Concept 40.3: Behavior Is Integrated with the Rest of Function

Escape running by pronghorn (Antilocapra americana) is one of the greatest spectacles of animal behavior on Earth (FIGURE 40.6). Although cheetahs (Acinonyx jubatus) achieve the highest speeds of all running animals (about 110 kilometers per hour), they can sustain those speeds for only a minute or two. Pronghorn can run at more than 50 kilometers per hour for tens of minutes—the highest sustained speeds known in running animals.

Figure 40.6: Pronghorn Run at the Fastest Sustained Speeds of Any Running Animals Their running behavior is possible only because they have also evolved systems of oxygen transport, ATP production, and muscular contraction that are up to the task. Pronghorn occur in grasslands from northern Mexico to southern Alberta and Saskatchewan.

Pronghorn strikingly illustrate that an animal’s behavior often depends on—and is integrated with—the animal’s other characteristics. For pronghorn to behave as they do, their muscles must be able to produce enormous forces over long periods. For exercise to be sustained in this way, it has to be based on aerobic, oxygen-based ATP production (see Concept 33.3). Investigators have discovered that almost all aspects of pronghorn are specialized so that the animals can deliver O₂ at very high rates to their muscles, use the O₂ at very high rates to make ATP in the muscle cells, and use the ATP at very high rates to perform intense muscular work. For example, compared with other mammals of their size, pronghorn have exceptionally large lungs and skeletal muscles, and their muscle cells are exceptionally tightly packed with mitochondria.

Toads and frogs have evolved contrasting behavioral specializations that depend on their biochemistry of ATP synthesis

When toads such as the western toad (Bufo boreas) are chased, they hop away at modest speeds that they can maintain for many minutes. In contrast, when leopard frogs (Rana pipiens) are chased, they leap away at lightning speeds—far faster than toads—but they become fatigued and stop leaping after a minute or two. These contrasting escape behaviors in toads and frogs reflect differences in the biochemical mechanisms that their leg muscles use to make ATP for escape running. Toads and frogs thus illustrate that there are often intimate links between escape behavior and the biochemistry of ATP synthesis.

APPLY THE CONCEPT: Behavior is integrated with the rest of function

When chased, both frogs and toads respond by hopping away until fatigue prevents them from going any farther—a behavior dependent on the synthesis and subsequent use of ATP during muscle contraction. Frogs hop away at high speeds using anaerobic glycolysis, a biochemical process that produces large amounts of ATP very rapidly but that can’t be sustained for long periods of time. Toads use aerobic respiration, a process that produces less ATP per unit of time but that can operate for a much longer period. Suppose you make records of speed and duration for two cases of escape behavior as in the figure (these data, although modeled on actual data, are hypothetical).

1. Which animal fatigued more quickly?
2. Which strategy of ATP synthesis allowed for the achievement of a higher maximum speed?
3. Which animal traveled a greater distance? (Hint: Determining the number of squares under each graph of speed versus time will allow you to integrate the information and estimate distance traveled.)

The usual rule, when comparing related animals, is that aerobic ATP production is slow and resistant to fatigue, whereas anaerobic ATP production is fast and subject to rapid fatigue (see Concept 33.3). Enzyme studies show that toads have evolved high levels of enzymes required for aerobic ATP production in their leg muscles. Frogs, by contrast, have an enzyme profile that emphasizes high rates of anaerobic ATP production. These evolved enzyme differences explain the differences in their hopping behavior.

Behaviors are often integrated with body size and growth

Many insects, such as crickets and katydids, sing to communicate with other members of their species. Among these insects there is a broad tendency for the tonal frequencies of their songs to vary regularly with body size. In a pipe organ, the large pipes produce the lowest frequencies of sound. For much the same reason, insect species of large body size tend to produce songs of lower frequency than species of small size. This same trend is observed in singing frogs. Body size helps determine how communication occurs in these animals.

In many species, individuals must grow to a large adult size before they can successfully consummate reproductive behaviors. Male elk (Cervus elaphus) in Scotland, for example, rarely father offspring before they are 5 years old, even though they are reproductively mature at 2 years of age. Being reproductively mature is not sufficient for successful reproductive behavior. A male must also be big enough and experienced enough to dominate other males.

When spotted hyenas (Crocuta crocuta) in a pack capture a zebra or antelope, they are famous for eating everything, including the skeleton, within minutes in a frenzy of feeding. For young hyenas to get food during this group feeding behavior, they need to be as effective as possible in grabbing bites to eat. Being able to bite through bone is a great advantage because it permits much faster feeding than trying to strip flesh off bone. A young hyena’s feeding behavior is transformed—in ways that raise its odds of survival—as its jaws, jaw muscles, and teeth mature, permitting faster and faster bone consumption (FIGURE 40.7).

Figure 40.7: Young Hyenas Must Mature to Feed on Bone Rapidly Spotted hyenas of various ages were given a standardized “bone-crunching test.” The amount of bone they could consume in 15 minutes increased dramatically as they aged and their jaws and jaw muscles became stronger. Young animals are limited in their ability to compete with adults during group feeding behavior at a kill. The line was fitted by linear least-squares regression (see Appendix B).

Let’s now turn our attention to travel behavior. Most animals move from place to place over the surface of Earth during their lives. Travel behavior of this sort can help animals find things they need, but also entails risks, including the risks of getting lost. Travel behavior is one of the important topics in the study of animal behavior.