Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What's the explination for why useless traits often disapear

  1. Aug 9, 2012 #1
    1. I mean for example why can't a kakapo fly? You could say flying is a waste of energy, but there's nothing wrong with a kakapo wasting his energy, flying is not a disadvantage, since he lives on an island with an abundance of food. So much food that to stop them eating it all they have somehow managed to evolve a system which stops them reproducing too often.
    Is there a Darwinian explanation for this? If not does it suggest epigenetics?

    I know quite often useless traits do not disappear. And I know sometimes you can evolve something that isn't necessarily helpful but that it keeps going anyway because it's not harmful. Then if you have a small population that turns into a large one (like with humans) you end up with lots of people having this useless trait because everyone is related to everyone else.

    2. That's another question. How on earth do you manage to evolve not to have too many offspringwhen you would think that if one parrot came along and had lots of chicks and the other came along and only had 4 in his whole lifetime that the one with lots would be the one that ended up taking over.. even though that's a really bad idea.

    Edit: I was just thinking maybe sometime in the past there wasn't enough food and flying therefore because a big disadvantage. That's probably it. Thoughts?
    Last edited: Aug 9, 2012
  2. jcsd
  3. Aug 9, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Gudduy and welcome to PF;
    Your examples suggest you are a kiwi :)

    Into each generation is born a range of characteristics and abilities ... it is not so much that some ancestor of the kakapo lost the ability to fly but more that there was no pressure to kill off those that were born with a reduced capacity to fly ... in NZ, a habitat existed that the lesser-abled flyers could exploit without getting killed off (until humans showed up).

    If you have a situation with few or no predators, then the creatures that can survive there will be those that do not reproduce so much that they exhaust their food supply.

    If they reproduce catastrophically - then the entire population is wiped out as well as anything else that is similar enough leaving an empty ecological niche for something else to radiate into. This creates a boom-bust cycle ... and the bust needn't wipe the animal out, that's just the extreme case.

    Over a very long time, the creature that ends up in that niche will be one that does not self destruct ... with lots of variety, many different strategies will end up being deployed and one of them is to be the one who reproduces slowly ... especially if there is something else which allows it to out-perform the faster breeders in the famine time like a slow metabolism say. Such a creature would be able to weather a lot of boom/bust cycles

    However, a low reproduction rate is not a likely thing to get selected for... and it is rare in nature.
    It is not a particularly successful strategy either - it looks to be in the process of being selected out of the creatures that employ it: they are going extinct remember?
    Predation and senescence are far more common approaches to the population problem ... and humans seem to be beating those restrictions.

    But I think the main conceptual problem here is the idea that creatures somehow "evolve into" other creatures. That is not what happens. What happens is, successive generations diverge (you look like your parents, but you are also different, and you are different from your siblings) and sometimes something happens to kill off a chunk of them or favor another chunk.
  4. Aug 9, 2012 #3
    Thanks that helped a lot!

    I think part of what happened is I forgot about niches, I was probably thinking in extremes, like the people on Easter island who ate absolutely everything on the island and cut down all the trees so almost nothing could live there ever again. I forgot that the birds who fly and the birds who hop around would end up eating very different food so the birds that flew could wipe themselves out without it having any effect on the birds that didn't. It seemed like even if you did end up not flying and having small families that you'd still end up dying out because your neighbor was flying around having chicks all over the place and eating all your food and his grandchicks eating the food of your grandchicks. So long as they had different niches they could all make their own mistakes without effecting one another.

    Am I on the right track?

    I'm not a Kiwi, though it's a great thing to be, I heard about Kakapos from Douglas Adams :) I'm from Ireland.
  5. Aug 9, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Oh I always thought it would be better to be Irish because you get to be drunk all the time... or some such cliche ;)

    You are on the right track - and even if the first species to show up did wipe itself out (and there must have been those that did... say they get to depend on a food supply that becomes scarce like Smilodon or Koalas?) those are not the only ones in the World ... other animals can migrate in.

    When Krakatoa erupted, it wiped out the entire wildlife on the island that had neglected to evolve traits for survival in red-hot flowing rock. But it was quickly repopulated.

    The Easter Island thing is a good example - the situation there was a big mystery for decades because nobody could figure out how it got like that. But it used to be very different and it got like that over a long time, then another long time intervened and everyone forgot. Evolution does stuff like that to us too ... there are loads of things that look weird when we see them today because something that used to be there is now gone.

    Dawkins talks about evolutionary scaffolding in his books.
    I think Stephen Fry got humped by a kakapo on camera.
    The local breeding program has taken off now that the researchers have realized that their mating behavior is not very instinctive: they have to be shown what to do! It's like they are trying to go extinct.
  6. Aug 9, 2012 #5
    Poor kakapo. Maybe they just want to lead a quiet, thoughtful existence free from carnal desire.

    Thanks for your help!
  7. Aug 9, 2012 #6

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Not if all the noise is anything to go by - their love songs can be heard for miles and miles. They are a bit like teenage guys down the pub - all bluster and bravado about how many birds they'll do and all shy and fumbles when it comes to talking to one.

    For that matter it's a wonder human beings reproduce.
    And on that bombshell ... time for bed. Cheers :)
  8. Aug 9, 2012 #7
    I'm sure he thinks he's writing sonnets of true love that are beautiful in and of themselves. Good night! Thanks again.
  9. Aug 9, 2012 #8
    Generally, evolution doesn’t completely eliminate physical features that are harmless but not useful. The function of the physical features can change or disappear, but the physical features remain in some form. The change in function of a physical feature isn’t strictly analogous to atrophy in the development of an individual.
    The analogy with atrophy is misleading. Sometimes, muscle starts shrink in an individual due to disuse. However, populations don’t lose the ability to grow muscles due to disuse. In general, evolution causes an unused feature to show more physical diversity. This was noted by Charles Darwin in “Origin of the Species.” Because the feature isn’t used, the exact shape and size of the organ doesn’t matter. So if the feature is truly harmless, but useless, there will be a wider variety of shapes for that feature. There is no natural selection of shapes and sizes. Therefore, a truly useless feature would show even more diversity than a useful feature. The useless feature doesn’t really disappear from the population.
    Changes in size, proportion and behavior could make the physical feature useless for its original function. The changes in size, proportion and behavior could be useful for some other function. Regulatory genes affect many physical features at the same time. Random changes in these genes produce disadvantages with regards to some functions, but advantages with respect to other functions. Even if the total affect is an increase in fitness, some functions will be removed.
    A good example would be the kakapo. You didn’t mention the important fact that the kakapo still has wings. The wings are useless for flying, but they are still there. The wings have reduced wing muscles, and diminished keels. However, the wings haven’t disappeared.
    The ancestors of the kakapo traded thermodynamic efficiency at the expense of flight abilities. With no predators, the main difficulty of a bird is finding food. Muscles use up energy, even when they are not being used. Wing muscles don’t get used by the kakapo, but they still make the bird hungry. So the reduction of wing muscles was a distinct advantage on this formerly isolated island.
    Here is a link that shows you that the kakapo does have wings. However, evolution has changed its body so that it is more efficient in using the food it eats.
    “The Kakapo is a large, rotund parrot; the male measures up to 60 cm (24 in) and weighs from 2 to 4 kg (4 to 9 lb) at maturity.[24] The Kakapo cannot fly, having short wings for its size and lacking the pronounced keel bone (sternum) that anchors the flight muscles of other birds. It uses its wings for balance, support, and to break its fall when leaping from trees. Unlike other land birds, the Kakapo can accumulate large amounts of body fat to store energy, making it the heaviest parrot.
    Its anatomy typifies the tendency of bird evolution on oceanic islands, with few predators and abundant food: a generally robust physique, with accretion of thermodynamic efficiency at the expense of flight abilities, reduced wing muscles, and a diminished keel on the sternum.”

    You didn’t mention that the kakapo needs a certain amount of land to call its own. The kakapo competes over land. They don’t fight over food. However, they do compete over mates. Juveniles often have foraging areas that overlap. So the resources aren’t unlimited as you suggest. So it is better for kakatos to be born heavy, and to be metabolically efficient, then to fly.

    Here is an article that shows that kakapo do have home ranges. The males fight over track and bowl areas. There stick to their own areas, with a little overlap in feeding areas. The juveniles stick together within feeding areas. Therefore, there is competition between kakato.
    Kakato didn’t have predators in the past, but they still had to compete for resources. Having loud voices to signal their location was obviously more important than flight, since this is how they mark out their turf. http://researcharchive.lincoln.ac.nz/dspace/bitstream/10182/640/4/Whitehead_MSc.pdf.txt
    Home ranges
    An animal’s home range is not the whole area traversed in its life-time but the area used in its normal day to day activities that contains all the resources required for its survival (Krebs & Davies, 1996). Both male and female kakapo generally stay within similar home ranges much of the year, for a number of years, with some overlap occurring between feeding areas of individuals (Merton et al., 1984; Moorhouse & Powlesland, 1991; Powlesland et al., 1992). Kakapo are known to vocalise their whereabouts, perhaps to signal their presence to other kakapo, but they do not fight to defend their foraging areas so are not considered territorial (Powlesland et al., 1992). Some fighting does occur however between male kakapo over track and bowl mating areas (Powlesland et al., 2006). Kakapo are generally considered solitary (Higgins, 1999; Heather & Robertson, 2000) but recent evidence suggests that juveniles especially may forage in close proximity to each other with considerable overlap between home ranges (Farrimond, 2003). Adult males have also been observed roosting within short distances of each other (D. Eason, pers. comm.).
    You seem to imply that if there are no predators, then the animal has no problems. However, food gets seriously limited.
    Kakapo do sometime starve. They don’t breed when the food supply is low. Therefore, you are wrong. Thermodynamic efficiency is more important than flight to the kakato. The extra energy needed for strong wings could very well cause a kakapo to starve or stop breeding when the food supply is low.
    Here is a link showing that kakapo stop breeding when their territories don’t have food.
    Breeding variation in female kakapo (Strigops habroptilus) on Codfish Island in a year of low food supply
    “ Multivariate analysis illustrates female kakapo were effectively partitioning available habitat, as breeders’ foraging locations were positively correlated with high-abundance rimu forest with a tall canopy, described as optimal breeding habitat. “
    Combined with the fact that juvenile kakapo have overlapping foraging areas, it is apparent that having small family sizes can be an advantage in terms of kinship selection. A gene that limits the size of a brood may have an advantage because the brood forages together. Competition between siblings could reduce the frequency of that gene in the population.

    There are a few exceptions where a feature disappeared in phylogenetic expression. However, very often the feature is still encoded in genes or chromosomes. The mutation may produce a regulatory gene that inhibits the growth of that feature, without changing the genetic code that contains the shape of the feature. The feature is hidden in the gene code, not eliminated. This is why there are atavistic mutations. A simple deletion of a regulatory gene can result in a physical feature coming back entirely. The genes didn’t go away, their expression was suppressed.
    Ceteceans (whales, dolphins) are an example that I know well. Ceteceans evolved from a land based mammal sometime in the Eocene. The hind legs were a disadvantage in swimming, as they ruin the stream lining. Hind legs started to disappear. Some whales as late as the Oligocene had diminished hind legs. Today, no cetacean species normally has a hind flipper. There are some traces of a pelvis, but the pelvis is practically unnoticeable. The hind legs have vanished without turning into flippers. Or at least the hind leg phenotype has vanished.
    There are mutant cetaceans that have a hind leg. A dolphin (Trusiops truncates) was recently found with hind flippers. The hind flippers were movable. The dolphin was sent to an aquarium in Japan. There have been whales caught with hind flippers.

    Here is a link to an article on “vestigial organs”. Note that the majority of cetaceans no longer have hind legs or hind flippers. However, sometimes a cetacean is born with “complete” hind flippers. Evolution added genes to suppress the growth of hind limbs. The hind limbs didn’t disappear because of disuse. The regulatory genes “hide” the hind limb code because hind limbs are a disadvantage. Sometimes, a single mutation can bring the hind limbs back.
    “In a few but well documented cases whales and dolphins have been found in the wild with larger hind legs.(Berzin 1972)(Hall 1984)(Sleptsov 1939)(Abel 1908)(Nemoto 1963)Some of these cases have even included feet and toe digits. To the left is a photo of the vestigial hind leg bones found in a humpback whale.(Andrews 1921) Additionally, as can be seen in the photo to the right, dolphins have also been found with full hind flippers. (Hidehiro and Ohsumi, 2008).”
    Note that by my logic, the muscles could have disappeared altogether. However, that would be a very unlikely mutation. Once a complicated feature has evolved, it takes a lot of unlikely mutations to bring it back to the former state.

    So you see that there is no "evolutionary atrophy." Sometimes, a feature is reduced because a reduced feature makes the offspring more fit.
  10. Aug 9, 2012 #9
    Such a detailed response! That was fascinating, thank you! Maybe I am studying the wrong subject.
  11. Aug 10, 2012 #10
    Reference: My other reply.
    I had never heard of a kakapo before your original post (OP). However, the minute after I read you described the island of the kakapo as being "free of predators," I knew both that there were poor hungry kakapo's somewhere and that there would be these more fortunate land-grabbing kakapos. The Google search that followed merely validated my hypotheses.
    If there was no challenge to kakapo life, then we would soon be able to walk to their island on the backs of living kakapos.
  12. Aug 10, 2012 #11
    Well, I might suggest a counterexample based on one your prior posts. The Antarctic penguins have no land based predators and they live in large colonies rather peacefully afaik. As long as they have access to open water, they have a virtually unlimited food supply, but their populations haven't exploded.
  13. Aug 10, 2012 #12
    This is not a good counterexample. Read the following links.
    “When penguins are in the water their predators are, leopard seals, fur seals, sea lions, sharks, or killer whales. Their predators on land are, foxes, snakes, and introduced predators such as feral dogs, cats, and stoats, prey on eggs and chicks of some penguin species, including the yellow-eyed and Galapagos penguins.”

    http://www.penguins-world.com/penguin-predators.html [Broken]
    “There are quite a few types of predators out there that find penguins to make the perfect meal. It is due to these predators that not all of them survive to become adults. Others are eaten after laying their eggs so they never make it back to care for their young. What predators a penguin has depends on the species. The larger it is the better chance it has of getting away from certain predators. Also, the location where they live may or may not have certain predators within the vicinity.
    Most of the time the predators that are animals are a risk to the penguins in the water. They tend to be safer on land. However, the smaller species can be attacked by snakes and by lizards when they are on land. There are other land animals too that have been known to attack penguins. They include various types of birds and even ferrets. Generally they will try to get the young penguins or the adults that are sickly. They know they will be easily enough to win the battle with.”

    "Well, penguins have pretty much ONE main predator: the leopard seal. Killer whales
    MAY eat a penguin, but only if they are really, REALLY hungry.
    Read more: http://wiki.answers.com/Q/Who_are_penguins_enemies#ixzz2rPCUesq2

    Penguins may not have many "land based predators". There are a few land based predators, but the real horrors in a penguin's life swim. Penguins have plenty of marine predators. Leopard seals love penguins. Not to mention sharks.
    Penguins don't have a good food source on land. Their food mostly comes from the open water. However, that is where their predators live!
    The predators include leopard seals and sharks. A lot of penguin parents, gathering food, are taken out by the predators. The predators live where the penguin food lives. There is no way for a penguin to obtain food without exposing itself to danger from predators. The predators are the reason their population hasn't exploded.
    Penguins breed on land despite the fact that there is no food on land. Emperor penguins have these elaborate migrations where they go to the most barren places on land. While they are breeding, they are starving. They wouldn't breed in such a barren landscape unless it provided protection from the predators.
    The kakapo is a slightly better example because they actually don't suffer much from predators. However, they have a mild competition with each other that makes up for their lack of predators. Penguins of all species have predators on the "open waters" where they hunt their food. Their population is kulled by marine predators. I suspect that is why they have social behaviors that involve cooperating with each other.
    If it wasn't for their predators, then penguins would quite possibly have evolved to be territorial. They would fight over mates, just like kakapo.
    Emperor penguins are on the verge of going extinct. They were in equilibrium with their predators for millions of years. Sadly, human beings are eating up their food supply. So at present, emperor penguins are over abundant in terms of the survival of their species. However, they were never free of predators. They are not breeding on a barren patch of ice for the scenery. They do not exchange care of the chicks because they think it is a fair thing to do. Their breeding behavior was shaped by the very real danger of predators.
    Penguins have existed for millions of years. If there was no check to their population in all this time, then we would be able to walk to the Antarctic on the backs of living penguins.
    Last edited by a moderator: May 6, 2017
  14. Aug 10, 2012 #13
    There were and are certainly checks on their population. I'm just saying that predatory marine animals alone probably aren't the most important or sufficient curb on their population. As you say, the current problem is not overpopulation but declining populations. We already talked about the challenges faced by oviparous flightless birds in very cold climates in terms of reproduction. Add to that the need for open water; they depend very much on sea ice conditions irrespective of predators.

    I would say that the stability of the Antarctic penguin populations depends partly on predation, but would tend to be self limiting without predation because of fickle sea ice conditions and the challenges they face in reproducing in very cold conditions.

  15. Aug 11, 2012 #14
    I agree that penguins die due to nonbiological threats, like climate and ice. I even agree that sudden changes in nonbiological conditions can by themselves bring on a species extinction. However, I disagree that the population would be self limiting due to nonbiological factors.
    Biological factors provide a negative feed back loop on penguin population because the agents of these limiting processes reproduce and die on their own. Biological factors that cause "self limiting" of a species include predators, parasites and internal competition within the species. When the penguin population goes up without any evolutionary changes, the populations of predator, parasite and other competitors increase. If the penguin makes some adaptation that prevents predation or parasitism, the predator or parasite can also make a counter adaptation. Therefore, there is never any danger of penguins filling up the seas with penguin bodies. As the population increases, the biological threats in penguin life increase.
    Suppose that penguins did not have any predators or parasites. Suppose that there was enough food to feed all penguins, no matter how many penguins there were. Suppose that the penguins only died of freezing in very cold winters or by being buried in ice avalanches.
    The penguins could adapt to the nonbiological threats by the standard evolutionary mechanisms. The penguin could evolve extra layers of fat to protect it from very cold temperatures. The penguin could develop a behavior where it avoids mountainous areas with avalanches. The nonbiological factor would not increase to limit the penguins growth. The winters won't get worse to check the penguins population. The mountains won't get less stable to check the penguins population.
    The penguins can make almost any adaptation necessary to adapt to nonbiological conditions. The penguin may be slowed down by extra layers of fat. However, there are no predators to run from. The penguin may avoid those mountains. However, there are no predators in the flat areas of the country to check the penguins.
    Very often, the nonbiological factors increase the threat of biological factors. If the ice melts due to global warming, the penguins will be more prone to predation. If there were no predators, the penguins could start reproducing in areas where there is food.
    The nonbiological factors provide a positive feedback loop on population. Every time the penguin makes an adaptation to nonbiological factors, the population goes up but the threat gets reduced. When a penguin makes an adaptation to a biological factor, the gain in population is temporary. The biological agent adapts to the penguins adaptation. Or the biological agent merely increases in numbers to make up for the new advantage the penguins have.
    Without a negative feedback loop, the penguin population would explode to the point that they are competing with each other. That will limit the penguin population. However, internal competition is a biological factor. It will produce another type of negative feed back loop.
    Therefore, I do not believe that the climate alone could cause the self limiting of the penguin population. If predators and parasites were completely eliminated, then the penguins would end up fighting each other.
    There is an episode of Futurama that makes that point. However, this probably isn't the forum to discuss it.
  16. Aug 12, 2012 #15
    Your arguments fail to address the particular circumstances of being the only year around land based natural fauna on the Antarctic continent. I'll only repeat this one more time. This and a previous reference that I linked clearly emphasize the importance of sea ice conditions as the major regulator of penguin populations on the continent. This link mentions predation (by marine mammals) only as a secondary factor mostly involving the young inexperienced penguins.

    Last edited: Aug 12, 2012
  17. Aug 12, 2012 #16
    You don't have to repeat it. I admit that you are right. The population of extant penguins is largely limited by changing conditions of sea and ice.
    I admit that changing sea conditions, including the motion of ice, are the main factors that limit the population of penguins from different species. I may have been wrong in thinking that predation and parasites directly limits the size of the penguin population. I don't know what the situation was in the case of the distant ancestors of today's penguins, but today's penguins fear changing sea conditions. However, the issue of predation alone is only partly relevant to the OP’s questions.
    The OP asked two questions. First, he asked how natural selection could remove a neutral trait like wings from a flightless bird. Second, he asked how natural selection could a limit the trait that determines the rate of reproduction, like the size of a bird’s brood.
    The OP referred to the specific example of a kakapo, a very large parrot that doesn’t fly. He claimed that it had limitless food and no predators. You brought up the penguin as another example of a flightless bird with large amounts of food and no predators. Right or wrong, I interpreted both your statements as claiming that natural selection did not limit the size of the bird’s broods. If this is what you both meant, then you were both wrong. If this is not what you meant, then I was wrong.
    The OP did not take into account the fact that individual kakapo compete over territory. Individual mothers graze with their young on their own territory. The males compete over females. They may not compete directly for food, on a fruit by fruit basis. However, not all the territories are equally provided with kakapo food. Therefore, competing for land and competing for mates is operationally the same as competing for food.
    If the brood is too large, the kakapo family strips their territory. The family dies. The other kakapo territories are unaffected by the death of this family. When the land recovers, another kakapo takes over the land. If a kakapo were to fly away from her territory for any significant length of time, another kakapo will take over her territory. Fruit comes with territory. If a male flies away for any length of time, then he loses the chance to find a female. So energy is limited at certain times. Food is not unlimited all the time.
    Once can think of this as group selection. Each family, in its own territory, competes with other families in other territories. If a kakapo can’t hold on to fertile territory, then natural selection removes its genes from the population. If a kakapo female has too many brood, her territory is stripped and her family dies. Another kakapo, that had a smaller family, takes over her territory. The kakapo hasn’t escaped natural selection.
    Your example of the penguins is also an example of natural selection. According to your claim, the penguin is occasionally separated from the ocean by changing ice conditions. When this happens, the food is limited. The penguin can starve. So the wings didn’t shrink to save energy. The wings adapted for better swimming. There was no “evolutionary atrophy”. The wings did not shrink from disuse.
    Brooding conditions change with the species of penguin. Therefore, I will focus on the “emperor penguin.” The female emperor penguin lays one egg at one time, and hands it off the male. The male juggles that one egg for months to keep it unfrozen. She comes back to take care of the chick. If she laid more than one egg at one time, the male would be unable to keep both eggs unforzen. The mother would be wasting energy on an egg that was doomed to die. If the egg freezes, then natural selection has not favored her or her mate. However, that does not change the chances of the other penguins.
    Evolution did not favor a one egg brood by anticipating global disaster. The family in penguins is like the territory in kakapatos. Atrophy is not part of evolution. Anticipation is not part of evolution.

    “Female Emperor Penguins lay a single egg in May or June. After the egg has been laid, the female must immediately feed in the ocean. First, the egg is carefully transported to the male, who incubates the egg under a brood patch that rests above the feet. Occasionally, the transfer is unsuccessful and the egg rolls onto the ice and instantaneously freezes. The male will incubate the egg for up to 65 days! During this time, he will not eat a single meal. On particularly cold days, hundreds of male penguins may gather together in a compact huddle to warm themselves in the pounding Antarctic winds. In about two months, the female returns and locates her mate and chick by sound. She regurgitates food stored in her stomach to feed the growing chick. The male then takes his turn feeding in the ocean and returns after about a month, at which point both parents tend to the chick by regurgitating food and keeping it off the ice. Once the chick is about seven weeks old, it joins other chicks in a creche (huddle) to keep warm.”

    The above behavior probably started to avoid predators. The entire point of living in such an isolated area is probably to avoid predation. Penguins live in areas which are subject to changing sea conditions precisely because it is a way to avoid predators. Tell me why a penguin would be living on the edge of icy disaster if it wasn’t to avoid predation.
  18. Aug 12, 2012 #17
    Here are a few questions.
    1) If kakapos want to be free of carnal desires, why to they fight over mates?
    2) How come the wings of kakapos are still useful even for a “flightless bird”?
    3) If their lives are so quiet thoughtful, how come evolution made these parrots so big?
    4) If these parrots are so thoughtful, why do they acquire territory?
    “The wings of the kakapo are also characteristic features. Their wings are relatively short in comparison to those of other birds, and are not used to fly in the traditional sense; however, they are still very useful to the bird. For instance, they use their wings to parachute from trees, to keep their balance, fight rivals, and attract mates.”
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook