1

Announcements

• Reminder: office hours Monday 1-2,Tuesday 9:30-10:30

• Collection station to be made in lab.• Reading for today’s class:Rank, N. E., K. Yturralde and E.P. Dahlhoff.

2006. Role of contests in the scramblecompetition mating system of a leaf beetle.Journal of Insect Behavior, 19:699-716.

Mating and Reproduction• How sexes find each other• Courtship• Nuptial gifts• Sexual Selection

Sexual selection– Occurs when a trait that is costlyfor survival, but

– confers greater mating success– My explain sexual dimorphism

• Males larger than females- rare in insects• Exaggerated traits in males

Types of sexual selection• Intrasexual selection

– Contest competition– Scramble competition– Sperm competition

• Intersexual selection– Male has exaggerated trait– Females choose males depending on traitvalue

– Sex roles reversed if males performmore parental care than females

Contest competition• Frequent fights over territory or access to

females• Males monopolize females• Often exaggerated traits in males• Positive assortative mating (large males

mate with large females) probablybecause large females lay more eggs

Large male wins Exaggerated structure

2

Scramble competition• Scramble to be first to mate• Little monopolization of females• No consistent body size effect

– small may be better for maneuverability– large may have advantages if it relates

to speed

Mixed strategy within dung beetles

• Contest- Largedhorned malesmonopolize tunnelsin which femaleslive

• Scramble- Smallhorned males sneakaround large malesor dig a side tunnel

Copulation and sexual selection• Copulation often prolonged in

insects– Female may try to break off

sooner if male is less desirable– Male may try to prolong

copulation to maximizefertilization

• Male accessory gland secretionmay– Repress female receptivity to

mating by others– Stimulate female oviposition

Sperm competition• What really counts

– Male: number ofoffspringfathered

– Female: highestquality mates

• Intersexual conflictmay occur overwhich sperm fertilizeeggs

Sperm competition• Last-in-first out systems

– Spermatophore closestto oviduct mostsuccessful

– Males push previoussperm higher intospermatheca

• Sperm removal– Scoop out previoussperm

– Flush out previoussperm

Outcomes of sexual selection

– Rapid evolution of genitalia

– High variation within and amongspecies, more than needed forcopulation

3

• Can prevent interspecies mating• Can also result from intersexual conflict

over control of sperm

Variation in female reproductive tract Morphological evolution in males

• Females of all three species look alike• Rapid evolution of male structures

associated with mating

Results of sexual selection– Further exaggeration of traituntil balance between cost andbenefit established

– May result in new species

Chrysomela aeneicollis

• Little sexual dimorphism,females larger thanmales

• Little evidence of strongsexual selection

• males frequently fight formates

• mating behavior is easyto study in the laboratory

Study Questions

• How does mating behavior in the fieldrelate to behavior in the laboratory?– with male competition– under female choice

• How does size affect mating successin males and females?

4

Field component

Laboratory Component

• field observations

• collect:

– mating pairs

– unmated males

• male competition trials

• female choice trials

• morphologicalmeasurements

MF

M

Morphological measurements• measured

– elytra length– femur length– thorax width

• compare size withmating success

femur

Male fights in the field

• males locked mandibles and bit each othersfront appendages

• interactions lasted 2 to 88 minutes• One male paired with female after 35% of

fights. Sometimes intruder, sometimesresident.

• females usually left fighting males (32 of 37)interactions

Most common behaviors in the field

Snapshot 2008-04-02 06-29-36

Large females mate with large males in the field

Female elytra length (mm)

Mal

e el

ytra

leng

th (m

m)

Mating males weren’t larger thansingle males

Mated males Unmated malesmean mean

Elytra length 4.52 4.59 P > 0.2

Femur length 1.29 1.29 P > 0.7

Thorax width 2.32 2.34 P > 0.6

5

Design of laboratory tests• half of field mating males were paired with

their field partner• other half of field mating males were paired

with new females

1 1

20 20

21 22

39 21

Female choice trial

Behaviors observed in thelaboratory

Behavior Competition Trial Choice Trialsitting 43% 64%feeding 8% 11%walking 21% 21%mating 14% 2%mate-guard 9% 1%fighting 7% 0%

Field female New female

Male competition trials

Mated male Unmated male

Prop

ortio

n of

tim

e m

atin

g

Female mate choice trials

Field female New female

Prop

ortio

n of

tim

e m

atin

g

Mated male Unmated male

Prop

ortio

n of

tim

e m

atin

g

Female elytra length (mm)

Female size and female mating success

0

0.1

0.2

0.3

0.4

0.5

0.6

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7

°°

°

°

°

°

°

°

°°

° °°

°°°

5.8

new female field female

6

Conclusions of beetle study

• Mostly likely a scramble mating system• Males do not monopolize females• Large males do not mate more than smaller ones

• Fighting and competition do occur• Females prefer ‘better’ competitors• Males prefer large or ‘new’ females