Category Archives: Behavior

Having few females turns male tortoises into rapists

by Piter Kehoma Boll

The war between the sexes and the endless conflicts that result from that are a common theme in behavioral and evolutionary research and have been addressed several times here too.

As we know very well, even from examples in our own species, males are usually not very good parents, being more interested in producing as many descendants as possible with little effort. Females, on the other hand, due to their great investment on eggs (and usually other resources for the offspring) are much more selective and will not accept any male to mate with them.

One of the most common solutions for males to resolve this sexual conflict is by forced copulation, or rape as it is called when it happens in our own species. Sometimes this forced copulation is extreme, with males heavily injuring females in order to make them surrender. One of those violent species is the Hermann’s tortoise, Testudo hermanni, a tortoise found around the Mediterranean areas of Europe.

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“I’m gonna bang you, bitch!” This photo of a juvenile male trying to mount on an adult female may look funny, but sex is no fun for female tortoises. Photo by Wikimedia user Palauenc05.*

Forced copulation is much more common in species in which males are bigger and stronger than females. This is not the case with tortoises, but male Hermann’s tortoises have found a way to deal with that. They pursue the females, sometimes for hours, pushing them, biting them, sometimes to the point of making them bleed, and eventually the poor females surrender. It is also common for the males to “stimulate” the cloaca of the females with their pointed tail, resulting in a swollen cloaca and sometimes severe injuries that let the females with horrible scars and deformities. Yes, it is not a nice face of nature.

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The tail of a male. Photo by Wikimedia user Bizarria.**

A recent study with two populations of the Hermann’s tortoise in Macedonia revealed that male aggressiveness is linked to female availability. The team of researchers studied one population in which the female:male ratio was close to 1:1 and other in which it was extremely male-biased to the point of 1 female to 17.5 males.

The results indicate that in the more balanced population forced copulation was less common and usually only adult females presented injuries caused by males, while in the male-biased population the lack of females made males go mad to the point that they forced copulation even with immature females. The situation as a whole is clearly maladaptive, as females end up injured and males end up exhausted and no offspring is generated.

I can only see two possible outcomes for such a population: either more resistant females will be selected or the population will go extinct after all females die by male violence.

As we see, sexual conflict is one of those deleterious side effects that natural selection created. Afterall, nobody is perfect, not even the fundamental laws of life.

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You may also like:

Male dragonflies are not as violent as thought

Gender conflict: Who’s the man in the relationship?

Badass females are unpopular among praying mantids

Having more females makes you gayer… if you are a beetle

Male resistance: when females disappear and hermaphrodites don’t like you

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Reference:

Golubović, A.; Arsovski, D.; Tomović, L.; Bonnet, X. (2018) Is sexual brutality maladaptive under high population density? Biological Journal of the Linnean Society 124(3): 394–402. https://doi.org/10.1093/biolinnean/bly057

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Male resistance: when females disappear and hermaphrodites don’t like you

by Piter Kehoma Boll

During the evolution of life, sex was certainly a great innovation. It allows organisms to reproduce while mixing their genes with that of another individual. Although it usually makes your offspring to have only half of your genes, which does not seem to be as great as an offspring that carries you as a whole into the next generation, there are certainly advantages in mixing. The most evident advantage is that your genes can combine with other versions and, as a result, produce a better team of genes than the one that you had. Even though each of your children carries only half of you, that half is more likely to survive than a child that carries you as a whole. In other words, sex gives the possibility for a population of genes (those that make up an individual) to get rid of some of the less efficient ones and replace them with better copies.

As you know, most sexual organisms make such a recombination by fusing two sexual cells, the gametes, and those are usually of two different kinds: a small one (the male) and a large one (the female).

In some species, each individual can only produce either male or female gametes, therefore being either a male organism or a female organism. In such species, sexual reproduction requires a male to mate with a female. This is the pattern found, for example, in most vertebrates and arthropods.

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A female (large) and a male (small) of the tick Ixodes ricinus mating. Image by Jana Bulantová.*

In other species, each individual can produce both male and female gametes, therefore being called a hermaphrodite. The advantage of such a system is that hermaphrodites can mate with any individual of their species, sometimes even with themselves! One of the main problems with hermaphroditism is when you decide to play only one role, which may lead to conflict during sex.

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Two snails Helix pomatia (hermaphrodites) making love. Photo by Wikimedia user Jangle1969.**

Now what evolved first? Dioecious species (those having male and female individuals) or hermaphrodites (allso called monoecious species)? It’s hard to tell, but we can be sure that during evolution many lineages switched from one system to the other and back. And the coolest part is that such switches still happen today.

You may know that most flowering plants are hermaphrodites. Flowers usually have both male and female organs, although they are rarely able to fertilize themselves (self-fertilization). Among plants, the cases of dioecious species seem to be mainly due to some mutation that ended up partially sterilizing an individual. For example, a mutation could appear that makes the plant unable to produce male organs, thus becoming only female. Other individuals in the population that lack this mutation continue to be hermaphrodites, so we have an “unbalanced” species with two sexes, females and hermaphrodites, but no males. Although unusual at first, such a system can remain stable if reproduction occurs through cross-fertilization and not self-fertilization. As both females and hermaphrodites need pollen (which produces the male gametes) from other plants, they can coexist as long as the pollinator carries pollen to both sexes. The same happens if the sexes are male and hermaphrodite. As long as the pollinator carries the male’s pollen to hermaphrodite flowers, both sexes can do just fine.

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The plant Geranium sylvaticum includes hermaphrodites and females, but no males. Photo by Enrico Blasutto.**

Species composed of males and hermaphrodites are called androdioecious (from Greek andro-, man, male + di-, two + oikos, home, house; therefore “male in two “houses”, i.e., in two different kinds of organisms), while those composed of females and hermaphrodites are called gynodioecious (from Greek gyno-, woman, female; therefore “female in two different kinds of organisms).

Androdioecious and gynodioecious species occur among animals as well, but in this case their existance indicates something happening in the other direction, i.e., it is a transition from a dioecious species (with males and females) to a hermaphrodite species. And this is much more complicated that the other way round. Actually, it can get really, really bad for the “single-sex sex”.

This unbalanced sexual system in animals usually happens like this. There is a happily dioecious species with male and female individuals, but one day a new mutation appears and allows one of the sexes to produce both male and female gametes, thus becoming an hermaphrodite. However, such hermaphrodites are usually unable to play the role of the new sex while mating, i.e., they have the gametes, but not the tool to mate using them. Thus, the only way to use both gametes is to fertilize themselves.

One problem that comes from doing that is inbreeding. When you fertilize yourself, you are not increasing genetic diversity. On the contrary, you have very high chances of producing offspring with two copies to the same gene, thus decreasing genetic diversity. In order to continue to have recombination, you must mate with the single-sex individuals, which means you can only play the role of your original sex and your hermaphroditism is irrelevant. You are producing useless gametes. Or are you?

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A male and a hermaphrodite of the nematode Caenorhabditis elegans an androdioecious species. Credit to Worm Atlas.

The problem with inbreeding happens when an organism ends up with two copies of a deleterious gene, which is fairly common in species where cross-fertilization is the rule and such deleterious genes are maintained in the population through individuals with a single copy that is not enough to cause any trouble. That is why having kids with your parents, children of siblings is usually a bad idea. When a species evolves from a system of cross-fertilization to one of self-fertilization, inbreeding can be a serious problem at first, producing many descendants that will die soon. However, eventually this will “purge” the set of genes. If individuals only mate with themselves, the number of deleterious genes will sharply decrease after some generations and inbreeding will not be such a big problem anymore.

When this happens in a species with unbalanced sex, the single-sex individuals will be in trouble. Two androdioecious animals have been studied regarding this conflict, the nematode and model organism Caenorhabditis elegans and clam shrimps of the genus Eulimnadia, such as Eulimnadia texana. In both groups, the hermaphrodites do not seem to be very interested in mating with males. They have even lost most phenotypic clues that help males identify them as potential mates. The only thing left for the males is to insist, to look for hermaphrodites and force them to mate with them, but it is a hard battle. Even when mating does occur, the hermaphrodite usually discards the male’s sperm.

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A hermaphrodite (left) and a male (right) of the clam shrimp Eulimnadia texana. Credits to arizonafairyshrimp.com

The persistence of males in the population depends basically on their ability to fertilize hermaphrodites against their will and the sex-determination system of the species. When hermaphrodites produce males by self-fertilization, they are destined to remain for at least some time even if they cannot fertilize that much. Now if self-fertilization only produce hermaphrodites, the poor males have to be really persistent or otherwise they will soon perish.

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You may also like:

Having more females makes you gayer… if you are a beetle

Endosperm: the pivot of the sexual conflict in flowering plants

Gender Conflict: Who’s the man in the relationship?

Male dragonflies are not as violent as thought

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References and further reading:

Chasnov JR 2010. The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp. Journal of Evolutionary Biology 23: 539–556.

Ellis RE & Schärer L 2014. Rogue Sperm Indicate Sexually Antagonistic Coevolution in Nematodes. PLoS Biol 12: e1001916.

Ford RE & Weeks SC 2018. Intersexual conflict in androdioecious clam shrimp: Do androdioecious hermaphrodites evolve to avoid mating with males? Ethology 124: 357–364.

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Having more females makes you gayer… if you are a beetle

by Piter Kehoma Boll

Homosexual behavior, as you may know, is a widespread phenomenon across the animal kingdom, especially male-male sexual behavior. Like many other aspects of life, this behavior likely evolved independently many, many times and plays different roles in different species.

A study recently published in the journal Animal Behaviour investigated the male homosexual behavior of the red flour beetle, Tribolium castaneum. The team wanted to verify whether a large supply of females or a large number of rival males would influence the occurrence of males having sex with other males.

The red flour beetle, Tribolium castaneum. Credits do CSIRO.*

In order to test that, the researchers directed the evolution of some populations of the red flour beetle in the lab. Some populations were maintained in a male:female ratio of 1:9 and others of 9:1, i.e., in the first group the population consisted of 10% males and 90% females, so males had a greater chance of finding a female than a male and competition between males was very week. In the second group, 90% of the population was made up of males, so females were harder to find and males had to fight for them.

After about 100 generations in which the sex ratios were maintained, the researchers compared the occurrence of male-male sex in both treatments. The results show that males that evolved in an environment where females were abundant and competition between males was low were more likely to engage in homosexual behavior than males that evolved in an environment were competition between males was high and the chances of finding a female were much lower.

The most likely explanation for this difference is that males in highly competitive enviroments need to be better in identifying female individuals to mate with, while in environment where there are plenty of females available, most of the encounters are with females, so the strategy to “have sex with whomever you find” is good enough. The few instances in which such males find other males and mate with them is not enough to reduce their reproductive fitness. In other words, when you live among few females, it is crucial for you to recognize someone as a female and mate with her, otherwise you may end up not passing your genes to the next generation. Now if there are plenty of females you may have sex whenever you want and you certainly will have some children, even if you sometims have fun with your male pals.

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You may also like:

Endosperm: the pivot of the sexual conflict in flowering plants

Gender Conflict: Who’s the man in the relationship?

Male dragonflies are not as violent as thought

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Reference:

Sales K, Trent T, Gardner J, Lumley AJ, Vasudeva R, Michalczyk Ł, Martin OY, & Gage MJG 2018. Experimental evolution with an insect model reveals that male homosexual behaviour occurs due to inaccurate mate choice. Animal Behaviour 139: 51–59. https://doi.org/10.1016/j.anbehav.2018.03.004

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Planarian: a living vessel built by unicellular organisms?

by Piter Kehoma Boll

Not long ago I talked about the peculiar genome of the freshwater planarian Schmidtea mediterranea and how it challenges our view of many cellular processes. Now what if I told you that planarians also challenge our view of what a multicellular organism is?

We all know that organisms may be either unicellular or multicellular, but sometimes it is hard to tell them apart, especially in what are called colonial organisms, in which clones of unicellular individuals may live together.

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Green algae of the genus Volvox are somehow at the boundary between unicellular and multicellular organisms. Although usually considered a colony of unicellular organisms, they behave somewhat like a multicellular organism. Photo by Frank Fox.*

So where lies the boundary between gathered unicellular organisms and true multicellular organisms? One of the ideas is that for a group of cells to be considered a single unit (an organism) they must have high levels of cooperation and low levels of conflict between each other and, perhaps more important than that, they have to be dependent on the association in order to survive.

As most recent evolutionary theories predict, cooperation increases with genetic similarity. As a result, multicellular organisms are (almost) always composed of cells having the exact same genetic material, i.e., they are all clones. We know, however, that during DNA replication mutations may occur, so that eventually at least some cells of an adult and many-celled organism may have become genetically distinct. This leads to a need to find a way to fix this problem by reincreasing genetic similarity, and the way most organisms found to do that is by allowing only one lineage of their cells, the germ cells (which produce the gametes) to generate the next generation. Thus, each transition from one generation to the next passes through a “zygotic bottleneck”, i.e., a new organism is always generated from a single original cell, the zygote, which assures that the genetic similarity is always brought back.

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A developing new egg, with an embryo that is created from a single original cell, the zygot, assuring a higher genetic similarity of the cells in the whole organism. Photo by Stéphanie Bret.**

The zygotic bottleneck is not a rule for a lot of species though. Many animals and plants are able to reproduce asexually by budding, fission or many other ways. In such cases, the new organism usually is built from several different lineages of the parent organism. For example, some succulent plants may generate a new organism from a dettached leaf and several cells of the original leaf start to reproduce and together they build the new plant and, as each lineage may have suffered different mutations, the offspring is not necessarily composed of genetically identical cells. Nevertheless, even such organisms, which are able to reproduce asexually, still retain the ability to generate zygotes through sexual reproduction, which eventually “cleans that mess”.

But in planarians things get really strange. First of all, let’s explain some basic things about planarians. They have, as you know, a remarkable regeneration ability. This happens due to the presence of stem cells called neoblasts that fill their bodies. Those neoblasts are able to generate all cell types that make up the planarian’s body. In fact, all cells in a planarian must come from neoblasts, because, as weird as it may be, all differentiated cells in a planarian body ARE UNABLE TO UNDERGO MITOSIS! Once a neoblast differentiates into any kind of cell, it is condemned to die in a few days without ever letting descendants. All cells in a planarian body are therefore constantly replaced by new ones coming from neoblasts. The only lineage of differentiated cells that is still able to reproduce is that of the germ cells, which, as in other multicellular organisms, assure that the next generation will consist of organisms with genetically homogeneous cells.

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Girardia tigrina, a freshwater planarian. Photo by Wikimedia user Slimguy.**

Several freshwater planarians, however, have lost their ability to generate sexual organs and, as a result, germ cells. In order to reproduce, they must rely on a form of asexual reproduction, which in this case happens by transversal fission of the body and posterior regeneration of the missing parts. In these populations, the zygotic bottleneck disappeared completely and, as a result, any non-lethal mutation in the neoblasts is retained in the organism, leading to a population of genetically distinct neoblasts inside a planarian.

Therefore, considering the fact that asexual planarians are not genetically homogeneous, having several different neoblast lineages in the same body, and that the neoblasts are the only cells able to reproduce and continue the species, a recent publication by Fields and Levin (see references) suggests that asexual planarians are nothing more than a very complex environment built by neoblasts in order to survive. Considering that each neoblast is an independent cell, which only needs the environment (the planarian) to survive, but does not need other neoblasts, planarians, at least the asexual ones, do not seem to have reached completely the requirements of high internal cooperation and low internal competition to be considered multicellular organisms.

We could interpret the neoblasts as unicellular organisms that live together, cooperating to build a complex environment, the planarian body, with their own sterile descendants (the differentiated cells), as if they were a group of queen ants living among sterile castes. Kind of mind blowing, huh? But it actually makes sense.

If you want to read more about it and understand every detail in this theory, read the references below.

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Maybe you’d like:

Endosperm: the pivot of the sexual conflict in flowering plants

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References:

Fields, C; Levin, M. (2018) Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity. Evolutionary Biology: 1–11.

West, S. A.; Fisher, R. M.; Gardner, A.; Kiers, E. T. (2015) Major evolutionary transitions in individuality. PNAS 112 (33): 10112-10119. https://doi.org/10.1073/pnas.1421402112

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The Dying Melody: Habitat fragmentation is killing the songs of our birds

by Piter Kehoma Boll

Birds, especially passerine birds (those of the order Passeriformes) are known for their ability to produce complex and melodic calls, or songs, used for a variety of purposes. Who doesn’t love to hear the birds singing beautifully in the forest?

Passerine birds include two main groups: the Passeri (songbirds), also called Oscines, and the Tyranni (tyrants), also called Suboscines. Both groups are able to produce complex calls, but those of the Oscines are usually more smooth and melodic and sound less mechanic.

However, there is one more difference between the calls of both groups. The calls of the tyrants are genetically transmitted from the parents to the offspring, i.e., they do not need to learn how to sing with adult birds. Among the songbirds, on the other hand, the complexity of the call is largely culturally inherited, i.e., they learn how to sing with other birds of the same species.

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The orange-billed sparrow is a songbird found in the Neotropical region. Photo by Francesco Veronesi.*

A recent study published in the journal Animal Behavior (see reference) analyzed the complexity of the calls of two passerine birds found in the forests of Costa Rica: a songbird, the orange-billed sparrow Arremon aurantiirostris, and a tyrant, the scale-crested pygmy tyrant, Lophotriccus pileatus.

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The scale-crested pygmy tyrant is a tyrant found in the Neotropical region. Photo by Chris Jimenez.*

The team compared the complexity of the calls in different populations of each species living in forest fragments of different sizes. The conclusion was that, while fragment size did not affect the complexity of the call in the tyrant bird, it significantly affected the call of the songbird.

Populations of the orange-billed sparrow that live in smaller fragments show a less complex song than those living in larger fragments. As the call in this species is culturally transmitted, this reduction in complexity is most likely a result of cultural erosion. As smaller fragments only support smaller populations, the birds do not interact that much with other individuals of the same species while they are growing up, and as a result their song becomes simpler and simpler.

We, humans, are the ones to blame for that, as you may already know. Our irresponsible practices are not only reducing population size in other species, but are destroying their culture as well.

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Reference:

Hart, P. J.; Sebastián-González, E.; Tanimoto, A.; Thompson, A.; Speetjens, T.; Hopkins, M.; Atencio-Picado, M. (2018) Birdsong characteristics are related to fragment size in a neotropical forest. Animal Behavior 137: 45–52. https://doi.org/10.1016/j.anbehav.2017.12.020

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Cat-handedness: can cats be left- or right-handed?

by Piter Kehoma Boll

In humans, as you may know, there is usually a preference for using one side of the body to perform a task, a thing called laterality. And we have a strong tendency to be right-handed, with about 90% of humans using their right side to perform most unilateral tasks. Several studies revealed that many other animals, at least among vertebrates, display laterality as well.

A recent study investigated laterality in the domestic cat during spontaneous behaviors in contrast with the more common experiments using forced behaviors, such as making the cat try to reach food. They looked for a side preference in cats during the behaviors of lying side, stepping down and stepping over.

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Photo by Juan Eduardo de Cristófaro.*

The result indicated that about one third of the cats is left-pawed, one third is right-pawed and one third is ambidextrous while moving up and down, but there is no clear preference for lying on their right or left side. Thus, we can see that, differently from humans, there is no strong bias to use one side of the body in cats, at least not when looking at cats in general.

When we consider sex, though, there was a significant difference: male cats tend to be left-pawed, while female cats are usually right-pawed. That would be very useful if cats danced the waltz.

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References:

McDowell, L. J.; Wells, D. L.; Hepper, P. G. (2018) Lateralization of spontaneous behaviours in the domestic cat, Felis silvestris. Animal Behavior135: 37–43. https://doi.org/10.1016/j.anbehav.2017.11.002

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The natural-unnatural fallacy: let’s stop with this bullshit

by Piter Kehoma Boll

This post may upset some people, perhaps people from very different backgrounds and very different points of view regarding society or human practices, but I feel that I have to discuss this subject that has been used as a way to justify or condemn some behaviors.

Everybody has already heard, and perhaps even uses, the argument of “this is not natural” as a justification to oppose something. You can hear homophobes use it to condemn homosexuality, vegans to condemn the ingestion of meat, or even old people to lament the death of a child. It is sometimes called “argumentum ad naturam” or “appeal to nature”, a fallacy that considers that something natural is always better than something unnatural.

When a homophobe says that he or she is against homosexuality because it is not natural, their oppositors, people who defend the freedom of sexuality, quickly present them with evidences of homosexuality in other species. I myself, being a gay man, used, in the past, examples of other gay animals to explain that homosexuality is indeed natural.

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Oh, yeah, babe. Two male lions having sex. Photo by Wikimedia user Rufus46.*

Regarding human diet, we can find the same thing, including two opposite groups using this very same argument. You can see vegetarians advocating against the consumption of meat because of our dentition or intestine, which are not typical of a carnivorous species. On the other hand, people who defend the so-called “paleo” diet try to reconstruct a diet that was common to our pre-historical ancestors, which would be our “natural” diet, and thus they consume large amounts of meat and fruits and avoid consuming grains, large amounts of sugar or other food that need to be domesticated. In the same way, other people, such as vegans (and even paleo lovers) are against consuming milk because this is a baby food and should not be ingested by adult mammals, afterall “it is not natural” to take the milk of a female of another species and drink it.

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Several ant species “milk” aphids to drink the sweet secretions (honeydew) that they release. Photo by Friedrich Böhringer.**

Now let me tell you something: this all means shit. This is not how things work in the real world, which by the way is the same as nature. When we use the argument of something being natural or unnatural, we are implying that there is a pre-established way for things to work, that there is a purpose in life. Well, there is not! Do you know natural selection, right? Well, it basically states that the most successful variations of something are the ones that will survive, regardless of what they are or how they are used. The mouth evolved primarily as an opening through which animals ingest food, but we use our mouths to many different things, such as to speak, breathe, kiss, hold things, or give blowjobs. Are those things unnatural? Who cares? As long as the mouth is good at doing it, it will continue to be used to do it.

And more than that, using other species as a comparison to things that are natural or not for humans is ridiculous. Just because we are the only species that write, is writing unnatural? If we consider that to be true, then we can apply it to find unnatural behaviors in any other species. As long as a behavior is only found in a single species, it would be unnatural.

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The satin bowerbird is the only species that uses blue objects to decorate its bower in order to attract a female. Is its behavior unnatural because of that? Photo by Joseph C Boone.***

As a social and (supposedly) scientific species, we should consider a behavior as good or bad based on its consequences to society or to individuals, as well as to the world as a whole. Murdering is bad not because it is unnatural, but because it kills people! Pollution is not bad because it is unnatural, it is bad because it pollutes! So more than only being stupid because of its assumption that natural things are good and unnatural things are not, the appeal to nature is also stupid because there is no way to separate things in natural and unnatural. Let’s grow up intellectually and stop using this bullshit of an argument at all.

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