Evolution of Parental Care • Parental care does not always take place. In many species (e.g. clams, barnacles, many fish) eggs are shed into the water and abandoned.
Costs and benefits of parental care • The decision to offer parental care depends on whether such care will increase the caregiver’s lifetime reproductive success. • Greater investment in individual young necessarily reduces the number of young that can be produced.
Costs and benefits of parental care • Consequently, species choose between producing many, small, uncared for young or fewer, larger, cared for young. • Whales and humans represent one end of the continuum and barnacles and clams the other.
Costs and benefits of parental care • If parental care enhances survival and growth of young enough to compensate for the reduction in young produced then we would expect parental care to evolve.
Costs and benefits of parental care • Obviously, one constraint of parental care is the ability the parent has to affect the offspring’s survival. • Barnacles produce many thousands of eggs which are shed into the water and drift away. They develop into larvae and one day settle permanently on a fixed substrate.
Costs and benefits of parental care • Barnacles are sessile and cannot do nothing to actively assist their young. • Not surprisingly, barnacles have not evolved parental care.
Costs and benefits of parental care • Parental care in organisms that can give it may significantly enhance the prospects of the offspring surviving to adulthood. • For example, higher bodyweight at fledging significantly increases a small birds chances of surviving to adulthood.
Costs and benefits of parental care • Extra investment (i.e. the parent’s working harder to supply food) comes at a cost though as it may reduce the parent’s prospects of surviving over the winter. • This effect has been documented in many studies in which brood sizes of parents were increased.
Costs and benefits of parental care • The costs associated with increased investment in a given offspring cause parents to limit the investment they make so as to increase their prospects of survival and also to allow them to invest in future offspring. • This decision causes parent offspring conflict.
Parent-offspring conflict. • In many species parents invest huge quantities of resources in their offspring. • Initially, both parent and offspring agree that investment in the offspring is worthwhile because it enhances the offspring’s prospects of survival and reproduction.
Parent-offspring conflict. • However, a parent shares only 50% of its genes with the offspring and is equally related to all of its offspring, whereas the offspring is 100% related to itself, but only shares 50% of genes with full siblings (and less with half-siblings).
Parent-offspring conflict. • As a result, at some point, a parent will probably prefer to reserve investment for future offspring rather than investing in the current one, while the current offspring will disagree. • When might parent be prepared to sink all its effort into a current offspring?
Parent-offspring conflict. • This leads to a period of conflict called weaning during which the offspring tries to acquire resources and the parent attempts to withhold them.
Parent-offspring conflict. • The period of weaning conflict ends when both offspring and parent agree that future investment by the parent would be better directed at future offspring rather than to the current offspring. • For full siblings, this is when the benefit to cost ratio drops below ½.
Parent-offspring conflict • In instances where parents produce only half siblings, we should expect weaning conflict to last longer because the current offspring is less closely related to future offspring. • This has been confirmed in various field studies.
Costs and benefits of parental care • In general, the willingness of a parent to invest in or take risk for an offspring should be influenced by (i) the parent’s future prospects of reproducing and (ii) the relative value of the current offspring. • This is borne out by studies of the behavior of long-lived versus short-lived birds.
Costs and benefits of parental care • In general, one would predict that long-lived birds should be less willing to risk their lives to protect their young, but that short-lived birds should be more willing to do so.
Costs and benefits of parental care • In general, North American birds are shorter lived than comparable South American species. • Ghalambor and Martin (2001) compared the behavior of matched pairs of North and South American birds to evaluate the birds’ willingness to take risks on behalf of their young.
Costs and benefits of parental care • E.g. compared American Robin to Argentinian Rufous-bellied Thrush. • When researchers played tapes of Jays (which raid nests) near the birds’ nests both species avoided returning to the nest, but robins reduced their activity more. Consistent with robins being less willing to risk the current brood.
Costs and benefits of parental care • When a stuffed Sharp-shinned Hawk (a predator of adults) was placed near the nest and calls played, again both species avoided visiting the nest, but this time the Rufous-bellied Thrushes reduced their visits more.
Costs and benefits of parental care • These results suggest that the thrushes were less willing to risk their lives by feeding the current brood. • Selection on robins and thrushes thus appears to have fine-tuned behavior to take account of costs and benefits of risk-taking behavior.
Maternal parental care • In general maternal parental care is more common than paternal care. • In some instances maternal care is a result of internal fertilization and the delay between mating and birth.
Maternal parental care • Other general reasons for maternal care being more common focus on the relative costs to the two sexes of being the caregiver. • For males there is uncertainty about paternity, which will reduce the benefit to cost ratio of engaging in parenting.
Maternal parental care • In addition, for males when there are opportunities to mate with multiple females, males that give up that opportunity to engage in parental care will pay too high a price. • Paternal care (either with the female or alone) would be selected for only when the payoff is sufficient to outweigh the costs.
Paternal Care • In fish male parental care is quite common. Many males mouth brood eggs or care for eggs in nests. • Costs of parental care seem to be lower for males than for females.
Paternal Care • Male sticklebacks can care for 10 clutches of eggs at once. • Males grow more slowly when caring for young, but because males are territorial and cannot range widely to look for food the additional cost of parental care is low.
Paternal Care • For a female stickleback parental care would severely limit her ability to forage and grow. • Because body size is closely correlated with egg production loss of foraging opportunities would have a significant effect on future reproduction.
Paternal Care • Because, in many fish, costs of parental care are higher for females than they are for males, paternal care may have evolved because males lose less from parental care than females do.
Discriminating Parental Care • Misdirecting parental care towards 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.
Fig 12.7 Young free-tailed bats at a creche.
Discriminating Parental Care • Mexican free-tailed bats use vocal and olfactory cues to identify their offspring from among thousands in the creche. • The bats do occasionally make mistakes but the benefits of leaving a baby in a creche (mainly thermoregulatory) appear to outweigh the cost.
Discriminating Parental Care • Cliff swallows, often nest in large colonies, and their young produce much more variable calls than do Barn Swallows, which generally nest solitarily. • Cliff Swallow parents are also much better at distinguishing between calls than are Barn Swallows.
Discriminating Parental Care • Similarly, the young of colonial Bank Swallows produce distinctive vocalizations that their parents can easily recognize, but the non-colonial Rough-winged Swallow does not.
Adoption • Obviously, it would appear beneficial to avoid adopting other individual’s offspring, but such adoptions sometimes happen. • In colonial nesting gulls chicks that have been poorly fed in their own nests sometimes leave their natal nest and join another brood, where they often are adopted.
Adoption • Moving is often a good decision for the chick because it may end up being better cared for in a different nest. • However, adoptive parents on average lose 0.5 young of their own as a result of the adoption so why do they tolerate the intruder?
Adoption • Most likely explanation is that any chick that begs confidently is accepted and fed.
Adoption • The reason that gulls do not discriminate more is probably that recognition errors would be too costly. • Errors in which a gull fails to feed or worse attacks and kills its own chick because it thinks it is a stranger would be very costly. • The cost of occasional adoptions appears to be low enough that selection has not favored higher levels of discrimination in gulls.
Adoption • In some instances adoption appears not to come with a cost and may be beneficial to the adopter. • It is common in ducks for females to accept extra eggs laid in their nests and to accept stray ducklings into their broods.
Adoption • In ducks there is little or no cost to adoption because chicks forage for themselves. • The benefit appears to be that there is a predator dilution effect with larger broods. Additional young in the brood reduce the odds of a chick taken from the brood being the parents own young.
Brood parasitism • There are several species of birds that are obligate interspecific brood parasites. • These include Old World Cuckoos, Old World Honeyguides and New World Cowbirds. • These birds lay their eggs in the nests of other birds and provide no parental care.
Brood parasitism • Based on phylogenetic analyses brood parasitism appears to have evolved independently three times in the cuckoos and a large number of cuckoos (53 of 136 species) are brood parasites.
Obligate brood parasites indicated in blue. Occasional parasites in red.
Brood parasitism • Interspecific brood parasitism is believed to have originated as intraspecific brood parasitism. • Intraspecific brood parasitism is common in birds and has been recorded in more than 200 species. • A plausible transition to interspecific brood parasitism would be for birds to begin laying eggs in the nests of closely related species.
Brood parasitism • Today cuckoos concentrate on species that are not closely related to them, but as parasitism in cuckoos may be 60 million years old this may simply reflect the long period of evolution that has occurred since the origin of the behavior.
Brood parasitism • In cowbirds, which much more recently evolved brood parasitism (in past 3-4 million years) the living species believed most like the ancestral parasite parasitizes only one other species and that belongs to its own genus. • Since then increasingly general brood parasitism appears to have evolved.