Download lecture 21 – mating systems ii

Matt Johnson
HSU Wildlife 365
Ornithology
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 21 – MATING SYSTEMS II
I.
Monogamy con’t
A. EPCs. Monogamy is not as monogamous as it sounds.
1. Many birds that appear monogamous, with overt pair bonds between 2
mates, actually copulate with other individuals.
2. Sometimes, these result from what are called "forced copulations." This is
especially common in waterfowl, where drake ducks will forcefully
copulate with hens. The males are behaving in a way to maximize their
reproductive output. The females may resist if they have chosen the best
mate they can, they their reproductive effort may be diluted by raising
offspring fathered by some other, less deserving male. [Note: book
erroneously calls this analogous to rape, but that’s horseshit. Human rape
is about male insecurity and wrongly perceived power, bird forced
copulations are about reproduction and maximizing fitness].
3. More often than these forced copulations, however, are EPC's between,
consenting adults, so to speak.
a. The advantages of EPCs to both sexes lies in the value of "riskspreading" and "free young"
i. Many nests just don't make it. Predators nail them. This is
especially bad for monogamous bird species that, quite literally,
have all their eggs in one basket. So, many birds of both sexes
seek extra-pair copulations.
ii. Also, remember that a bird can determine a baby birds genetic
lineage by looking at it. So, they have no idea which chicks really are
their own in the nest. So, by getting its genes into the nests of other
parents, a bird’s young may be raised with very little of its own
parental investment, but with significant investment from the
unknowing foster parents. Thus, both sexes benefit from these “free
young” – so both sexes seek extra-pair copulations.
b. But....they sneak them. Why? First, they need to get the genes into
the nest without both parents knowing. So a female will senak her egg
in the nest of another’s (called egg dumping), and a male will sneak a
copulation with another female (that “hopefully” will result in an egg –
this is called cuckoldry). In addition, birds attempt to make such
EPC’s unknown to their won mate…why? It all comes down to
investment.
c. Monogamy evolves when the parental effort of both parents is required
for the successful raising of young. Both sexes must contribute to nest
building, egg incubation, and/or chick rearing. They maximize their
reproductive effort by cooperating. The more a single mate in a pair
cheats, the more and more it is investing into other satellite nests and
the less and less into its primary nest.
d. Yet because cooperation is required to raise a brood, both males and
females seek to ensure that they choose a mate that will indeed invest
sufficiently into their reproductive effort. Thus, females "size up" the
males.....and males also must judge during courtship how likely he
alone will be the father of the chicks the cares for. So neither sex is
benefited by choosing mates that obviously cheats.
e. In short, there is a conflict. For BOTH sexes, an individual maximizes
its reproductive output by simultaneously doing two things: (1)
ensuring as much energy as possible is invested by itself and its mate
into the primary nest, but also (2) getting its genes into some other
nests just in case. Thus, natural selection operates such that each sex
is benefited by choosing a faithful mate, to which it is somewhat
unfaithful.
4. Cuckoldry and egg dumping.
a. Cuckoldry is the case in which a male raises a chick that he has not
fathered. It is common due to EPCs. Despite efforts to keep tabs on
his mate, his neighbors are trying to mate with her, and studies have
demonstrated that most nests contain one or more eggs not fathered by
the male of the nest.
b. Is there a benefit to the female in permitting cuckoldry? Depends on
her mate. She does not benefit much by risk-spreading....regardless of
who fathers her eggs in her nest...if it gets nailed by a predator, its
gone. However, if she has not been able to pick a male of very high
quality....then in behooves her to allow neighbors, which may be better
fit than her mate, to copulate with her.
5. A good example of cuckoldry is in Purple Martins.
a. Adult males return to breed first and choose the best sites in the
"apartments."
b. But then they start attracting inexperienced males as well as females.
c. They cuckold the young males like crazy!!!
d. The young males still benefit because without tagging along with the
elder males, they likely not reproduce at all.....and also they keep
returning to this apartment complex as a way to inherit the best spots
when the elders die....then THEY start cuckolding.
6. So a female benefits from permitting cuckoldry only if her neighboring
males are clearly better males than her own mate. But what can she do to
spread her risk and get free young via EPC’s? – Egg Dumping.
a. But she CAN spread her risk by laying eggs in the nests of other
individuals. This is called egg dumping, and is not to be confused
with nest parasitism (the former occurs within a species, that latter
between species).
b. Females often lay their eggs in the nests of other females when they
aren't watching......regardless of who is the father of those eggs, it
benefits the female.
c. Is there an advantage to males of letting neighboring female lay her
egg in his nest? If he’s mated with her you bet there is! Thus, females
in some species foster covert relationships with heir male neighbors so
that they can then better their chances of successfully egg dumping.
d. Is there an advantage to females in letting their nests gets dumped into
– hell no, they stop it if they can!
II.
A.
B.
C.
D.
E.
III.
A.
B.
IV.
A.
B.
C.
D.
Role of ecology on mating system determination.
Variations in mating systems reflect differences in the availability of resources
-- food, nest sites, etc.
Great example of this with African Weavers. Overhead.
Generally, where both sexes’ participation is essential for raising young -- and
if one sex cannot get enough resources by itself to support multiple nests -monogamy is the system.
As a female’s ability to care for her young alone increases, polygny becomes a
more viable option.
So, under what conditions can females raise offspring on their own?
1. If resources are super abundant -- they don't need males, and thus males
can maximize the repro output by mating with as many females as
possible.
a. Birds that nest in productive marshes are an example -- redwing
blackbirds are polygynous. Each male defends a territory -- many
females choose that territory, and the male fathers most of their
offspring. Of the 14 species of songbirds that are polygamous in NA,
11 nest in freshwater marches.
b. Tropical fruit-eating birds often have abundant resources, or at least
predictable and easy to find. They are also often polygynous.
2. Also, if the young are able to feed themselves, then they require little
investment (after hatching that is), and polygyny may result. 17% of
species precocial young are polygynous or promiscuous, only 7% of
species with altricial young are polygynous.
Polygyny.
Occurs often where territory quality varies dramatically, as in freshwater
marshes. Males with the best territories attract the most females.
But why should a female mate with an already paired male? Because she can
do better mating with a polygynous male on a good territory than a faithful
monogamous male on a poor territory -- food is so abundant that they can feed
their own young on a good territory, they don't NEED the male.
Polyandry.
How these systems have evolved remains mysterious.
The sandpipers are a good example. 15 species are monogamous, 3 are
polyandrous, and 3 are polygynous.
Linking these differences to species differences in ecology, length of breeding
season, predictability of food, etc. have not been convincing.....so why?
Still not sure. One possible explanation is that an evolutionary quirk in their
history makes it impossible, physiologically, for a sandpiper to lay more than
four eggs in a single clutch, whereas other birds' clutch sizes are more flexible
over evolutionary time. Consequently, although many have evolved variances
in clutch size in correspondence to environmental variation, sandpipers have
not been able to do so. So, natural selection has operated instead to produce a
bewildering variation in mating systems that, ultimately, is linked to
ecological differences in the species, but we have yet to articulate them.
V.
Leks. -- Leks are courtship arenas that contain no important resources, but
serve to congregate the population.
A. What are they? -- Leks are areas where females come for one thing -fertilization. The males, in turn, compete intensively for opportunity to
copulate, creating a conspicuous dominance hierarchy. In many situations, a
handful of males will achieve 70-80% of al the matings.
B. Evolution of leks. Why should males gather in leks such that only a few males
wind up reproducing? Three hypotheses.
1. Reduced predation hypothesis. In species where competition for females
has resulted in elaborate display dances, the males may put themselves at
risk by dancing and not remaining vigilant of predators. They can reduce
this risk by congregating, so that predator detection and avoidance is
maximized in the group. Some support of this in that open country
promiscuous birds are lekers, forest birds are not as often.
2. Hot Spot hypothesis -- males need to be "chosen" to maximize the
breeding success. So they congregate in areas frequently used by females
to maximize their chances of being seen. Perhaps, but in many cases the
lekking grounds are nearly devoid of resources....so females would not be
frequenting these areas w/o the males.
3. Hot shot hypothesis -- females are more likely to choose mates from large
aggregations of males because they can more easily compare them. Thus,
to even have a chance, the solitary males need to join leks.
4. Satellite displayers -- There is a wide variation in display abilities in
males. The subordinate (often young ones) just don't do it well. So, they
probably won't get chosen. But by joining a lek on the periphery as a
satellite male, the benefit for two reasons:
a. They can watch and learn - to maximize future repro opportunities.
b. They can perhaps get lucky and engage in a forced or sneak
copulation.
C. Cooperative leks.
A. This satellite male hypothesis is most prevalent in what are called
cooperative leks, where a group of males perform a single display dance,
but only one males actually copulates.
B. Why do the dancers even bother? Because without cooperation, they have
NO chance of breeding at all. With cooperation they can (1) learn the
dance for future...and (2) if the lead dancer gets nailed by a predator, they
can take over his position. Often the secondary dancers are young birds,
and in cooperative lekking species, they often take several years to
develop adult plumage. They wear their age.