Morphology and development of the comb

The comb of birds is generally vacillated during different behaviours (Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). In male Gallus domesticus the comb is controlled by their behaviour but in some species like black grouse, the comb size changes rapidly (in few second) by controlling blood circulation (Holder and Montgomeri, Reference HOLDER and MONTGOMERI1993). Research indicates that red and white light stimulates the growth of the comb and testes in broilers, confirming the effect of red light in stimulating reproduction via the hypothalamus (Foss et al., Reference FOSS, CAREW and ARNOLD1972). The comb of the male is a naked head appendage and red in colour (Yoshioka et al., Reference YOSHIOKA, WATAHIKI, KANIE, TSUJIO, IKADAI, KASHIMOTO and MUTOH2010), with haemoglobin from the blood showing through the epidermis providing the colour intensity (Hardesty, Reference HARDESTY1931). As all avian species lack sweat glands, the comb acts as a cooling agent and increases the flow of blood towards the comb and wattle. The comb of the male is composed of the epidermis, dermis and central layers. The first epidermis has the structure similar to stratum corneum and stratum germinativum. The second dermis has the micro-physiological structure resembling the peripheral and intermediate layers. Beneath the epidermis, the narrow and fibrous peripheral layer is found which is attached via a network of capillaries. The last intermediate layer consists of collagen fibres and fibroblasts which are filled with an acidic mucopolysaccharide fluid, mainly hualuronan. The central layer is comprised of collagen fibres in bundled form and adipose tissue and is attached to the many arteries and veins together with some nerves (Hodges, Reference HODGES1974; Yoshioka et al., Reference YOSHIOKA, WATAHIKI, KANIE, TSUJIO, IKADAI, KASHIMOTO and MUTOH2010). In the development of the comb, the hormone androgen is responsible for important cell functions such as proliferation and differentiation of keratinocytes and acid mucopolysaccharide synthesis by fibroblasts as well as the synthesis of fibres (Yoshioka et al., Reference YOSHIOKA, WATAHIKI, KANIE, TSUJIO, IKADAI, KASHIMOTO and MUTOH2010). Testosterone is secreted from the testes (Sanford and Baker, Reference SANFORD and BAKER2010) and comb size is associated with testes weight (McGary et al., Reference MCGARY, ESTEVEZ and RUSSEK-COHEN2003b). Male comb size is testosterone dependent and comb colour is due to epidermal pigmentation by carotenoids in avian species (Mougeot, Reference MOUGEOT2008). Comb development is influenced by high hormonal levels in maturing hens (Tufvesson et al., Reference TUFVESSON, TUFVESSON, SCHANTAZ, VAN, JOHANSSON and WILBELMSON1999). Comb size can be increased by testosterone administration which ultimately increases the production of acid mucopolysaccharides which extends the capillaries of the peripheral layer. Comb growth can be induced in chickens by administering androgen (Moss et al., Reference MOSS, KOLB, MARQUISS, WATSON, TRECA, WATT and GLENNIE1979; Owens and Short, Reference OWENS and SHORT1995; Etches, Reference ETCHES1996). The main activities of androgen take place in the comb and this hormone helps to maintain the structure and size of the comb and controls its growth (Ludwig and Boas, Reference LUDWIG and BOAS1950). The activities of androgen in the male comb have been validated through the discovery of the androgen receptor in the comb tissue (Shanbhag and Sharp, Reference SHANBHAG and SHARP1996). The nuclei of the cells in the base of the comb, fibroblasts of the dermis layer and intermediate layers of the epidermis, such as the stratum germinativum, are important places for androgen receptor immunoreactivity in the male comb (Yoshioka et al., Reference YOSHIOKA, WATAHIKI, KANIE, TSUJIO, IKADAI, KASHIMOTO and MUTOH2010). In the peripheral layers, androgen controls the diameter of the capillaries, whereas testosterone elongates the tiny blood vessels and fuses these blood vessels with the neovascular network in the vascular network in the outer layers. Androgen has the ability to increase surface vascularisation. Neovascularisation is initiated by disruption of the extracellular matrix of the blood vessels basement membrane in a mature individual and via proteases produced by endothelial cell and vascular cells going through the processes of migration and proliferation (Patan, Reference PATAN2000). A neovascular framework is developed by the formation of a luminal structure, which goes through repeated sprouting and anastomoses (Patan, Reference PATAN2000). In the domestic fowl comb size is under the influence of testosterone (Gilbert, Reference GILBERT1971; Zuk et al., Reference ZUK, JOHNOSEN and MACLARTY1995). In the red jungle fowl male comb size is a renowned predictor of females for mate choice in the flock (Parker et al., Reference PARKER, KNAPP and ROSENFIELD2002). Testosterone has a very strong effect on comb development, and dictates the physical aggressiveness of birds. The presence of females in the flock has a strong effect on the comb size of male birds because this enhances testosterone secretion which in turn increases the size of the male comb (Alatalo et al., Reference ALATALO, HOGLUND and LUNDBERG1988; Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). The testosterone 5α-dihydrotestosterone (DHT) adrostenedione is equally active in inducing comb development (Johnsen, Reference JOHNSEN1986). It was found that 5 β (testosterone metabolites produced by neutral tissue of male chick) alone or blended with oestradiol reduce androgen concentration and affect comb size. Consequently, the comb converts testosterone into 5 α reduced compounds which further effect comb growth. However, in accordance with the handicap hypothesis, a negative relationship exists between testosterone and the immune system (Folstad and Karter Reference FOLSTAD and KARTER1992). Male jungle fowl with larger combs and higher levels of circulating testosterone have fewer circulating lymphocytes (Zuk et al., Reference ZUK, JOHNOSEN and MACLARTY1995). Thus, in agreement with the arguments of both Møller and Pomiankowski (Reference MØLLER and POMIANKOWSKI1993) and Hill (Reference HILL1994), male red jungle fowl appear to possess both low cost-low information traits (plumage ornaments) and a high cost–high information trait (the comb). The most direct evidence that large-combed males may be considered genetically superior is the positive relationship between sire's comb size and growth rates of offspring (Johnson et al., Reference JOHNSON, THORNHILL, LIGON and ZUK1993).

Snedecor (Reference SNEDECOR1968) concluded that testes weight and comb weight increased by application of Pregnant Mare Serum (PMS) gonadotrophin in normal chicks. In hypothyroid chicks only the gonad showed a significant increase in weight as the result of PMS injection. The lack of significance in comb response in the latter case suggests that Leydig cells in the testes of hypothyroid chicks do not respond to thyroxin. In radio-thyroidectomised chicks, PMS causes a significant increase in the weight of the testes (74%) but the increase in comb weight (30%) was not significant (Snedecor, Reference SNEDECOR1968). Egg production, egg quality and reproductive traits of flocks were significantly associated with the number of cusps on the comb. Additionally, rapid growth of the comb is associated with prepubertal luteinizing hormone concentration (Sharp, Reference SHARP1975; Eitan et al., Reference EITAN, SOLLER and ROZENBOIM1998).

Regression or atrophy of the comb

The castration of the male consequently reduces the size of the comb and reduces body weight along with thickening the dermis layers and reducing hyaluronic acid concentrations (Robert and Carr, Reference ROBERT and CARR1985). Induced caponization by testosterone reduces the comb weight, height and length in male birds (Chen et al., Reference CHEN, CHI and CHIOU2005; Reference CHEN, TSAY, CHIOU, CHEN and WENG2009). A study was conducted by Wilson et al. (Reference WILSON, WOODARD, NORSTRON and SMITH1958) to observe the regression changes in external indicators (comb) following the cessation of laying conditions. They concluded that the period required for regression of the comb is 39 days. The comb was slowest to react to non-laying conditions. The period that a hen must be out of production is 11 days for comb index changes to be observed. Any change in the comb index results from decreases in androgen which is produced by the active ovary which becomes inactivated under non-laying conditions (Wilson et al., Reference WILSON, WOODARD, NORSTRON and SMITH1958). Both testosterone and diethyl stilbestrol treatments had a depressive effect on testes weight, comb weight, comb length and comb height (Falconer, Reference FALCONER1981). Oestradiol increased body weight and relative weight but suppressed the growth of the comb and testes. Growth of the testes and comb was suppressed in progesterone implanted chickens (Chand et al., Reference CHAND, ARNEJA, GEOGRIE and ARORA1982). Prolactin treated cocks exhibited a decrease in size of both the testes and the comb, cessation of growth and lack of sex drive (Eisner, Reference EISNER1960). The administration of large doses of thyroxin causes a sudden moult with a reduction in egg laying and atrophy of the comb and wattles. In both sexes, comb weight was reduced in the phyoestrogen treated group and a number of effects were seen on the pituitary or adrenal glands (Rath et al., Reference RATH, HUFF, BALO and BAYARRI1996). Prolactin doses ranging from 90 IU to 150 IU does not affect body weight, testes weight or comb dimension (Eisner, Reference EISNER1960).

Comb and mating behaviour

In most avian species, the females choose the male whereas cockerels in breeder houses often appear to mate forcibly and aggressively with hens (Jones and Prescott, Reference JONES and PRESCOTT2000). Comb size increases during mating and decreases during incubation (Rydmel, Reference RYDMEL2010). In laying flocks, male birds highlight their dominance through well-developed bigger combs (Pagel and Dawkins, Reference PAGEL and DAWKINS1997; D'Eath and Keeling, Reference D'EATH and KEELING2003) and larger comb size has a positive correlation with copulatory success in birds (Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). Larger combs may reliably indicate roosters with greater semen production (Burrows and Titus, Reference BURROWS and TITUS1939), higher androgen levels (Gilbert, Reference GILBERT1971), or increased mating activity (Ligon et al., Reference LIGON, THORNHILL, ZUK and JOHNSON1990). Better quality males always possess larger ornaments (Goransoon et al., Reference GORANSOON, VON SCHANTZ, FROBERG, HELGEE and WITTZELL1990; Hill, Reference HILL1991). The ornaments traits present the males’ instant condition which ultimately has a positive effect on the fertility and viability of females such as less chance of disease transmission (Kirkpatrick and Ryan, Reference KIRKPATRICK and RYAN1991). In a mate choice experiment, males preferred by females had larger comb size with longer spur length (Schantz et al., Reference SCHANTZ, TUFVESSON, GORANSOON, GRAHN, WILHELMSON and WITTZELL1995) because larger combs are a cue for females as to the quality of the male (Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). The degree of contact with males during rearing affected the females’ physiological measures of reproductive function (Widowski et al., Reference WIDOWSKI, DANILO, WONG and DUNCAN1998). Males with a larger comb size always showed a successful display (Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). The large size ornamental traits and their intensity is an important criteria for females to select their male for mating because these traits provide an unfailing indicator of sire quality in the flock. A larger comb is positively correlated with the number of copulations in birds (Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000). According to the immune-competence theory, male ornament traits are the best indicator of resistance to different diseases and parasite infestation and birds with a larger comb have fewer parasitic infestations and have more immunity and better health status (Ligon et al., Reference LIGON, THORNHILL, ZUK and JOHNSON1990: Zuk et al., 1990 a; 1990b). Display traits have a positive relationship with successful mating, which are important stimuli to initiate copulation for females (Andersoon, Reference ANDERSOON1991). Removal of the comb from adults’ cockerels lowered the dominance of the individual which can affect mate choice if social status is implicated in choosing a male. Eliminating secondary sexual characteristics such as plumage colour or comb may influence the sexual contact between a hen and a cockerel, which may influence behaviour and success of mating. If the immune competence handicap mechanism is operating, wattles, comb and spur will be used by hens to assess males rather than plumage. However, it is difficult to see why a jungle fowl cockerel should possess a more ornamental plumage than jungle fowl hens.

Ultra violet light may be used to commercially differentiate comb and wattle texture. Comb size has an insignificant relationship with male-male fighting competitions (Gratson, Reference GRATSON1993; Rintamaki et al., Reference RINTAMAKI, HOGLUND, KARVONEN, ALATALO, BJORKLUND, LUNDBERG, RATTI and VOUTI2000) but is very reliable in male-male signalling (Zuk, Reference ZUK1991). During male-male fighting, each contender attempts to damage the comb of his challenger (Owens and Short, Reference OWENS and SHORT1995) to reduce his dominance and aggressiveness (Holder and Montgomeri, Reference HOLDER and MONTGOMERI1993).

Comb as a reliable indicator for selection

Inbreeding without selection decreases egg number, egg weight, shell thickness and increases age at first egg, haugh unit score and body weight (Ibe, Reference IBE1982). Inbreeding increases the homozygosity by 2% per generation (Cywa and Wezyk, Reference CYWA and WEZYK1985), so improvements for productive and reproductive potential through selection and breeding need rigorous studies. Avian appearance plays a key role in successful selection procedures in breeding programs. Sexual ornaments are variable for repeatability and coefficients of variation for morphological character (Zuk et al., Reference ZUK, THORNHILL, LIGON and JOHNSOON1990a; Wittzell, Reference WITTZELL1991). In females, the comb is a moderate to highly heritable trait but in males the comb size has high heritability (Tufvesson, Reference TUFVESSON1998). Comb has a high heritability estimate and genetic changes such as egg laying performance are due to comb size. Evidently it is suggested that male comb size at 16 weeks can be used to select for early maturity and comb size at 29 weeks can be used to select for egg mass. The comb size measurement at an early age has a higher correlation with egg traits (Tufvesson, Reference TUFVESSON1998). Internal parasite genetic correlation was found between comb size and body weight in lines selected for comb size at 29 weeks. There is a negative correlation between comb size and age at first lay and egg number and a positive correlation with egg weight in the flock. There exists a negative correlation between comb size and testes weight and spur length.

Effect of comb size on productive and reproductive traits

Females that have larger, redder combs lay eggs earlier compared to females with smaller size combs (Eitan et al., Reference EITAN, SOLLER and ROZENBOIM1998). Comb index in female breeders increases rapidly before five weeks of point of lay, and maximum growth occurs between the third and fifth week of first oviposition (Fattori et al., Reference FATTORI, WILSON, HARMS, MATHER, MILES and BUTCHER1993). In meat-type pullets, comb size is a very helpful display of sexual maturation (Eitan et al., Reference EITAN, SOLLER and ROZENBOIM1998). The sexual maturity of breeders can be assessed by regular analysis of oestrogen concentration because the oestradiol-17β concentration has a positive correlation with sexual maturation (Itoh et al., 1998; Joseph et al., Reference JOSEPH, ROBINSON, RENEMA and THORSTEINSON2003) and decreases once maturity is attained (Wilson and Sharp, Reference SHARP1975). A delay in the development of the comb always delays the egg production process in flocks. Sexual maturation in breeder flocks can be assessed by comb growth (Biswas et al., Reference BISWAS, RANGANATHA and JAG2010) and changes can be used as a potential tool for assessing sexual maturation within a breeder flock (Joseph et al., Reference JOSEPH, ROBINSON, RENEMA and THORSTEINSON2003). Quantitative trait loci are found on chromosome no.1 for comb, the phenotypic measurement in chickens indicates that genes for comb size and egg production are linked closely and broken in F8 generation (Rydmel, Reference RYDMEL2010). Androgen levels and comb size increase before the onset of lay (Eitan et al., Reference EITAN, SOLLER and ROZENBOIM1998) and generally comb size is a substitute measure for the development of reproductive function in chickens (Jacoby et al., Reference JACOBY, SNAPIR, ROZENBOIM, ARNON, MEIDAN and ROBINZON1992; Rozenboim et al., Reference ROZENBOIM, SNAPIR, ARNON, BEN-ARYEH, BURKE, SHARP, KOCH and ROBINZON1993). Comb size, comb weight and comb length have a correlated response with total egg production, egg number and mean egg weight in chickens (Rydmel, Reference RYDMEL2010). However, all egg laying traits have a negative correlation with male comb size at 29 weeks of age and egg weight at first egg (Haq et al., Reference HAQ, MUKHTAR, SHAHID-UR-REHMAN, RAMZAN and AHMAD2003). Other researchers reported (McGary et al., Reference MCGARY, ESTEVEZ, BAKST and POLLOCK2002; Reference MCGARY, ESTEVEZ and BAKST2003a) that male fertility has a positive correlation with secondary sexual characters in Gallus domesticus.

Sire comb size has a significant effect on hatchability whereas dam comb size has a significant effect on egg weight. Female comb size has a non-significant effect on hatchability, fertility, age at first lay, albumen quality, haugh units and egg production in layers (Haq et al., Reference HAQ, MUKHTAR, SHAHID-UR-REHMAN, RAMZAN and AHMAD2003). The strong selection for increased comb size does not affect male mortality but does indicate higher mortality in females. Larger and heavier dam combs have a significant effect on egg weight but larger male combs exert a significant effect on day old chick weight (Haq et al., Reference HAQ, MUKHTAR, SHAHID-UR-REHMAN, RAMZAN and AHMAD2003).