Lecture 5 ADL's Lactate and Depth

ADL

Aerobic Dive Limit

What is ADL?

Length of time an animal can nominally dive using oxygen stores

Animals with a higher metabolic rate underwater will reduce their ADL accordingly

How is ADL calculated?

1. Calculate O₂ stores

2. Divide by basal metabolic rate

= cADL (calculated)

Calculates a maximum ADL

Basal metabolic rate

Minimum amount of energy needed to expend to stay alive

Why speed important?

If speed is doubled 8 times the amount of oxygen is required, need to spend less time underwater - why most animals travel slow

How often do ADL’s get exceeded (Case Study)?

New Zealand and Australian Sea lion (deep divers): dived for longer than they should

Antarctic fur seal dived within ADL, feeds shallower

The deep dives are benthic foragers, requiring them to exceed or meet their ADL

What happens when an animal exceeds its ADL?

Move into energy production without oxygen (Anaerobic dives)

Overall, produces less energy and creates lactic acid build up

Requires much longer surface recovery time

How to remove lactate?

1. Oxidise to create energy - requires 17 times the aerobic period, i.e. a long surface period

2. Reconvert to glucose - requires energy

What is the difference between ADL and cADL?

ADL (Diving Lactate Threshold, DLT) - dive duration after which there is an increase in post-dive concentration of lactate, at ADL there is some usable oxygen remaining

cADL - divides usable oxygen by rate of oxygen consumption during diving, assumes that all of the usable oxygen stores have been consumed (not always true)

Animals go anaerobic before all their oxygen has been consumed

What do animals do to forage efficiently?

Don’t go anaerobic because it takes a longer time to recover

On a depth/time graph how to identfy an aerobic dive?

TDR data shows short surface intervals

How is it possible that a number of diving animals exceed their cADL (without long surface intervals)?

1. Underestimate oxygen stores

2. Overestimate metabolic rate

Oxygen stores, what do the best animal divers have?

• Expanded blood volume and RBC mass (high hematocrit)

• High muscle myoglobin concentrations

• Collapsible lungs

Where are oxygen stores?

• Blood volume

• Blood pigment (haemoglobin)

• Muscle pigment (myoglobin)

• Lungs

Mass specific metabolic rate explained

• Larger animals use oxygen more slowly per unit mass compared to smaller animals, but use more overall

• Looks at amount of oxygen used per gram of tissue

• Larger animals have a lower mass-specific metabolic rate

• Amount of oxygen per kg body mass is not affected by size

• Larger animals can spend longer underwater - smaller animals use oxygen faster

• Oxygen stores scale linearly with mass

• Metabolic rate = total energy used by the whole body

• Mass-specific metabolic rate = energy used per unit mass

• As body mass increases it gives space to store more oxygen

What is the relationship between oxygen stores and mass?

• Scale linearly

• Amount of oxygen a small animal can store per gram in its body is the same as a large animal

• Small animals just use oxygen up faster

Elephant Shrew curve (Mass Specific Metabolic Rate)

• Amount of oxygen used per gram of tissue, shrews use much more than elephants

• Why: Per unit mass, small animals use oxygen very fast, large animals use it slowly

Mass Specific Metabolic Rate definition

How quickly animals use oxygen per kg body mass depends on animal mass

Allometry (oxygen store/usage hypothesis)

• Small animals use oxygen faster, so can’t spend as long underwater

• If animals store about the same oxygen per gram:

  • 1 kg animal → small total oxygen store

  • 100 kg animal → much larger total oxygen store

• So bigger animals have more total oxygen available

• Dive duration increases with body mass, as oxygen is not used as fast (reason for animals to be big)

Dive duration limits, paper example

• Halsey et al., 2006

• Oxygen stores/ usage hypotheisis suggests that larger animals can dive for longer and deeper because oxygen storage scales isometrically (goes up linearly) with body mass

• Whereas, oxygen usage scales allometrically with an exponent <1 (uses oxygen less quickly)

• True when compared with birds and mammals, but birds tend to dive deeper than mammals of equivalent mass

Time under

Oxygen stores/metabolic rate

Why does body mass not explain everything?

• E.g. Sperm whale and Grey whale - same mass, 10x difference in dive duration

• Body mass only approximates O2 storage and use, differences in ecology and activity

• E.g. Grey whales mainly feed on krill, so don’t need to deep dive

Muscle O₂ stores

Muscle mass (kg) x Mb concentration (g Mb kg^-1) x 1.34 (ml O₂ g Mb^-1)

Mb = myoglobin

What’s the difference between endotherms and ectotherms dive duration, paper example?

• Wilco et al., 2020

• Ectotherms can remain submerged for longer

• Mass scaling relationship for dive duration is steeper in endotherms

• Explains why the largest extant vertebrate divers are endothermic (not reptiles)