Chapter 9 The Flexible Phenotype

Metabolic ceiling: the maximum energy output that can possibly be sustained by an organism that eats enough to stay in energy balance.

Basal metabolic rate (BMR): standard conditions, baseline before any extra energy exerted.

Maximal sustained working levels: the aerobic capacity for work of animals as it is limited by evolutionarily shaped physiological constraints.
- Animals that choose to ignore this ‘physiological warning level’ would lose body condition and face precipitous increases in mortality risk.

Optimal working capacity: energetic performance level of parents beyond which they would suffer from increased risks due to physical fatigue, infection, and predation.

‘Capital’ energy spender: rely on stored resources
‘Income’ spender: rely on concurrent intake of nutrients or energy

One can argue that ‘maximum sustained working levels’ are only achieved by animals that are ‘income’ spenders
- b/c they maintain energy balance over the period of peak performance.

‘Sustained metabolic scope’ (factorial scope):

What is true endurance?

explain the below figure without caption for exams

FP-1

Cold-induced factorial scope vs. exercise-induced scope

Importance of energy balance during sustained higher metabolic rates.

Weiner’s barrel model: metabolic ceilings

Five constraints on energy (bow-ties):
Bow-tie A: availability of energy in the environment
- But it is unlikely that this restrains maximal performance
- The rate at which energy can be harvested often exceeds the rate at which it can be processed.

Bow-tie C: limited by capacity to excrete metabolic end-products.
- i.e. kidneys to excrete urea

For awhile 2 main hypotheses existed to explain maximal performance.
1.) central limitation hypothesis (Bow-tie B): related to the capacity of the alimentary tract to absorb energy (food-processing chain)
2.) peripheral limitation hypothesis (Bow-tie D): related to the capacity to expend energy at the different sites of utilization (i.e. respiratory chain, with muscles, mammalian brown adipose tissue and mammary glands)

Performance limits are not set centrally:
- Mice that increased food-intake rates when muscles and BAT were made to work harder in the cold, suggested that it was actually the capacity of the mammary glands that set the upper limit – so back to the hypothesis #2.

heat-dissipation limits > come back to this..

fig2-FP

allometric scaling constant
- in mammals this is consistently higher in maximal metabolic rate compared to BMR

Long term fitness assets

Increase body size, increase in energy required to maintain, increase in BMR levels.

Symmorphosis: a single site does not limit aerobic performance.

If very hard work precipitously increases the likelihood of death (b/c of free radical derived oxidative DNA and tissue damage), without leading to compensatory increases in reproductive output, evolutionary trade-offs would select for animals that are not prepared to work harder than what is called the ‘optimal working capacity’. Emphasizes the cost of high performance.

allostatic loads > come back to this

mass-specific lifetime energy expenditure > come back to this

lifetime energy expenditure

In birds: The mechanisms for a lower rate of ageing and bodily decline, which relate to mitochondrial functioning and the ways that electron leakage and free-radical production are balanced under conditions of high and low energy demands.

animals are reluctant habitually to spend as much as they are physiologically capable of.

how would you measure metabolic ceiling in a coral? See Barott papers?

Phenotypic plasticity: matching phenotypes to environmental demands

Plastic barnacle penis example of environment influencing phenotype and function.

Use it or lose it:
- heart morphology based on usage: atrophy vs. hypertrophy
- in space calf muscle decreases by 20% and in on earth weightless scenarios. positive correlation of decrease in leg muscles (%) and duration of unloading experiment

After losing it, it is possible to regain it:
- astronauts 6 mos post flight can regain % muscle

Dynamic gut: snakes go through veritable ‘gastrointestinal rebirths’: going without meals for so long that stonachs, intestines, and accesory organs shrink and become ‘dormant’
- when they eat again, it is a huge meal and there is a burst of physiological activity by orders of magnitude
- there is a higher cycling of cells post meal while preparing for another meals

Classical phenotypic plasticity: developmental reaction norms

developmental plasticity: where environmental conditions during ontogeny determine the size, shape, construction, and behavior of the mature phenotype.
- water fleas Daphnia show hoods, helmets, spines, and longer tails in response to predators or to the chemical identifiers (‘kai hormones’). also called induced defenses
- density levels drive urchin size
- water flow drives scleractinian corals to be denser or more spaced out structures
- seasonal polyphenism: the tropical butterfly Precis almana has either angular wing shapes and a dull brown color (resembling a dead leaf) in the dry season or rounded and colorful wings with eye spots in the wet season

Size, age, condition, context-dependent sex change occurs in plants, annelids, echinoderms, crustaceans, molluscs, and fish

Seasonal phenotypic change:
- plumage of birds and antlers of deer
- Bicyclus butterflies show what is typical of ‘classic’ developmental plasticity: hormonal effects in larva and pupa and physiological acclimatization: induced in the reproductive adult stage and maternally contributed to the eggs

Seasonal polyphenism in insects is called life-cycle staging in birds and animals..

Mismatches of environment and phenotype can happen in an unpredictable environment and this can be a large cost of developmental plasticity. If organisms are capable of fast and reversible phenotypic change then that cost isn’t as big of a deal or risk.

Below is a conceptual diagram of types of plasticity defined in 2-D space. Variability: degree of environmental variability (relative to the organism) and Predictability: the degree to which this variability is predictable.

fp3

Between individuals/generations would include short-lived organisms and within individuals/generations would include long-lived organims (corals are long-lived).

Lower predictability means it would be better for an organism to respond opportunistically rather than seasonally scheduled.

Variable responses: organisms encountering unpredictably variable environments in the course of their lifetime would benefit from plasticity being reversible; i.e. showing phenotypic flexibility.

In a low predictability environment, organisms could bet-hedge and development phenotypic changes at random in short-lived organisms. Long-lived organisms could deal with this by not changing at all and show robustness.

why couldn’t bet-hedging be true for long-lived organisms? and vice versa with robustness?

Costs and limits of plasticity

FP4

Degrees of flexibility

Cell types could have differing responses to functionality
- Sled dogs in the summer when not in use, had thinner muscle and decreases in mitochondrial numbers, lipid droplet size, and number of contractile myofibrillar elements but capillary networks remained the same

In theory for phenotypic plasticity to evolve, no single phenotype can be optimal in all environments experienced by the organism.
- Therefore, there must be trade-offs. and reliable cues to inform the organism about the state of its variable environment.

Plasticity itself is a trait with a genetic basis

Traits are only so plastic, there is likely to be a leveling off point where the organism isn’t able to continuously increase their plasticity even if the signal is continued or increased.
- organizational limit or functional limit that reflects the high cost of a phenotype i.e. a defensive phenotype (frog with predator presence example)

Reductions in metabolically active tissue dramatically reduce basal energy requirements.

Reproductively successful phenotypes will satisfy ecological demands by optimizing the balance between various cost and benefit functions of specific kinds of phenotypic (trait) variation.

Chapter 9: Population consequences: conservation and management of flexible phenotypes

Information other than genes influence phenotype, but there is a limit to coping to environment pertubations.