Hormones II

Question 1

What makes circannual cycles adaptive? Why do tests of circannual rhythms have to take at least two years?

Answer 1

1. Changing of seasons throughout the year
2. Because the cycle lasts for one year, so to test the cycle you would need a control year than then the test year (or possibly that it would take two years to see any effect)

Question 2

What are two examples of circannual rhythms? Elaborate.

Answer 2

1. Golden-mantled ground squirrels blinded after birth and kept in the dark at constant temperatures for 4 years - all showed circannual cycle of hibernation followed by activity
2. Nestling stonechats from Kenya –> Germany - reared in constant photoperiod and temperature led to circannual patterns of molt –> gonads –> regression of gonads (mating season) over 7 years

Question 3

What are two examples of a species with a lunar cycle? Is this really a good example of a lunar cycle, why or why not?

Answer 3

1. Kangaroo rats - avoid being out during day and when moon is full (predators) but when food runs short in springtime, they will go out regardless of whether the moon is full or not.
2. Lions hunt more efficiently without moonlight (leaner when moon is out)
3. These are not evidence of lunar cycles because it’s dependent on the amount of light each day (especially for Kangaroo rats) and not by the month.

Question 4

What is the purpose of annual cycles and breeding in songbirds? What is an example? Elaborate. Is this circannual?

Answer 4

1. ANTICIPATE breeding (not respond to a breeding opportunity) because gonads have to grow themselves
2. White-crowned sparrows - overwinters in Mexico –> northern US in spring (these birds MUST anticipate changes in the environment because territories are set up as soon as males arrives to each site)
3. No, it is circadian (by day, not by year, sad)

Question 5

What makes the white-crowned sparrow example of breeding a circadian clock and not a circannual clock (i.e. what is the mechanism)? What happens if you rear the birds with very long nights? What hormone stimulate this growth? What part of this cycle IS a circannual rhythm?

Answer 5

1. Mechanism - clock that is sensitive to certain number of hours in the day being light

This resets at dawn (insensitive to light during the day). Then sensitive again (in other words, the clock is sensitive to the times of dawn and dusk, which measure the beginning and end times of the day). If days are long, these sensitive areas light up, and testes growth occurs, short days = nonsensitive = no growth.

2. Gonads will eventually form because the every other day there will be light during the photosensitive period of time even if there’s more dark than light total (not circannual)
3. LH hormone is released in peak amounts at the same time every day.
4. After breeding season is over, there is a refractory period of no growth (insensitive to light) that ends in late winter. This is presumed to be circannual.

Question 6

What are two conditions that effect circadian cycles or where cycles are based on these things? What is an example of each?

Answer 6

1. Rain - Rufus-winged sparrows - food is likely after summer rains so LH spikes only after it rains because that is the best time to breed

**Bonus: In these birds, the LH hormone increases the brain part that encodes song and causes also an increase in the song rate itself

2. Food - Crossbills - some years pinecones are available all year long (mast years) and in others they can be hard to find at any time. This causes foraging rates to be the determinant for breeding

**Bonus: This is still an annual pattern though too because no matter what there is no growth in testes in December/January (think freezing temps)