Why Male Ground Squirrels Disperse

Question 1

General hypothesis: an increase in circulating gonads hormones triggers dispersal in juvenile males
Prediction: levels of circulating androgens should be elevated in juvenile males at the time of natal dispersal
Results?
Support?

Answer 1

Results: no increase in circulating androgens was detected at the age when dispersal typically occurs
Support? No the hypothesis is not supported

Question 2

Hypothesis: an increase in circulating gonadal hormones triggers dispersal in juvenile males
Prediction: in the absence of androgens, males should not disperse
Results?
Support?

Answer 2

Results: castration had little effect on natal dispersal
Support: no this hypothesis is not supported

Question 3

Hypothesis: androgen exposure early in life leads to developmental changes that allow later dispersal
Prediction: exposing perinatal or neonatal females to androgens should masculinize subsequent behavior, including natal dispersal
Results?
Support?

Answer 3

Results: 75% of females exposed to androgens dispersed
Support: yes

Question 4

Aggression hypothesis: increased aggression toward juveniles pushes them away
Prediction: prior to the time of dispersal, the frequency of severity of agonistic behavior between adults and juvenile males should increase
Results?
Support?

Answer 4

Results: adults neither attacked or chased juvenile males more frequently or vigorously than juvenile females, no increase in aggression toward juvenile males at the time of dispersal
Support: no

Question 5

Litter size/litter mate avoidance hypothesis: males leave to avoid contact with litter mates
Prediction: the bigger the litter, the more likely they are to disperse. Or the higher number of sisters, the more likely they are to disperse
Results?
Support?

Answer 5

Results: no significant relationship between litter size or sex ratio and dispersal behavior
Support: no

Question 6

Ontogenetic switch hypothesis: makes wait until they’re energy stores are sufficient to help them survive
Prediction: juvenile males will disperse when they attain a threshold body mass and that disperses should be heavier or exhibit different patterns of weight gain, than pre dispersed males of any ages
Results?
Support?

Answer 6

Results: emigration dates were correlated with the time at which males reached a minimum body weight of about 125 grams
Support: yes

Question 7

Food shortage hypothesis: avoidance of food shortages
Prediction: juveniles whose natal burrow is surrounded by abundant food should be more philopatric than those from food poor areas; immigration to food rich areas should exceed emigration from them
Results?
Support?

Answer 7

Results: juvenile male immigration equaled emigration every year
Support: no

Question 8

Food shortage hypothesis: avoidance of food shortages
Prediction: dispersing individuals should be in poorer condition (perhaps weigh less) than males if the same age residing at home
Results?
Support?

Answer 8

Results: dispersing males were significantly heavier than pre-dispersal males
Support: no

Question 9

Food shortage hypothesis: avoidance of food shortages
Prediction: food requirements of young males and females should differ
Results?
Support?

Answer 9

Results: juvenile males and females are similar amounts of the same plants and at similar rates
Support: no

Question 10

Burrow hypothesis: increased chance at finding a suitable burrow
Prediction: dispersers should emigrate from areas of high population density or low burrow quality to areas where unoccupied holes of high quality are available
Results?
Support?

Answer 10

Results: neither the probability of juvenile male dispersal nor its timing was significantly related to population density or burrow availability near home, dispersers did not settle in areas of higher burrow density
Support: no

Question 11

Ectoparasite hypothesis: avoidance of ectoparasites
Prediction: is parasites build up in the natal nest and if juvenile males are more affected by them than are juvenile females, then more males in particular might emigrate to avoid them
Results?
Support?

Answer 11

Results: low levels of ectoparasitism throughout the animals’ natal summer, and no consistent differences between infestations in males and females
Support: no

Question 12

Incest hypothesis: avoidance of incest
Prediction: incest will decrease fitness; the more mates a male has, the further he should move from his breeding area
Results?
Support?

Answer 12

Results: not tested because never found any cases of males breeding with relatives; confirmed
Support: no, yes