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Using Genetic Evidence to Examine Group Behavior Among Harri's Hawks
Using Genetic Evidence
to Evaluate Group Behavior
Using Genetic Evidence to Examine Group Behavior
In this lesson, students use molecular and behavioral data to answer an ecological question: why do Harris' hawks (Parabuteo unicinctus) live in cooperative breeding groups?. Biologists have observed that the Harris' hawk forms cooperative groups in Arizona and New Mexico when most other predatory birds do not. Students considers reasons animals form groups and learn more about Harris' hawk behavior. They interpret actual DNA profiles and behavioral data from several nests to form hypotheses about why Harris' hawks form cooperative breeding groups. This activity is based upon Robert Sheehy's Ph.D. dissertation and research in the Department of Genetics, University of Arizona.
Behavioral observations of Harris' hawks indicate that they are social predators--they hunt and live together. Nesting groups generally consist of between two and seven adult birds. Individual birds have defined roles . Within an established hierarchy, some birds scare off predator and collect foods for the young but are not allowed near the nest, other birds have a more active role in rearing the young. They may gather nesting materials, feed the nestlings, or incubate the eggs.
Among Harris' hawks, parentage cannot be determined solely by observation
because defined social roles limit access to the nests by most group members.
Furthermore, matings occur away from the nest and are often not witnessed
by field biologists. Insights into the nesting behavior of the Harris'
hawk must be sought through other means. The application of DNA profiling
and molecular biology techniques provides insights into the parentage
and genetic relatedness of the group members.
Biologists were able to safely trap the birds, band the birds (for identification) and collect blood samples. They extracted the DNA from the blood and created DNA profiles for each bird at the nests. The biologists were able to determine which birds were reproducing.
Interpreting DNA Profiles
Bands in a DNA profile are similar to phenotypes because they give an indication of the individual's underlying genotype. They can be thought of as "alleles" which are inherited according to the principles of Mendelian genetics. In this system, there are two alleles: the "marker allele", which produces a band, and the "null allele" which does not. It is not possible to tell whether an individual has one or two copies of the "marker allele".
Every individual has an unique combinations of bands (or DNA profile)
which he inherited from his parents. Siblings also have distinct banding
patterns (DNA profiles) because they inherited different combinations
of bands from their parents. Only identical twins, which share the same
genetic material, have the same DNA profile. Unrelated individuals may
share some bands by chance (just as not all people with brown hair are
related). All bands present in a child's profile are inherited from his
parents. DNA profiles can be used to establish identity or paternity.
Keep in mind:
- An individual has a 50% chance of inheriting a specific allele from one of his parents.
- All bands present in the offspring must be present in one of the parents.
- Unrelated individuals may share some bands by chance.
- If both parents and the offspring have the same band, it is impossible to tell from which parent it was inherited.
- The absense of a band is not informative. An offspring may have inherited a "null allele".
- Thick bands are difficult to interpret; they may represent two close bands or a single thick band.
Group Behavior in Animals
Animals form groups for many reasons, which often are not exclusionary. Groups may form because of environmental, behavioral or genetic factors. In some species of birds, mating is a group behavior; many males will act together to attract females (ex. lecking behavior). Groups also form because of an environmental influence, such as a lack of suitable habitat; group living becomes advantageous (ex. calving areas for Gray whales or colonial nesting for sea birds). Individuals may leave and try to establish territories in a less desirable area or they may join a group as a subordinate. By joining an established group, there is the possibility of mating opportunities, inheriting the territory, or surviving until a suitable territory can be established. There may be a behavioral component which makes group living advantageous, such as increased hunting success (ex. wolf packs and lion prides) or an increased ability to detect predators (ex. schooling fish and herding deer). Juveniles may stay with a group while they gain life skills necessary for survival on their own. When they are older and more likely to survive on their own, they will leave the group and establish their own territory. Animals also form groups for genetic reasons (kin selection). Our relatives share some of our genes. If an individual helps to raise related young, it ensures the survival of at least some of its own genes. An individual shares 50% of its genes with its own offspring, and 25% of its gene's with a sibling's offspring. Thus, helping to raise a few of a sibling's offspring is almost like raising one's own.
Group behavior may be evaluated in terms of an individual's genetic fitness. For an individual, survivorship is not enough. An organism must pass on his/her genes in order to have evolutionary success. Genes of individuals who do not reproduce are lost from the gene pool. Those individuals who survive and produce many offspring have a higher genetic fitness, transmitting many genes to the next generation. Group living results in an increased competition for mates. It may limit an individual's opportunity to reproduce decreasing his/her overall fitness. To overcome this, some organisms stay with a group only until they are able to survive on their own, others may share mates, or they may help to raise a sibling's offspring.
Research on Harris' hawks in the Sonoran desert indicates that their
habitat is most likely saturated. Habitats that appear "good" seem to
support larger numbers of Harris' hawks than habitats that are "degraded".
Among that disperse, there is a high mortality rate. Juvenile birds who
stay with the group and gain life skills are more likely to survived to
reproduce later. Juvenile females are more likely to disperse the first
year, probably founding groups as siblings.
Key:
1. Biologists hope to learn why the Harris' hawks live in groups.
2. The behavioral roles suggest that the alpha birds are reproducing
because they have the greatest access to the nest and hold dominant social
positions.
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3. There are eight bands in n1's profile.
4. Six of these bands may have been inherited from n1's mother (A). (From the top, the 1st, 3rd, 4th and 7th bands could have been contributed only by the mother; the 6th and 8th band may have been contributed by the mother.)
5. Four of these bands may have been inherited from n1's father (B). (From the top, the 2nd and could have been contributed only by the father; the 6th and 8th band may have been contributed by the father.)
6. Yes. Two of the bands (the 6th and 8th) were shared by both parents.
7. No. All of the bands in n1's profile were inherited from his parents.
8. Some traits are common among a population and are shared by chance.
9. No, the behavioral evidence suggests that only the dominant birds
are reproducing while the genetic evidence shows that all of the birds
(even the betas) are reproducing.
10. Answers will vary.
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11. Answers will vary. Harris' hawks probably form cooperative groups for a number of reasons including limited suitable habitat, ability to gain life skills, access to mates, increased hunting success in a desert environment, and kin selection.
12. Answers will vary. No, most students probably thought that only the
dominant birds were reproducing.
13. Using many types of data can provide a clearer insight into ecological
and evolutionary questions; behavioral and molecular data can complement
one another.
Department of Biochemistry and Molecular Biophysics
General Biology Program for Secondary Teachers
warder@email.arizona.edu
http://biology.arizona.edu/sciconn/lessons2/lessons.html
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