Parental Care

BIOL 3100

1) The evolution of Parental Care

2) When to provide care

Evolution of Parental Care1) No care is the ancestral state2) When parental care does arise,

either male or female care evolves (usually female)

3) After single-parent care evolves, biparental care may evolve if two parents substantially increase offspring success.

Costs/Benefits of Parental CareFor care to evolve, the fitness benefits of providing care must outweigh the costs and increase the fitness of the young.

Generally, fitness is increased through increased offspring survival or improved quality of surviving offspring.

Remember, both parents equally receive the benefits of parental care, even if only one provides

Northern, migratory bird species tend to have lower survival rates and larger clutch sizes than their southern counterparts.

Prediction: Under increased predation risk, northern species should invest more heavily in offspring survival than southern species.

When nest predators are presented, northern species decrease the number of trips they take to the nest more so than southern species (reducing the amount of attention drawn to the nest)

When sharp-shinned hawks are presented, northern species continue to feed offspring, whereas southern species drop the level of care much more (investment in own survival).

Evolution of Female Care

Evolution of Female Care

In females of Membracinae (treehoppers), maternal care has evolved independently at least 3 times, but paternal care has never evolved (as far as we know)

5 explanations for female care1) Anisogamy selects for continued investment in offspring they’ve already invested in heavily. Problem is that past investment doesn’t require future investment.

The Concorde Falacy

http://www.flickr.com/photos/roninphotography/4769491005/

Spotted sandpipers immediately stop caring for offspring once they hatch and leave care up to the males

5 explanations for female care2) When females are limited in egg production, regardless of whether she provides care, it should benefit her to stay and care for offspring, especially if she has little to gain from mating with additional males.

5 explanations for female care3) It’s too costly for males to stick around, so they’re likely to abandon, leaving females with the care. Problem: skewed operational sex ratio means it’s likely quite difficult for males to find another mate and they may end up losing more.

5 explanations for female care4) In systems with multiple matings, females are likely to have sired all their own offspring, while males may have lower levels of paternity, which devalues the benefit of male care.

5 explanations for female care5) Gamete release hypothesis: care should evolve such that the sex that releases its gametes first can abandon the offspring and leave care to the other sex. May help explain high incidence of care in internally fertilized species, but can’t explain high prevalence of male care in externally-fertilized species.

What about cases where males do all the work?

Belostomatid males provide exclusive care of the young – a characteristic common to fishes, but rare to vertebrates and invertebrates.

Females glue eggs directly to the males back and males spend hours near the water surface pumping water up and down to aerate the eggs.

Without male care, the eggs automatically die.

Male care in belostomatids1) Belostomatid eggs are much larger

than other aquatic insects, requiring lots of oxygen

2) Even though oxygen content is higher on land, dessication is a problem

3) Best solution is to repeatedly moisten eggs and expose to air

4) So, why not just lay smaller eggs?

Belostomatids are among the largest insects and prey on large items like fish frogs and tadpoles.

Growth only occurs from one instar to the next and halts at maturity. Size change between instars is only 50-60% per moult and no member of the family has more than 6 moults.

In other words, moult is a constraint on development. In order to reach a large size, the eggs must be sufficiently large.

Fine, but why am I the one taking care of the babies?

Costs of parental careIn St. Peter’s fish, both males and females can orally incubate fertilized eggs. Both parents lose weight while incubating and the interspawn interval increases when incubating.

Costs: Interspawn interval, subsequent offspring

Discriminating Parental CareCaring for your own offspring is good, caring for the offspring of others…not so good.

http://www.birdingisfun.com/2009_06_01_archive.html

The parents that care for these brown-headed cowbird chicks (the brown eggs) will suffer reduced fitness compared to birds that can discriminate

Mexican free-tailed bats form colonies numbering in the millions.

Females leave their pups in a creche in the cave that may contain 4,000 pups/m2.

Do the females actually manage to feed their own offspring?

Mexican free-tailed bats form colonies numbering in the millions.

Females leave their pups in a creche in the cave that may contain 4,000 pups.

Do the females actually manage to feed their own offspring?

Turns out that the females do – well over 80% of the time and seem to rely on both vocal and olfactory signals.

- Female fur seals give birth on crowded island beaches

- Females remain with offspring for 1 week before going off to fish for 3 weeks

- Takes offspring less than 5 days to learn mother’s call; females also learn pups calls quickly

- When a mother seal returns, she calls out and her infant calls back.

- Reunions take less than 15 minutes.

Similarly, in highly colonial cliff swallows, offspring produce highly structured, distinctive calls that help parents recognize them as individuals.

Not seen in more solitary species.

Adoption of offspring clearly offers no fitness advantage, so why do it?

Some colonial gulls, like ring-billed gulls, occasionally adopt unrelated chicks.

Occasionally, offspring will leave the nest if they are not being fed well by parents and beg for food from potential adopters and crouch submissively when threatened.

This tactic often works and the offspring are much more likely to survive than if they’d remained with the genetic parents that weren’t feeding them well.

Gulls have a less-than-perfect rule of thumb: accept a chick that begs confidently for food (even though there’s a chance it’s not your own) rather than risk rejecting your own offspring

http://www.flickr.com/photos/kevcole/2435138583/

Dickinson & Weathers removed nesting males, experimentally “widowing” the females

Soon after, replacement partners arrived, but only ~ half of the males provided parental care to the chicks.

http://www.flickr.com/photos/kevcole/2435138583/

All-or-nothing rule of thumb:

1) If a male joins a female during egg-laying (when she’s fertile), he’ll provide full parental care.

2) If a male joins a female after egg-laying, he provides no care.