Question

Small animals tend to expire sooner than related large ones if forced to live on stored supplies. For instance, suppose you have a mouse and a dog that both start with body stores of fat equal to \(20 \%\) of body weight. Explain why the mouse would be likely to die sooner if these animals could not find any food and thus had to live on their fat reserves. Which one would die sooner if they were trapped underwater and had only their stores of \(\mathrm{O}_{2}\) to live on while trying to escape?

Short answer

Due to higher metabolic rates, the mouse would die sooner than the dog in both scenarios. Firstly, fat reserves would be used up quicker due to faster calorie burning. Secondly, the mouse's oxygen reserves would also be consumed quicker in comparison to the dog when trapped underwater.

Step-by-step solution

Understand the Metabolic Rate

The metabolic rate is the amount of energy an animal uses per unit of time. Smaller animals have a higher metabolic rate than larger ones. This means they use up energy (or burn calories) at a faster rate.

Living on Stored Fats

When an animal begins to live on its stored fats, it uses these reserves for energy. Because smaller animals, like the mouse in this instance, have higher metabolic rates, they will use up their fat reserves more quickly than a larger animal, like a dog, even if both start at the same percentage of body weight.

Survival Underwater Using Oxygen Reserves

Similarly to fat reserves, the rate at which an animal uses its oxygen reserves is determined by its metabolic rate. Smaller animals consume oxygen faster than larger ones due to their high metabolic rate. Thus, the mouse would likely run out of its oxygen reserves first.

Key concepts

Energy Consumption

Energy consumption is a vital concept in understanding how animals survive on stored resources. When animals consume food, they convert it into energy to sustain various bodily functions such as movement, growth, and homeostasis.

• Metabolic rate, which indicates the energy used by an animal per unit of time, varies significantly between species and sizes.
• Smaller animals have a higher metabolic rate compared to larger animals.

This means that a smaller animal like a mouse will burn calories at a much faster rate than a larger animal such as a dog.
As a result, when forced to rely solely on internal sources like fat reserves for energy, smaller animals deplete these reserves quicker. This rapid consumption due to a high metabolic rate is the reason why smaller animals may die sooner on stored supplies compared to larger animals with lower energy demands.

Fat Reserves

Fat reserves play a significant role in an animal's survival in environments where food is scarce. These reserves are essentially the energy storage within the animal's body, packed with calories that can be used when external food is unavailable.

• Both the mouse and the dog, in this context, have fat reserves that make up 20% of their body weight.
• Due to the mouse's higher metabolic rate, it will utilize these fat reserves much faster than the dog.

Fat reserves are like backup energy; once the primary sources (daily food intake) are gone, the organism turns to this stored energy.
The faster utilization of fat reserves leads to quicker depletion, meaning that animals with smaller body size and higher metabolic rates, such as mice, are at a disadvantage during periods of no food availability.

Oxygen Consumption

Oxygen consumption is critical, especially when animals must rely on oxygen reserves temporarily, such as during underwater escapes. Oxygen is essential for cellular respiration, the process where cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products.

• Similar to energy and fat reserves, oxygen consumption is higher in smaller animals.
• This is because their higher metabolic rates demand more constant and rapid oxidation of stored energy sources.

The mouse, with its high metabolic rate, would consume available oxygen much faster than a larger animal like the dog. Thus, when trapped underwater and dependent on internal oxygen reserves, the mouse is expected to exhaust these supplies sooner, reducing its chances of survival compared to larger animals. This ties back to the broader concept that size and metabolic demands influence how long an animal can survive without external resources.