How the Siamese Fighting Fish Got His Name- Territoriality of B. splendens

Siamese Fighting Fish, or Betta splendens (also known as just ‘Bettas’) are a member of the gourami family that are very common in the pet trade. The colorful fish are well known for living up to their fighting name. Males can’t be housed together without harming each other, and are known to show aggression to their reflection in mirrors. The calmer females even spar sometimes.

Wild B. splendens are quite hardy and often are found in shallow waters, such as rice paddies. They can tolerate low water oxygen water levels because they can take in oxygen from the air, thanks to an organ called the labyrinth. They eat insects and larvae. The species (like the domestic counterpart) is highly territorial. This is likely due to the fact that space, as well as access to the surface (for air and food) can be tight in unstable and already shallow environments. Space for nesting is also needed. Male bettas make bubble nests on the surface of the water in which they place fertilized eggs and guard ferociously.

To defend their turf, bettas engage in a number of behaviors. Ritualized aggression in bettas includes flaring of the gill covers (opercula) and spreading the rays of their fins apart in attempt to make themselves look larger. They may nip and chase one another. Encounters can get nasty, but usually the stronger and more dominant male will push out the subordinate.  Both sexes are territorial, but males more so than females.

The little fish were domesticated many years ago. People fought them for sport; admiring them for their tiny tenacity. While the fish were being bred for looks to create a wide array of color and fin shape seen today, they were also being bred for aggressive behavior. Highly dominant individuals were bred more frequently. Both length of aggressive display before backing down and fighting ability were selected for. Instead of a threat display and small squabble followed by one individual backing off, neither will back down and they may fight to the death.  Being placed together in small containers will only enrage the fish more, as they cannot escape one another.

Today, bettas aren’t bred so much for their aggression anymore; appearance has become the focus for artificial selection. Submissive fish usually aren’t selected against. A range of aggression still exists in the domestic lines, from violently aggressive to very passive.

I’ve kept bettas for many years and have been able to observe a wide variety of behaviors all over the spectrum. Some male bettas I have had never showed any aggressive displays. Others often did. One notably aggressive individual was a male called Oscar whom I kept for 4 years. Any objects or people near his tank were regarded as threats and he would flare his gills in ritualized aggression. Over time, he learned that I was not a threat (fish can recognize faces!), and he would only flare at me if I snuck up on him, and then subsequently stop once he realized it was me. Other members of my family were tolerated to a point, and complete strangers were met with a flurry of fins and flaring. This is an example of the “Dear Enemy Effect”, in which territorial animals treat their familiar neighbors with less aggression over time as the boundaries are well set. Unfamiliar animals are still treated with a high level of aggression.

(More information and on the Dear Enemy Effect in cichlids and skylarks.)

Bubble nesting also seems to be correlated with dominance and aggression. Oscar built the largest nests the and built most frequently. Others who made them more occasionally were less aggressive. Submissive and non-combative males didn’t usually create nests, or did so infrequently.

I also have 2 female siblings who both display high levels of aggression. They do not flare their gills, but they will swiftly attack and bite any foreign object put into their tank. They carefully patrol the edges of their tank territories as well. Typically, female domestic bettas can be housed together, as long as they have ample space to establish their own territories (people have also claimed to have kept 2 non-aggressive males in a large tank together). There also usually needs to be a large enough group of them so that they can establish a pecking order between themselves. Too few fish, and the submissive ones will be bullied and could perish from stress or wound infection.

Natural and artificial selection have shaped this species into the explosive array of behavior, color, and form. While they are “commonplace pets”, they exhibit a wide range of interesting behaviors and can be quite rewarding to observe and keep.


Cichlid link: Aires RF, Oliveira GA, Oliveira TF, Ros AFH, Oliveira RF (2015) Dear Enemies Elicit Lower Androgen Responses to Territorial Challenges than Unfamiliar Intruders in a Cichlid Fish. PLoS ONE 10(9): e0137705. doi:10.1371/journal.pone.0137705 under Creative Commons Liscence Attribution 4.0 International

Skylark link: Briefer E, Rybak F, Aubin T (2010) Are Unfamiliar Neighbours Considered to Be Dear-Enemies? PLoS ONE 5(8): e12428. doi:10.1371/journal.pone.0012428 under Creative Commons Liscence Attribution 4.0 International

Images are mine (© Emily Thorpe)

Dianne J. Bray, 2011, Siamese Fighting Fish, Betta splendens, in Fishes of Australia, accessed 04 Oct 2016,


6 thoughts on “How the Siamese Fighting Fish Got His Name- Territoriality of B. splendens

  1. love everything about this topic!! I never even knew these fish were bred for aggression I was always just told they were aggressive and battle to the death so don’t put two in the same tank. I feel like everyone should read this post before buying a beta fish, maybe to provide them with as little a stressful life as they can have since they are so aggressive. When did you start noticing these aggressive behaviors? Or did you know about them before you bought the fish? and how long did it take for the fish to recognize you?


    1. I knew a bit about their aggressive behavior beforehand. In some individuals its immediately evident. Others are super mellow. Usually it takes a couple of months for the fish to really get to know you. I usually spend some time each day watching them (to make sure they are healthy and whatnot,– plus it is so relaxing), and they can get to know you too!

      Liked by 1 person

  2. Awesome post ! I have had several bettas as well (they were always named after jewels: Jade, Ruby, etc…). I was always intrigued by their beauty and violent reputations. The ones I had seemed to be pretty passive in nature and I never observed any bubble nesting, I regret to say. I have been thinking recently about getting another one (or two!) -your post has solidified that intention !


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