Mane

From Freepedia

The mane is the term, when speaking of a horse or lion, used to describe the line of hair along the spine of the neck, starting behind the ears and ending just above the withers. When the horse/lion is born, the mane is a short, stiff, upright row of hair, not unlike a zebra's, but as the horse/lion ages the mane grows out and softens.

Pictures of Manes

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Mane is the name of several communes in France:

How did the lion get its mane? an essay Discussing evolutionary adaptations in response to natural and sexual selection

Lions are social, territorial animals. As such the reproductive success of the males is directly correlated with their ability to compete with other males for females. There is much evidence that the lions mane is instrumental in this competition: Having a mane is a liability in the hot climates inhabited by lions. Evidence for this has been accumulated using thermal imaging cameras (West and Packer, 2002), which showed that the mane acts as a strong insulator, limiting heat exchange. The effect of this insulation is increases by the thickness and length of the hairs. Furthermore, darker hairs will absorb more heat then lighter hairs. So a long, thick, dark mane causes a great deal of heat stress. So why would male lions have such an extravagance? The evidence points to the mane being used primarily as an advertisement of fitness, since mane length (affected by the hair thickness, thicker hairs being less likely to break so ending up longer) and darkness are directly correlated with fitness for several reasons: • Manes use up valuable resources, so a healthy long mane indicates good nutrition, which indicates good feeding abilities. Thus males with severe injuries have significantly reduced manes as resources are allocated elsewhere. • Manes cause significant heat stress, and the ability of the male (or the males direct ancestors in regard to fertility) to cope with this heat stress is reflected in the mane length and darkness. The mane’s length and darkness are elastic, reflecting climate, nutrition and short term health much more strongly then genetic predisposition. As the temperature experienced by a cohort of lions from the same area will be the same, it makes the relative length and pigmentation of manes a good indication of the other variables. Poor nutrition, health, or genetic factors helping to cope with heat stress will be reflected in the plasticity of manes with shorter, lighter manes. Furthermore, hotter lions tend to eat less, making them more susceptible to sudden lack of food. And lastly, high heat levels have a negative effect on fertility as seen in malformed sperm and testis shape. • Mane length and darkness are correlated with age; thus a lion that has a long, dark mane must have what it takes to survive longer. • Mane length and darkness is correlated with testosterone levels. High levels of testosterone will make a male more aggressive. This reflects fitness, as A) male lions, when ousting another male from a pride in order to take over the reproductive rights to the females, will kill all the cubs of the original male. Thus a more aggressive father will confer on his offspring a better survival rate by being a more persistent defender. Also, more aggressive males are more likely to oust other males and take over their breeding privileges. B) Testosterone has an adverse affect on lions immune system; a male that can afford a relatively high level of testosterone must have a relatively strong immune system or general levels of fitness to be able to make such a sacrifice.

The length of the mane then is a compromise between the sexual selection pressure bestowed by having a large, dark mane (thereby advertising fitness) and the negative natural selection pressure incurred by heat costs, nutrient costs and testosterone. Hence the mane only appears during puberty, as there is no need to make sacrifices for advertisement if you are not in a position to mate. Let us look at both aspects of selection in more detail, so that we may better understand both the biotic and abiotic environmental factors in this equation.

Firstly, advertising fitness is aimed at males (from within or without of the resident coalition of males laying claim to a group of females) and at the females themselves. When a female enters estrous, the first male to reach her establishes a consortship. Although observations of this occurrence are rare, it is likely that there is competition amongst males for said female. Experiments with dummies have shown that male lions will be a lot more cautious of lions with longer, darker maned modals. This must be because the mane is a reflection of aggressiveness and fitness; it is much riskier to contest a male with a longer, darker mane. This has similar implications when a male is deciding whether to challenge a current cohort for their females or whether to attempt to defend his cohort against an invading male. As females tend to come into estrous at the same time, there is occasionally an excess of males to females. An unclaimed female will then have the opportunity to choose a mate. It is in the females interest (and she has been naturally selected to) choose a male that will give her cub the highest rate of survival. Males with longer, darker manes sire cubs with higher survival rates than shorter, lighter maned companions. This may be a reflection of general genetic fitness, but is also undoubtedly related to the ability of the male to defend his cubs and females against marauding males, and to bring down larger game, which would allow the cubs more food at the kill. Smaller game tends to be dominated by the dominant male, which leaves the cubs malnourished. Although in areas where large game is rare the male will dominate the kills irregardless, and thus cubs sired by darker maned lions in those areas have a slightly lower survival rate.

Now let us discuss the natural selection pressures imposed by the mane. Firstly, there are the points about fitness mentioned about in the bulletpoins. There is also the possibility that manes are used to defend the neck and shoulders from attacks by other males. However, there doesn’t seem to be a propensity amongst lions to attack the neck and shoulders specifically, and wounds in those areas do not seem to be disproportionately damaging, and so this is an unsatisfactory claim. However, that is not to say that the mane does not offer increased protection; perhaps a serendipitous side effect. The mane may not be pre adapted for defence, but may also serve that role. The evidence, though, is still insufficient. As discussed previously, the mane is highly elastic in its properties, reflecting current states much more then genetic predispositions. In fact, a study by West and Packer (2002) showed no heritability of either mane length or pigmentation. I believe this elasticity is itself an adaptation. Perhaps originally the length ad pigmentation of manes was directly affected by genes. In such a situation there would be fitter and less fit males with short, light manes and only fit males with long dark manes, as the unfit ones would be dead. Long, dark maned males would gain an advantage (both in female selection and inter male competition) as they would be sure to be fit, so a better bet to mate with or avoid. If a mutation arose that imparted elasticity on the mane length and darkness, even the unfit males, long dark maned males would be selected for in times of plenty. Furthermore, the unfit males would survive better if they could make sacrifices in mane length and darkness, as a low chance of reproduction is better then none (which you get if you’re dead because you’ve spent vital resources on a mane instead of survival). Thus the gene would be passed on, and though the other poor fitness genes would be weeded out in subsequent generations, the gene for plasticity will find its way (probably) into a body with high fitness. This would be more then enough selection pressure for the gene to rapidly spread throughout the population, and shortly the plasticity gene would be fixed. Seen in this way, the elasticity of the mane can be seen as a canalised adaptation; once the genes for longer, darker manes came into being the elasticity would follow. However, we have not yet considered the lions mane as a selection pressure for anything other then the individual organism. Is it possible that the mane has selection advantage in group selection or kin selection? Well, for group selection it is doubtful; secondary sexual ornamentation such as these are costly, and reduce the species/groups overall survival. However, kin selection may add to the selection advantage of having a long, dark mane as cohorts of males tend to be related, often brothers (as the father will evict his adolescent male offspring from his territory). Thus, a kin group of males with one or more long, dark manes ought to have a better chance at claiming a territory, though I have no evidence to support this. Lastly, it is possible that the mane was pre-adapted to colder climates in the past. Females were selected to choose long manes, and now the sexual selection for long manes outweighs the natural selection for reduction of heat stress; it is possible the mane is a historical accident, as any lion with no mane (except in exceptionally hot areas such as the Tsavo National Park in Kenya, where the male is maneless) would fail to breed even if they were slightly better adapted to the heat. Thus elasticity may have arisen, but the seemingly more sensible and less wasteful loss of the mane maybe an overall step backwards selectively when coupled with sexual selection, and evolution cannot make such short term sacrifices.

And so to conclude, the mane truly seems to be a compromise between natural selection against manes to reduce heat stress and sexual selection promoting manes as an advertisement of fitness. As this aspect of the lions genome is poised on a constant razor edge between these two pressures, the compromise of elasticity was strongly selected for and has arisen as an integral, if not immediately obvious, part of the lions mane.

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