Mating Systems and Sexual Selection

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Mating Systems and Sexual Selection

• Sexual Dimorphism

veiled chameleon
African reedfrog, Hyperolius argus
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Mating Systems and Sexual Selection
Sexual Selection

• Directional selection that acts on genetically
  variable phenotypic traits that affect the
  reproductive success of the individuals of a
  particular sex

- secondary sexual characteristics
Coloration Size Crests

• Usually in the male
• b/c females are the limiting resource

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Mating Systems and Sexual Selection
Sexual Selection
male
Darwin 1871 as cited in Pough et al. 2001
male
male
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Mating Systems and Sexual Selection
Sexual Selection
Two components
1. Male male competition for access to females
2. Female choice (of which male to mate with)
Pough et al. 2001
Pough et al. 2001
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Males more affected by sexual selection than
females

• Females spend energy on egg production ability
• Males a whole lot of little cheap sperm
• Magnitude of sperm production favors
  fertilization of eggs from many females
• Males do not need to find quality mates, rather
  quantity mates

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Mating Systems and Sexual Selection
Mating Systems
Most herps polygynous
- Individual male reproductive success variable
- Some males mate with more than one female
- Many males dont mate at all
Females Choose By?
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Why do females prefer certain phenotypic traits?

• Direct Benefit Certain male characters (nuptial
  gifts of food or defensive compounds, care
  provided to offspring)
• Good genes Male characters are "indicators" of
  "good genes", i.e.,
• Sensory bias or sensory drive some aspect of the
  sensory world biases females

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Natural Selection vs. Sexual Selection

• Sexual Selection and Natural Selection may
  actually oppose each other
• Increased color/ behavior that attracts predation
• Increased cost of maintaining a territory
• Heavy weaponry

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Mating Systems and Sexual Selection
Mating Systems
Behavioral tactics used by males depends on the
spatial and temporal distribution of females
1. Many clumped briefly explosive scramble
2. Few dispersed males go searching
3. Intermediate many options

• Mate guarding

• Signalling to attract
• - dispersed
• - lekking

Grade into each other Intraspecific variation

• Defend territories
• - food resources
• - nesting sites

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Mating Systems and Sexual Selection
Mating Systems
Explosive Mating Aggregations
- Temporary ponds Spea, Scaphiopus, Rana

• Flood the world with sperm
• some ambystomatid salamanders
• and frogs

• Spatial aggregation
• some Thamnophis, Natrix (spring at den)

Males outnumber females (e.g., 10-1)
male-male competition
Pough et al. 2001
including
Sexual Interference
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Rana sylvatica Bufo bufo,
Scaphiopus
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Sperm Competition
Chiromantis xeramplina African gray treefrog

• Multiple paternity
• the red-eyed tree frog (Agalychnis callidryas),
  the Australian frog (Crinia georgiana), and the
  common frog (Rana temporaria).
• Genetically superior sperm more likely to survive
  (better to mate with several males)

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Thamnophis sirtalis parietalis males swamp the
few females that emerge
High numbers of male garter snakes die soon after
emerging from hibernation because they are
attacked by crows. She-males at the center of a
mating ball, however, are less exposed to
predators also remain warmer
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Explosive
Male
1980
Female
1981
Male
Female
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Mating Systems and Sexual Selection
Mating Systems
Mate Searching
- some salamanders
- common in reptiles
- turtles and tortoises - snakes tend to
be solitary follow pheromones
Thamnophis sirtalis
- populations may vary density
fewer, widely dispersed more search time and
more waiting
- widely foraging lizards
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Mating Systems and Sexual Selection
Mating Systems
Mate Searching
- Crotalus viridis
Overwinter in communal dens Disperse in spring
and forage widely Males search for females in
midsummer Most males dont find a female Long
term strategy
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Mating Systems and Sexual Selection
Mating Systems
Mate Guarding If searching for mate is costly,
then it may be worthwhile to invest energy in
guarding her from other males
- amplexus in anurans and salamanders
salamander - male may physically carry away
female
- male with enlarged teeth used to deter other
male
Atelopus (Bufonidae)

• male may amplex weeks/months
• before breeding season

- costs to female and male
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Mating Systems and Sexual Selection
Mating Systems
Mate Guarding
Gopherus agassizii
- Male searches for female - Stays with her many
days - Wards off other males use of gular
Territorial lizards - male will guard female
- forfeit other females
photos by Roger A. Repp
Tiliqua rogusa - multi-year pair bonds
Crotalus atrox
Some Snakes - often wrestling contests -
topping
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Mating Systems and Sexual Selection
Mating Systems
Leks aggregations of males that gather in sites,
defend small territories, and display for females

Females choose males based on their traits
-color, size, vigor, display structures
Triturus cristatus
Marine iguana
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• Chorus anuran males calling from particular
  perches

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Mating Systems and Sexual Selection
Mating Systems
Resource Defense
Resources attractive to females - oviposition
sites - feeding areas
Male mating success depends more on resource
quality than on the characteristics of the male
per se
Male size and vigor may be correlated
Not common in salamanders, but
Cryptobranchids with external fertilization
- defend territories with nesting sites
Red-backed salamanders (Plethodon cinereus) -
female judges territory quality based on male
feces (termites better than ants)
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Mating Systems and Sexual Selection
Mating Systems
Resource Defense
Resources attractive to females - oviposition
sites - feeding areas
Rana clamitans (green frogs) Rana catesbeiana
(bullfrog)
Territories guarded for 2 months (during
breeding season) appropriate oviposition sites
(water temperature)
Territorial Lizards
Pough et al. 2001
Male with large quality territory likely to
encompass many female territories
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Mating Systems and Sexual Selection
Male Reproductive Success
Some males mate many times Many males dont mate
at all
Variance in reproductive success leads to strong
sexual selection and sexual dimorphism
- Search time - Competition - Attractiveness -
Handling time - Parental care
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Some males mate many times Many males dont mate
at all
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Mating Systems and Sexual Selection
Male Persistence and Allocation of Resources
Stamina
e.g. Amount of time male anuran spends in chorus
positively correlated with mating success
Sacrifice energy reserves and foraging
opportunities
Salamander lesson
Explosive aggregations (Ambystoma) males
deposit many, many spermatophores most not
picked up by a female
Courtship (Salamandridae, Plethodontidae)
males court extensively, deposit few
spermatophores female picks up
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Sacrifice energy reserves and foraging
opportunities
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Mating Systems and Sexual Selection
Male Competitive Ability
- explosive breeding aggregations - mate
searching - mate guarding
Larger body size
Also important for territorial species
(indirectly)
Pough et al. 2001
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Mating Systems and Sexual Selection
Male Competitive Ability/ Sexual Dimorphism
- Frog fangs - Frog wrestling - Frog trunk
muscles - Newt tail fins - Lizard biting -
Snake wrestling
Male-male assessment - size - color -
repeat encounters
Pough et al. 2001
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Male
Males wrestling
Female
Amplexus
Pough et al. 2001
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Mating Systems and Sexual Selection
Alternative Mating Tactics
- Sneaking/Satellite - Female Mimicry - Sexual
Interference
Uta stansburiana orange super male blue
mate guarder yellow sneaker
Pough et al. 2001
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Small satellites
Alternative Mating Tactics
Small males more likely to behave as satellites
density and satellites
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Mating Systems and Sexual Selection
Sperm Competition - amount - vigor -
longevity
Uta stansburiana
Male vs. Female size - sexual selection -
clutch size - ecological roles (diet)
Atelopus