today's class: synchrony in flashing fireflies

Today’s class: Synchrony in flashing fireflies

“The Glowworms ... represent another shew, which settle on some Trees, like a fiery cloud, with this surprising circumstance, that a whole swarm of these Insects, having taken possession

of one Tree, and spread themselves over its branches, sometimes hide their Light all at once, and a moment after

make it appear again with the utmost regularity and exactness”

- Engelbert Kaempfer’s (1727) account of traveling in Thailand

Today’s class: Synchrony in flashing fireflies

The firefly lifecycle

larva pupa beetleegg

The firefly lifecycle

larva pupa beetleegg

all stages produce light

Functions of glowing

to teach predatorsthey're disgusting

to locate potential mates

Firefly courtship

male flies around, flashing

lets him find her

female responds

Species-specific flash codes

(but watch out)

Photuris females flash like Photinus females....

then eat the Photinus males that respond

aggressive mimicry

So, what causes the synchronized flashing?

early theories:

‘it’s just an illusion’

‘there’s a leader beetle’

‘there’s some outside trigger’

So, what causes the synchronized flashing?

modern theory:

self-organisation!

to understand how it works, we first need to understand the mechanism behind rhythmic firefly flashing

the beetles cause each other to flash in unison, without any leader or external trigger

Mechanism: The ‘sawtooth model’

neural oscillator, affected by light

time

flash

motor delay

oscillator

threshold

basal level

Why we know light affects the oscillator

fireflies will respond to any

kind of light

makes their oscillator easy

to study

Entrainment

by studying the light’s effect on the firefly’s flashing, it’s possible to deduce the workings of its oscillator

light, flashing in a rhythm

firefly, who flashes back

The problem

light actually affects different species in different ways

one dimension that matters here is mating system

Synchrony and mating systems

‘rovers’

individuals are solitary, and distributed

‘leks’

males and females congregate in trees

occassional, brief synchrony

habitual, mass synchrony

Synchrony in rovers

the mechanism in rovers tends to be simpler

so we’ll start with one of those

Photinus pyralis,a North American firefly

Reminder: The ‘sawtooth model’

neural oscillator, affected by light

now the question is: how does light affect a Photinus pyralis oscillator, and how does this allow them to synchronize?

time

flash

motor delay

oscillator

threshold

basal level

Entrainment in Photinus pyralis

time

flash

regular flashing is a single flash every six seconds


time

flash light continuation of the regular flash pattern

no effect of the light

flash a light early in the firefly’s regular cycle,then the firefly’s flashing is unaffected

threshold 8, latency 2, window 2, start-firefly-0 1, light-cycle 10


time


effect of the light

flash a light late in the firefly’s regular cycle,then the next flash is advanced



time

flash

flash lights at a frequency at the firefly’s natural rhythm, then the firefly usually matches it perfectly after a few flashes

light



time

flash light

flash lights at a frequency faster than the firefly’s natural rhythm, then the firefly matches it, but is always late


time

flash light

flash lights at a frequency slower than the firefly’s natural rhythm, then the firefly flashes after every light, but also in between



Mechanism: Any ideas?

time


effect of the light

Mechanism: Phase advance resetting

when the firefly sees a light in its late window, its pacemaker is raised to the threshold level, triggering a flash

time

flash

motor delay threshold

basal level

oscillator

Mechanism: Phase advance resetting

when the firefly sees a light in its late window, its pacemaker is raised to the threshold level, triggering a flash

time

lightlate window

Phase advance resetting: NetLogothreshold 8, latency 2, window 2, start-firefly-0 3, light-cycle 10

Phase advance resetting: Summary

• a flash resets the pacemaker to the threshold level

• there is a critical window where resetting can occur

properties

• allows the firefly to synchronize with a light flashing at its natural rhythm

• ...unless the light's flashes always occur outside of window

effects

Synchrony in phase advance resetters: NetLogo

number 1500, threshold 8, latency 2, window 2 (waves) or 4 (sychrony)

Synchrony in phase advance resetters: Summary

• almost perfect (with some never synchronizing due to window)

• very brief, quickly settling back into seemingly random flashing

• 'wave like', with groups of fireflies flashing in sequence

depending on the pacemaker's properties and the spatial situation, synchrony can be:

(threshold 8, latency 2, window 4)

(threshold 8, latency 2, window 4, movement on)

(threshold 8, latency 2, window 4, movement off)

Synchrony in habitual synchronizers

Pteroptyx cribellata,a South Asian firefly

Entrainment in Pteroptyx cribellata

time

flash

regular flashing is a single flash every second

time

flash

flash a light almost anywhere in the firefly’s regular cycle,then next flash is delayed

continuation of the regular flash patternlight

effect of the light



time

flash

exception: flash a light just before a regular flash, and then the next flash is regular - but the one after that is advanced

light

effect of the light



time

flash

flash lights at a frequency at the firefly’s natural rhythm, then the firefly always matches it perfectly from the next flash

light



time

flash light

flash lights at a frequency a little faster than the firefly’s natural rhythm, then the firefly matches it, but is always late



time

flash light

flash lights at a frequency a lot faster than the firefly’s natural rhythm, then the firefly never flashes at all



time

flash light

flash lights at a frequency slower than the firefly’s natural rhythm, then the firefly flashes before the next light




time

flash continuation of the regular flash patternlight

effect of the light

Mechanism: Phase delay resetting

time

flash

motor delay

oscillator

threshold

basal level

when the firefly sees a light, its pacemaker is immediatelyset back to the basal level


time

light

in almost all parts of a firefly’s flashing rhythm,this causes a delay in the next flash


time

light

however, if the light occurs when a flash has already been triggered, then the next flash is advanced

Phase delay resetting: NetLogothreshold 8, latency 2, window 0, start-firefly-0 4, light-cycle 10 (for synchrony) or start-firefly-0 1, light-cycle 9 (for advancement)

Phase delay resetting: Summary

• a flash resets the pacemaker to the basal level, anywhere in its cycleproperties

• generally, this causes a delay in the timing of the next flash

• but, if a flash has already been triggered, the next flash may seem to be advanced

• allows the firefly to synchronize with a light flashing at its natural rhythm

effects

Synchrony in phase delay resetters: NetLogo

• always perfect synchrony within a few cycles

number 1500, threshold 8, latency 2, window 0

Synchrony in habitual synchronizers

Pteroptyx malaccae,a South Asian firefly

Entrainment in Pteroptyx malaccae

flash light

unlike the other two species, Pteroptyx malaccae can match a rhythm that is slightly slower or faster than its natural rhythm

however, this process is very slow, taking many cycles

Mechanism: 'Perfect' resetting

a variant of phase delay resetting

• a flash early in the cycle raises its threshold slightly, a flash late in the cycle lowers its threshold slightly, within limits of a minimum and maximum threshold

• in this way, Pteroptyx malaccae can adjust to rhythms 15% faster or slower than its own

what explains the slow entrainment:

• a flash lowers the level of the pacemaker by a small amount, rather than to zero

what explains its ability to adjust to different rhythms:

Mechanism: Perfect resetting

• allows the firefly to synchronize with a light at or close to its natural rhythm

• entrainment is always very slow; it takes many cycles to reach synchrony

effects

• a flash lowers the level of the pacemaker by a small amount, rather than to zero

• in addition, the pacemaker can adjust its period as well as its phase

properties

Other similarly synchronizing species

frogskatydids fiddler crabs

Just for fun: Fiddler crabs

Functions of synchrony

the 'beacon' hypothesis

synchronous flashing helps females find the mating trees


the 'female recognition' hypothesis

if males don't flash in synchrony, they can't detect female response flashes

the 'male recognition' hypothesis

if males don't flash in synchrony, the female can't detect the species specific flash

or, conversely:


the 'interloping' hypothesis

if males synchronize with each other, they can hope to steal each other's mates


the 'predator avoidance' hypothesis

if males synchronize with each other, it makes it more difficult for predators to locate them

And now for something completely different (or not?)

phase alternation

many species of katydid, grasshopper, and frog

true katydid

Entrainment in alternating species

time

regular chirping is a series of phrases, each consisting of several pulses

chirp


time

to simplify a bit, assume that every phrase consists of just one pulse

chirp


time

chirp continuation of the regular chirp patternbeep

produce a beep early in the katydid’s regular cycle,then the next chirp is advanced

effect of the beep



time

chirp

produce a beep late in the katydid’s regular cycle,then the next chirp is delayed


beep

effect of the beep


time

chirp

produce regular beeps in the katydid’s natural rhythm, and alternation ensues


beep


time

chirp continuation of the regular chirp patternbeep

effect of the beep

Mechanism: Accelerated resetting

time

beep

motor delay

oscillator

threshold

basal level

a variation of phase delay resetting, where the oscillator rebounds quickly after inhibition by a beep


time

beep

produce a beep early in the katydid’s cycle, and rapid resetting causes the next flash to advance


time

beep

threshold

produce a beep late in the katydid’s cycle, and rapid resetting isn’t enough to prevent a delay

Accelerated resetting: NetLogothreshold 8, latency 2, window 2, start-firefly-0 3, light-cycle 10

Accelerated resetting: Summary

• a beep resets the pacemaker to the basal level

• after resetting, the pacemaker rebounds rapidly

properties

• can produce advancement or delay of the katydid’s next chirp

• allows the katydid to alternate with a chirp flashing at its natural rhythm

effects

Function of alternation

females of many species prefer the leading of two chirps

phase delay resetting helps a male avoid chirping right after another male

rapid resetting helps a male avoid being silenced completely if the other male chirps faster

Synchrony in flashing fireflies: Conclusions

• what seems like a massive act of coordination can in fact be explained by self-organisation➡ e.g., the habitual synchrony of South Asian fireflies

• what seems like similar behavior can in fact be caused by subtly different mechanisms➡ e.g., the phase-delay resetting of Pt. cribellata vs. the

perfect resetting of Pt. malaccae

• what seems like very different behavior can in fact be caused by very similar mechanisms➡ e.g., the firefly synchrony vs. katydid alternation

today's class: synchrony in flashing fireflies

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