Download Mating Systems 1

Mating Systems 1
• Darwin claimed that the complex structures and elaborate behaviors surrounding animal
reproduction were shaped by two mechanisms:
> Competition Between Males
> Female Choice
• The idea of female choice was not widely accepted until the 1970’s
• Revived by Gordan Orians and Robert Trivers, among others (Trivers’ Social Evolution)
• There are several possible explanations for why females make the choices they do…
• Good genes model - male’s appearance, as well as his ability to survive, suggests that he
would add these positive traits to your offspring
• He must have really good genes
• Physiological basis for good genes model
• Male bobolinks in good physical condition (measured by body mass/wing length) have longer
flight displays
• Number of mates correlates with length of flight display (Mather and Robertson 1992)
• Clipped wings of males, altered males got fewer mates…
• Female barn swallows prefer males with longer tail streamers (Smith et al. 1991)
• Tail length in males inversely correlated with ectoparasite load
• UV intensity in male plumage of Satin Bowerbird inversely correlated with blood parasite
load
• Provides an obvious external marker for females to judge mate quality
• The handicap model of Amotz Zahavi is a variation on the good genes model
• If a male can carry around that enormous tail and still avoid predators, and feed and groom
himself, - he must really have his act together
• Difference is that in the good genes model, the male’s traits correlate positively with fitness
• In the handicap model, the male’s traits correlate negatively with fitness
• Female’s choice may be an aesthetic choice
• Why else might a female prefer a bright yellow plume on a male bird’s head, or the particular
staring eye pattern of the male peacock’s plumes?
• This was Darwin’s hypothesis…
• Whatever the basis for the female’s choice, once it is made it is self-reinforcing
• Birds with a prominent yellow feather on their heads would mate more often than those that
lacked one
• Might lead to “runaway selection”
• Trait is reinforced generation after generation, until it becomes so exaggerated it could
become a dangerous burden (R.A. Fisher)
• Emlen and Oring (1977) define mating systems as behavioral strategies used to obtain mates,
which include
> number of mates acquired
> way in which mates are acquired
> formation and characteristics of pair
> patterns of parental care
• There are four basic mating systems:
> monogamy
> polygyny
> polyandry
> promiscuity
• In monogamy, one male mates with one female, forming a pair bond that can endure for a
single season or for a lifetime (>90%)
• In polygyny, one male mates with several females, while each female only mates with one
male (2%)
• In polyandry, one female mates with more than one male, while each male only mates with
one female (1%)
• In promiscuity, males mate indiscriminately with any female, and no pair bond is formed
(6%)
• Mating systems seem to be shaped by the way in which resources and mates are distributed
and exploited
• Most birds are monogamous, a mating system that is relatively rare in mammals
• Lack (1968) estimates over 90% of bird species are monogamous
• Prevalence of monogamy in birds vs. mammals relates to different energetic needs, different
roles of the sexes re: young
• Traditional viewpoint - monogamy is widespread in birds because birds have such high
energetic needs
• Female bird, unlike a female mammal, cannot hope to supply those needs by herself, but
needs the full-time help of her mate to provide for their young
• Birds spend several weeks or months building nests and raising young, unlike fish or turtles or
frogs who dump their eggs and leave
• Raising baby birds requires a high parental investment
• Several forms of monogamy
• Some birds actually mate for life - swans, geese, eagles, albatross, petrels, some owls and
parrots
• Some birds keep the same mate for several seasons before choosing another mate, -robins,
tree swallows, mourning doves
• Most species, and most passerines, practice serial monogamy - the pair bond only lasts for a
single breeding season
• Maintaining a pair bond significantly enhances reproductive success
• Coulson (1966) conducted an intensive 12 year study on monogamy in kittiwakes (Rissa
tridactyla)
• Females who kept their mates from prior breeding seasons
> bred earlier than other females
> laid more eggs
> were more successful in fledging young
• Mills (1973) found the same to be true for Red-billed Gulls (Larus novaehollandiae)
• Changing mates resulted in lower hatching and fledging success
• Females are assumed to make the larger parental investment
• Making large eggs is energetically more costly than producing sperm
• Relative investment of the male varies greatly in monogamous species
• Females usually take on primary nesting duties, while the male guards the territory to preserve
the resources she will need
• Most males also share in provisioning, bringing food to the nest for the incubating female and
the young
• In herons, egrets, and a few species of woodpeckers, males also share incubation duties
• In ground-nesting birds like ducks and geese, males even risk their lives by defending the nest
against predators
• Genetic analyses of offspring have revealed some interesting facts about supposedly
monogamous birds
• Many of the young they are raising together are of mixed parentage, representing infidelity on
the part of one of the partners
• Extra pair copulations (epc’s) are relatively common
• EPC = 30-40% of the young of Indigo Buntings and White-crowned Sparrows
• EPC = 20% of Red-winged Blackbirds and over 50% of Tree Swallows
• Morton (1990) studied purple martins Progne subis, living in 24 compartment martin house in
his backyard
• Older males got there first in migration, settled into the upper floors - why?
• Birds defend 2 or 3 compartments, though only nest in one (male sleeps alone)
• Younger males arrive later in the season
• Older males take off at dawn, sing a special song whose only purpose is to lure in the juvenile
males
• Get them to take a lower berth, attract a mate…
• Older males averaged 4.5 eggs with their own mates
• Luring inexperienced males and moving in on their females = another 3.6 eggs!
• 43% of yearling offspring were from EPC’s with older males
• Yearlings only managed 8% EPC’s on older male’s mates
• Yearlings will mature, move uptown, start the same scam with the next crop of juveniles…
• Females seem to be willing participants
• If paired with older male, female will tend to avoid EPC’s
• If paired with a younger male, will readily accept EPC’s with older males…
• Only about 14% of monogamous birds are faithful partners in the usual sense…
• Rest resort to EPC’s to greater or lesser extent
• Genetic monogamy vs. social monogamy
• Although both partners cooperate to raise the young, there are both selfish and altruistic
aspects to monogamy
• Being selfish, each partner seeks to maximize their individual reproductive success by mating
with other partners
• Being altruistic, the pair cooperates to rear all the young in their own nest, regardless of
parentage
• Birds often incubate clutches that contain eggs of mixed parentage
• Cliff Swallows, Savannah Sparrows, Eastern Bluebirds, and Common Goldeneyes ex…
• Egg dumping - females lay their eggs in other nests of their own species
• Monogamy now viewed as less of a sentimental bond between lovers, more of a temporary
truce between selfish individuals coming together for a common goal
• Males are unable to monopolize females, and so must cooperate with a mate to ensure their
own reproductive success
• Selecting a mate for a long commitment requires the same sort of critical judgment about mate
quality that we use to select our lifelong partners
• And, like birds, we sometimes make bad choices and have to start all over
• Nothing lasts forever, not even diamonds, and divorce is as much a fact of life among
monogamous birds as it is among humans
• Fulmars have a divorce rate of about 4% per year, Mute Swans of less than 5% a year
• LeResche and Sladen (1970) tagged 37,000 Adelie Penguins, and observed that
> younger birds changed mates 44% of the time
> older birds only exchanged mates 16% of the time
• Not all birds are monogamous…
• Many species are polygamous, choosing more than one mate
• Of the two forms of polygamy, polygyny is relatively common and polyandry is relatively
rare
• Both polygyny and polyandry are expected to occur when
> either sex is able to monopolize mates
> either sex is able to monopolize critical resources that mates need
• Evolution of polyandry is an interesting unsolved problem
• Polyandry is found in less than 1% of bird species, mainly in some Gruiformes (Button
Quails, Roatelos, and a few rails) and in some Charadriiformes (Jacanas, Painted Snipes,
phalaropes, a few plovers)
• In some polyandrous species, role reversal is nearly complete
> females are larger with bright plumage
> females compete for males
> females are the aggressors in courtship
• In some polyandrous species, role reversal is nearly complete
> males are smaller and cryptically colored
> males incubate the eggs
> males drive females away from the nest
• Even Audubon was confused, and incorrectly labeled male and female phalaropes in his plates
• In North America, Red Phalaropes, Red-necked Phalaropes and Spotted Sandpipers show the
usual pattern = sequential polyandry
• Females end their relationship with a given male after the eggs are laid, to pursue a new male
• Lewis Oring’s long-term study of a population of Spotted Sandpipers (Actitis macularia)
nesting on an island in Minnesota
• Either sex can evolve the ability to monopolize access to mates or resources those mates need
(Oring et al. 1983)
• Female Spotted Sandpipers are the dominant sex, outweighing their wimpy mates by 25%
• Sandpipers, like many shorebirds, breed in the Arctic, taking advantage of the brief but
abundant populations of insects in Spring and Summer
• But Spotted Sandpipers breed in the temperate zone
• They take advantage of the much longer breeding season to lay additional clutches of eggs
• Maximum clutch size for these birds is four eggs per clutch, which equals about 80% of the
females’ body weight
• So females can’t increase reproductive success by increasing clutch size
• Additional young can only come from additional clutches
• Even worse, females only live about 3.7 years, and have low nesting success
• Because breeding ends in early July when insect populations start to decline, the total
breeding season is only 6-7 weeks
• Females could lay up to five clutches if they didn’t have to stop and incubate
• Females arrive first on the breeding grounds
• Females compete for arriving males with intense displays and frequent fights with other
females
• Once mated, they defend a territory that contains the territories of their male partners
• Males are in short supply to begin with
• Once males start to incubate, more and more males are tied up incubating eggs
• Eventually get up to 6 or 7 females per available male
• Even though females could potentially lay up to 20 eggs per season, they are lucky to lay 8
• The older and more experienced females get more mates, lay more eggs, and fledge more
young
• It’s harder to see what the payoff is for males in polyandry
• (Ligon 1993) maybe males, by freeing up females to lay extra clutches, hope to be able to
increase their own reproductive success by fertilizing some of those additional eggs
• Polygyny is more common in birds, though still limited to about 2% of all species
• Many Galliformes are polygynous (grouse, pheasants), ostriches, many species of Icteridae,
like Red-winged Blackbirds
• Many polygynous birds live in marsh habitats
• 11 of 14 North American polygynous species live in marshes or meadow habitats
• Females must make a very substantial energetic investment in eggs
• Females are also limited in how many eggs they can produce over their lifetime
• It is imperative that females select the very best male, and these are usually the males with the
highest quality territories
• But what if the best males are already taken?
• Then females are faced with a tough decision…
• Should they select an inferior male and hope for the best, or share a superior male with
someone else?
• Classic model of polygyny was formulated by Gordon Orians (1969)
• Orians proposed that there is a polygyny threshold (#2 on graph), a point past which it is in
the female’s best interests to share a male
• Polygyny threshold represents the minimum difference between the quality of male territories
• If the difference in territory quality is great enough, she will be better off sharing a male in a
better territory than pairing with an unmated male on an inferior territory
• Verner (1964, 1965) observed female Marsh Wrens frequently choose to share a male, even
though there are many bachelor males available
• The number of females held by each male correlates with the amount of emergent vegetation
on the male’s territory
• Amount of emergent vegetation correlates with insect abundance (54% ev = unmated males,
80% ev = monogamous, 95% ev = polygynous)
• Dickcissels, Red-winged Blackbirds, and Lark Buntings show similar relationships between
territory quality and mating success
• Lark Buntings prefer to nest in deep shade
• Shade is scarce in their grassland habitats, young birds often die from heat stress
• Male Lark Bunting’s with better territories can easily monopolize prime nesting spots
• Will accept a second female, who is allowed to mate and nest in the territory
• Meanwhile the male spends most of his time and energy helping his primary mate
• Polygyny also makes good sense if little effort is needed to raise the young
• Gallinaceous birds, with precocious young, are often polygynous
• American House Wrens are seldom polygynous (about 6% of matings), but European Wrens
are often polygynous (~50%)
• European Wrens commonly breed in English gardens, where food is abundant
• But on the Scottish island of St. Kilda, where food is very scarce, all the European Wrens are
monogamous
• It takes both parents to find enough food to raise chicks (Armstrong 1955)
• Savannah Sparrows are monogamous in the Arctic tundra, where the breeding season is too
short for one parent to find enough food
• But they are polygynous in temperate habitats where breeding seasons are longer, and there is
a longer window of food abundance
• Polygynous males form a temporary pair bond, and make at least some contribution, if only to
defend the territory while the female raises the young
• But in many polygynous birds, even this fragile pair bond is nonexistent
• The male contributes only his sperm and the energetic cost of his display
• Such males are promiscuous - about 6% of all bird species (Sage Grouse, Black Grouse, some
Birds of Paradise, bowerbirds)
• In promiscuous mating systems, males mate with any available female, but females still
choose their mates carefully