than mammals. If the lighter color in ventral helping the cubs finding the papilla, this sexual dichromatism is more possible to happen in mammals rather than in birds as there is no lactation in birds. In other words, if the countershading only serves as crypsis, is that means male birds need less protection than females? I would like to say yes to this question but more evidence is needed.
So, the real function of countershading needs to be discovered in the future. But we have a better understanding on the molecular mechanism of countershading development. The main pigment in mammals and in birds is melanin, which can form to kinds of particles: eumelanin and pheomelanin. Eumelanin shows a black/brown color and pheomelanin shows a yellow/red color. Melanocortin 1 receptor (MC1R) plays a critical role in the synthesis of melanin. MC1R is a G protein-coupled receptor that activates the cAMP signaling pathway, after binding with α -melanocyte-stimulating hormone (α-MSH) or adrenocorticotropic hormone (ACTH), eumelanin will be synthesized. The agouti signaling protein(ASIP) has a competitive effect on bingding MC1R, when MC1R is coupled with ASIP, the synthesis of eumelanin will be repressed and pheomelanin will be synthesized. In mice, there are five kinds of ASIP mRNA variants and are controlled by two kinds of promoters: the hair cycle-specific promoter and the ventral-specific promoter. The hair cycle-specific promoter acts at the midpoint of the hair growth cycle to produce hairs with a black base. The ventral-specific promoter directs expression throughout the entire hair growth cycle of ventral but not dorsal hair follicles. In this way countershading is developed. In rabbits, only two kinds of ASIP mRNA variants are expressed by the action of two promoters to produce countershading pattern.
Scientists focused on the function of ASIP just in recent years. The chicken feather follicles express at least seven kinds of ASIP mRNA variants using three promoters. And the chicken ASIP gene is expressed in a wide variety of tissues, which contrasts with the expression of the wild-type mouse agouti (the murine ASIP) gene which is limited only to skin which suggests that ASIP in birds may play more roles than that in mammals. In 2012, scientists found that the distal ASIP promoter not only acts to produce countershading in chicks and adult females, but also plays an important role for creating sexual plumage dichromatism controlled by estrogen. What surprised me is that the countershading and sex dichromatism is associated again in molecular level! I believe this is kind of evidence that no countershading in male birds is a kind of benefit for the whole population in evolution. If we understand this benefit, we will be able to solve the problem of why countershading.
In addition, it is the melanocytes that produce the melanin, so the presence of melanocytes is also a key factor that determines the pattern of coloration. What I can tell now is that the melanoblasts (precursor of melanocytes) are derived from neural crest cells (NCCs), a multipotent population of cells emigrating from the dorsal neural tube. Considering countershading, I believe it is a benefit at least in chicken that the melanoblasts migrate from dorsal to ventral not ventral to dorsal. Because some time the migration will not complete at the time of hatching (See Pic5), so countershading is formed. In other words, the migration from dorsal to ventral gives the pigment to the chick at least in the dorsal part and somehow protects the chick (although the countershading function is still in debate). I would like to talk about more about NCCs and chicken pigmentation next week after digging deeper in the articles.
Pic.4 Chick display countershading(Photo taken by me).
H. Vrieling, D.M. Duhl, S.E. Millar, K.A. Miller, G.S. Barsh, Differences in dorsal and ventral pigmentation result from regional expression of the mouse agouti gene, Proc. Natl. Acad. Sci. USA 91 (1994) 5667–5671.
L. Fontanesi, L. Forestier, D. Allain, E. Scotti, F. Beretti, S. Deretz-Picoulet, E. Pecchioli, C. Vernesi, T.J. Robinson, J.L. Malaney, V. Russo, A. Oulmouden, Characterization of the rabbit agouti signaling protein (ASIP) gene: transcripts and phylogenetic analyses and identification of the causative mutation of the nonagouti black coat colour, Genomics 95 (2010) 166–175.
C. Yoshihara, A. Fukao, K. Ando, Y. Tashiro, S. Taniuchi, S. Takahashi, S. Takeuchi, Elaborate color patterns of individual chicken feathers may be formed by the agouti signaling protein, General and Comparative Endocrinology 175 (2012) 495–499.
Eri Oribe, Ayaka Fukao, Chihiro Yoshihara, Misa Mendori, Karen G. Rosal, Sumio Takahashi, Sakae Takeuchi, Conserved distal promoter of the agouti signaling protein (ASIP) gene controls sexual dichromatism in chickens, General and Comparative Endocrinology 177 (2012) 231–237.
Ben Dorshorst, Anna-Maja Molin, Carl-Johan Rubin, Anna M. Johansson, Lina Stromstedt, Manh-Hung Pham, Chih-Feng Chen, Finn Hallbook, Chris Ashwell, Leif Andersson, A Complex Genomic Rearrangement Involving the Endothelin 3 Locus Causes Dermal Hyperpigmentation in the Chicken, PLoS Genetics, 12, 2011, Volume 7, Issue 12, e1002412.
Wikipedia – Countershading http://en.wikipedia.org/wiki/Countershading#cite_note-42