Lecture 19. Parental Care in Animals

Parenting and Parental Care

• Parenting is an investment, and thus can be 'expensive'.
• Some species do not provide any parental care.

Definitions of Parental Care

• Parental care is any form of parental behavior that appears likely to increase the fitness of a parent's offspring.
  • Preparation of nests and burrows
  • Production of large, heavily yoked eggs
  • Care of eggs (inside or outside the body)
  • Providing for young (before or after birth)
  • Care of offspring after nutritional independence

Economics of Parental Care

• Depreciable care (sharable): The benefits of parental expenditure decline as brood size increases (e.g., food provisioning).
• Non-depreciable care (non-shareable): The benefits of parental expenditure remain the same despite brood size increases (e.g., nest defense).

Parental Care, Investment, and Effort

• Parental investment: Characteristics or actions of parents that increase the fitness of offspring at a cost to any component of the parent's fitness.
  • Impacts: parental care to other offspring, survival, fecundity, growth
• Parental effort: The total costs of caring for ALL progeny.

Factors Affecting Parental Investment

• Resources available to parents
• Effects of expenditure on survival
• Parents’ reproductive value
• Value of offspring sex
• Value of offspring
• Balance between parents’ parental behaviors
• Relatedness between parents and offspring
• Whether you are a mother or a father
  • Certainty of parentage

Why is parental care usually maternal?

• The common explanation that females have already invested more energy in making eggs, giving them a special incentive to ensure this investment is not wasted, is flawed; Many males DO care for young.

• This flawed logic commits the Concorde fallacy.

  • Organisms protect their offspring based on future potential and not on past investment

• In most species, the potential reproductive rate of males is greater than that of females.

• This makes the cost of parental care by males greater than for females, thus decreasing the likelihood that it would evolve.

Exceptions to Maternal Care

• Care of young may not reduce male reproductive success.
  • Example: Male sticklebacks can brood the eggs of up to 10 females, and females are more likely to lay eggs for a male already brooding. A female stickleback could only care for one brood at a time, and she would produce fewer eggs during her lifetime as a result of brooding.

Costs of Parental Behavior: St Peter’s Fish

• St Peter’s fish is a mouth-brooding cichlid. Either the male or female may brood.

• Both sexes lose weight when brooding, because it is hard to eat with a mouth full of baby fish.

• For both male and female St Peter’s fish, the inter-spawn interval is greater if they brood young, compared with fish that have their clutches experimentally removed.

• The effect is greater for females than for males.

  • Also, subsequent clutches are smaller for females, whereas males fertilize the same number of eggs

Changing Male Paternal Behavior

• Populations are usually male biased.
• A female-biased operational sex ratio means males can easily find new partners.
• For St Peter’s fish, males respond by increasing the frequency of brood desertion.

Male Care by Giant Water Bugs

• Role reversal: Females actively search for males to mate with, and males rear the young.
• Female giant water bugs lay their eggs on males' backs, and males can mate with multiple females, adding eggs until the space is full.
• Female giant water bugs preferentially lay eggs on males that already had eggs on their backs instead of egg-free males.
• Females prefer males that provide parental care.

Offspring Recognition

• Cliff swallows and barn swallows are in the same family and both build mud nests on similar vertical surfaces. Their range is sympatric over much of North America.

• Mandy Medvin and Michael Beecher examined why the begging calls of cliff swallows and barn swallows differ.

• Barn swallow chicks produce undefined and similar calls.

• Cliff swallow chicks produce highly structured and distinctive calls, enabling their parents to recognize them as individuals. The calls contain sixteen times more information about individual identity.

Why Adopt Genetic Strangers?

• If species can evolve mechanisms for offspring recognition, why do adults sometimes care for non-kin offspring?
• The answer is that a simple, robust rule of thumb for recognizing offspring may sometimes result in a parent caring for non-kin.
• The alternative might be a more complex rule that eliminates all care of non-kin, but often results in rejection of offspring by mistake.

Interspecific Brood Parasitism

• The occasional mistaken identity of conspecific non-kin offspring is understandable.
• The acceptance of heterospecific offspring, often very different in appearance from conspecifics, seems positively careless.
• How did this evolve, and why is it maintained?

Brood Parasitism Evolution

• Brood parasites started out as intraspecific parasites, shifting with time to parasitize a few closely related species and eventually becoming a generalist.

• This scenario is consistent with Scott Lanyon’s cladistic analysis of cowbirds, an interspecific brood parasite group of species.

• Many brood-parasitic species are larger than their hosts. This suggests that brood parasitism may have originated with a sudden shift to laying eggs in the nests of smaller hosts.

Why Are Parasites Accepted?

• Often, they are not. Eggs may be removed from a nest, or the nest abandoned.
• If parasitism is rare, chances are a discriminator will eject its own egg.
• It may be physically impossible to eject an egg.
• It may be hard to find an alternative nest site.
• Lisa Petit tested this with prothonotary warbler hosts and cowbird parasites.

Abandonment and Nest Availability

• Hosts were more likely to abandon parasitized nests when alternative nest sites were available.

Can Adoption Benefit Foster Parents?

• Occasionally.
  • Dilution effect: Eider duck crèche
  • Regular nursing induces ovulation: Elephant seal
  • Practice for own reproduction

Evolution of Parental Favoritism

• Why do parents favor some offspring?
• Why do parents stand by while offspring try to kill each other (siblicide)?
• Do parents establish the asymmetries between offspring that ensure some are more likely to die as a result of sibling conflict?

Spotted Hyena Twins

• Pups fight in the den, sometimes resulting in the death of one.
• Aggression is greater amongst same-sex litters.
• The effect of siblicide on parental fitness is not known.

Boobies and Siblicide

• In fights between siblings, blue-footed booby (BFB) parents intervene, while masked booby (MB) parents do not.
• Cross-fostering of chicks shows that parental behavior can reduce siblicide.

Do Parents Promote Siblicide?

• If parental behavior in boobies can control behavior, why don’t masked boobies control siblicide?
• Could parents promote and benefit from a degree of siblicide?

Siblicide in Cattle Egrets

• The first two eggs in a three-egg clutch have higher levels of androgens.
• Incubation starts with the laying of the first egg.
• As a result, the first eggs hatch sooner, the chicks are bigger, and they are more aggressive.
• Cattle egret parents rear more offspring more efficiently due to siblicide.

Environmental Uncertainty in the Egret’s Habitat

• Hatching asynchrony ensures that in good years more chicks are reared, but in bad years, the brood is efficiently reduced to a number that matches the available conditions.

Do Parents Favor the Best Offspring?

• Parents may use cues about the health of their offspring to bias their parental care towards those most likely to survive and reproduce.
• High carotenoid levels in nestling birds are associated with good immune function.
• Does this explain the red gapes of many begging nestling birds?

Colour of the Mouth Gape

• Nicola Saino used food dyes to color the mouths of nestling barn swallows to see if it altered parental care.

• Saino also tried challenging the immune system of barn swallow nestlings by injecting them with sheep red blood cells.

• As a result, the color of the birds’ gapes became dull, and sometimes yellow or green.

• There was a direct relationship between the gape color and immune system response.

• Honest signal

• Parents need to make choices to secure reproductive fitness when resources are limited.

Take-Home Message

• Parental care is probably associated with protecting previous investment but also with potential gain.
• Male parental care is not uncommon, and it happens when it brings an advantage to male reproductive success. In species where male parental care exists, females select males on the basis of their parenting potential.
• Parental care is costly (both for males and females); therefore, recognizing offspring is important.
• Fostering is also common. It may be accidental, but it may also have advantages for the fostering parent.
• Brood parasitism is uncommon, but it may occur for a number of reasons.
• Unpredictable environments favour parental favouritism.