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natural sciences

Suicide is unique to humans and sets us apart from other animal species

Author/Source:bubblegumgirlz/Creative Commons
Author/Source:bubblegumgirlz/Creative Commons
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Suicide is a distinctive feature of humankind and to find its reasons, we need to look at how we differ from other species of animals, writes animal ecologist Tuul Sepp in the Sirp newspaper.

The characteristics of living organisms have developed as a result of evolution with the aim of improving the likelihood of survival or successful reproduction of organisms, says Tuul Sepp in Sirp. It is therefore difficult to find evolutionary explanations to self-destructive behaviour. The genes that cause suicidal behaviour should have been eliminated by natural selection long ago.

Nevertheless, suicide and deep depression leading to it has never been a more serious problem in modern society than now. The estimated number of suicides committed each year is around one million. How can evolutionary biology explain suicide?

Is suicidal behaviour genetically determined, or is it purely the unfavourable environment that leads to suicide? Indeed, studies have shown that suicidal behaviour is hereditary, just as susceptibility to depression. For example, first-degree relatives of individuals who have committed suicide have more than twice the probability of killing themselves than the average of the general population, and for identical twins, the relative risk is 11 times higher than average1 in the case of one of them committing suicide.

Suicidality is estimated to be approximately 43% heritable or in other words, the role of the environment makes up more than the half. For comparison, the heritability of alcohol use is about 50%.2 As with other behavioural traits, it is probable that a lot of genes are involved in the development of suicidality, with each gene having its own small role. In the future, however, each of us could know the genetic risk of suicidal behaviour and prevent it, if possible.

Most studies on suicidality focus on its direct causes and identification of risk factors. Genetic predisposition, personality traits (impulsiveness and aggressiveness), development conditions before and after birth, traumatic events in the early years of life and disorders of the nervous system have been identified as long-term risk factors, and in a shorter perspective, use of substances, psychological crises, the availability of a means to take one’s life and suicidal examples have been suggested as short-term predictors of suicidal behaviour.3

Indeed, suicide seems to be catching. Within four months after the death of the actor Robbie Williams, for example, the suicide rate increased in the United States by 10%. The same has been observed in schools, in the army, in families and groups of friends. Considering the heritability, kinship can be easily explained, but the other aspects are probably not. While the mentioned factors may account for individual deaths by suicide, none of them explain why natural selection has not eliminated the so-called suicide genes from the gene pool of different species.

 Do animals commit suicide?

When trying to understand the evolutionary background of suicide, we naturally wonder how widespread it is in the world of animals. Although the stories about animals having committed suicide abound in the internet, most of the described events turn out to be a myth (e.g., lemmings do not actually jump off the cliff) or accident (e.g., the dogs seemingly jumping into a cleft may have smelled a prey). Animal suicide is a contradictory subject in the scientific world and the approved opinion is that other species besides humans do not deliberately take their own life. However, some animals (mostly laboratory mice) are used in animal models to study behavioural traits (aggressiveness, impulsiveness, anxiety, hopelessness) that lead to suicide, and to examine associated neural mechanisms.5

Consequently, development of suicidal behaviour in animals seems possible, but in the human species these characteristics have evolved into a type of behaviour, i.e. causing one’s own death, which is wrong in evolutionary terms.

The scientifically proved suicide events by animals are mainly related to parasites affecting the behaviour of animals. Namely, eating up the host is in the interests of some parasite and makes it possible for the parasite to spread from one intermediate host into another. The protozoan parasite Toxoplasma gondii, for example, not only makes rodents to serve as brave and aggressive intermediate hosts, but the animals also start to show strong preference for cat scent.6 As a result, the mice and rats with Toxoplasma infection actually try to find their predators themselves and help the parasite to move on in its life cycle.

Schistocephalus solidus is a tapeworm that infects fish and causes them swim up to surface waters where the fish passively wait for their predators, being the fish-eating birds. A well-known example is also a parasite-infected grasshopper that drowns itself – the parasite needs water for the next stage of its life cycle.

Can manipulation with parasites also cause suicidal behaviour in humans? Indeed, there are some facts suggesting that. Toxoplasma, for example, also infects people, although we are the dead-end or incidental hosts for it. No matter how much a Toxoplasma-infected person likes cat scent, we are still unlikely to become the prey of a cat (or some larger feline). In our evolutionary past as Savannah animals, this strategy may have worked to a certain degree for Toxoplasma.

At the same time, it has been shown that Toxoplasma also manipulates the brain of humans and affects their behaviour.7 For example, infection of Toxoplasma increases the risk of car accidents and part of these crashes are also related to suicidal behaviour.1

There is, however, reason to believe that manipulations of parasites are not the major cause for the suicide epidemics as the events of suicide seem to be a problem of modern society, while the number of parasites affecting people has decreased significantly due to the development of health care and hygiene.

Cry for help and need to feel needed

If suicide is not a trick of parasites playing with our brain, there are two possible explanations. First, suicidality is still useful for something and this behavioural trait has evolved as a result of natural selection. The other possibility is that suicidality is not adaptive or, in other words, does not increase the fitness of humans and some ecological features of our current living environment push the behavioural traits leading to suicide over the edge.

Considering the first explanation, a suicide attempt is often seen as a cry for help directed to your mates and it may often help those close to us to understand how serious your problems are. In many cases, a suicide attempt results in closer attention from the family or friends and in finding solutions to the problems. Since humans have been social beings through their evolutionary history, the signals indicating a need for help may have actually evolved in us. It has been found, for example, that a great part of characteristics associated with symptoms of illnesses are targeted to seeking attention and signalling the need for help.8

For a signal to be efficient, it has to be honest – in an evolutionary sense, then, a suicide attempt that has no chance of being successful would not be effective enough. According to this approach, suicide is a risk that may pay off, but not necessarily. If a suicide attempt is unsuccessful, it is a success from the evolutionary perspective, because your mates will notice that and help you. In this case, a successful act of taking one’s own life is, considering natural selection, a well-weighed risk which is worthwhile taking into account the number of individuals needing help. The hypothesis is supported by the fact that most of the attempts of suicide do not result in death.1

Can suicidality also be explained by kin selection? Lots of people who try to kill themselves feel that they are useless and their family or friends are better off without them.9 The relatives, however, carry the same genes. Suicide may then function in the same way as apoptosis, a regulated self-destruction mechanism of the body cells in the interests of the overall functioning of the organism.

This hypothesis is known as altruistic suicide1, although trying to benefit your relatives cannot be considered true altruism. Similar mechanisms have been described in animals, e.g., social insect colonies where self-sacrifice in defence of the nest helps to increase the fitness of the specimens capable of reproduction. Historically, this reason for suicide has been reported to exist among some tribes of Inuits where very old or poorly family members asked the others to kill them or did it themselves as they did not want to be a burden.

From the evolutionary perspective, not wishing to be a burden to the family may be an adequate justification of suicide. It would also explain, in part, why suicide has become a greater problem in today’s world than it was in the past. Until very late, humans were evolving as creatures living in a group, in which the contribution of each individual was essential for survival. Also, some chemicals produced in the brain that make us feel happy and satisfied are associated to the feeling of being needed and useful.10

Helping your relatives and friends is physiologically rewarding because it increases the success of our genes. In the modern resourceful society, each individual is not essential for the survival of the family or associates, and the range of activities for what we are rewarded by our brain with feelings of joy and satisfactions has diminished. In other words, we may feel that we are expendable and redundant.

Something is wrong with the world

Now we have come to the theory according to which suicidal behaviour is not adaptation, but has evolved (or intensified) as a result of a conflict between the evolutionary background of humankind and the modern living environment. We possess a number of features which show that we have not adapted to the modern environment. A large part of our body has still adapted to the way of life of the hunter-gatherer.

There is nothing wrong with us, but with the world. Our craving for sugar, which once helped to provide the body with necessary energy, makes us ill and fat. We are short-sighted because instead of spending time outside we sit in with indoor lighting conditions and our eyes are fixed on computer screens and books.

Because of air pollution, we cannot smell the scents that might send us signals that indicate our mates or food. The need for energy-saving recreation makes us passive, indifferent and weak. In terms of energy, modern people are more and more overfed but malnourished, sedentary, sunlight-deficient, sleep-deprived, and socially isolated.11 Modern lifestyles also lead to the development of chronic diseases (such as diabetes and cancer). Combined with feeling that we are not needed, these factors may drive a person into a deep established depression.

However, there is no reason to think that depressiveness is only a modern-day condition. The signs of the disease may be found throughout the known human history, although the rate of occurrence is difficult to estimate. Short-term studies seem to indicate rising prevalence and an increased risk for younger cohorts.11 One of the reasons may be the escalating use of substances.

Although historically the probability of suicide has increased with age and the largest risk group has been elderly men, the number of self-inflicted deaths among the youth, in particular among young men, started to grow rapidly after 1970 especially in developed countries.1 Also, depression has become a larger problem in developed countries than in less developed parts of the world.11

On the one hand, it may be that the richer have more time to take care of their mental health and see the doctor if they have problems. On the other hand, depression may, paradoxically, become more prevalent in societies where the vital needs of people are satisfied. If you struggle for the survival of yourself and your family, you have no time for depression. This is also in line with the view that to feel happy you need to feel needed by the others.

Depression and anxiety disorders share features that are adaptive in an evolutionary sense.12 If our goal pursuit is unsuccessful, we should stop to think and find a new strategy. In a more lenient form, dissatisfaction and low mood may contribute to making changes. But if the environment does not transmit signals that would help to overcome low mood and depression, this physiological adaptation becomes detrimental to daily functioning.

Similarly, in a less stable or more dangerous environment moderate anxiety may help detect danger in good time and avoid it. As all behavioural traits, depression and anxiety vary within the population and extreme forms are incongruous. It is worth mentioning that the extremely reduced form of anxiety and depressiveness is certainly more fitting than the other end of the scale, because bravely pursuing our opportunities may provide new solutions and possibilities, while by retreating and fearing everything, we may find ourselves stuck in a difficult situation, and unable to change it without external help.

More joy

Although we may point out a number of negative aspects in our modern environment, we should not ignore the positive side. We live longer than ever and suffer less from infectious diseases and parasites. Our children do not die young one after another like they did in the past. We do not suffer from hunger, and we have a roof over our heads. Our life is safe.

It may seem controversial that signals of this abundance and well-being are often ignored by our body. The lines transmitting satisfaction signals between the environment and the body are designed for a different environment, for signals of different “wave-length”. Leaving a supermarket with a bag full of food does not make us as happy as coming from the wood with a basket full of mushrooms, no matter that the first has a greater nutritional value and has taken less energy from us.

For many of us, lying on the couch and watching television is less satisfactory than taking care of family members or helping friends. These signals can be induced artificially, by taking anti-depression drugs. We should remember, however, that there is a social creature inside us who needs the closeness of our mates, approval, physical activity, sunshine, and the feeling of being needed. The creature has to be helped, even if it disturbs our way of life in which our personal comfort, so easily achieved in the modern world, seems to take priority.

Evolution has not provided suicide as a solution to our problems. It is quite a characteristic problem in humans and to find its reasons, we should consider how we differ from other species of animals. One the one hand, suicidal behaviour results from the amplification of some common traits among animals in humans, and on the other hand, from the feeling that we are not needed, which is connected to our easy and comfortable life. If we realise what depression and loss of the will to live tell us about the shortcomings of the present day living environment, it may be easier for us to rearrange our ways so that we could enjoy life with all the abundance, security and individuality it offers.

References:

1 P. McGuffin, A. Marusic, A. Farmer, What Can Psychiatric Genetics Offer Suicidology? Crisis. 22: 61-5, 2001.

2 B. Verhulst, M. C. Neale, K. S. Kendler, The Heritability of Alcohol Use Disorders: A Meta-Analysis of Twin and Adoption Studies. Psychol Med. 45: 1061-72, 2015.

3 K. Hawton, K. van Heeringen, Suicide. Lancet 373:1372–1381, 2009.

4 D. S. Fink, J. Santaella-Tenorio, K. M. Keyes, Increase in Suicides the Months after the Death of Robin Williams in the US. PLoS ONE 13(2): e0191405, 2018.

5 O. Malkesman, D. Pine, T. Tragon, D. R. Austin, I. D. Henter, G. Chen, & H. K. Manji, Animal Models of Suicide Trait-Related Behaviors. Trends in Pharmacological Sciences, 30(4), 165–173, 2009.

6 J. P. Webster, Rats, Cats, People and Parasites: The impact of Latent Toxoplasmosis on Behaviour. Microbes Infect. 2001 Oct; 3(12):1037-45, 2001.

7 Tuul Sepp, Isiksusi ja kultuure kujundav parasiit – Sirp 27. III 2015.

8 P. C. Lopes, J. Adelman, J. C. Wingfield, G. E. Bentley, Social Context Modulates Sickness Behavior. Behavioral Ecology and Sociobiology 66: 1421–1428, 2012.

9 H. Blasco-Fontecilla, J. Lopez-Castroman, A. Gomez-Carrillo, E. Baca-Garcia, Towards an Evolutionary Framework of Suicidal Behavior. Med Hypotheses. 73:1078-9, 2009.

10 P. J. Zak, A. A. Stanton, S. Ahmadi, Oxytocin Increases Generosity in Humans. PLoS One 2(11): e112, 2007.

11 B. H. Hidaka, Depression as a Disease of Modernity: Explanations for Increasing Prevalence. Journal of Affective Disorders, 140(3), 205–214, 2012.

12 C. T. Bergstrom, & F. Meacham, Depression and anxiety: Maladaptive byproducts of Adaptive Mechanisms. Evolution, Medicine, and Public Health, 2016, 214–218, 2016.

The translation of this article from Estonian Public Broadcasting science news portal Novaator was funded by the European Regional Development Fund through Estonian Research Council.

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