Evolution of parental care
 Parental
care does not always take place.
In many species (e.g. clams, barnacles,
many fish) eggs are shed into the water
and abandoned.
 Similarly,
turtle young are on their own
once they hatch
Costs and benefits of parental care
 The
decision to offer parental care
depends on whether such care will
increase the caregiver’s lifetime
reproductive success.
 Greater
investment in individual young
necessarily reduces the number of young
that can be produced.
Costs and benefits of parental care
 Consequently,
species choose between
producing many, small, uncared for young
or fewer, larger, cared for young.
 Whales
and humans represent one end of
the continuum and barnacles and clams
the other.
Costs and benefits of parental care
 If
parental care enhances survival and
growth of young enough to compensate
for the reduction in young produced then
we would expect parental care to evolve.
Costs and benefits of parental care
 Obviously,
one constraint of parental care
is the ability the parent has to affect the
offspring’s survival.
 Barnacles produce many thousands of
eggs which are shed into the water and
drift away. They develop into larvae and
one day settle permanently on a fixed
substrate.
Costs and benefits of parental care
 Barnacles
are sessile and can do nothing
to actively assist their young.
 Not
surprisingly, barnacles have not
evolved parental care.
Costs and benefits of parental care
 Parental
care in organisms that can give it
may significantly enhance the prospects of
the offspring surviving to adulthood.
 For
example, higher bodyweight at
fledging significantly increases a small
birds chances of surviving to adulthood.
Costs and benefits of parental care
 Extra
investment (i.e. the parent’s working
harder to supply food) comes at a cost
though as it may reduce the parent’s
prospects of surviving over the winter.
 This
effect has been documented in many
studies in which brood sizes of parents
were increased.
Costs and benefits of parental care
 The
costs associated with increased
investment in a given offspring cause
parents to limit the investment they make
so as to increase their prospects of
survival and also to allow them to invest in
future offspring.
 This decision causes parent offspring
conflict.
Parent-offspring conflict.
 In
many species parents invest huge
quantities of resources in their offspring.
 Initially,
both parent and offspring agree
that investment in the offspring is
worthwhile because it enhances the
offspring’s prospects of survival and
reproduction.
Parent-offspring conflict.
 However,
a parent shares only 50% of its
genes with the offspring and is equally
related to all of its offspring, whereas the
offspring is 100% related to itself, but only
shares 50% of genes with full siblings (and
less with half-siblings).
Parent-offspring conflict.
 As
a result, at some point, a parent will
probably prefer to reserve investment for
future offspring rather than investing in the
current one, while the current offspring will
disagree.
 When
might parent be prepared to sink all
its effort into a current offspring?
Parent-offspring conflict.
 This
leads to a period of conflict called
weaning during which the offspring tries to
acquire resources and the parent attempts
to withhold them.
Parent-offspring conflict.
 The
period of weaning conflict ends when
both offspring and parent agree that future
investment by the parent would be better
directed at future offspring rather than to
the current offspring.
 For full siblings, this is when the benefit to
cost ratio drops below ½.
Fig 11.18
Figure shows B/C benefit to cost ratio of investing in the current offspring.
Benefit is measured in benefit to current offspring and cost is measured
in reduction in future offspring.
Parent-offspring conflict
 In
instances where parents produce only
half siblings, we should expect weaning
conflict to last longer because the current
offspring is les closely related to future
offspring.
 This
has been confirmed in various field
studies.
Costs and benefits of parental care
 In
general, the willingness of a parent to
invest in or take risk for an offspring
should be influenced by (i) the parent’s
future prospects of reproducing and (ii) the
relative value of the current offspring.
 This
is borne out by studies of the
behavior of long-lived versus short-lived
birds.
Costs and benefits of parental care
 In
general, one would predict that longlived birds should be less willing to risk
their lives to protect their young, but that
short-lived birds should be more willing to
do so.
Costs and benefits of parental care
 In
general, North American birds are
shorter lived than comparable South
American species.
 Ghalambor
and Martin (2001) compared
the behavior of matched pairs of North and
South American birds to evaluate the birds’
willingness to take risks on behalf of their
young.
Costs and benefits of parental care
 E.g.
compared American Robin to
Argentinian Rufous-bellied Thrush.
 When
researchers played tapes of Jays
(which raid nests) near the birds’ nests
both species avoided returning to the nest,
but robins reduced their activity more.
Consistent with robins being less willing to
risk the current brood.
Costs and benefits of parental care
 When
a stuffed Sharp-shinned Hawk (a
predator of adults) was placed near the
nest and calls played, again both species
avoided visiting the nest, but this time the
Rufous-bellied Thrushes reduced their
visits more.
Costs and benefits of parental care
 These
results suggest that the thrushes
were less willing to risk their lives by
feeding the current brood.
 Selection
on robins and thrushes thus
appears to have fine-tuned behavior to
take account of costs and benefits of risktaking behavior.
Maternal parental care
 In
general maternal parental care is more
common than paternal care.
 In
some instances maternal care is a
result of internal fertilization and the delay
between mating and birth.
Fig 12.1A
Maternal parental care
 Other
general reasons for maternal care
being more common focus on the relative
costs to the two sexes of being the
caregiver.
 For
males there is uncertainty about
paternity, which will reduce the benefit to
cost ratio of engaging in parenting.
Maternal parental care
 In
addition, for males when there are
opportunities to mate with multiple
females, males that give up that
opportunity to engage in parental care will
pay too high a price.
 Paternal
care (either with the female or
alone) would be selected for only when the
payoff is sufficient to outweigh the costs.
Paternal Care
 In
fish male parental care is quite
common. Many males mouth brood eggs
or care for eggs in nests.
 Costs
of parental care seem to be lower
for males than for females.
Paternal Care
 Male
sticklebacks can care for 10 clutches
of eggs at once.
 Males
grow more slowly when caring for
young, but because males are territorial
and cannot range widely to look for food
the additional cost of parental care is low.
Paternal Care
 For
a female stickleback parental care
would severely limit her ability to forage
and grow.
 Because
body size is closely correlated
with egg production loss of foraging
opportunities would have a significant
effect on future reproduction.
Paternal Care
 Because,
in many fish, costs of parental
care are higher for females than they are
for males, paternal care may have evolved
because males lose less from parental
care than females do.
Discriminating Parental Care
 Misdirecting
parental care towards nonoffspring obviously would be a costly
mistake for any organism.
 Many
animals rear their young in colonies
and there is plenty of opportunity for
confusion.
Fig 12.7
Young free-tailed bats at a creche.
Discriminating Parental Care
 Mexican
free-tailed bats use vocal and
olfactory cues to identify their offspring
from among thousands in the creche.
 The bats do occasionally make mistakes
but the benefits of leaving a baby in a
creche (mainly thermoregulatory) appear
to outweigh the cost.
Discriminating Parental Care
 Cliff
swallows, often nest in large colonies,
and their young produce much more
variable calls than do Barn Swallows,
which generally nest solitarily.
 Cliff
Swallow parents are also much better
at distinguishing between calls than are
Barn Swallows.
Fig 12.9
Discriminating Parental Care
 Similarly,
the young of colonial Bank
Swallows produce distinctive vocalizations
that their parents can easily recognize, but
the non-colonial Rough-winged Swallow
does not.
Adoption
 Obviously,
it would appear beneficial to
avoid adopting other individual’s offspring,
but such adoptions sometimes happen.
 In
colonially nesting gulls chicks that have
been poorly fed in their own nests
sometimes leave their natal nest and join
another brood, where they often are
adopted.
Adoption
 Moving
is often a good decision for the
chick because it may end up being better
cared for in a different nest.
 However,
adoptive parents on average
lose 0.5 young of their own as a result of
the adoption so why do they tolerate the
intruder?
Adoption
 Most
likely explanation is that parents use
an imperfect behavioral when deciding
who to feed.
 Any chick that begs confidently is
accepted and fed.
Adoption

The reason that gulls do not discriminate more is
probably that recognition errors would be too
costly.
 Errors in which a gull fails to feed or worse
attacks and kills its own chick because it thinks it
is a stranger would be very costly.
 The cost of occasional adoptions appears to be
low enough that selection has not favored higher
levels of discrimination in gulls.
Adoption
 In
some instances adoption appears not to
come with a cost and may be beneficial to
the adopter.
 It
is common in ducks for females to
accept extra eggs laid in their nests and to
accept stray ducklings into their broods.
Adoption
 In
ducks there is little or no cost to
adoption because chicks forage for
themselves.
 The
benefit appears to be that there is a
predator dilution effect with larger broods.
Additional young in the brood reduce the
odds of a chick taken from the brood being
the parents own young.
Brood parasitism
 There
are several species of birds that are
obligate interspecific brood parasites.
 These
include Old World Cuckoos, Old
World Honeyguides and New World
Cowbirds.
 These
birds lay their eggs in the nests of
other birds and provide no parental care.
European Cuckoo removing host’s egg
Brood parasitism
 Based
on phylogenetic analyses brood
parasitism appears to have evolved
independently three times in the cuckoos
and a large number of cuckoos (53 of 136
species) are brood parasites.
Obligate brood parasites
indicated in blue.
Occasional parasites
in red.
Brood parasitism

Interspecific brood parasitism is believed to have
originated as intraspecific brood parasitism.
 Intraspecific brood parasitism is common in birds
and has been recorded in more than 200
species.
 A plausible transition to interspecific brood
parasitism would be for birds to begin laying
eggs in the nests of closely related species.
Brood parasitism
 Today
cuckoos concentrate on species
that are not closely related to them, but as
parasitism in cuckoos may be 60 million
years old this may simply reflect the long
period of evolution that has occurred since
the origin of the behavior.
Brood parasitism
 In
cowbirds, which much more recently
evolved brood parasitism (in past 3-4
million years) the living species believed
most like the ancestral parasite parasitizes
only one other species and that belongs to
its own genus.
 Since
then increasingly general brood
parasitism appears to have evolved.
Brood parasitism
 Brood
parasites have a significant effect
on the reproductive success of the hosts.
 Baby
cuckoos eject the eggs and young of
the host so the host rears no young of its
own.
Brood parasitism

Brood parasites exploit the host parents
tendency to feed the largest young in a brood
the most food and to reward the young that can
reach highest for food.

By laying in the nests of smaller birds, cuckoos
give their young an advantage in the competition
for food. So do cowbirds whose eggs hatch
after a shorter incubation period which allows
them to hatch before the host’s young.
Brood parasitism
 The
advantage of laying in the nests of
smaller species has been shown in
experiments in which nestlings of nonparasitic Great Tits and Blue Tits were
switched between nests.
 The smaller Blue Tits did badly in Great Tit
nests, but Great Tits prospered in Blue Tit
nests.
Why tolerate parasite’s eggs?
 Given
the heavy costs of rearing a
parasite, why don’t hosts reject parasitic
eggs?
 Rejection
also comes with costs so costbenefit analysis is needed.
Why tolerate parasite’s eggs?
 Some
birds do recognize parasitic eggs
and remove them from the nest. However,
there is a risk that the host will discard one
or more of its own eggs in error.
 Reed
Warblers have been shown to make
this mistake.
Why tolerate parasite’s eggs?
 Accepting
a parasite’s egg is even more
likely to be adaptive when the host is too
small to remove the parasitic egg.
 Such
hosts must either accept the egg or
abandon the nest, which is an expensive
option, especially if nest sites are scarce
(e.g. as in cavity nesters).
Why tolerate parasite’s eggs?
 Consistent
with this hypothesis,
Prothonotary Warblers parasitized by
cowbirds are much more likely to abandon
their nest if there are alternative nest sites
on the male’s territory.
12.18
Why tolerate parasite’s eggs?
 Similarly,
Yellow Warblers parasitized near
the end of the breeding season tend to
accept parasitic eggs, presumably
because there is too little time to start
over.
Why tolerate parasite’s eggs?
 Another
reason for hosts to tolerate
parasite eggs is that the parasite may
monitor the nest and harm the host’s nest
if its egg is removed.
 This
“Mafia hypothesis” has been
supported by studies of Great Spotted
Cuckoos and their Magpie hosts.
Why tolerate parasite’s eggs?
 Magpie
nests from which cuckoo eggs
were ejected suffered a much higher rate
of predation (87%) than nests that
accepted cuckoo eggs (12%).
 Threatening
the clutch of the hosts
appears to be an effective strategy
because renesting is costly in the magpies’
seasonal environment.
Arms race between hosts and
parasites.
 As
selection operates on both hosts and
parasites the differing selection pressures
have resulted in an arms race between
hosts and parasites.
 In the case of the European cuckoo and its
hosts selection has led to extremely good
mimicry of host eggs.
Arms race between hosts and
parasites.
 Individual
cuckoos specialize on one host
species and lay eggs that closely mimic
only that species’ eggs.
 Historical
interactions between cuckoos
and some hosts appear to have resulted in
victory for the host.
Arms race between hosts and
parasites.
 E.g.,
European blackbirds are rarely
parasitized by cuckoos and even though
under no current selection pressure, these
birds reject parasitic eggs at a very high
frequency.
 Apparently, blackbirds evolved rejection
behavior in the past and cuckoos have
moved on to other host species.
Arms race between hosts and
parasites.

With many other species race arms-race
between parasites and hosts is ongoing.
Horsfield’s Bronze-cuckoo parasitizes the
Superb Fairy Wren.
 Fairy-wrens respond to cuckoo eggs laid before
they have started laying by abandoning nest or
building over the egg. They also abandon if
cuckoo lays egg after incubation has begun

Arms race between hosts and
parasites.
 Bronze-cuckoos
have responded by
inserting eggs during fairy-wren laying
period. Such eggs are generally accepted
and incubated.
 However,
when young cuckoo pushes
young wrens out of nest, fairy-wrens
abandon the nest about 40% of the time
and cuckoo starves.
Arms race between hosts and
parasites.
 In
other cases cuckoo appears to fool
parents into believing their sole chick is a
fairy-wren.
 An
important factor in the chick’s ability to
fool the fairy-wren parents is its ability to
mimic the begging call of young fairywrens.
Chicks of Horsfield’s Bronze-cuckoo
closely mimic calls of host fairy-wrens.
Shining Bronze Cuckoo which rarely
parasitizes fairy wrens does not
resemble fairy-wren chick and does not
sound like it either.
Arms race between hosts and
parasites.

Another example of the use of calls in the arms
race between parasites and hosts is that of calls
by European Cuckoo chicks in Reed Warbler
nests.

The rate at which cuckoos call simulates that of
a whole brood of Reed Warblers which
encourages parents to feed at a much higher
rate than they otherwise would.
Blackbird chick placed in nest. Control is delivery rate when
no tape played. Cuckoo is delivery rate when cuckoo begging call
played. Reed warbler brood is delivery rate when tape of
a brood of reed warblers in played.
Parental favoritism and siblicide
 Parents
may be related to all of their
offspring equally, but often do not treat
them equally well.
 In
many cases parents actively
discriminate against certain offspring and
either allow them to starve or allow their
siblings to kill them.
Parental favoritism and siblicide
 For
example, in African Black Eagles the
first hatched of two chicks attacks its
younger sibling as soon as it hatches and
pecks it to death.
 Similarly
in egrets, boobies, pelicans and
other birds older siblings attack and drive
younger offspring out of the nest where
they starve to death.
Young great egrets
fight while their parent
ignores the behavior.
At a Brown Booby nest the
older chick (under its parent)
has driven its smaller sibling from
The nest where it will die of
exposure and starvation.
Parental favoritism and siblicide
 Despite
the fact some of their offspring are
being killed, parents seem not only to
tolerate the behavior, but to actively
encourage it.
Parental favoritism and siblicide
 For
example, in Black Eagles incubation
begins as soon as the first egg is laid.
 As
a result the first egg hatches 3-7 days
before the second and so the older
offspring has a huge size advantage over
its younger sibling and can easily kill him.
Parental favoritism and siblicide
 Such
hatching asynchrony is very
common among birds and results in
hatching asynchrony, which establishes an
age and size hierarchy within the brood.
 Birds
do not have to hatch their young
asynchronously and many birds (e.g.
ducks) even though they lay large clutches
hatch their young synchronously.
Parental favoritism and siblicide
 In
cattle egrets (and other birds) in
addition to promoting hatching
asynchrony, parents spike the earlier laid
eggs with high doses of androgens (male
hormones).
 The hormones make the earliest hatched
chicks more aggressive and gives them an
extra advantage over later hatched chicks.
Parental favoritism and siblicide
 Why
do parents play favorites and
facilitate siblicide?
 There


are two major reasons:
Insurance against failure
Environmental uncertainty
Insurance
 The
most extreme form of brood reduction
is obligate brood reduction in which
younger offspring essentially always die.
 Examples
of obligate brood reducers
include: Black Eagles, Harpy Eagles,
Giant Pandas, and Hooded Grebes.
Insurance
 These
animals have no intention of rearing
more than a single offspring.
 The
second offspring represents an easily
cancelled insurance policy against the
failure of the first offspring to hatch or
develop normally.
Insurance

When the first offspring arrives it kills its sibling
(Black Eagle), the parents cover over the second
egg (Harpy Eagles), the parents abandon the
second egg (Hooded Grebes) or abandon the
second born cub (Giant Pandas).
 Thus, the parents avoid prolonged investment in
a back-up offspring. However, if the first
offspring fails the second can step in and take its
place.
Insurance
 Why
don’t these animals go ahead and
rear the second baby once it arrives?
 In
many cases parents would appear to be
capable of rearing two young, but don’t do
so? Why not?
Trade-offs

Because there are trade-offs between offspring
number and quality as well as between offspring
number and parental future reproductive
success.

For these species it is usually not possible to
provide enough food to rear two high-quality
young. Pandas feed on low quality food and the
burden of providing milk for two cubs is too
much for most mothers. Two weakling offspring
are worse than a single sturdy cub.
Trade-offs
 In
addition, extra effort invested in trying to
rear two young in a season generally
reduces future reproductive success by
reducing lifespan and ability to produce
eggs or babies.
Environmental uncertainty
 Many
other species are facultative brood
reducers which means that brood
reduction does not always occur.
 These species practice a policy of parental
optimism.
 They lay a clutch size that can be reared
in a good year, but in a bad year will result
in brood reduction.
Environmental uncertainty
 In
these species the brood contains two
classes of offspring: core and marginal
offspring.
 Marginal offspring are handicapped by the
parents and as in obligate brood reducers
have insurance value, but mainly are
produced so that parents can take
advantage of a good year if one occurs to
rear bonus offspring.
Environmental uncertainty
 Consistent
with this idea, in facultative
brood reducers, the handicap the parents
create is enough to create a clear
hierarchy in the brood but not so great that
it cannot be overcome.
Environmental uncertainty
 Thus,
in cattle egret broods the effects of A
and B chicks (the eldest chicks)
aggression towards younger C and D
chicks is moderated by food supply.
 If food is plentiful, the younger chicks can
tolerate the beating and may survive to
fledge. If food is scarce the younger
chicks quickly starve or are driven out of
the nest and die.
Environmental uncertainty
 The
cattle egret parents’ policy of hatching
asynchrony thus creates a situation in
which in good years conditions can be
taken advantage of and extra babies
reared, but in bad years the brood can be
efficiently reduced to what foraging
conditions will support.
Environmental uncertainty
 The
amount of asynchrony in cattle egret
broods appears to have been tailored by
natural selection to maximize parents
reproductive success and efficiency in
rearing babies.
 Artificially synchronized nests produced
fewer survivors and required more food
because offspring fought more and so
expended more energy.
Environmental uncertainty
 Nests
in which asynchrony was
exaggerated produced similar numbers of
young as normally asynchronous nests,
but brood reduction took place at younger
ages, which may limit the ability of the
parents to rear large broods in good years.
Brood reduction and humans
 Discussions
of brood reduction are
applicable to humans also.
 Twin
births are rare, but twin conceptions
are much commoner and only one in ten
to one in fifty twin conceptions produce
twins. The other pregnancies result only
in singleton births.
Brood reduction and humans
 This
phenomenon has been dubbed the
“vanishing twin” syndrome.
 Part
of the phenomenon may be that
producing extra eggs is an insurance
strategy against pregnancy failure due to
defective embryos. Some of these early
embryos have chromosomal defects and
are quietly aborted by the mother.
Brood reduction and humans
 This
idea is supported by the fact that
older women are more likely to give birth
to twins than younger women.
 Apparently
this is because older women
polyovulate more than younger women
and older women are much more likely to
produce chromosomally defective eggs.
Brood reduction and humans
 It
is possible that other cases of
disappearing twins may not be caused by
an embryo failing because it is defective,
but from a “decision” by the mother to
reduce her brood, although a mechanism
by which this might occur has not been
identified.
Evaluating the reproductive
value of offspring
 As
we have seen not all offspring are
created equal and even in the absence of
parental manipulation of quality we would
expect parents to assess offspring quality
when deciding how to allocate scarce
resources.
Evaluating the reproductive
value of offspring
 It
has been suggested that the gape color
of baby birds may signal the quality of their
immune system and thus offspring quality
 Red
gape color is produced by carotenoid
pigments in the blood and these are
believed to enhance immune function.
Evaluating the reproductive
value of offspring
 In
an experiment on barn swallows in
which chicks gapes were colored with food
coloring chicks whose gapes were
reddened received more food, although
chicks whose gapes were yellowed did not
receive less food.
Evaluating the reproductive
value of offspring
 Alternative
explanations for the role of
gape coloration have been put forward,
however.
 An
obvious alternative is that parents are
not assessing offspring quality, but just
feeding those chicks whose gapes are
more conspicuous under the prevailing
lighting conditions.
Evaluating the reproductive
value of offspring
 Consistent
with this idea Great Tit chicks
whose mouths were painted yellow
received more food than chicks whose
mouths were painted red and were less
conspicuous in a dark nest box.
 When
a plexiglass lid was placed on the
nest box however, both sets of chicks
were equally well fed.
Evaluating the reproductive
value of offspring
 Although
gape color may not signal
offspring quality or value other traits may.
 Coots
have an unpleasant way of reducing
brood size. They peck certain babies in
their brood when they beg for food and
these ones quickly die.
Evaluating the reproductive
value of offspring
 Baby
coots have prominent long orange
tipped plumes on their backs and throats
and these may be a cue parents use in
deciding which chicks they wish to feed.
 When
these plumes were trimmed from
half the members of a brood the unaltered
members of the brood received more food
and grew faster than the trimmed birds.
Evaluating the reproductive
value of offspring
 Control
broods in which all birds were
trimmed survived as well as broods in
which no chicks were trimmed.
 Coots thus appeared to discriminate
against trimmed chicks because they
lacked orange plumes not because they
could not recognize them.
 Thus, it may be that the orange plumes
are a signal of offspring quality.
Magpie assessment of offspring
value
 Magpie
young are increasingly likely to
survive as they age. Thus their value
increases and one would expect parents
to value them more.
 Consistent
with this parents are more
likely to engage in defensive behavior
when a predator approaches a nest if the
brood is older.