Behavioral plasticity

Behavioral plasticity refers an animal’s ability to produce more than one alternative form of behavioral phenotype (observable characteristic) from a single genotype in response to changing environmental conditions.^[1] Behavior is one of the most variable traits and can be influenced by many factors.

Background

The concept of behavioral plasticity began to gain prominence in the 1970s under the guise of behavioral flexibility.^[1] Flexibility in this sense referred to an organism’s ability to alter their behavior in response to the environment. Although, behavioral plasticity has become a more widely used hypothesis over the last decade, demonstrating true plasticity without assuming genetic variance remains controversial and difficult in many studies.

Types

Behavioral plasticity can be broadly organized into two types: exogenous and endogenous. Exogenous plasticity refers to the changes in behavioral phenotype caused by an external stimuli, experience or environment.^[2] Endogenous plasticity encompasses plastic responses that result from changes in internal cues (circadian rhythm, menstruation, etc.) These two broad categories can be further broken down into two other important classifications. When an external stimuli elicits or ‘activates’ an immediate response, then the organism is demonstrating contexual plasticity.^[2] However, if an organism's current behavior is altered by past experiences then the animal is said to be exhibiting developmental plasticity.

Contextual plasticity occurs when there is an immediate effect on behavior by a stimuli. This form of plasticity highlights the concept that external stimuli in a given context activate neural and hormonal mechanisms or pathways which already exist inside the organism.^[2] Developmental or ‘innate’ behavioral plasticity,^[3] on the other hand, encompasses the effects stimuli from an organism's past has on an organism’s current behavior. This form of plasticity is generally thought to require new neuronal pathways to form.

Further Distinguishing between Developmental and Contextual

Developmental behavioral plasticity corresponds to the commonly used of definition of plasticity: a single genotype can expresses more than one behavioral phenotype as a result of different developmental routes triggered by differences in environments. Developmental plasticity thus envelops what is referred to as ‘learning’. However, developmental plasticity also includes developmental changes in morphology and physiology relevant to a particular behavior, such as changes in muscles, limbs or bones that influence foraging or locomotion throughout and organism's life.^[2]

A major difference between developmental and contextual plasticity is the inherent trade-off between the time of interpreting a stimuli and exhibiting a behavior. Contextual plasticity is a near immediate response to the environment. The underlying hormonal networks/neuronal pathways are already present so it is only a matter of activating them. Developmental plasticity, however, requires actual changes to occur during developmental stages on the hormonal networks and neuronal pathways. Developmental plasticity is a slower process than contextual plasticity but has the potential to result in a much wider range of behavioral phenotypes. The environmental cues received early in development can influence the rest of the subsequent development of multiple traits.

Examples

Contextual plasticity includes mate preference and other stimulus-response relationships. As an example, ants can alter their running speed in response to the current environmental temperature.^[4] Another example of contextual plasticity is when birds change their vocalizations in response to compensate for loud background noise. Both of these changes are immediate and do not require any new chemical pathways to form within the organism.

Developmental plasticity encompasses the effects of past experience on an organism’s learning, habituation, flexibility, and acclimation abilities. For example, when larvae are raised in different densities the courtship signals they produce as adults changes ^[5]

Endogenous plasticity includes circadian rhythms, circannual rhythms, and other age-dependent changes in behavior. A good example of endogenous plasticity are the zebrafish (Danio rerio). Larval zebrafish exhibit circadian rhythms in their responsiveness to light. The contextual plasticity caused by the endogenous stimuli is much higher during the day than during the night.^[6] Another example, involves changes in an individual’s behavior and hormonal profile during and after sexual maturity being affected by physiological changes that occurred months to years earlier in life.^[2]

Potential vs. realized plasticity

A useful distinction to make when looking at behavioral plasticity is between potential and realized plasticity. Potential plasticity refers to the ability of a given phenotypic trait to vary in its response to variation in stimuli, experiences, or environmental conditions. Thus, potential plasticity is the theoretical range in behavioral plasticity that could be expressed. This value is never truly known but serves more as a baseline in plasticity models. Realized plasticity, on the other hand, refers to the extent to which a given phenotype actually varies in response to changes in a specific stimulus, experience, or environmental condition.

Evolutionary causes and consequences

Behavioral plasticity can have major impacts on the evolutionary fitness of an individual. Both developmental and contextual plasticity influence the fitness of an animal in a novel environment by increasing the probability that the animal will survive in that environment. Developmental plasticity is particularly important in terms of survival in novel environments because trial-and-error processes such as learning (which encompass both phenotype sampling and environmental feedback) have the ability to immediately shift an entire population close to a new adaptive norm. As such, the ability to express some level of behavioral plasticity can be very advantageous. In fluctuating environments, animals that can change how they respond to differences in stimuli would have a leg up over animals that were set in a rigid phenotype. However, this would only be the case if the costs of maintaining the ability to change phenotype was lesser than the benefit conferred to the individual.

References

1. West-Ebberhard, MJ (2003). "Developmental Plasticity and Evolution". Oxford University Press.
2. Snell-Rood, EC (2013). "An overview of the evolutionary causes and consequences of behavioural plasticity". Animal Behaviour 85: 1004. doi:10.1016/j.anbehav.2012.12.031.
3. Mery, F; Burns, J (May 2010). "Behavioural plasticity: an interaction between evolution and experience". Evolutionary Ecology 24 (3): 571–583. doi:10.1007/s10682-009-9336-y.
4. Andrew, N.R. (2013). "Can temperate insects take the heat? A case study of the physiological and behavioural responses in a common ant, Iridomyrmex purpureus(Formicidae), with potential climate change.". Journal of Insect Physiology 59: 870–880.
5. Zhou, Y (2008). "Reaction norm variants for male calling song in populations of Achroia grisella (Lepidoptera : Pyralidae): toward a resolution of the lek paradox". Evolution 62: 1317–1334.
6. Emran, B.J (2011). "Zebrafish larvae lose vision at night". Proceedings of the National Academy of Sciences of the United States of America 107: 6034–6039.