Darwin in his magnus opus The origin of Species mentioned under his revolutionary theory of evolution the premise of sexual competition. Darwin defined sexual competition as the struggle between the sexes of a particular species for the reproductive right to pass on their traits unto the succeeding generation. (Darwin, On the origin of species by means of natural selection, 1859) He further elaborated upon the concept of sexual selection on his second book entitled The Descent of Man, and Selection in Relation to Sex. Darwin, The descent of man, and selection in relation to sex. , 1871)
In recent years sexual selection has become a popular subject of interest in various fields, from animal behavior, behavioral ecology, and evolutionary science and even up to parasitology. Sexual selection is in evidence in almost all forms of organisms within the animal kingdom, from the preference of flies, to reptile and avian reproduction and even up to human sexual preference. (Merry, 1998) Avian sexual selection has been one of the subjects more thoroughly researched in recent years.
This is perhaps due to the large number of secondary anatomical and behavioral characteristics in which birds, particularly the males, use for attracting members of the opposite sex. In this paper, I hope to give an overview of sexual selection in birds, its behavioral and morphological components. In particular, it is my intent to delve into the role of snood size in male wild turkeys in attracting females for reproduction. In this paper, I aim to prove that snood size serves as an attractant to female turkeys because it serves as a morphological indicator of the viability of the male turkey as a potential mate.
To prove this, I would obtain literature regarding the relationship of snood size with testosterone level, disease resistance and parasite resistance. Background Wild turkey (Meleagris gallopavo) is a member of the Galliformes. It is a native of the Americas and exists as five distinct subspecies. Turkeys usually stand up to 3 feet tall. Their head and neck are usually devoid of feathers. Their feet are equipped with sharp spurs and their tails are capable of being raised for mating or aggressive displays. Males are more colorful than females, having iridescent yellow feathers covering their body.
Males also have a variety of secondary anatomical features. Flaps of skin line the male turkey’s neck up to the base of its beak. These are called wattles. They also have brightly colored growths on the throat region called caruncles. Male turkeys also have a protuberance atop their beak called a snood. Wild turkeys can be found in a variety of habitats, from mountains, forests, swamps, prairies, woodlands and savannahs. Wild turkeys are voracious foragers. They usually prefer insects when young and plant food as they mature. Mating season for turkeys usually start at spring and is usually heralded by gobbling.
It is used to attract potential females in the vicinity. After attracting potential hens nearer, the gobbler begins a courtship display. The courtship display consists of a combination gobble and dance, with the gobbler strutting by raising his body feathers, fanning its tail and dropping his wings to the ground. The display also induces blood to rush to the gobbler’s head, making his snood elongate and his wattles turn into a bright red color. Sexual Selection In his second book, Darwin identified two distinct forms of sexual selection.
The first form consists of the male to male interaction of the same sex in obtaining reproductive access to the females of the opposite sex. (Darwin, The descent of man, and selection in relation to sex. , 1871) This is defined as intra sexual sexual selection. This usually comes in the form of competition and usually consists of combat among males of a species. (Merry, 1998) The combat does not usually result in the death of the losing side, the outcome generally results in the inability of the loser to pass on its genome unto the next generation.
This male to male completion is thus considered to be a selective process which ensures that the best genes which would manifest itself in combat would result in reproduction and increase the chance that those same genes would appear in the offspring. Examples of these include the breeding season fights between deer, which use their antlers as an indicator of their combat prowess. Thus, deer with larger antlers tend to have a greater chance of reproducing, and passing on genes for increased antler size unto the next generation.
The use of anatomical features to fight over females also leads to a hierarchal dominance among males of the species. Early reptiles like Paracephalosaurs also theoretically exhibited this kind of behavior. These dinosaurs had thick skulls, which it may have used for head butting other males of the species to establish dominance and thus an increase in chance of reproductive success. (Carpenter, 2001) In birds, these behavioral and anatomical characteristics are also evidenced. Galliforms (fowls) possess spurs on either wings or feet. These spurs are used in contests between males of a species.
Aside from direct combat, birds have also developed other mechanisms to establish male to male dominance regarding reproduction. The evolution of birdsong can be considered as an indirect form of combat, as its pitch and volume is usually vital in establishing male territoriality since strong calls are usually sufficient to dominate weaker voiced individuals from one’s territory. The loss of territory means a loss of area to acquire potential mates, thus resulting in a loss of reproductive chance for the loser. The second form of sexual selection according to Darwin is referred to as the mate choice model.
This is usually defined as epigamic or intersexual sexual selection. Under this theory, an individual of one sex, usually female, selects a member of the opposite sex based upon a morphological characteristic or a particular behavior that distinguishes that member from other individuals of the same sex. Darwin in The Descent of Man, and Selection in Relation to Sex put forth the hypothesis that animals, particularly females had an aesthetic sense, therefore, males were selected based upon characteristics which made them beautiful to females of the species. (Darwin, The descent of man, and selection in relation to sex. 1871)
This hypothesis was likely made upon the observation of male fowl species such as the peacock, whose extravagant plumage is displayed in the presence of females. The females would in turn select upon the males in the area the best plumage, entitling the male the opportunity to pass on their genome unto the next generation. The two forms of sexual selection, the male- male and the mate choice model are at times difficult to differentiate. As an example, male ring necked pheasants possess large sized spurs on their legs which are utilized in male to male combat.
Thus it falls upon the category of male-male competition. However, it has also been shown that females of the species prefer those males with longer spurs, regardless of the outcome of combat, thus placing this in the category of female choice model. It could be that females regard the presence of large spurs as an indicator of a mate’s potential prowess in combat, regardless of whether that combat actually occurs. The use of birdsong is also a case in point since aside from being used as a means to obtain dominance for territoriality, females also select upon potential mates based on the quality of their bird song.
There are various theoretical models that further delve into the mate choice model. In almost a majority of species exhibiting mate choice, it is the male which goes to extra lengths to impress the females, on whose choice resides the possibility of a successful mating. It seems that the female is better of in selecting high quality mates as opposed to preferring a multitude of mates, which may or may not be reproductively viable. Bateman in 1994 observed that male offspring increases with the number of females mated, as opposed to the constant fecundity observed in females regardless of the number of mates.
Thus a polygamous male is at advantage and is better to pass on their genes to the next generation while females are at an disadvantage when it comes to promiscuity in terms of the energy and effort spent obtaining and mating with multiple mates. Thus, females would have a tendency to seek out more qualified mates and ensure that the effort of bearing offspring is not wasted on males with lesser quality. (Andersson & Iwasa, 1996) The definition of more qualified mates is a broad term which may have many existing definitions.
Fitness is the capability of an organism to survive and be able to reproduce and yield viable offspring. (Mills & Beatty, 1979) Fitness in females is henceforth dependent on the ability of their male offspring to be able to successfully reproduce upon reaching maturity. This therefore leads to females of a species choosing males which have traits they wish to pass on to their male offspring in order to ensure its reproductive success. (Weatherhead & Robertson, 1979) Of particular interest in sexual selection is the good genes model.
In this model, females of a species are said to select those males which are healthy and dominant based on the fact that these are phenotypic expressions of potential genomes which could more likely make their offspring also strong and dominant. The preference for health is derived from the likelihood that a healthy species can better be able to reach reproductive age amidst the presence of parasites and diseases. The preference for strong and dominant mates also ensures that the offspring can better reach reproductive age amidst the presence of competitors, predators and environmental conditions which would select against weaker individuals.
The propensity of males to fight over the females of their species is a show of strength in order for the females to better select those males with the increased likelihood of producing better offspring. Males which have particular characteristics which enable them to win over other males would then have an increased chance of passing on those same characteristics to their offspring, thus ensuring an increase in expression for that particular trait in the population. Thus a buck with large antlers would have an increased chance of winning contests against other bucks and getting to mate with a female.
Their offspring would then have a higher chance of inheriting the trait for large antlers, thus ensuring that it itself would have a better chance of reproducing. There are various mechanisms by which animals select for good genes in their potential mates. The presence of large colorful tails in peacocks is, for the individual, a handicap since the tail limits its movement, as well as catching the eye of predators. Thus, for a male peacock to reach reproductive age while in possession of a large and colorful tail would mean that it probably has good genes which ensure its survival.
Consequently, the larger and brighter the tail, the more handicapped the male peacock is in terms of survival, and yet the probable presence of good genes which enable it to reach sexual maturity is a sign for females that that particular male is a viable mate. And although the presence of having such handicaps is a detriment for survival, the female preference for those handicaps and the increased likelihood of matings which result in offsprings with the said handicap ensure that the genes for those handicaps remain in the population gene pool. In birds, the presence of bright coloration is an attractant for females in some species.
The bright coloration serves to highlight a particular male individual from the environment, making them easier for females to perceive. However, the presence of bright colors which stand out from the surrounding environment is also perceived to be a handicap since being conspicuous also means being easily visible to predators. The presence of bright colors therefore signal an individual of higher fitness since for them to have that coloration, their ancestors would have been exceptionally fit to be able to reach reproductive age while being highly visible to predators.
This however would have the effect of reducing predator preference for individuals with brighter colors since it would signal a higher probability that the individual could have a greater chance of avoiding the predator. Another mechanism by which individuals select against others is through the recognition of morphological markers which act as indicators for that individual’s resistance to disease or parasitism. Females look for morphological markers which highlight a particular individual’s disease or parasite resistance.
This also indicates the presence of disease or parasite resistant genes which would be ideal to be passed on to their own offspring. In rats, the presence of a particular odor acts as an indicator for females in selecting mates. Female rats are able to detect olfactory signals in the urine of male rats. These olfactory signals indicate the presence of parasites in the males. Thus the female rats are able to avoid the males whose urine indicates the presence of parasites, consequently lowering the chance of them having a parasite susceptible offspring.
Some birds have also been observed to avoid making parasite laden males their mates. Birds of paradise have been observed to avoid males with high blood parasite content. However, the same study also noted a correlation between bright coloration and parasite load, thus leading to the conclusion that males which are more likely to reproduce are also more parasite intensive. This shows that although parasite load is an indicator of low fitness, birds of paradise rely on other indicators such as plumage as signals for preferable mates.
Also, it shows that although birds of paradise avoid birds with high parasite content, they select for birds which they deem fit due to coloration and look upon parasites as additional handicaps which augments the marker of having bright coloration. (G. Pruett-Jones, Pruett-Jones, & Jones, 1990) In 1982, Hamilton and Zuk proposed a hypothesis which correlates mate selection on morphological patterns which show resistance to parasitism. They showed that parasite presence adversely affects morphological markers like coloration and song which are deemed attractive by females.
This was further investigated and supported in 1990 in a study on red jungle fowl. The study showed that the presence of blood parasites inversely affected morphological markers in fowl. The presence of parasites resulted in roosters having duller and shorter combs, reduced tail feather length and coloration, and paler hackle feathers. The study also showed that females prefer unparasitized males over those with parasites in a ratio of 2;1. Snood size as a morphological indicator of fitness and mate viability in male turkeys
Although the courtship display consists of a variety of features, it seems that female turkeys look in particular at snood length in males. Female turkeys (hens) prefer longer snood lengths on male turkeys (toms). Richard Buchholz studied the effect of snood length on male competition. Buchholz placed two decoy male turkeys in a small area and place some birdseed in front of the decoys. The decoy’s appearance was identical except for the snood length. One decoy had twice the snood length of the other.
He then placed 21 male turkeys one at a time into the area and observed their reactions. 17 of the male turkeys approached only the decoy with the shorter snood while only 4 approached the decoy with the longer snood. Buchholz found that in fights between male turkeys, those with large snood size were often the winners. (Buchholz R. , Female choice, parasite load and male ornamentation in the wild Turkey. , 1995) In wild turkey, snood size has been linked to testosterone level. The longer the snood, the higher the testosterone level of the tom.
Bulchholz also tested whether snood size is linked to parasite resistance. Coccidia is a deleterious, intestinal protozoal parasites of wild turkey. (Apicomplexa: Eimeriidae). It adversely affects turkey growth, and severely retards the development of a male turkey’s ornamental features. (Buchholz R. , Female choice, parasite load and male ornamentation in the wild Turkey. , 1995) Bulcholz theorized that female turkeys infected with coccidian would show a preference for male turkeys with longer snoods.
However, his results showed that there was no significant preference in infected females for males with longer snoods. He attributed this to three factors. First is that the presence of the parasite may have impaired the hens’ neurological functions, making their mate selection process unreliable. Second was that the hens were avoiding mating due to avoidance of acquiring additional parasites.
And third was that the parasites were manipulating the hens’ behavior and inducing them to select more susceptible toms to mate , thereby increasing the parasite’s chance to survive in the offspring. Buchholz R. , Effects of parasitic infection on mate sampling by female wild turkeys (Meleagris gallopavo): should infected females be more or less choosy? , 2004) Bucholz also investigated the relationship of disease resistance on snood size. It was found that specific restriction fragments appear to be favourable to both immunocompetence and ornament size. However male turkey snood length is not a valuable predictor of wing web swelling because of phytohemagglutinin (PHA) injection.
Buchholz conluded that there is no direct relationship between snood size and disease resitance, however, due to the correlation of snood size to testosterone, it could be that male turkey’s with larger snood size are more capable of adapting to environmental conditions that would have otherwise weakened other turkeys with shorter snood length and made them more susceptible to disease. Conclusion Although experimental evidence shows that females prefer large snood size in males, there is still no indication aside from the correlation of testosterone to snood length to state that snood size is a sign of fitness.
Tests for correlation of snood length to parasite and disease resistance have returned inconclusive results. It would be ideal to research more on the physiology of the snood as well as to a direct correlation between snood size and disease and parasite load. Also, an investigation on the cumulative effects of the other factors in the male turkey’s courtship display should be looked into to find whether there is an interaction in the effects of the different factors.