A Case Study in Evolutionary Biology and Animal Behavior

Belding’s ground squirrels (Spermophilusbeldingi) are diurnal rodents that live in sub-alpine meadows in the far western United States. Due to the extreme weather, the squirrels hibernate for seven or eight months of the year. They must enter hibernation with sufficient fat stores to survive this long hibernation. The squirrels spend their short active period by initially mating, then eating large quantities of food. They are primarily herbivorous, eating mostly seeds, flowers, and vegetation.

Adult females mate shortly after they emerge from hibernation. After mating, some males disperse to new groups; the others often return to hibernation before the young are born. The females establish territories within the social group and have between three and six pups. The pups emerge from their burrows when three to six weeks old, and the juvenile males disperse (leave to join new groups) shortly thereafter. The females typically remain in their natal (birth) group for life.

Paul Sherman (1977; 1981) studied Belding’s ground squirrel behavior. The squirrels are subject to many dangers, including predation by coyotes, weasels, and raptors. Often, if a squirrel spots a predator, it will stand up on its hind feet and call out an alarm. When others hear the alarm, they quickly retreat to their burrows. Not all squirrels are equally likely to call.

Part I-Experimental Results

Not all squirrels call equally. Examine the following figures and then answer the question.

Question

1. What conclusions can you draw from the data shown in the above figures?

Part II-Sherman’s Conclusions

Females call disproportionately more often than predicted by their abundance. Adult females call more often than one-year-olds or juveniles. Males call disproportionately less than predicted by their abundance.

Questions

2. Why might this be?

1. How do these data compare to your predictions?
2. Why would females call more than males?
3. How should the proximity of relatives influence whether it is cost-effective to call?

Part III-Kin Recognition Mechanism

Kin recognition mechanisms are the mechanisms squirrels might use to recognize that an individual is kin. Kin selection, by contrast, favors related individuals evolutionarily. The kin selection hypothesis requires that individuals can recognize kin. Sherman’s data demonstrate that females are more likely to call when their kin are nearby.

Questions

How might individuals recognize kin?

Can you think of ways to test whether a particular modality (call, smell, taste, and so forth) is important in kin recognition?