Neural mechanisms in Animal Behavior

Simple behaviors – phenomenonological description

Sign stimulus or releaser -- screening out of extraneous stimuli
Innate releasing mechanism – releaser is recognized and processed
Result = fixed action pattern (FAP) = a relatively fixed sequence of motor patterns –> ‘instinct’ = a behavior performed essentially perfectly the first time it is performed.

examples: spot on gull’s bill, egg retrieval by greylag geese
inflexibility of response allows manipulation or parasitism by other species. Example of rove beetle mimicking smell and begging behavior of ants.
Supernormal stimuli such as giant eggs (preferred by geese over their own eggs).

Neural description of a more complex behavior – moths evading bats.

Bats ‘see’ with sound: use reflections of high-frequency sound (why high frequency?) returning to their ears (how do they know which ‘echo’ they are listening to?) and judge the distance from the time it takes to return. Review Griffin’s expts. to prove sonar.
Moths can evade bats, apparently by hearing the bats approach. But how does this work in detail?

receptor – simple membrane (like eardrum) hooked up to two nerve cells, A1 and A2.
How do nerves cells work? Difference in ion concentration between the inside and the outside of the cell, which gives rise to potential energy, which can be used for work (e.g., battery). In this case, the cell membrane is said to be ‘polarized’. Need to work (chemical pump) to maintain a difference in ion concentration. Electrical charge on cell membrane creates a series of channels in the membrane through which the ions flow rapidly from the higher to the lower concentration – this is called a ‘depolarization’ (DP) of the cell membrane.
NOW, nerve cell membranes are very sensitive locally to DP. Channels at the edge of a site where the membrane has been depolarized switch from closed to open, but only very briefly before they close up again. From wherever the DP first occurs, it travels along the neuron. This traveling ‘wave’ of DP is called the action potential (AP). It travels very rapidly, but NOT at the speed of electricity because reaction is chemical. At the end of the axon (or ‘arm’) of the neuron, AP has to stop. There the DP may allow a chemical (neurotransmitter) to escape, that changes the permeability of cell membrane of different nerve cell close to the tip of the axon. An increase in permeability of next neuron is called ‘excitation’, which makes the neuron more likely to start a new AP. A decrease in permeability is called ‘inhibition’, reducing likelihood of new AP.

Back to the moth example. What patterns of APs do these neurons give? Note that most nerve cells 'fire' APs at a low 'background rate' even if they are not being stimulated.

A1 = increases APs when membrane is struck by low-->high intensity sound, of high frequency (> 25 KHz), and of short duration (if sound continues, APs decline in frequency => 'habituation').
A2 = increases APs only for high-intensity sound.

How do these function to help moth escape bat? Two problems: figuring out where bat is and deciding how best to escape.

Where is bat? Distance proportional to intensity of sound (prop. to firing rate of AP's in A1 and to shorter delay in starting to fire). Right-left distinguished by intensity of sound arriving at the two sides of body. Up-down can be coded by changes in intensity of sound as the moth's wings flap, if the source is above the moth (but not if below or behind the moth). What about Back-Front?
Response. If bat is far away, moth need only turn toward side with weaker signal until the signal strength in A1 is the same on both sides (i.e., moth is moving directly away from bat). If bat is very close (stimulates A2 nerve), moth should try to be as unpredictable as possible.

Adaptive value of this system: bat detector nerve systems found only where they are needed (mobile sex = males, in habitats with bats, in species that are tasty to bats). Evidence for ‘arms race’ between moths and bats: jamming by moths, shifts in frequency by bats.