Hummingbirds are tiny birds that have an extremely high metabolism and must consume a lot of energy in the form of nectar or insects each day in order to survive. Some species of hummingbirds migrate incredibly long distances each year, such as Ruby-throated Hummingbirds that travel across the Gulf of Mexico between their breeding grounds in North America and their wintering grounds in Central America. This journey across 500 miles of open water is a grueling test of endurance for such small birds. In preparation for this journey, Ruby-throated Hummingbirds nearly double their body mass by storing up fat reserves to fuel their flight across the Gulf. But why do they store fat rather than carbohydrates as their main energy source for this long migration? There are several key reasons.
Fat Provides More Energy Per Unit of Weight
Fat contains significantly more energy per unit of weight compared to carbohydrates. Fat provides about 9 calories per gram, while carbohydrates and proteins provide only around 4 calories per gram. This means fat has over twice the energy density of carbohydrates. Given their tiny body size, hummingbirds need to pack in as much potential energy as possible into a minimal amount of weight to fuel their migration. As they cannot physically carry much weight, fat is the most efficient and dense form of stored energy.
Fat is More Energy Efficient
Not only does fat provide more calories per gram, but it actually requires less energy to metabolize than carbohydrates. The metabolic conversion of fat into usable energy requires less oxygen than the breakdown of carbohydrates or proteins. This is extremely beneficial during the sustained vigorous exercise of flying over 500 miles nonstop across the Gulf, when a hummingbird’s oxygen supply may be limited. Burning fat requires less oxygen for more energy, improving energetic efficiency.
Fat Fuels Muscles Directly
Unlike carbohydrates, fat can be directly taken up and oxidized by muscles as an energy source. Muscles have transporters that allow them to import fatty acids from the bloodstream. This means that fat can fuel muscle contraction without having to go through the intermediate steps of glycogen synthesis and breakdown like carbohydrates. Once again, this improves energetic efficiency. Direct utilization of fat reserves may be especially useful towards the end of migration, when liver and muscle glycogen stores have dwindled.
Fat Does Not Require Water for Storage
Carbohydrates, such as glycogen, have a bound water molecule for each individual glucose unit. This means storing carbohydrates also requires allocating water for storage. Fat, however, does not bind water molecules when it is stored in adipose tissue. This makes fat a more efficient form of dry storage. Bringing along excess water weight would be a disadvantage for hummingbirds making an overwater flight.
Fat Provides Buoyancy and Insulation
The layer of fat built up by hummingbirds before migration helps make them more buoyant, an advantage when flying over water. The fat also provides insulation from cold temperatures and wind exposure during long distance travel. These ancillary benefits may further enhance migration success and survival.
Dietary Sources of Fat are Abundant Before Migration
In the weeks leading up to migration, Ruby-throated Hummingbirds increase their food intake substantially to put on weight for the journey. At this time of year, flower nectar and tree sap are particularly rich in lipids and fatty acids. Eating lots of nectar and sap allows the birds to easily increase their fat stores. Fat makes up about 30-40% of their body weight prior to migration.
How Do Hummingbirds Store Fat Before Migration?
In order to nearly double their body weight before migration, Ruby-throated Hummingbirds have evolved a clever strategy to rapidly accumulate fat stores:
They Preferentially Store Unsaturated Fat
While saturated fats would theoretically be more energy dense, hummingbirds preferentially store unsaturated fats. This is because unsaturated fat molecules contain double bonds that make them slightly “bent”. This shape prevents the fat molecules from packing tightly together, keeping them fluid at avian body temperatures. This unsaturated fat is therefore less likely to solidify when the birds are flying in cold temperatures.
They Store Fat All Over Their Bodies
Hummingbirds do not store fat in a single centralized location. They distribute fat broadly under the skin and between muscle fibers throughout the chest, abdomen, back and even neck. This allows for streamlined body profiles and prevents too much abdominal fat from unbalancing the bird during flight.
They Ramp Up Fat Synthesis and Storage
In the liver, hummingbirds enhance both the absorption of dietary fat as well as de novo lipogenesis from carbohydrates to make new fatty acids. More fat is created and absorbed from the diet. In adipose tissue, lipid synthesis and accumulation are increased through mechanisms like upregulated LPL activity. Fat storage outpaces breakdown.
They Suppress Fat Breakdown
At the same time, hummingbirds suppress fat metabolism pathways like beta-oxidation to conserve lipids. For example, carnitine palmitoyltransferase I, the enzyme that allows fat breakdown, is downregulated in muscle leading up to migration. This allows newly synthesized and consumed fat to be retained.
They Become Insulin Resistant in Fat Cells
Hummingbirds develop tissue-specific insulin resistance in their adipocytes, preventing insulin from suppressing fat accumulation. At the same time, they maintain insulin sensitivity in skeletal muscle to promote carbohydrate utilization for energy over fat. This clever balancing act lets them maximize pre-migratory fattening.
What Are Key Differences Between Fat and Carbohydrate Metabolism During Exercise?
There are several important differences in how fat and carbohydrates are metabolized to provide energy during sustained vigorous exercise like hummingbird migration:
Carbohydrate Metabolism
- Glycogen broken down into glucose
- Glucose metabolized through glycolysis to pyruvate
- Pyruvate enters mitochondria for oxidation in Krebs cycle
- Electrons fed into electron transport chain to generate ATP
Fat Metabolism
- Triglycerides broken into fatty acids and glycerol
- Fatty acids transported into mitochondria by carnitine
- Fatty acids undergo beta-oxidation to generate acetyl-CoA
- Acetyl-CoA enters Krebs cycle for oxidation
- Electrons fed into electron transport chain to generate ATP
Key differences:
- Carbohydrates require more intermediary steps
- Carbohydrates generate less ATP per gram metabolized
- Carbohydrates require more oxygen to metabolize
- Fatty acids can directly fuel muscle contraction
So fat provides more energy with better efficiency and oxygen economy.
What Are the Advantages and Disadvantages of Using Fat Versus Carbohydrates to Fuel Migration?
Advantages of Fat
- Higher energy density – 9 kcal/g versus 4 kcal/g
- Greater energy efficiency and oxygen economy
- Directly utilized by muscles
- Does not require water for storage
- Provides insulation and buoyancy
Disadvantages of Fat
- Heavier weight per unit of energy than carbohydrates
- Can only provide ATP through aerobic metabolism, limited anaerobic capacity
- Higher risk of dehydration due to metabolic water produced during carbohydrate metabolism
Advantages of Carbohydrates
- Lighter weight per unit of energy than fats
- Can provide ATP through both aerobic and anaerobic metabolism
- Generate metabolic water during breakdown
Disadvantages of Carbohydrates
- Lower energy density – only 4 kcal/g
- Less efficient metabolism, requires more oxygen
- Cannot directly fuel muscles
- Requires water for glycogen storage
- No insulation or buoyancy benefits
Overall, the advantages of using fat clearly outweigh those of carbohydrates for fueling hummingbird migration.
Conclusion
In summary, hummingbirds preparing for migration across the Gulf of Mexico accumulate substantial fat stores because fat provides over twice the energy density of carbohydrates for a minimal increase in weight. Fat is more efficiently metabolized with lower oxygen requirements. It can also directly power muscle contraction. Moreover, fat does not bind water when stored and contributes useful insulation and buoyancy. While carbohydrates have some advantages like their ability to provide fast anaerobic energy, these benefits are outweighed by the superior energetic properties of fat for fueling sustained vigorous exercise like 500 mile migration. Given their extreme metabolic demands and tiny body size, Ruby-throated Hummingbirds have therefore evolved to preferentially store fat as their main energy reserve for crossing the Gulf of Mexico.
