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  • The Evolution of Parental Care

    Chapter 12 Alcock (Animal Behavior)

    Tom WenseleersEthology & Behavioural Ecology

  • Plan of lectureCosts and benefits of parental careParent-offspring conflictMaternal, paternal & biparental careParental favoritism & siblicide

  • 1. Costs and benefitsof parental care

  • Evolution of parental careParental care does not always take place. In many species (e.g. clams, barnacles, many fish) eggs are shed into the water and abandoned. Similarly, turtle young are on their own once they hatch.The decision to offer parental care depends on whether such care will increase the caregivers lifetime reproductive success.Greater investment in individual young necessarily reduces the number of young that can be produced.Consequently, species choose between producing many, small, uncared for young or fewer, larger, cared for young.Whales and humans represent one end of the continuum and barnacles and clams the other.If parental care enhances survival and growth of young enough to compensate for the reduction in young produced then we would expect parental care to evolve.

  • Costs and benefits of parental careObviously, one constraint of parental care is the ability the parent has to affect the offsprings survival.Barnacles produce many thousands of eggs which are shed into the water and drift away. They develop into larvae and one day settle permanently on a fixed substrate. Barnacles are sessile and can do nothing to actively assist their young. Not surprisingly, barnacles have not evolved parental care.Parental care in organisms that can give it may significantly enhance the prospects of the offspring surviving to adulthood. For example, higher bodyweight at fledging significantly increases a small bird's chances of surviving to adulthood. Extra investment (i.e. the parents working harder to supply food) comes at a cost though as it may reduce the parents prospects of surviving over the winter.This effect has been documented in many studies in which brood sizes of parents were increased.

  • Costs and benefits of parental careIn general, the willingness of a parent to invest in or take risk for an offspring should be influenced by (i) the parents future prospects of reproducing and (ii) the relative value of the current offspring.This is borne out by studies of the behavior of long-lived versus short-lived birds.In general, one would predict that long-lived birds should be less willing to risk their lives to protect their young, but that short-lived birds should be more willing to do so.In general, North American birds are shorter lived than comparable South American species.Ghalambor and Martin (2001) compared the behavior of matched pairs of North and South American birds to evaluate the birds willingness to take risks on behalf of their young.

  • Fig 12.1AE.g. Am. Robin (roodborstlijster, short lived) vs. Arg. Rufous-bellied Thrush (roodbuiklijster, long lived).When researchers played tapes of Jays (which raid nests, vlaamse gaai) near the birds nests both species avoided returning to the nest, but robins reduced their activity more, meaning they were less willing to risk the current offspring. When a stuffed Sharp-shinned Hawk (a predator of adults) was placed near the nest and calls played, again both species avoided visiting the nest, but this time the Rufous-bellied Thrushes reduced their visits more, meaning they were less willing to risk their lives by feeding the current brood.Hence selection has fine-tuned behavior to take account of costs and benefits of risk-taking behavior.

  • 2. Parent-offspring conflict

  • Parent-offspring conflictIn many species parents invest huge quantities of resources in their offspring. Initially, both parent and offspring agree that investment in the offspring is worthwhile because it enhances the offsprings prospects of survival and reproduction.However, a parent shares only 50% of its genes with the offspring and is equally related to all of its offspring, whereas the offspring is 100% related to itself, but only shares 50% of genes with full-siblings (and less with half-siblings).Robert Trivers predicted that this should lead to parent-offspring conflict over the amount of food provisioned to young. At some point, a parent will prefer to reserve investment for future offspring rather than investing in the current one, while the current offspring will disagree.

  • Figure shows B/C benefit to cost ratio of investing in the current offspring. Benefit is measured in benefit to current offspring and cost is measured in reduction in future offspring. Parent-offspring conflict leads to a period of conflict called weaning during which the offspring tries to acquire resources and the parent attempts to withhold them.The period of weaning conflict ends when both offspring and parent agree that future investment by the parent would be better directed at future offspring rather than to the current offspring. For full siblings, this is when the benefit to cost ratio drops below .Parent-offspring conflictperiod of weaning conflict

  • In instances where parents produce only half siblings, we should expect weaning conflict to last longer, until the B/C ratio drops to 1/4, because the current offspring is less closely related to future offspring. This has been confirmed in various field studies.Parent-offspring conflictperiod of weaning conflict

  • Test: effect of relatednesson begging loudnessBegging calls are louder in species with lower chick-chick relatedness and this results in more frequent predation.a b c d eSpecies pairVolume of begging calls (dB)-40-30-20-101 23 46 78 910 11Black: high relatedness (monogamous) Red: low relatedness (frequent extrapair copulations or socially parasitic)Lower relatedness results in louder callsbrown headed cowbird

  • Other possible consequence of young only being related by 1/2 (full-siblings) or 1/4 (half-siblings): siblicideProcess whereby some young kill brothers or sisters.Siblicide

  • Siblicide Masked booby Kittiwake gulls Indian rosewood Spadefoot toads Sand tiger sharks Piglets

  • 3. Maternal, paternal& biparental care

  • Maternal parental careMaternal parental care is more common than paternal care.In some instances maternal care is a result of internal fertilization and the delay between mating and birth.Other general reasons for maternal care being more common focus on the relative costs to the two sexes of being the caregiver.For males there is uncertainty about paternity, which will reduce the benefit to cost ratio of engaging in parenting.In addition, for males when there are opportunities to mate with multiple females, males that give up that opportunity to engage in parental care will pay too high a price.Paternal care (either with the female or alone) would be selected for only when the payoff is sufficient to outweigh the costs.

  • Maternal care: Membracinae treehoppers(boomcicade)

  • Paternal Care: fishesIn fish male parental care is quite common. Many males mouth brood eggs or care for eggs in nests.

    Costs of parental care in these cases seem to be lower for males than for females. E.g. because females prefer males that engage in parental care or because males can take care of several egg clutches.

  • Paternal Care: sticklebackMale sticklebacks can care for 10 clutches of eggs at once.Males grow more slowly when caring for young, but because males are territorial and cannot range widely to look for food the additional cost of parental care is low.For a female stickleback parental care would severely limit her ability to forage and grow.Because body size is closely correlated with egg production loss of foraging opportunities would have a significant effect on future reproduction.

  • Paternal Care: fishesBecause, in many fish, costs of parental care are higher for females than they are for males, paternal care may have evolved because males lose less from parental care than females do. E.g. St. Peter's fish.

    (mouth brooder)

  • Paternal Care: male water bugsMale water bugs guard and moisten eggs above the water (Lethocerus) or carry eggs on back (Abedus, Belostoma).Abedus eggs do not develop unless aerated by male.Because water bugs are predatory insects (catching fish, frogs and tadpoles) they are large and consequently their eggs are too. This is why oxygenation is necessary.Why only male care? Male water bugs with one clutch of eggs sometimes attract a second female. Also costs of parental care may be disproportionally great for females in terms of lost fecundity.

  • 3. Intra- and interspecificbrood parasitism

  • Discriminating Parental CareMisdirecting parental care towards non- offspring obviously would be a costly mistake for any organism.

    Many animals rear their young in colonies and there is plenty of opportunity for confusion. Yet, as predicted, parental care is usually very discriminating.

  • Fig 12.7Young Mexican free-tailed bats at a creche containing 4000 pups per square meter. Females give birth to a single pup. They use vocal and olfactory cues to identify their offspring from among thousands in the creche. The bats do occasionally make mistakes but the benefits of leaving a baby in a creche (mainly thermoregulatory) appear to outweigh the cost (accuracy from allozyme data: 80%).

    Discriminating Parental Care

  • Fig 12.9Discriminating Parental CareCliff Swallows often nest in large colonies and their young produce much more variable calls than do Barn Swallo