REPRODUCTION

MATING SYSTEMS

SEXUAL SELECTION

Ovis canadensis

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 PRECOCIALrequire no further maternal provisioning

Rattlesnakes (Crotalus)

Protection of young-- frequency of ovoviviparity increaseswith increasing environmental stress

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)

Young are PRECOCIAL independent at birthrequiring no further

maternal provisioning

Extra provisioning-- some degree of “viviparity” (internal transfer of nutrients from mother to young)

Oviparity in birds

In general, birds produce relatively large eggs(with large yolk)

Bald eagle 1-3

Gray partridge 10-20

American Robin 3-4

Blue-winged teal 4-16

Clutch size is highly variable

Active birds – small clutchesMore sedentary birds – larger clutches

Oviparity in birds

Reproductive constraints imposed by flight

Limitations on egg size & clutch size

A solution :

Barn Owl

egg 8x the mass of a similar-sized flying bird

Flightless birds have larger relative egg mass

Kiwi

sequential laying AND sequential hatching

Oviparity in birds

Relative development of young

Precocial

In birds, even relatively precocial young require continued parental investment

Altricial

Oviparity in monotremes

Young are extremely Atricial at hatching

Require extendedpostnatal provisioning

Postnatal care isstrictly maternal(protection & lactation)

Prehatching incubation period is very short compared to birds

Small clutch (1-2); eggs very small relative to maternal size.

Life history of Mesozoic prototherians

Conjecture based on comparison to living groups

Altricial young

Extended post-hatching maternal care

Lactation

Oviparous – some possibly ovoviviparous

Morganucodon

Ptilodus

Marsupials“ovoviviparous” or “semi-viviparous”

Retention of egg shell membranes

Limited placentation

Postnatal care is strictly maternal(protection and lactation)

Neonate extremely altricial, requiring extended postnatal provisioning

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

Neonatal development is variable, from:

Placentals – viviparity

but ALWAYS requiring extended postnatal provisioning

highly altricial--Ailurops

-- to highly precocial

Equus

Postnatal provisioning is exclusively maternal(lactation)

Dyacopterus spadiceus

Francis. et al; "Lactation in Male Fruit Bats," Nature, 367:691, 1994.

Lactation

Pre-weaning provisioning by females

Why not male lactation?

What limits PATERNAL care in mammals?

Monogamy is rare in mammals (only 3 to 5% of all species)

Dolichotis

Monogramy is OBLIGATEOften involves mating for life

Generally FACULTATIVE (involving mate guarding, or due

to scarcity of potential mates)

Not OBLIGATE(involving direct paternal

investment)

IN CONTRAST

Monogamy is common in birds (90% of all species)

Why the difference?

Males can directly increasetheir fitness through paternal care

Campephilus

Parental Care in Mammals

Cooperative female kin(the foundation of

social behavior)

LACTATION makes maternal care

absolutely essential inALL mammals

(to ensure direct fitness)

Females may also help female kin rear

offspring(indirect fitness)

Paternal care is restricted

even in species that are monogamous

Haliaeetus

Lasiurus

Most bats have small litters of 1-2 young (in a few species up to 4 young)

Reproductive constraints of flightin bats

Constraints of flight are much greater in viviparous mammals compared to oviparous birds

Pteropus

Females have an unavoidable and prolonged mass increase during pregnancy

Altricial young require protection and nursing (are often are carried in flight)

Bats have very low reproductive output compared to other mammals of similar size (e.g. rodents)

BUT they are much longer-lived

RESULT:

Sexual strategies

The sexes have different fitness perspectives:

Females – mate quality, resource availability

Males – mating opportunities

Result:

Female choice of mates (and often associated resources)

Male-male competition for mating opportunities

Mating systems

Polygyny

uncommon in birds (2% of species)

Single female mating with multiple males, ANDmales take on rearing responsibilities (i.e., “role reversal”)

Harem polygyny -- male “control” of female groups to achieve exclusive matings

Impossible (?) in mammals due to lactation

Polyandry

Single male mating with multiple females

Promiscuity

Both sexes mating with multiple partners

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 femalesFor multiple mates

(i.e., advantages of multiple-sired litters)

Many (most?) mammal species are promiscuous

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