Evolution of Parental Care

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					    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.
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
Costs and benefits of parental care
Barnacles are sessile and cannot 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
      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

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.
      Parent-offspring conflict
In instances where parents produce only
  half siblings, we should expect weaning
  conflict to last longer because the current
  offspring is less closely related to future

This has been confirmed in various field
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 long-lived
  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

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.
Fig 12.1A
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 risk-
 taking 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.
       Maternal parental care
Other general reasons for maternal care
 being more common focus on the relative
 costs to the two sexes of being the

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

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 non-
 offspring obviously would be a costly
 mistake for any organism.

Many animals rear their young in colonies
 and there is plenty of opportunity for
              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
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.
Obviously, it would appear beneficial to
 avoid adopting other individual’s offspring,
 but such adoptions sometimes happen.

In colonial 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
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

Most likely explanation is that any chick that
 begs confidently is accepted and fed.
The reason that gulls do not discriminate more is
  probably that recognition errors would be too
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.
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.
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

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
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
            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 non-
 parasitic 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
 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 cost-
 benefit 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 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.
 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
 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

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 re-
 nesting is costly in the magpies’ seasonal
   Arms race between hosts and
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
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
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
Apparently, blackbirds evolved rejection
  behavior in the past and cuckoos have
  moved on to other host species.
    Arms race between hosts and
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
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
In other cases cuckoo appears to fool
  parents into believing their sole chick is a

An important factor in the chick’s ability to
 fool the fairy-wren parents is its ability to
 mimic the begging call of young fairy-
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
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

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
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
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

There are two major reasons:
     Insurance against failure
     Environmental uncertainty
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.
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.
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.
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?
Because there are trade-offs between offspring
  number and quality as well as between offspring
  number and parental future reproductive

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.
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
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
     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
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.
   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
   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,

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
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.

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