When an organism adapts its physical and behavioral characteristics as per its surrounding environment in order to survive better is known as ‘adaptation’. Living organisms always try to adapt to their habitat as they have special features, which arise as a result of evolution. Evolution is a slow process that brings inheritable changes in the population and aids in survival and reproduction. It results due to gene mutation. One example of such adaptation is named ‘Bergmann’s Rule’, which was given by a German biologist ‘Karl Bergman’ in the 19th century.

What is Bergmann's Rule?

Bergmann's rule tells us that the animals or organisms residing at a higher altitude should be larger and have a thicker coat than those that are living at lower altitudes and found close to the equator. Let's say, for example, in Canada, you will find a lot more white-tailed deer than you will find in Florida, this is because of its altitude and climate. This rule that was proposed is known as Bergmann's law in the 19th century because of Karl Bergmann, a German Biologist. The latest study also reports that turtles and salamanders also live by Bergmann's rule.

Bergmann Rule Ecology

Bergmann said that the total surface area of an animal's body plays a major role in the dissipation of heat, and in producing heat, the volume is considered The animals which are large usually have a larger surface area than compared to the smaller animals who obviously have a smaller surface area. This larger surface area prevents the larger animals from emitting a lot of heat into the environment and out of its body and, therefore, does a good job in keeping them warm and cozy during the winters. There are some species like insects and tapeworms who do not require lungs to expand their surface area. Whereas, the larger animals need specific body functions to transport food and oxygen from the exterior to the insides.

Deep-Sea Gigantism

According to deep-sea gigantism, the animals and organisms found residing in the depths of the sea are larger in size when compared to the ones found in the surface water. This increment found in the sizes of the organisms residing in the deep seas can be elucidated with the help of Bergmann's rule. As per Bergmann's rule, as there is a drop in the temperature, there is an increment in the size of the animals. The cold climatic conditions give rise to the expanded cells that make them up and also their lifespan. Let's take, for example, the colossal squid. It resides 7,200 below the sea. Due to the depth of the sea, the squid is capable of being large in weight and height.

Bergmann's Rule Exceptions

The birds residing in California are excluded from Bergmann's rule. The large population of birds found in California is quite the opposite of what Bergmann's rule states and contradicts it. The rule states that animals found in colder climates and higher altitudes are larger in size and have a large surface area; however, these birds are small. The birds in California, therefore, must be assumed are decreasing in size due to the increasing global warming. However, studies show us that the birds found in California are expanding in weight and have an increased wingspan from 2% to 5% in the time span of 40 years.

It was also claimed that these birds, to protect themselves from the cold weather, are now storing more fat in their tissues. Naturally, larger birds suited best for the climatic conditions and the cold environment are being chosen and are, therefore, is found in larger numbers now. Due to the variations in the climatic conditions, these birds have started to change their diets. No longer do they feed on the same insects and worms that they would once feed on. These birds change their feeding habits according to how warm or cold the climate is. Therefore, the birds residing in the colder climatic regions would not feed on the same insects as the birds in the warmer climatic regions. This could have resulted in a significant increase in the mass of the bird's body. However, the concept pertaining to the increase in sizes in birds is still not clear even amongst scientists.

Conclusion

Bergmann’s rule is one of the popular generalizations in zoology, which states that size of animals tend to increase with increase in latitude and decrease in temperature. We discuss examples of animals that follow this rule and some of the exceptions as well.

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FAQs on Bergmann's Rule: Definition, Exceptions & Biological Significance

1. What is Bergmann's rule in ecology?

Bergmann's rule is an ecogeographical principle that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. This rule primarily applies to endotherms (warm-blooded animals) like mammals and birds.

2. How does a larger body size help an animal survive in a colder climate?

A larger body size provides a survival advantage in colder climates due to the relationship between an animal's surface area and its volume. Larger, more compact animals have a smaller surface-area-to-volume ratio. This means they lose body heat more slowly relative to their mass, making them more efficient at conserving heat in a cold environment. Conversely, smaller animals have a larger surface area relative to their volume, leading to faster heat loss.

3. What are some classic examples of animals that follow Bergmann's rule?

Several animal species demonstrate Bergmann's rule clearly across different latitudes. Key examples include:

Bears: Polar bears, which live in the Arctic, are the largest species of bear, while species in temperate or warmer climates, like sun bears, are much smaller.
Penguins: The Emperor penguin, found in the extreme cold of Antarctica, is the largest penguin species. Smaller species like the Galápagos penguin are found in much warmer climates near the equator.
White-tailed Deer: Deer populations in colder northern latitudes (like Canada) are significantly larger and heavier than their counterparts in warmer southern latitudes (like Florida).

4. What is the main difference between Bergmann's rule and Allen's rule?

Both rules relate to thermoregulation but focus on different aspects of body form. The key difference is:

Bergmann's Rule deals with overall body size (mass). It predicts that animals in cold climates will have larger, bulkier bodies to conserve heat.
Allen's Rule deals with the length of appendages (limbs, ears, tails). It predicts that animals in cold climates will have shorter appendages to minimise heat loss, while animals in hot climates will have longer ones to help dissipate heat.

They are often considered complementary principles explaining how animals adapt to different temperatures.

5. Does Bergmann's rule apply to humans?

Yes, human populations generally conform to Bergmann's rule. Populations native to colder, high-latitude regions, such as the Inuit of the Arctic or the Sami of northern Europe, tend to have a heavier and stockier build. This body type helps to conserve heat. In contrast, populations native to hot, tropical regions often have a more linear build with less body mass, which is more efficient for cooling.

6. What are the major exceptions to Bergmann's rule?

Bergmann's rule is not a universal law and has several important exceptions. It often does not apply to:

Ectotherms (cold-blooded animals): Reptiles and amphibians may show an inverse trend, where larger body sizes are found in warmer climates, as they depend on external heat for their metabolic activity.
Hibernating Animals: Some mammals that hibernate may not follow the rule, as their survival strategy is based on avoiding the cold rather than enduring it.
Invertebrates and Plants: Many invertebrate groups and vascular plants show the greatest size and diversity in the tropics, directly contradicting the rule.

Other ecological factors like food availability and predation pressure can also override the influence of temperature on body size.

7. How is the concept of 'deep-sea gigantism' related to Bergmann's rule?

Deep-sea gigantism is the tendency for species of invertebrates and other deep-sea dwelling animals to be larger than their shallower-water relatives. This phenomenon is consistent with Bergmann's rule because the deep sea is an extremely cold environment. According to the rule, a larger body size would be advantageous for conserving heat in these frigid conditions. While other factors like high pressure and food scarcity also play a role, the temperature-size relationship described by Bergmann's rule is considered a significant contributing factor.

8. Why is Bergmann's rule considered a generalization and not a universal law?

Bergmann's rule is a generalization because an animal's body size is influenced by a complex interplay of factors, not just temperature. Other selective pressures can be more important in certain environments. For example, food availability can limit an animal's potential size, regardless of how cold it is. Similarly, high predation pressure might select for smaller, more agile bodies, or conversely, for very large, defensively robust bodies, independent of climate. The rule is most reliable when comparing closely related, widespread endothermic species.