evolution of parental care parental care does not always take place. in many species (e.g. clams,...

Evolution of Parental CareEvolution of Parental Care

Parental care does not always take place. Parental care does not always take place. In many species (e.g. clams, barnacles, In many species (e.g. clams, barnacles, many fish) eggs are shed into the water many fish) eggs are shed into the water and abandoned.and abandoned.

Costs and benefits of parental careCosts and benefits of parental care

The decision to offer parental care depends The decision to offer parental care depends on whether such care will increase the on whether such care will increase the caregiver’s lifetime reproductive success.caregiver’s lifetime reproductive success.

Greater investment in individual young Greater investment in individual young necessarily necessarily reducesreduces the number of young the number of young that can be produced.that can be produced.


Consequently, species choose between Consequently, species choose between producing many, small, uncared for young producing many, small, uncared for young or fewer, larger, cared for young.or fewer, larger, cared for young.

Whales and humans represent one end of Whales and humans represent one end of the continuum and barnacles and clams the continuum and barnacles and clams the other. the other.


If parental care If parental care enhancesenhances survival and survival and growth of young enough to compensate growth of young enough to compensate for the reduction in young produced then for the reduction in young produced then we would expect parental care to evolve.we would expect parental care to evolve.


Obviously, one constraint of parental care is Obviously, one constraint of parental care is the ability the parent has to affect the the ability the parent has to affect the offspring’s survival.offspring’s survival.

Barnacles produce many thousands of eggs Barnacles produce many thousands of eggs which are shed into the water and drift which are shed into the water and drift away. They develop into larvae and one away. They develop into larvae and one day settle permanently on a fixed day settle permanently on a fixed substrate.substrate.


Barnacles are sessile and cannot do nothing Barnacles are sessile and cannot do nothing to actively assist their young. to actively assist their young.

Not surprisingly, barnacles have not evolved Not surprisingly, barnacles have not evolved parental care.parental care.


Parental care in organisms that can give it Parental care in organisms that can give it may significantly enhance the prospects of may significantly enhance the prospects of the offspring surviving to adulthood.the offspring surviving to adulthood.

For example, For example, higher bodyweight at higher bodyweight at fledgingfledging significantly increases a small significantly increases a small birds chances of surviving to adulthood. birds chances of surviving to adulthood.


Extra investment (i.e. the parent’s working Extra investment (i.e. the parent’s working harder to supply food) comes at a cost harder to supply food) comes at a cost though as it may though as it may reduce the parent’s reduce the parent’s prospects of surviving over the winterprospects of surviving over the winter..

This effect has been documented in many This effect has been documented in many studies in which brood sizes of parents studies in which brood sizes of parents were increased.were increased.


The costs associated with increased The costs associated with increased investment in a given offspring cause investment in a given offspring cause parents to limit the investment they make parents to limit the investment they make so as to increase their prospects of so as to increase their prospects of survival and also to allow them to invest in survival and also to allow them to invest in future offspring.future offspring.

This decision causes This decision causes parent offspring parent offspring conflict.conflict.

Parent-offspring conflict.Parent-offspring conflict.

In many species parents invest huge In many species parents invest huge quantities of resources in their offspring. quantities of resources in their offspring.

Initially, both parent and offspring agree that Initially, both parent and offspring agree that investment in the offspring is worthwhile investment in the offspring is worthwhile because it enhances the offspring’s because it enhances the offspring’s prospects of survival and reproduction.prospects of survival and reproduction.


However, a parent shares only 50% of its However, a parent shares only 50% of its genes with the offspring and is equally genes with the offspring and is equally related to all of its offspring, whereas the related to all of its offspring, whereas the offspring is 100% related to itself, but only offspring is 100% related to itself, but only shares 50% of genes with full siblings (and shares 50% of genes with full siblings (and less with half-siblings).less with half-siblings).


As a result, at some point, a parent will As a result, at some point, a parent will probably prefer to reserve investment for probably prefer to reserve investment for future offspring rather than investing in the future offspring rather than investing in the current one, while the current offspring will current one, while the current offspring will disagree. disagree.

When might parent be prepared to sink all When might parent be prepared to sink all its effort into a current offspring?its effort into a current offspring?


This leads to a period of conflict called This leads to a period of conflict called weaningweaning during which the offspring tries to during which the offspring tries to acquire resources and the parent attempts acquire resources and the parent attempts to withhold them.to withhold them.


The period of weaning conflict ends when The period of weaning conflict ends when both offspring and parent agree that future both offspring and parent agree that future investment by the parent would be better investment by the parent would be better directed at future offspring rather than to directed at future offspring rather than to the current offspring. the current offspring.

For full siblings, this is when the benefit to For full siblings, this is when the benefit to cost ratio drops below ½.cost ratio drops below ½.

Parent-offspring conflictParent-offspring conflict

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

This has been confirmed in various field This has been confirmed in various field studies.studies.


In general, the willingness of a parent to invest in In general, the willingness of a parent to invest in or take risk for an offspring should be influenced or take risk for an offspring should be influenced by (i) the parent’s future prospects of by (i) the parent’s future prospects of reproducing and (ii) the relative value of the reproducing and (ii) the relative value of the current offspring.current offspring.

This is borne out by studies of the behavior of This is borne out by studies of the behavior of long-lived versus short-lived birds.long-lived versus short-lived birds.


In general, one would predict that long-lived In general, one would predict that long-lived birds should be birds should be lessless willing to risk their willing to risk their lives to protect their young, but that short-lives to protect their young, but that short-lived birds should be more willing to do so.lived birds should be more willing to do so.


In general, North American birds are shorter In general, North American birds are shorter lived than comparable South American lived than comparable South American species.species.

Ghalambor and Martin (2001) compared the Ghalambor and Martin (2001) compared the behavior of matched pairs of North and behavior of matched pairs of North and South American birds to evaluate the South American birds to evaluate the birds’ willingness to take risks on behalf of birds’ willingness to take risks on behalf of their young.their young.


E.g. compared American Robin to E.g. compared American Robin to Argentinian Rufous-bellied Thrush.Argentinian Rufous-bellied Thrush.

When researchers played tapes of Jays When researchers played tapes of Jays (which raid nests) near the birds’ nests (which raid nests) near the birds’ nests both species avoided returning to the nest, both species avoided returning to the nest, but robins reduced their activity more. but robins reduced their activity more. Consistent with robins being less willing to Consistent with robins being less willing to risk the current brood. risk the current brood.

Fig 12.1A


When a stuffed Sharp-shinned Hawk (a When a stuffed Sharp-shinned Hawk (a predator of adults) was placed near the predator of adults) was placed near the nest and calls played, again both species nest and calls played, again both species avoided visiting the nest, but this time the avoided visiting the nest, but this time the Rufous-bellied Thrushes reduced their Rufous-bellied Thrushes reduced their visits more.visits more.


These results suggest that the thrushes These results suggest that the thrushes were less willing to risk their lives by were less willing to risk their lives by feeding the current brood.feeding the current brood.

Selection on robins and thrushes thus Selection on robins and thrushes thus appears to have fine-tuned behavior to appears to have fine-tuned behavior to take account of costs and benefits of risk-take account of costs and benefits of risk-taking behavior.taking behavior.

Maternal parental careMaternal parental care

In general In general maternalmaternal parental care is more parental care is more common than paternal care.common than paternal care.

In some instances maternal care is a result In some instances maternal care is a result of internal fertilization and the delay of internal fertilization and the delay between mating and birth.between mating and birth.


Other general reasons for maternal care Other general reasons for maternal care being more common focus on the relative being more common focus on the relative costs to the two sexes of being the costs to the two sexes of being the caregiver.caregiver.

For males there is For males there is uncertaintyuncertainty about about paternity, which will reduce the benefit to paternity, which will reduce the benefit to cost ratio of engaging in parenting.cost ratio of engaging in parenting.


In addition, for males when there are opportunities In addition, for males when there are opportunities to mate with multiple females, males that give up to mate with multiple females, males that give up that opportunity to engage in parental care that opportunity to engage in parental care will will pay too high a price.pay too high a price.

Paternal care (either with the female or alone) Paternal care (either with the female or alone) would be selected for only when the payoff is would be selected for only when the payoff is sufficient to outweigh the costs.sufficient to outweigh the costs.

Paternal CarePaternal Care

In fish male parental care is quite common. In fish male parental care is quite common. Many males mouth brood eggs or care for Many males mouth brood eggs or care for eggs in nests.eggs in nests.

Costs of parental care seem to be lower for Costs of parental care seem to be lower for males than for females.males than for females.


Male sticklebacks can care for 10 clutches Male sticklebacks can care for 10 clutches of eggs at once.of eggs at once.

Males grow more slowly when caring for Males grow more slowly when caring for young, but because males are territorial young, but because males are territorial and cannot range widely to look for food and cannot range widely to look for food the the additionaladditional cost of parental care is low. cost of parental care is low.


For a female stickleback parental care would For a female stickleback parental care would severely limit her ability to forage and severely limit her ability to forage and grow.grow.

Because body size is closely correlated with Because body size is closely correlated with egg production loss of foraging egg production loss of foraging opportunities would have a significant opportunities would have a significant effect on future reproduction. effect on future reproduction.


Because, in many fish, costs of parental Because, in many fish, costs of parental care are higher for females than they are care are higher for females than they are for males, paternal care may have evolved for males, paternal care may have evolved because males lose less from parental because males lose less from parental care than females do. care than females do.

Discriminating Parental CareDiscriminating Parental Care

Misdirecting parental care towards non- Misdirecting parental care towards non- offspring obviously would be a costly offspring obviously would be a costly mistake for any organism.mistake for any organism.

Many animals rear their young in colonies Many animals rear their young in colonies and there is plenty of opportunity for and there is plenty of opportunity for confusion.confusion.

Fig 12.7

Young free-tailed bats at a creche.


Mexican free-tailed bats use vocal and Mexican free-tailed bats use vocal and olfactory cues to identify their offspring olfactory cues to identify their offspring from among thousands in the creche.from among thousands in the creche.

The bats do occasionally make mistakes but The bats do occasionally make mistakes but the benefits of leaving a baby in a creche the benefits of leaving a baby in a creche (mainly thermoregulatory) appear to (mainly thermoregulatory) appear to outweigh the cost.outweigh the cost.


Cliff swallows, often nest in Cliff swallows, often nest in large colonieslarge colonies, , and their young produce much more and their young produce much more variable callsvariable calls than do Barn Swallows, than do Barn Swallows, which generally nest solitarily.which generally nest solitarily.

Cliff Swallow parents are also much better at Cliff Swallow parents are also much better at distinguishing between calls than are Barn distinguishing between calls than are Barn Swallows.Swallows.

Fig 12.9


Similarly, the young of colonial Bank Similarly, the young of colonial Bank Swallows produce distinctive vocalizations Swallows produce distinctive vocalizations that their parents can easily recognize, but that their parents can easily recognize, but the non-colonial Rough-winged Swallow the non-colonial Rough-winged Swallow does not.does not.

AdoptionAdoption

Obviously, it would appear beneficial to Obviously, it would appear beneficial to avoid adopting other individual’s offspring, avoid adopting other individual’s offspring, but such adoptions sometimes happen.but such adoptions sometimes happen.

In colonial nesting gulls chicks that have In colonial nesting gulls chicks that have been poorly fed in their own nests been poorly fed in their own nests sometimes leave their natal nest and join sometimes leave their natal nest and join another brood, where they often are another brood, where they often are adopted.adopted.

AdoptionAdoption

Moving is often a good decision for the chick Moving is often a good decision for the chick because it may end up being better cared because it may end up being better cared for in a different nest.for in a different nest.

However, adoptive parents on average lose However, adoptive parents on average lose 0.5 young of their own as a result of the 0.5 young of their own as a result of the adoption so why do they tolerate the adoption so why do they tolerate the intruder?intruder?

AdoptionAdoption

Most likely explanation is that any chick that Most likely explanation is that any chick that begs begs confidentlyconfidently is accepted and fed. is accepted and fed.

AdoptionAdoption

The reason that gulls do not discriminate more is The reason that gulls do not discriminate more is probably that recognition errors would be too probably that recognition errors would be too costly. costly.

Errors in which a gull fails to feed or worse attacks Errors in which a gull fails to feed or worse attacks and kills its own chick because it thinks it is a and kills its own chick because it thinks it is a stranger would be very costly.stranger would be very costly.

The cost of The cost of occasionaloccasional adoptions appears to be adoptions appears to be low enough that selection has not favored higher low enough that selection has not favored higher levels of discrimination in gulls. levels of discrimination in gulls.

AdoptionAdoption

In some instances adoption appears not to In some instances adoption appears not to come with a cost and may be beneficial to come with a cost and may be beneficial to the adopter.the adopter.

It is common in ducks for females to accept It is common in ducks for females to accept extra eggs laid in their nests and to accept extra eggs laid in their nests and to accept stray ducklings into their broods. stray ducklings into their broods.

AdoptionAdoption

In ducks there is little or no cost to adoption In ducks there is little or no cost to adoption because chicks forage for themselves. because chicks forage for themselves.

The benefit appears to be that there is a The benefit appears to be that there is a predator dilution effectpredator dilution effect with larger broods. with larger broods. Additional young in the brood reduce the Additional young in the brood reduce the odds of a chick taken from the brood being odds of a chick taken from the brood being the parents own young. the parents own young.

Brood parasitismBrood parasitism

There are several species of birds that are There are several species of birds that are obligate interspecific brood parasites.obligate interspecific brood parasites.

These include Old World Cuckoos, Old These include Old World Cuckoos, Old World Honeyguides and New World World Honeyguides and New World Cowbirds.Cowbirds.

These birds lay their eggs in the nests of These birds lay their eggs in the nests of other birds and provide no parental care. other birds and provide no parental care.

European Cuckoo removing host’s egg


Based on phylogenetic analyses brood Based on phylogenetic analyses brood parasitism appears to have evolved parasitism appears to have evolved independently three times in the cuckoos independently three times in the cuckoos and a large number of cuckoos (53 of 136 and a large number of cuckoos (53 of 136 species) are brood parasites.species) are brood parasites.

Obligate brood parasitesindicated in blue.Occasional parasites in red.


Interspecific brood parasitism is believed to have Interspecific brood parasitism is believed to have originated as intraspecific brood parasitism.originated as intraspecific brood parasitism.

Intraspecific brood parasitism is common in birds Intraspecific brood parasitism is common in birds and has been recorded in more than 200 and has been recorded in more than 200 species.species.

A plausible transition to interspecific brood A plausible transition to interspecific brood parasitism would be for birds to begin laying parasitism would be for birds to begin laying eggs in the nests of closely related species.eggs in the nests of closely related species.


Today cuckoos concentrate on species that Today cuckoos concentrate on species that are not closely related to them, but as are not closely related to them, but as parasitism in cuckoos may be 60 million parasitism in cuckoos may be 60 million years old this may simply reflect the long years old this may simply reflect the long period of evolution that has occurred since period of evolution that has occurred since the origin of the behavior.the origin of the behavior.


In cowbirds, which much more recently evolved In cowbirds, which much more recently evolved brood parasitism (in past 3-4 million years) the brood parasitism (in past 3-4 million years) the living species believed most like the ancestral living species believed most like the ancestral parasite parasitizes only one other species and parasite parasitizes only one other species and that belongs to its own genus. that belongs to its own genus.

Since then increasingly general brood parasitism Since then increasingly general brood parasitism appears to have evolved.appears to have evolved.


Brood parasites have a Brood parasites have a significantsignificant effect on effect on the reproductive success of the hosts.the reproductive success of the hosts.

Baby cuckoos eject the eggs and young of Baby cuckoos eject the eggs and young of the host so the host rears no young of its the host so the host rears no young of its own.own.


Brood parasites exploit the host parents tendency Brood parasites exploit the host parents tendency to feed the largest young in a brood the most to feed the largest young in a brood the most food and to reward the young that can reach food and to reward the young that can reach highest for food. highest for food.

By laying in the nests of smaller birds, cuckoos By laying in the nests of smaller birds, cuckoos give their young an advantage in the competition give their young an advantage in the competition for food. So do cowbirds whose eggs hatch for food. So do cowbirds whose eggs hatch after a shorter incubation period which allows after a shorter incubation period which allows them to hatch before the host’s young.them to hatch before the host’s young.


The advantage of laying in the nests of The advantage of laying in the nests of smaller species has been shown in smaller species has been shown in experiments in which nestlings of non-experiments in which nestlings of non-parasitic Great Tits and Blue Tits were parasitic Great Tits and Blue Tits were switched between nests. switched between nests.

The smaller Blue Tits did badly in Great Tit The smaller Blue Tits did badly in Great Tit nests, but Great Tits prospered in Blue Tit nests, but Great Tits prospered in Blue Tit nests.nests.

Why tolerate parasite’s eggs?Why tolerate parasite’s eggs?

Given the heavy costs of rearing a parasite, Given the heavy costs of rearing a parasite, why don’t hosts reject parasitic eggs?why don’t hosts reject parasitic eggs?

Rejection also comes with costs so cost-Rejection also comes with costs so cost-benefit analysis is needed.benefit analysis is needed.


Some birds do recognize parasitic eggs and Some birds do recognize parasitic eggs and remove them from the nest. However, remove them from the nest. However, there is a risk that the host will discard one there is a risk that the host will discard one or more of its own eggs in error.or more of its own eggs in error.

Reed Warblers have been shown to make Reed Warblers have been shown to make this mistake.this mistake.


Accepting a parasite’s egg is even more Accepting a parasite’s egg is even more likely when the host is too small to remove likely when the host is too small to remove the parasitic egg.the parasitic egg.

Such hosts must either accept the egg or Such hosts must either accept the egg or abandon the nest, which is an expensive abandon the nest, which is an expensive option, especially if nest sites are scarce option, especially if nest sites are scarce (e.g. as in cavity nesters).(e.g. as in cavity nesters).


Consistent with this hypothesis, Consistent with this hypothesis, Prothonotary Warblers parasitized by Prothonotary Warblers parasitized by cowbirds are much more likely to abandon cowbirds are much more likely to abandon their nest if there are alternative nest sites their nest if there are alternative nest sites on the male’s territory.on the male’s territory.


Similarly, Yellow Warblers parasitized near Similarly, Yellow Warblers parasitized near the end of the breeding season tend to the end of the breeding season tend to accept parasitic eggs, presumably accept parasitic eggs, presumably because there is too little time to start because there is too little time to start over.over.


Another reason for hosts to tolerate parasite Another reason for hosts to tolerate parasite eggs is that the parasite may monitor the eggs is that the parasite may monitor the nest and harm the host’s nest if its egg is nest and harm the host’s nest if its egg is removed.removed.

This “Mafia hypothesis” has been supported This “Mafia hypothesis” has been supported by studies of Great Spotted Cuckoos and by studies of Great Spotted Cuckoos and their Magpie hosts.their Magpie hosts.


Magpie nests from which cuckoo eggs were Magpie nests from which cuckoo eggs were ejected suffered a much higher rate of ejected suffered a much higher rate of predation (87%) than nests that accepted predation (87%) than nests that accepted cuckoo eggs (12%). cuckoo eggs (12%).

Threatening the clutch of the hosts appears Threatening the clutch of the hosts appears to be an effective strategy because re-to be an effective strategy because re-nesting is costly in the magpies’ seasonal nesting is costly in the magpies’ seasonal environment.environment.

Arms race between hosts and Arms race between hosts and parasites.parasites.

As selection operates on both hosts and As selection operates on both hosts and parasites the differing selection pressures parasites the differing selection pressures have resulted in an arms race between have resulted in an arms race between hosts and parasites.hosts and parasites.

In the case of the European cuckoo and its In the case of the European cuckoo and its hosts selection has led to extremely good hosts selection has led to extremely good mimicry of host eggs.mimicry of host eggs.


Individual cuckoos specialize on one host Individual cuckoos specialize on one host species and lay eggs that closely mimic species and lay eggs that closely mimic only that species’ eggs.only that species’ eggs.

Historical interactions between cuckoos and Historical interactions between cuckoos and some hosts appear to have resulted in some hosts appear to have resulted in victory for the host. victory for the host.


E.g., European blackbirds are rarely E.g., European blackbirds are rarely parasitized by cuckoos and even though parasitized by cuckoos and even though under no current selection pressure, these under no current selection pressure, these birds reject parasitic eggs at a very high birds reject parasitic eggs at a very high frequency. frequency.

Apparently, blackbirds evolved rejection Apparently, blackbirds evolved rejection behavior in the past and cuckoos have behavior in the past and cuckoos have moved on to other host species.moved on to other host species.


With many other species race arms-race between With many other species race arms-race between parasites and hosts is ongoing.parasites and hosts is ongoing.

Horsfield’s Bronze-cuckoo parasitizes the Superb Horsfield’s Bronze-cuckoo parasitizes the Superb Fairy Wren. Fairy Wren.

Fairy-wrens respond to cuckoo eggs laid before Fairy-wrens respond to cuckoo eggs laid before they have started laying by abandoning nest or they have started laying by abandoning nest or building over the egg. They also abandon if building over the egg. They also abandon if cuckoo lays egg after incubation has beguncuckoo lays egg after incubation has begun


Bronze-cuckoos have responded by Bronze-cuckoos have responded by inserting eggs during fairy-wren laying inserting eggs during fairy-wren laying period. Such eggs are generally accepted period. Such eggs are generally accepted and incubated.and incubated.

However, when young cuckoo pushes However, when young cuckoo pushes young wrens out of nest, fairy-wrens young wrens out of nest, fairy-wrens abandon the nest about 40% of the time abandon the nest about 40% of the time and cuckoo starves.and cuckoo starves.


In other cases cuckoo appears to fool In other cases cuckoo appears to fool parents into believing their sole chick is a parents into believing their sole chick is a fairy-wren.fairy-wren.

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

Chicks of Horsfield’s Bronze-cuckoo closely mimic calls of host fairy-wrens.Shining Bronze Cuckoo which rarely parasitizes fairy wrens does notresemble fairy-wren chick and does notsound like it either.


Another example of the use of calls in the arms Another example of the use of calls in the arms race between parasites and hosts is that of calls race between parasites and hosts is that of calls by European Cuckoo chicks in Reed Warbler by European Cuckoo chicks in Reed Warbler nests.nests.

The rate at which cuckoos call simulates that of a The rate at which cuckoos call simulates that of a whole brood of Reed Warblers which whole brood of Reed Warblers which encourages parents to feed at a much higher encourages parents to feed at a much higher rate than they otherwise would.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 callplayed. Reed warbler brood is delivery rate when tape of a brood of reed warblers in played.

Parental favoritism and siblicideParental favoritism and siblicide

Parents may be related to all of their Parents may be related to all of their offspring equally, but often do not treat offspring equally, but often do not treat them equally well.them equally well.

In many cases parents actively discriminate In many cases parents actively discriminate against certain offspring and either allow against certain offspring and either allow them to starve or allow their siblings to kill them to starve or allow their siblings to kill them.them.


For example, in African Black Eagles the first For example, in African Black Eagles the first hatched of two chicks attacks its younger sibling hatched of two chicks attacks its younger sibling as soon as it hatches and pecks it to death. as soon as it hatches and pecks it to death.

Similarly in egrets, boobies, pelicans and other Similarly in egrets, boobies, pelicans and other birds older siblings attack and drive younger birds older siblings attack and drive younger offspring out of the nest where they starve to offspring out of the nest where they starve to death. death.

Young great egretsfight while their parentignores the behavior.

At a Brown Booby nest theolder chick (under its parent)has driven its smaller sibling fromThe nest where it will die of exposure and starvation.


Despite the fact some of their offspring are Despite the fact some of their offspring are being killed, parents seem not only to being killed, parents seem not only to tolerate the behavior, but to actively tolerate the behavior, but to actively encourage it.encourage it.


For example, in Black Eagles incubation For example, in Black Eagles incubation begins as soon as the first egg is laid.begins as soon as the first egg is laid.

As a result the first egg hatches 3-7 days As a result the first egg hatches 3-7 days before the second and so the older before the second and so the older offspring has a huge size advantage over offspring has a huge size advantage over its younger sibling and can easily kill him. its younger sibling and can easily kill him.


Such hatching asynchrony is very common Such hatching asynchrony is very common among birds and results in hatching among birds and results in hatching asynchrony, which establishes an age asynchrony, which establishes an age and size hierarchy within the brood.and size hierarchy within the brood.

Birds do Birds do notnot have to hatch their young have to hatch their young asynchronously and many birds (e.g. asynchronously and many birds (e.g. ducks) even though they lay large clutches ducks) even though they lay large clutches hatch their young synchronously.hatch their young synchronously.


In cattle egrets (and other birds) in addition In cattle egrets (and other birds) in addition to promoting hatching asynchrony, parents to promoting hatching asynchrony, parents spike the earlier laid eggs with high doses spike the earlier laid eggs with high doses of androgens (male hormones).of androgens (male hormones).

The hormones make the earliest hatched The hormones make the earliest hatched chicks more aggressive and gives them an chicks more aggressive and gives them an extra advantage over later hatched chicks.extra advantage over later hatched chicks.


Why do parents play favorites and facilitate Why do parents play favorites and facilitate siblicide? siblicide?

There are two major reasons:There are two major reasons: Insurance against failureInsurance against failure Environmental uncertaintyEnvironmental uncertainty

InsuranceInsurance

The most extreme form of brood reduction is The most extreme form of brood reduction is obligate brood reductionobligate brood reduction in which in which younger offspring essentially always die.younger offspring essentially always die.

Examples of obligate brood reducers Examples of obligate brood reducers include: Black Eagles, Harpy Eagles, include: Black Eagles, Harpy Eagles, Giant Pandas, and Hooded Grebes.Giant Pandas, and Hooded Grebes.

InsuranceInsurance

These animals have no intention of rearing These animals have no intention of rearing more than a single offspring.more than a single offspring.

The second offspring represents an The second offspring represents an easily cancelled insurance policy easily cancelled insurance policy against the failure of the first offspring against the failure of the first offspring to hatch or develop normally.to hatch or develop normally.

InsuranceInsurance

When the first offspring arrives it kills its sibling When the first offspring arrives it kills its sibling (Black Eagle), the parents cover over the second (Black Eagle), the parents cover over the second egg (Harpy Eagles), the parents abandon the egg (Harpy Eagles), the parents abandon the second egg (Hooded Grebes) or abandon the second egg (Hooded Grebes) or abandon the second born cub (Giant Pandas).second born cub (Giant Pandas).

Thus, the parents avoid prolonged investment in a Thus, the parents avoid prolonged investment in a back-up offspring. However, if the first offspring back-up offspring. However, if the first offspring fails the second can step in and take its place.fails the second can step in and take its place.

InsuranceInsurance

Why don’t these animals go ahead and rear Why don’t these animals go ahead and rear the second baby once it arrives? the second baby once it arrives?

In many cases parents would appear to be In many cases parents would appear to be capable of rearing two young, but don’t do capable of rearing two young, but don’t do so? Why not?so? Why not?

Trade-offsTrade-offs

Because there are trade-offs between offspring Because there are trade-offs between offspring number and quality as well as between offspring number and quality as well as between offspring number and parental future reproductive number and parental future reproductive success.success.

For these species it is usually not possible to For these species it is usually not possible to provide enough food to rear two high-quality provide enough food to rear two high-quality young. Pandas feed on low quality food and the young. Pandas feed on low quality food and the burden of providing milk for two cubs is too burden of providing milk for two cubs is too much for most mothers. Two weakling offspring much for most mothers. Two weakling offspring are worse than a single sturdy cub.are worse than a single sturdy cub.

Trade-offsTrade-offs

In addition, extra effort invested in trying to In addition, extra effort invested in trying to rear two young in a season generally rear two young in a season generally reduces future reproductive success by reduces future reproductive success by reducing lifespan and ability to produce reducing lifespan and ability to produce eggs or babies.eggs or babies.

Environmental uncertaintyEnvironmental uncertainty

Many other species are facultative brood Many other species are facultative brood reducers which means that brood reducers which means that brood reduction does not always occur.reduction does not always occur.

These species practice a policy of parental These species practice a policy of parental optimism.optimism.

They lay a clutch size that can be reared in They lay a clutch size that can be reared in a good year, but in a bad year will result in a good year, but in a bad year will result in brood reduction.brood reduction.


In these species the brood contains two classes of In these species the brood contains two classes of offspring: offspring: corecore and and marginalmarginal offspring. offspring.

Marginal offspring are handicapped by the parents Marginal offspring are handicapped by the parents and as in obligate brood reducers have and as in obligate brood reducers have insurance value, but mainly are produced so that insurance value, but mainly are produced so that parents can take advantage of a good year if parents can take advantage of a good year if one occurs to rear bonus offspring.one occurs to rear bonus offspring.


Consistent with this idea, in facultative brood Consistent with this idea, in facultative brood reducers, the handicap the parents create reducers, the handicap the parents create is enough to create a clear hierarchy in the is enough to create a clear hierarchy in the brood but not so great that it cannot be brood but not so great that it cannot be overcome.overcome.


Thus, in cattle egret broods the effects of A and B Thus, in cattle egret broods the effects of A and B chicks (the eldest chicks) aggression towards chicks (the eldest chicks) aggression towards younger C and D chicks is moderated by food younger C and D chicks is moderated by food supply. supply.

If food is plentiful, the younger chicks can tolerate If food is plentiful, the younger chicks can tolerate the beating and may survive to fledge. If food is the beating and may survive to fledge. If food is scarce the younger chicks quickly starve or are scarce the younger chicks quickly starve or are driven out of the nest and die.driven out of the nest and die.


The cattle egret parents’ policy of hatching The cattle egret parents’ policy of hatching asynchrony thus creates a situation in asynchrony thus creates a situation in which in good years conditions can be which in good years conditions can be taken advantage of and extra babies taken advantage of and extra babies reared, but in bad years the brood can be reared, but in bad years the brood can be efficiently reduced to what foraging efficiently reduced to what foraging conditions will support.conditions will support.


The amount of asynchrony in cattle egret The amount of asynchrony in cattle egret broods appears to have been tailored by broods appears to have been tailored by natural selection to maximize parents natural selection to maximize parents reproductive success and efficiency in reproductive success and efficiency in rearing babies.rearing babies.

Artificially synchronized nests produced Artificially synchronized nests produced fewer survivors and required more food fewer survivors and required more food because offspring fought more and so because offspring fought more and so expended more energy.expended more energy.


Nests in which asynchrony was exaggerated Nests in which asynchrony was exaggerated produced similar numbers of young as produced similar numbers of young as normally asynchronous nests, but brood normally asynchronous nests, but brood reduction took place at younger ages, reduction took place at younger ages, which may limit the ability of the parents to which may limit the ability of the parents to rear large broods in good years.rear large broods in good years.

Evaluating the reproductive Evaluating the reproductive value of offspringvalue of offspring

As we have seen not all offspring are As we have seen not all offspring are created equal and even in the absence of created equal and even in the absence of parental manipulation of quality we would parental manipulation of quality we would expect parents to assess offspring quality expect parents to assess offspring quality when deciding how to allocate scarce when deciding how to allocate scarce resources.resources.


It has been suggested that the gape color of It has been suggested that the gape color of baby birds may signal the quality of their baby birds may signal the quality of their immune system and thus offspring qualityimmune system and thus offspring quality

Red gape color is produced by carotenoid Red gape color is produced by carotenoid pigments in the blood and these are pigments in the blood and these are believed to enhance immune function.believed to enhance immune function.


In an experiment on barn swallows in which In an experiment on barn swallows in which chicks gapes were colored with food chicks gapes were colored with food coloring chicks whose gapes were coloring chicks whose gapes were reddened received more food, although reddened received more food, although chicks whose gapes were yellowed did not chicks whose gapes were yellowed did not receive less food.receive less food.


Alternative explanations for the role of gape Alternative explanations for the role of gape coloration have been put forward, coloration have been put forward, however.however.

An obvious alternative is that parents are not An obvious alternative is that parents are not assessing offspring quality, but just assessing offspring quality, but just feeding those chicks whose gapes are feeding those chicks whose gapes are more conspicuous under the prevailing more conspicuous under the prevailing lighting conditions.lighting conditions.


Consistent with this idea Great Tit chicks Consistent with this idea Great Tit chicks whose mouths were painted yellow whose mouths were painted yellow received more food than chicks whose received more food than chicks whose mouths were painted red and were less mouths were painted red and were less conspicuous in a dark nest box.conspicuous in a dark nest box.

When a plexiglass lid was placed on the When a plexiglass lid was placed on the nest box however, both sets of chicks nest box however, both sets of chicks were equally well fed.were equally well fed.


Although gape color may not signal offspring Although gape color may not signal offspring quality or value other traits may.quality or value other traits may.

Coots have an unpleasant way of reducing Coots have an unpleasant way of reducing brood size. They peck certain babies in brood size. They peck certain babies in their brood when they beg for food and their brood when they beg for food and these ones quickly die.these ones quickly die.


Baby coots have prominent long orange Baby coots have prominent long orange tipped plumes on their backs and throats tipped plumes on their backs and throats and these may be a cue parents use in and these may be a cue parents use in deciding which chicks they wish to feed.deciding which chicks they wish to feed.

When these plumes were trimmed from half When these plumes were trimmed from half the members of a brood the unaltered the members of a brood the unaltered members of the brood received more food members of the brood received more food and grew faster than the trimmed birds. and grew faster than the trimmed birds.


Control broods in which all birds were Control broods in which all birds were trimmed survived as well as broods in trimmed survived as well as broods in which no chicks were trimmed. which no chicks were trimmed.

Coots thus appeared to discriminate against Coots thus appeared to discriminate against trimmed chicks because they lacked trimmed chicks because they lacked orange plumes not because they could not orange plumes not because they could not recognize them.recognize them.

Thus, it may be that the orange plumes are Thus, it may be that the orange plumes are a signal of offspring quality. a signal of offspring quality.

Magpie assessment of offspring Magpie assessment of offspring valuevalue

Magpie young are increasingly likely to survive as Magpie young are increasingly likely to survive as they age. Thus their value increases and one they age. Thus their value increases and one would expect parents to value them more.would expect parents to value them more.

Consistent with this parents are more likely to Consistent with this parents are more likely to engage in defensive behavior when a predator engage in defensive behavior when a predator approaches a nest if the brood is older. approaches a nest if the brood is older.

evolution of parental care parental care does not always take place. in many species (e.g. clams,...

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