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Big Cold Animals

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A new explanation for why animals grow larger at higher latitudes.


Transcript

A new look at an old rule. I'm Bob Hirshon and this is Science Update.

Scientists have long known that animals grow larger at high latitudes than they do closer to the Equator. Now,
University of Houston ecologists Chuan-Kai Ho and Steven Pennings have a new explanation. Pennings had already learned that herbivorous insects prefer plants from high latitudes over the same plants from low latitudes.

Pennings:
It turns out that their preferences actually are revealing differences in the quality of the food—that the high-latitude food is better.

Now, Pennings and Ho report that three different herbivores all grew bigger on a diet of high-latitude plants than on a lower-latitude diet. Pennings says the plants from cold climates appear to have more nitrogen, which makes them more nutritious, and have fewer toxic defense chemicals. They're also softer and easier to eat. And it may be that bigger herbivores make the predators that eat them bigger too. I'm Bob Hirshon for AAAS, the Science Society.


Making Sense of the Research

The idea that animals at colder, higher latitudes tend to grow larger than similar animals at warmer, lower latitudes is called Bergmann's Rule, after the German biologist Christian Bergmann, who formulated the rule in 1847. The rule is actually somewhat controversial; not all scientists agree with it at all, and others debate whether the rule applies when comparing different species, or only different populations of the same species. 

However, the shakiest aspect of Bergmann's Rule has been its most popular explanation. According to the traditional view, larger animals have a lower surface-to-body mass ratio than smaller animals. As a result, they retain heat better. Since animals in cold climates have to hold onto their body heat, while animals in warm climates have to release it quickly, it has been argued that the correlation between body size and altitude reflects adaptations to the local climate. But this argument has never been wholly satisfying, in part because many extremely large mammals, like elephants, thrive in tropical climates. 

Pennings and Ho decided to look at another possible factor: the nutritiousness of plants. It may seem surprising that plants in colder climates can be more nutritious than similar plants found in warmer climates. Certainly, warm climates often have a richer variety of edible plants than cold ones. However, when Pennings and Ho fed insects plants from different latitudes in carefully controlled conditions, they found that the bugs not only preferred plants from high latitudes to the same plants picked from lower latitudes, but that they also grew larger on the high-latitude diet.

It turns out that living in a highly diverse, warm climate has its drawbacks. Plants in these ecosystems tend to face a greater number and variety of predators than plants in chillier regions. As a result, plants in warmer climates tend to develop more defenses, including tougher skins and poisonous chemicals. Plants also develop more nitrogen in cooler climates, and nitrogen is an essential nutrient for creatures that eat them.

Although this study looked only at herbivores, it stands to reason that if herbivores get bigger, the carnivores that eat them might get bigger too. Also, it's likely that plant nutrition isn't the only factor driving the correlation between latitude and body size. However, it could be a missing piece of a puzzle that has eluded scientists for over 160 years.

Now try and answer these questions:

  1. What is Bergmann's Rule?
  2. What is the body-temperature explanation for Bergmann's Rule? What are the problems with it?
  3. In order to reach their conclusions, what kinds of controls did the researchers have to put on their experiment? 
  4. Suppose the herbivores had preferred the plants from higher latitudes, as they did in this study, but that they hadn't grown any bigger eating high-latitude plants than eating low-latitude plants. Would the conclusion of the study be the same? Why or why not?

You may want to check out the March 26, 2010 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: reindeer lack internal clocks, why animals from higher latitudes are larger, and the relationship between size, intelligence, and longevity in dogs.


For Educators

The Science NetLinks lesson Abrupt Climate Change delves into the consequences of a rapidly changing climate to various ecosystems.

The Science NetLinks lesson Introduction to Natural Selection helps students to develop an understanding of natural selection, specifically, how it unfolds from generation to generation.


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