Provisioning young

Lecture 3

Optimal load size when collecting food

optimal load size when collecting food

• Trade-off between collecting food and travelling to the food

The central place foraging model

• slope: rate of food delivery to the nest

Maximising the tangent

The Max load size is lower than the optimal load size:

• This is because you need to factor in the time to get there/ spent collecting

• By collecting a little less food, you can make more trips coz it takes you less time to do so

How optimal load size changes with increased travel time?

the optimal load size increases with travel time

Testing this: starlings

Observed= dots

predicted= stepped line

Shows data matches prediction

But why do some experiment not fit? (3)

1. Costs and benefits not measured correctly

   • e.g energy costs may be better currency than ‘time’

2. Model incomplete- other costs and benefits

   • e.g have to return to nest for protect= less foraging time

3. Animal might be behaving sub optimally

   • e.g change in environment or manipulated by others

How much food to give offspring?: Brood hierarchies

When The parents give more food to one of their offspring

• e.g can use hating asynchonry so the older chicks get more food and are bigger

Advantage of brood hierarchies

Selective starvation hypothesis:

• If food short

• Will have efficient brood reduction

  • cos smaller chicks can quickly die off

• Without hierachies

  • limited food spread across offspring

  • so probs all die anyway

  • waste of time and energy

Testing this hypothesis

When food is short, asynchronous must be more productive than synchronous?:

Results: e.g blackbirds

Note: synchronous birds do slightly better when food is available

But from the chick’s point of view

There are genetic conflicts of interest between:

• offspring and parent

Offspring:

• more interested in its own welfare (r=1), than their sib

Parent

• has same genetic stake in all its offspring (r=0.5)

Therefore:

The offsprings will be selected to demand more food than their parents want to give it (more than their fair share)

Intra-brood conflict

One of the offspring wanted more than its ‘fair share’ of the food

• It is selfish

• r=1 to itself and 0.5 to the offspring

Inter-brood conflict

• Offspring wants more food than sibling

  • to increase chances of individual survival

  • deamnds care beyond partental optimum

• But parent wants to give equal amounts to each offspring

  • so can have many offspring

  • lower parental care than offspring demand

    • because it wants to optimise lifetime reproductive success

Graphical model of parent-offspring conflict

Graph the optimal parental offspring from

• parent’s point of view

• offsrping’s point of view

• Offspring benefit is x2 of the parent (relatedness to self if x2)

• Assuming they are full-sibs: costs to parent/offspring is the same

  • i.e the cost of not helping the other sibling coz there is some genetic relatedness

What the graph shows

e.g in a situation where offspring should descide to kill sibling to ensure its own survival- when food is scarse?

• Threshold of offspring wanting to kill sibling is lower than

• When the parent would want to kill it

THEREFORE: there is a ‘battle ground’ between optima

Parent offspring conflict

When can the offspring siblicide threshold be lowered?

If they are not as related:

• half siblings

• even r=0, cuckoos not related at all: get rid of all the ‘siblings’

As food gets scarcer…

Stages:

1. Siblicide (cox offspring threshold lower)

2. Infanticide (parental threshold reached)

3. Suicide (agree to die to save life- rare it gets to this point though!)

Evidence for sibling rivalry: Intra-brood killing

Large chicks kill younger siblings when food is short

• kingfishers

• bee-eaters

• egrets

Evidence: Inter-brood conflict

Galapagos fur-seals

• older almost-weaned pup

• demand continued feeding

• even when the next pup is born

Sibling rivarly should increase as r decreases (half siblings)

Chicks are more selfish

• Louder begging

As % of extra-pair mating increases

Cainism- Black eagles and boobies

• Chick always kills its younger sibling

Then why parent lay second egg?

• insurance against infertility

Parent- offspring conflict: yellow-billed hornbills

Assuming that food is not scarce

When female present for feeding:

• controls food allocation

• both chicks can get food

When female parent not there

• Bigger chick gets more food

• why?

• Parent not there to stop attacks on smaller chick

What these results show

When food scarce:

• brood hierarchy: useful

When food is not scarce

• brood hierarchy: cost

Why is brood hierarchy bad when food is abundant?

• Bigger chicks still take more food

• Take more than they need: wasted

• Smaller chicks could have survived if better allocation

This is why synchronous broods do better (e.g in black birds) than asynchronous when food is around.

Adapting to differing food availbility

e.g American coots

• Food scarce:

  • brood reduction

  • favour larger chicks

• Food available

  • punish greedy larger chicks

  • makes sure all chicks get fair share

  • decrease sibling rivalry

Resolution of parent-offspring conflict

Matching Parental food allocation to offspring demand

How do this?

Mothers customise their chicks so the level of offspring demand matches their capacity to care

• Changed the food quality available

• Mothers varied the androgens in the eggs

• Related to the food quality available when making the eggs

  • better food → increased androgens→ more beggin

  • co-adaptation: of maternal capcity and offspring demand

Test if the resolution is beneficial?

Made with cross-fostering

• Optimum chick growth is when the demand matched the supply

• When chicks begged less than expected

  • less food given

  • less growth

• When chicks begged more than expected

  • more energy wasted begging

  • grow less

We are left with a question…

If the resolution is benefical

• why have a battleground?

  • When would you expect a battleground

  • When would you expect a resolution?