How does the reward system make animals want to reproduce?

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Main Question or Discussion Point

When in history did our reward system develop, in order to ensure human reproduction? And what drives lower animals to reproduce?

Instinct
The reward is the survival of the lineage. but do animals know this as well? Do they have some kind of consciousness that they (well, actually not them, just their offspring) will "survive" if they manage to reproduce, or is it rather an instinctive, unconscious attraction towards the other sex, which has nothing to do with their brain? Animals with a neurological circuit do need some kind of stimulus to get them to reproduce, but that stimulus is built instinctively into their genes and/or brain, but if so, where exactly?

Reward
Neurological systems underlying reward seeking behaviour are widespread in the animal kingdom, from worms, to molluscs, to arthropods to vertebrates. In most cases, these reward pathways are mediated by biogenic amines, specifically, dopamine in worms, molluscs and vertebrates (including humans) and octopamine in arthropods. Thus, these reward systems are phylogenetically ancient, long predating mammals, in fact probably going back to at least the Cambrian era. But, are biogenic amines in all these cases also inherently related with reproduction? It is indeed well-known that dopamine/octopamine is present as a neurotransmitter in the nervous systems of lower animals, but does the act of reproduction in these cases also affect the increase of dopamine activity? Has this been measured? I'm questioning this correlation because dopamine is also known to have a rather neutral function, which has nothing to do with sexual arousal, since it is also used in the basal ganglia, where it motivates physical movement of the limbs (and can lead to Parkinson's disease in humans). A given neurotransmitter can signal in multiple functions, but what determines the function is the pattern of connections, not the signalling molecule itself.

Natural selection
There has always been a reward system, called "natural selection". It's where the fittest compete for food and mates, win, and survive. Generally, animals are driven to reproduce by a general primal urge to do so. But what is this "drive"? And where is located? All animals have different neurological circuits compared to humans, yet they all seem to have the same desire to reproduce, so where does this drive to sexually reproduce come from? Is it merely a physical attraction towards the other sex, or a neurological/genetic message which ensures an animal's survival?
 

Answers and Replies

  • #2
.Scott
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For questions about the brain, this seems to be a good article:
http://neurosciencefundamentals.unsw.wikispaces.net/Sex+and+the+Brain.+What+parts+are+involved?

As for when did the reward system develop: immediately. Nothing genetic is going to persist past one generation if it does not reproduce.

Clearly, reproduction had to precede "knowing".
As far as intellectual abilities are concerned, a fair question may be whether reproduction was the primary Darwinian push to improve IQ or if IQ was promoted by other survival factors and courtship just went along for the ride?
 
  • #3
BillTre
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You have posted a lot of claims of what is known neurologically about wide ranges of animals that are not supported by any references.
I am thinking a lot of your statements are likely true, but since there are no references I (and I assume others) am not strongly moved to wade your the many statements and sort out what might not be. This is a loss to you in that your would not be getting possible corrections on the assumptions underlying you questions.

That said, reproduction is going to be strongly selected for because as you and @.Scott stated it is basic to continued biological existence. Kind of tautological.
The results of this will be the survival of reproductive systems the work well and the extinction (the ultimate price evolutionarally) of reproductive systems that don't.
This can simply depend on mechanisms that will work, regardless of the reward system, as long as those mechanisms can be genetically encoded in some way.
A sensory system designed to draw opposite sexes could be important.
Of the timed release of gametes into the ocean at a particular time and place (broadcast spawners in the ocean). Probably driven by hormonal factors that could be controlled by sensory inputs of some kind (for example circadian (day/night) inputs).
Reward systems are not really required for these kinds of things to happen, although they might be involved.

Reward systems would be important when learning is involved (rewarding the correct choice to increase its likelihood in subsequent behavior) or in cases where the intricacies of generating the behavior appropriate to the situation an animal finds itself in, and/or the rapidity at which it needs to be established, exceeds the rate at which it could be evolved as a behavioral generating mechanism of a hardwired nervous system, through repeated cycles of mutation and selection. In this case the "hardwired" aspects of the nervous system would get supplemented by the directing actions of a reward system.

dopamine is also known to have a rather neutral function, which has nothing to do with sexual arousal
This is too be expected.
It is probably safe to say that all neural transmitters will be used in a variety of neural pathways (underlying different functions), not all of which are going to be directly involved in reproduction or a reward system. As people on the forum like to say, if a process or mechanism exists in an organism, it is likely to be put to use for some new or additional purpose (being co-opted for an additional purpose).

The question of when the reward system evolved is quite interesting.
Clearly there are successful organisms that have little or no nervous system and reproduce just fine: plants, fungi, sponges, bacteria, etc.
In addition, some animals probably don't have sufficient nervous system capabilities to make much sophisticated use of a reward system, such as: comb jellies, coelenterates, flat worms.
This question of reward system origins would be best considered by looking at them from a phylogenetic (evolutionary history) perspective.
Different evolutionary lines could have evolved different reward systems independently.
Here is a recent open access article on the phylogenetic distribution of peptides in animals.
In some cases, these peptides would be components of rewards systems, but perhaps, sometimes not.
There are many aspects to a functional reward system. The existence of some peptides alone is not indicative of a functional reward system.
 
  • #4
Laroxe
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There is a great deal we know about human sexual behaviour that are not well explained by its links to rewards, while reproduction is the biological function this doesn't mean the function is achieved by any motive to reproduce. I would suggest that sex is perhaps a more direct motivator and both this and the social systems to support reproduction are the driving forces. In humans the effects of sexual arousal, which has a clear biological basis and induces marked changes in physiology, has all sorts of other effects, it is easily associated with other forms of emotional arousal, it is dis-inhibiting and sensitises people to associative learning. This means humans quickly learn a variety of behaviours, gas masks were a popular fetish item, something that was arousing in itself but this was only common in countries where people had to shelter, in the dark often wearing gas masks.
A big problem with evolution is the way in which our biology and the environments we create evolve to support certain aims, humans have developed a society in which it is the social rules that control sexual behaviour and support the reproductive functions with rules of marriage, fidelity etc, rewards and sanctions are largely social. Animals also have developed a variety of social systems that achieve the same ends, individuals really don't need awareness of reproductive functions of sex and its suggested that in some societies people were generally unaware of the link between sex and pregnancy.
 
  • #5
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@BillTre You're absolutely right, I should have provided references to my claims. Here are already some quotes from papers which support my biggest claims, let me know if you need any additional substantiation for anything I've said. I haven't read these papers from top to bottom, but I will do so tomorrow, as well as @.Scott's link, and I'll let you know ASAP what the content was about, alright?

Thanks for the comment you made anyway. This remark in particular I found very interesting:
The results of this will be the survival of reproductive systems the work well and the extinction (the ultimate price evolutionarally) of reproductive systems that don't.
This can simply depend on mechanisms that will work, regardless of the reward system, as long as those mechanisms can be genetically encoded in some way.
So the question is whether animals need a reward for their behaviour in the first place, since reproduction is their only goal. But still, if that is the case, where is this knowledge coming from? And why are they reproducing, what's triggering them if it's not a reward? It must be a memory of some kind, because a certain instinctive anticipation is involved, and lies probably within their genes, but where does this motivation reside?

- - -

The role of octopamine in locusts and other arthropods (link).
Quote: "Octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems".

The Roles of Dopamine and Related Compounds in Reward-Seeking Behavior Across Animal Phyla (link).
Quote 1: "The neurobiology of reward-seeking behavioral systems is less well understood in invertebrates, but in many diverse invertebrate groups, reward learning and responses to food rewards also involve dopamine. The obvious exceptions are the arthropods in which the chemically related biogenic amine octopamine has a greater effect on reward learning and reinforcement than dopamine".
Quote 2: "By contrast, in the arthropods, octopamine is a major regulator of behavior and physiology (Roeder et al., 2003; Roeder, 2005). The similarities between the octopamine receptor subtypes in protostomes and adrenergic receptor subtypes in vertebrates suggest these two systems may have diverged from a common evolutionary origin".

Neuronal Reward and Decision Signals: From Theories to Data (link).
Quote 1: "Some of the stimuli and events that are pleasurable in humans may not even evoke pleasure in animals but act instead through innate mechanisms. We simply do not know".
Quote 2: "The absence of dedicated receptors that by themselves signal reward value may not reflect evolutionary immaturity, as rewards are as old as multicellular organisms".
 
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  • #6
russ_watters
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So the question is whether animals need a reward for their behaviour in the first place, since reproduction is their only goal.
It sounds like you are saying you think animals are consciously aware of the need to reproduce and have sex in furtherance of that goal. I can't imagine why you would think that when it is clearly seen that the most consciously aware animals (humans) don't have sex primarily because they consciously choose to reproduce. The vast majority of human sex happens with the participants hoping that reproduction will not occur and even after going to great lengths to avoid reproduction while having sex, nearly half of all pregnancies - in the US anyway - are unplanned.

So I think it should be clear that our biology has driven us to want to have sex without a necessary, much less conscious connection being made between sex and reproduction.
 
  • #7
BillTre
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So the question is whether animals need a reward for their behaviour in the first place, since reproduction is their only goal. But still, if that is the case, where is this knowledge coming from? And why are they reproducing, what's triggering them if it's not a reward? It must be a memory of some kind, because a certain instinctive anticipation is involved, and lies probably within their genes, but where does this motivation reside?
Lots of behavior came happen without expectation of a reward.
Even if there is a reward waiting too be bestowed up on an organism as a result of a particular behavior, the behavior would have to happen without the expectation of the reward in the first instance of the behavior.
Plants (without even a nervous system) can have behaviors in their slow manner, perhaps produced by like differential rates of growth, or differences in internal pressures.

As I said above, behaviors can be hardwired into the structure and functioning of the nervous system.
This knowledge (concerning what is the appropriate behavior for survival (reproduction in this discussion)) is embedded in the structure/function of the nervous system. Because this knowledge is genetically encoded and directs development in the nervous system, it could be compared to memory (of earlier events in evolution).
This genetically encoded information:
  • Has been accumulated over generations of selection
  • The selection was on a population with a diversity of genetically encoded developmental programs, that generated a diversity of nervous systems that generated different behaviors in different conditions.
  • The various nervous systems would produce reproductive behavior in slightly different forms and in response to slightly different sensory inputs.
From these, selection (each and every generation) would select the winners (those breeding and making more off-spring in the next generation):
  • The winners's genes (now more common in the next generation) would the generate a nervous system, that generates behaviors, that would more frequently result in successful breeding.
  • Repeat millions of time.
This can result in the evolution of a wide variety of behaviors directed at any perceivable (by the animal) goal.

However, none of this has to be done with any awareness.
Hardwired behaviors need not be thought out by the organism. It is like a control mechanism that is throwing out a series of hardwired commands to a physically competent body.

The reward would be required when a selection pressure changes and waiting the long time for natural selection to have its effects would not work fast enough.
This doesn't mean it can't happen, just that is not required in order to explain what is going on.
In this case something that selects the closest to goal (reward) behavior out of an animal's behavioral repertoire.

Evolution, which drives changes like these in hardwired behaviors, is not planned before hand.
It is:
  • a selection on a population
  • with genetic variation in nervous system development.
  • the most adaptive result (breeding in this discussion) is that which best fulfills the requirements for generating the behavior the best leads to successful breeding.
No awareness is required, just a lot of time and reps applied to a genetically variable population.

These are the kinds of interactions that mindlessly generate that knowledge, and can end up directing a behavior, driven by a some hardwired function in a nervous system.
 
  • #8
Laroxe
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"Lots of behavior came happen without expectation of a reward."

Looking through the responses I think there are some key points, to achieve the end result of reproduction doesn't need any motive or goal to reproduce, it can be achieved by controlling proximate behaviours.
I think we can safely dismiss learning as the primary driving force so thinking about rewards in terms of reinforcement, doesn't help

"As I said above, behaviors can be hardwired into the structure and functioning of the nervous system."

Its becoming increasingly common to discuss humans is if they were simply a nervous system, I wonder how many nervous systems have reproduced independently. Natural selection operates at the level of the organism and while the brain has the executive function it is subject to and acts upon its environment.
We have to consider that the development of various behaviours is interlinked with other behaviours, reproductive behaviours are not discrete entities the fact that we are a social animal, that communicates, that learns, is self aware etc all directly impact on our reproductive behaviours.
  • "The selection was on a population with a diversity of genetically encoded developmental programs, that generated a diversity of nervous systems that generated different behaviors in different conditions.
  • The various nervous systems would produce reproductive behavior in slightly different forms and in response to slightly different sensory inputs."
Unlike most animals humans appear to have few inbuilt, innate patterns of behaviour, evolution has equipped humans with abilities that allow flexibility of responses, it would be impossible for genetics to carry the information needed for all the possible situations we face. In fact the fact that we have taken control of our environment, and only recently doesn't support the idea of fine control.

"From these, selection (each and every generation) would select the winners (those breeding and making more off-spring in the next generation):"
  • "The winners's genes (now more common in the next generation) would the generate a nervous system, that generates behaviors, that would more frequently result in successful breeding. " Again there is the problem that genetic control is always context dependent, selection occurs in response to very specific environmental cues. Humans have the nasty habit of moving to or modifying their environments, natural selection couldn't possibly keep up if we were to depend on specific predispositions in behaviour that were genetically encoded.
"This can result in the evolution of a wide variety of behaviors directed at any perceivable (by the animal) goal."

This could only occur if the evolutionary changes were in the abilities which had a pervasive effect on a wide range of behaviours, its quite possible for a single innate motivator acting on the much broader abilities could drive all reproductive behaviour.

I think its worth revisiting drive theory, which suggests that instinctual needs generate an "excitatory state produced by a homeostatic disturbance" which has the power to drive behaviour. I still think genetic control is introduced at the level of sex and indeed this is often described as a drive. The idea would be that innate needs that are not satisfied create a negative state of tension, which increases over time, this is what we call sexual frustration. I think as long as people are motivated to have sex, they will use all the other skills available to them to achieve this. This would represent one specific genetic driver capable of generating a huge number of different behaviours which will inevitably lead to reproduction. Parsimony in action.
 
  • #9
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Lots of behavior came happen without expectation of a reward.
Plants (without even a nervous system) can have behaviors in their slow manner, perhaps produced by like differential rates of growth, or differences in internal pressures.
True, but aren't plants growing towards the light? Isn't that considered a reward for their movements?
I think its worth revisiting drive theory, which suggests that instinctual needs generate an "excitatory state produced by a homeostatic disturbance" which has the power to drive behaviour. I still think genetic control is introduced at the level of sex and indeed this is often described as a drive. The idea would be that innate needs that are not satisfied create a negative state of tension, which increases over time, this is what we call sexual frustration. I think as long as people are motivated to have sex, they will use all the other skills available to them to achieve this. This would represent one specific genetic driver capable of generating a huge number of different behaviours which will inevitably lead to reproduction. Parsimony in action.
Very interesting, especially the part about "sexual frustration".
"Primary food and liquid rewards serve to correct homeostatic imbalances. They are the basis for Hull's drive reduction theory that, however, would not apply to rewards that are not defined by homeostasis. Sexual behavior follows hormonal imbalances, at least in men, but is also strongly based on pleasure".
This sounds like the withdrawal effect that has been observed in addictions, since drugs can excite (the nucleus accumbens electrically excites the VTA with dopamine), but if you're stimulate it too much, you generate so many receptors that you no longer need drugs (or sex) to get excited (from pleasure seeking...), but just to survive, to feel neutral, i.e. behavioral dependence on dopamine (...to aversion avoidance). Could this be why sex and aggression often go hand in hand? Because a high level of aggression is linked to a high level of testosterone.

Why are only humans/dolphins/apes said to have sex because of pleasure, and all other animals don't (because male animal "want" to impress the females, in order to get sex/rewarded). Is this even a right statement to make, since dopamine (and octopamine) is involved in the animal reward system?
"The neurobiology of reward-seeking behavioral systems is less well understood in invertebrates, but in many diverse invertebrate groups, reward learning and responses to food rewards also involve dopamine".

Primary drives are innate drives (e.g. thirst, hunger, and sex), whereas secondary drives are learned by conditioning (e.g. money), according to the drive reduction theory. My point is that humans (especially men) want to reproduce because of the pleasure feeling, NOT because we want to reproduce, that’s the effect, but we do it because of the feeling. There’s a mechanism built inside of us that gives a dopamine release. It's a mechanism that has been built at some point in evolution, I presume, to make us have sex, with the goal to reproduce. It hasn't been built in our body just to feel pleasure.
"To ensure gene propagation, the primary rewards mediate the survival of the individual gene carrier and her reproduction".
Yes, we have selfish genes, we already knew that, but how/when were they responsible for the development of this reward system in our brain? How is male sperm (that "wants" to reproduce) able to influence brain structures?
 
  • #10
BillTre
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True, but aren't plants growing towards the light? Isn't that considered a reward for their movements?
Not in the sense of your original question about being conscious of a reward and how it might involve mechanisms in the brain:
Do they have some kind of consciousness that they (well, actually not them, just their offspring) will "survive" if they manage to reproduce, or is it rather an instinctive, unconscious attraction towards the other sex, which has nothing to do with their brain? Animals with a neurological circuit do need some kind of stimulus to get them to reproduce, but that stimulus is built instinctively into their genes and/or brain, but if so, where exactly?
 
  • #11
Laroxe
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You might find this useful;
http://www.csun.edu/~vcpsy00h/students/sexmotiv.htm
Remember the focus on the brain will not really help, testosterone, clearly very important in animal behaviour, isn't produced in the brain though it clearly effects its function. In humans it seems the actual level is less important, it seems we need a minimum amount, though sex itself actually increases production.
When dopamine is used as a drug in the treatment of Parkinsons disease an increased desire for sex is described as an adverse effect, its worth considering that while this might be described as reward seeking, its not a reward response. In sex itself, if we consider the reward to be the orgasm, why do so many people actively prolong the the act and effectively defer the reward.
 

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