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What determines lek size? Cognitive constraints and per capita attraction of
females limit male aggregation in an acoustic moth.
S. Alem, C. Clanet, V. Party, A. Dixsault, & M. D. Greenfield. 2015
Animal Behaviour 100: 106-115
Leks
Classically – form because of strong ♀ choice
♀ gains - improved choice comparisons
- reduce time and energy in mate
seeking
Why do ♂s aggregate?
- ♂s at leks have a higher /♂ mating
success than alone
- ♀s prefer aggregations
- location of lek ⟶ better signal
transmission
- lower /♂ predation risk
Leks
If leks offer ♂ increased /♂ matings - what determines lek size?
- Never get huge leks
Why is there a limit?
Costs to male
Aggression
Disease
Stress
-increase faster than attractiveness
of lek to female
Costs
Attractiveness
Leks
If leks offer ♂ increased /♂ matings - what determines lek size?
- Never get huge leks
Why is there a limit?
Constraints on Female
-rate of arrival of ♀s could decelerate benefit
- /♂ mating success drops
-too expensive for ♀s to travel far
-in larger leks - difficult for ♀s to find ‘best’ male
Perceptual Constraints
-saturation of female receptors
-if saturated,
n+1 or n+2 lek = n lek
& no benefit to a male from changing groups
∴ there will be a limit on lek size
Which influences are responsible for lek size?
Do influences coincide?
Does the number of ♂s on a lek = maximum benefit ♂ : ♀ ratio?
OR
Is there a sexual conflict?
- ♂ avoiding predation in large groups
- ♀ wanting access to more ♂s
This study
1. Measure per capita male attractiveness as a function of lek size
2. Measure tendency of males to aggregate in leks
3. Mechanisms by which females distinguish leks and assess lek size
Methods
Species
Achroia grisella
Lesser wax moth
Symbiont of bee hives
Methods
Eclose in hive
Species
Remain in vicinity of hive
Males in groups of 2 - 10
Achroia grisella
Lesser wax moth
Sing for 6 -10 hrs each
night
Die in ≤ 10 days
Methods
Upstroke = 2 pulses
(one on each side)
Downstroke = 2 pulses
(one on each side)
Methods
Upstroke = 2 pulses
(one on each side)
Downstroke = 2 pulses
(one on each side)
Time
Methods
95 dB
95 dB
95 dB
95 dB
Ultrasonic - 70 – 130 kHz
Methods
Protocol
Raised in lab
Isolated by sex (to ensure virginity)
Used in mating experiments
(♀s mate once only)
Experiment 1 – Female preference for leks of singing males
Release point
Cage with male
Empty cage
20 females tested
Groups of 2, 3, or 4 males vs 1 male
Experiment 1 – Female preference for leks of singing males
Calculations (for each lek size)
Number of ♀ choosing a lek
= per capita attractiveness
Number of ♂ in a lek
Per capita attractiveness
Number of ♀ choosing single male
= relative per capita attractiveness
= proportion of females choosing a lek
= attractiveness of a lek relative to single male
Female choice
Per capita
attraction = 1
Lek size
Relative per capita attraction
Random choice
Experiment 2 – Marginal female preference for larger leks
Release point
Cage with male
Empty cage
Groups of males, one with n males, other with n+2
(1 vs 3, 2 vs 4, 3 vs 5, 4 vs 6, 5 vs 7, 6 vs 8, 7 vs 9)
Prediction: Female preference for larger leks decreases with larger leks
Experiment 2 – Marginal female preference for larger leks
= proportion of females choosing a lek
= attractiveness of a lek relative to single male
Per capita
attraction = 1
Lek size
Relative per capita attraction
Female choice
Random choice
Experiment 3 – Distinguishing acoustic features of leks
How do females detect and distinguish leks from solitary males?
Recorded males in groups and alone
Experiment 3 – Distinguishing acoustic features of leks
How do females detect and distinguish leks from solitary males?
Recorded males in groups and alone
Solitary
How often these pulses
are repeated is “pulse
pair rhythm”.
Grouped
Difficult to detect
rhythm
Experiment 3 – Distinguishing acoustic features of leks
How do females detect and distinguish leks from solitary males?
Recorded males in groups and alone
Grouped
Difficult to detect
rhythm
Considered only pulse pairs that were ≥ 2 msec after previous pair
Also considered pulse pairs that were 5 msec after previous pair
(refactory period of axons in moth)
Experiment 3 – Distinguishing acoustic features of leks
How do females detect and distinguish leks from solitary males?
Results
1. Males increased pulse pair rate about 10% when in groups
2. No change in mean peak amplitude
3. Pulse pair rhythm of a lek >> that of solitary male
4. Not a linear increase with 3 males - overlap
Experiment 4 – How do females discriminate leks from solitary males?
Playback experiments:
1. Can ♀s distinguish leks by acoustic features alone?
2. Do ♀s use the rhythm broadcast by leks to distinguish them?
3. Do ♀s attend to additional features in overall song broadcast
by leks?
Experiment 4 – How do females discriminate leks from solitary males?
1. Can ♀s distinguish leks by acoustic features alone?
♂
♂
♀
♂
♂
Results: 1. Females can distinguish leks on acoustic cues alone
2. Females use pulse pair rate to distinguish leks
3. Females do not use acceleration of rhythm to distinguish leks
Experiment 5 – Do singing males join other male singers?
Release point
Cages with males (1 → 5)
Empty cage
Experiment 5 – Do singing males join other male singers?
Discussion
1. Females prefer leks over solitary males – based on per capita attractiveness
2. Females judged leks based on pulse pair rhythm.
Role of neuroethology?
Results suggest that neural constraints (ability to distinguish pulse pair
rate) dictate maximum lek size
Sound intensity not a factor
Discussion
Why do females ignore rhythm accelerations?
-duty cycle and chances of simultaneous sounds
Matching lek size and female preference
-compare ♂ and ♀ responses
-roughly matched on lek size – about 3
Inter-male aggression may play a role
May be a product of similar neural constraints
Questions??