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.

Similarly, turtle young are on their own Similarly, turtle young are on their own once they hatchonce they hatch

Costs and benefits of parental careCosts and benefits of parental care

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

Greater investment in individual young Greater investment in individual young necessarily reduces the number of young necessarily reduces 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 enhances survival and If parental care enhances 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 Obviously, one constraint of parental care is the ability the parent has to affect the is the ability the parent has to affect the offspring’s survival.offspring’s survival.

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


Barnacles are sessile and can do nothing Barnacles are sessile and can do nothing to actively assist their young. to actively assist their young.

Not surprisingly, barnacles have not Not surprisingly, barnacles have not evolved parental care.evolved 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, higher bodyweight at For example, higher bodyweight at fledging significantly increases a small fledging 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 reduce the parent’s though as it may reduce the parent’s prospects of surviving over the winter.prospects 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 parent offspring This decision causes 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 Initially, both parent and offspring agree that investment in the offspring is that investment in the offspring is worthwhile because it enhances the worthwhile because it enhances the offspring’s prospects of survival and offspring’s prospects of survival and reproduction.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 weaning during which the offspring tries to weaning 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.

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

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


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


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

Ghalambor and Martin (2001) compared Ghalambor and Martin (2001) compared the behavior of matched pairs of North and the 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.


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

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

Fig 12.1A


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

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

Paternal CarePaternal Care

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

Costs of parental care seem to be lower Costs of parental care seem to be lower for males than for females.for 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 For a female stickleback parental care would severely limit her ability to forage would severely limit her ability to forage and grow.and grow.

Because body size is closely correlated Because body size is closely correlated with egg production loss of foraging with 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 The bats do occasionally make mistakes but the benefits of leaving a baby in a but the benefits of leaving a baby in a creche (mainly thermoregulatory) appear creche (mainly thermoregulatory) appear to outweigh the cost.to outweigh the cost.


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

Cliff Swallow parents are also much better Cliff Swallow parents are also much better at distinguishing between calls than are at distinguishing between calls than are Barn Swallows.Barn 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 colonially nesting gulls chicks that have In colonially 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 Moving is often a good decision for the chick because it may end up being better chick because it may end up being better cared for in a different nest.cared for in a different nest.

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

AdoptionAdoption

Most likely explanation is that parents use Most likely explanation is that parents use an imperfect behavioral when deciding an imperfect behavioral when deciding who to feed.who to feed.

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

The cost of occasional adoptions appears to be The cost of occasional 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 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. accept stray ducklings into their broods.

AdoptionAdoption

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

The benefit appears to be that there is a The benefit appears to be that there is a predator dilution effect with larger broods. predator dilution effect 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 Today cuckoos concentrate on species that are not closely related to them, but as that 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 In cowbirds, which much more recently evolved brood parasitism (in past 3-4 evolved brood parasitism (in past 3-4 million years) the living species believed million years) the living species believed most like the ancestral parasite parasitizes most like the ancestral parasite parasitizes only one other species and that belongs to only one other species and that belongs to its own genus. its own genus.

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


Brood parasites have a significant effect Brood parasites have a significant effect on the reproductive success of the hosts.on 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 Brood parasites exploit the host parents tendency to feed the largest young in a brood tendency to feed the largest young in a brood the most food and to reward the young that can the most food and to reward the young that can reach highest for food. reach 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 Given the heavy costs of rearing a parasite, why don’t hosts reject parasitic parasite, why don’t hosts reject parasitic eggs?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 Some birds do recognize parasitic eggs and remove them from the nest. However, and 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 to be adaptive when the host is too likely to be adaptive when the host is too small to remove the parasitic egg.small to remove 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 Another reason for hosts to tolerate parasite eggs is that the parasite may parasite eggs is that the parasite may monitor the nest and harm the host’s nest monitor the nest and harm the host’s nest if its egg is removed.if its egg is removed.

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


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

Threatening the clutch of the hosts Threatening the clutch of the hosts appears to be an effective strategy appears to be an effective strategy because renesting is costly in the because renesting is costly in the magpies’ seasonal environment.magpies’ seasonal 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 Historical interactions between cuckoos and some hosts appear to have resulted in and 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 With many other species race arms-race between parasites and hosts is ongoing.between parasites and hosts is ongoing.

Horsfield’s Bronze-cuckoo parasitizes the Horsfield’s Bronze-cuckoo parasitizes the Superb Fairy Wren. Superb 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 The rate at which cuckoos call simulates that of a whole brood of Reed Warblers which a 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 In many cases parents actively discriminate against certain offspring and discriminate against certain offspring and either allow them to starve or allow their either allow them to starve or allow their siblings to kill them.siblings to kill them.


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

Similarly in egrets, boobies, pelicans and Similarly in egrets, boobies, pelicans and other birds older siblings attack and drive other birds older siblings attack and drive younger offspring out of the nest where younger offspring out of the nest where they starve to death. they starve to 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 Such hatching asynchrony is very common among birds and results in common among birds and results in hatching asynchrony, which establishes an hatching asynchrony, which establishes an age and size hierarchy within the brood. age and size hierarchy within the brood.

Birds do not have to hatch their young Birds do not 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 In cattle egrets (and other birds) in addition to promoting hatching addition to promoting hatching asynchrony, parents spike the earlier laid asynchrony, parents spike the earlier laid eggs with high doses of androgens (male eggs with high doses of androgens (male hormones).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 Why do parents play favorites and facilitate siblicide? facilitate 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 The most extreme form of brood reduction is obligate brood reduction in which is obligate brood reduction 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 easily The second offspring represents an easily cancelled insurance policy against the cancelled insurance policy against the failure of the first offspring to hatch or failure of the first offspring to hatch or develop normally. 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 Thus, the parents avoid prolonged investment in a back-up offspring. However, if the first a back-up offspring. However, if the first offspring fails the second can step in and take its offspring fails the second can step in and take its place.place.

InsuranceInsurance

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


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

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


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


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

If food is plentiful, the younger chicks can If food is plentiful, the younger chicks can tolerate the beating and may survive to tolerate the beating and may survive to fledge. If food is scarce the younger fledge. If food is scarce the younger chicks quickly starve or are driven out of chicks quickly starve or are driven out of the nest and die.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 Nests in which asynchrony was exaggerated produced similar numbers of exaggerated produced similar numbers of young as normally asynchronous nests, young as normally asynchronous nests, but brood reduction took place at younger but brood reduction took place at younger ages, which may limit the ability of the ages, which may limit the ability of the parents to rear large broods in good years.parents to rear large broods in good years.

Brood reduction and humansBrood reduction and humans

Discussions of brood reduction are Discussions of brood reduction are applicable to humans also.applicable to humans also.

Twin births are rare, but twin conceptions Twin births are rare, but twin conceptions are much commoner and only one in ten are much commoner and only one in ten to one in fifty twin conceptions produce to one in fifty twin conceptions produce twins. The other pregnancies result only twins. The other pregnancies result only in singleton births.in singleton births.


This phenomenon has been dubbed the This phenomenon has been dubbed the “vanishing twin” syndrome.“vanishing twin” syndrome.

Part of the phenomenon may be that Part of the phenomenon may be that producing extra eggs is an insurance producing extra eggs is an insurance strategy against pregnancy failure due to strategy against pregnancy failure due to defective embryos. Some of these early defective embryos. Some of these early embryos have chromosomal defects and embryos have chromosomal defects and are quietly aborted by the mother.are quietly aborted by the mother.


This idea is supported by the fact that This idea is supported by the fact that older women are more likely to give birth older women are more likely to give birth to twins than younger women.to twins than younger women.

Apparently this is because older women Apparently this is because older women polyovulate more than younger women polyovulate more than younger women and older women are much more likely to and older women are much more likely to produce chromosomally defective eggs.produce chromosomally defective eggs.


It is possible that other cases of It is possible that other cases of disappearing twins may not be caused by disappearing twins may not be caused by an embryo failing because it is defective, an embryo failing because it is defective, but from a “decision” by the mother to but from a “decision” by the mother to reduce her brood, although a mechanism reduce her brood, although a mechanism by which this might occur has not been by which this might occur has not been identified. identified.

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 It has been suggested that the gape color of baby birds may signal the quality of their of 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 In an experiment on barn swallows in which chicks gapes were colored with food which 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 Alternative explanations for the role of gape coloration have been put forward, gape coloration have been put forward, however.however.

An obvious alternative is that parents are An obvious alternative is that parents are not assessing offspring quality, but just not 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 Although gape color may not signal offspring quality or value other traits may.offspring 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 When these plumes were trimmed from half the members of a brood the unaltered half 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 Coots thus appeared to discriminate against trimmed chicks because they against trimmed chicks because they lacked orange plumes not because they lacked orange plumes not because they could not recognize them.could not recognize them.

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

Magpie assessment of offspring Magpie assessment of offspring valuevalue

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

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

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

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