REPRODUCTION
Ovis canadensis
MATING SYSTEMS
SEXUAL SELECTION
Oviparity
Ancestral reproductive mode of amniotes
Turtles & Crocodilians
Large clutches of small eggs (relative to adult size)
Sea Turtle – no maternal care
Aligator – basic maternal care (nest guarding)
Hatchlings are PRECOCIAL
require no further maternal provisioning
Ovoviviparity (egg retention)
lizards and snakes
Live birth of small well-developed young,
Small number of eggs hatch internally
(maternal protection)
Horned lizard (Phrynosoma)
Rattlesnakes (Crotalus)
Protection of young-- frequency of ovoviviparity increases
with increasing environmental stress
Extra provisioning-- some degree of “viviparity”
(internal transfer of nutrients from mother to young)
Young are PRECOCIAL
independent at birth
requiring no further
maternal provisioning
Rattlesnakes (Crotalus)
Oviparity in birds
In general, birds produce relatively large eggs
(with large yolk)
Clutch size is highly variable
Bald eagle
1-3
Blue-winged teal 4-16
American Robin 3-4
Gray partridge
Active birds – small clutches
More sedentary birds – larger clutches
10-20
Oviparity in birds
Reproductive constraints imposed by flight
Limitations on
egg size & clutch size
A solution :
sequential laying AND sequential hatching
Barn Owl
Kiwi
Flightless birds have
larger relative egg mass
egg 8x the mass of
a similar-sized flying bird
Oviparity in birds
Relative development of young
Altricial
Precocial
In birds, even relatively precocial young require continued
parental investment
Oviparity in monotremes
Small clutch (1-2);
eggs very small
relative to maternal size.
Prehatching incubation period
is very short compared to birds
Young are extremely
Atricial at hatching
Require extended
postnatal provisioning
Postnatal care is
strictly maternal
(protection & lactation)
Life history of Mesozoic prototherians
Conjecture based on comparison to living groups
Morganucodon
Ptilodus
Oviparous – some possibly ovoviviparous
Altricial young
Extended post-hatching maternal care
Lactation
Marsupials
“ovoviviparous” or “semi-viviparous”
Retention of egg shell membranes
Limited placentation
Neonate extremely altricial, requiring extended
postnatal provisioning
Postnatal care is strictly maternal
(protection and lactation)
Little developmental flexibility – fixed short gestation
and extended lactation
Wide variation in litter size
Placentals – evolution of complete viviparity
Elaborate placentation
Prolonged gestation period
Considerable developmental flexibility –
gestation and lactation periods are variable
Placentals – viviparity
Neonatal development is variable, from:
Ailurops
highly altricial--
Equus
-- to highly precocial
but ALWAYS requiring extended postnatal
provisioning
Postnatal provisioning is exclusively maternal
(lactation)
Lactation
Pre-weaning provisioning by females
Why not male lactation?
Dyacopterus spadiceus
Francis. et al; "Lactation in Male Fruit Bats," Nature, 367:691, 1994.
What limits PATERNAL care in mammals?
Monogamy is rare in mammals
(only 3 to 5% of all species)
Generally FACULTATIVE
(involving mate guarding, or due to
scarcity of potential mates)
Not OBLIGATE
(involving direct paternal
investment)
Dolichotis
IN CONTRAST
Monogamy is common in birds
(90% of all species)
Monogramy is OBLIGATE
Often involves mating for life
Why the difference?
Campephilus
Males can directly increase
their fitness through paternal care
Parental Care in Mammals
LACTATION
makes maternal care
absolutely essential in
ALL mammals
(to ensure direct fitness)
Females may also
help female kin rear
offspring
(indirect fitness)
Cooperative female kin
(the foundation of
social behavior)
Paternal care is
restricted
even in species that are
monogamous
Reproductive constraints of flight
in bats
Pteropus
Haliaeetus
Lasiurus
Constraints of flight are much greater in viviparous
mammals compared to oviparous birds
Females have an unavoidable and prolonged
mass increase during pregnancy
Altricial young require protection and nursing
(are often are carried in flight)
RESULT:
Most bats have small litters of 1-2 young
(in a few species up to 4 young)
Bats have very low reproductive output compared
to other mammals of similar size (e.g. rodents)
BUT they are much longer-lived
Sexual strategies
The sexes have different fitness perspectives:
Females – mate quality, resource availability
Males – mating opportunities
Result:
Male-male competition for mating opportunities
Female choice of mates (and often associated resources)
Mating systems
Polygyny
Single male mating with multiple females
Harem polygyny -- male “control” of female groups
to achieve exclusive matings
Polyandry
Single female mating with multiple males, AND
males take on rearing responsibilities
(i.e., “role reversal”)
uncommon in birds (2% of species)
Impossible (?) in mammals due to lactation
Promiscuity
Both sexes mating with multiple partners
Many (most?) mammal species are promiscuous
Multiple mating, but NOT necessarily indiscriminate mating
May reflect male-male competition (“sperm competition”)
e.g. – “copulatory plugs” in rodents
May involve active selection by females
For multiple mates
(i.e., advantages of multiple-sired litters)
Sexual Selection
Differential selection for traits in one sex (males)
through the action of male-male competition
and female choice
Size dimorphism
Sexual combat structures
Display behavior and structures