Long term cycles

Beyond circadian rhythms Environment independent Difficult to test (study must run at least 2 years) Circannual rhythms run on a 365 day cycle Hibernation Migration Mating proclivity/ability Examples As circadian rhythms help animals prepare for daily changes in light, risk of predation, source of food, etc. can animals prepare for annual changes in habitat or food availability that are seasonal?

Hibernation 5 golden mantled ground squirrels were born in captivity and blinded. They grew up in constant temperature with abundant access to food. Still hibernated at regular intervals.

Reproductive physiology The stonechat lives in Kenya and is subject to a spring rainy season during which there is an abundant supply of insects available to feed the young birds. In anticipation of this richness, males undergo seasonal changes in testicular growth. The free running cycle shown above under constant conditions in a German lab is slightly faster than 1 year, as shown by the leftward shift of the grey lines over time.

How does the bird know spring is coming? White-crowned sparrow Winters in Mexico or SW United States Flies north in spring to NW US, Canada, or Alaska Courts receptive females, defends territory, fights rivals, etc.: Reproductive activity Gonads grow from the winter state, which is 1% of the spring/summer size. How are these behavioral and physiological changes coordinated at the right time of the year?

Hypothesis The clock exhibits a daily change in sensitivity to light that is reset every day at dawn Clock is insensitive for the first 12 hours of light, then steadily rises to a peak around 16-20 hours after the start of the cycle, then fades again If the days are less than 12 hours, the system will not be activated because no light is present during the photosensitive period.

Experiment If the photosensitive hypothesis is correct, you should be able to trick it by manipulating day/night length. Start with 8H light, 16H dark: no stimulation during photosensitive period (days are short, testes don’t grow) Shift to 8H light, 28H dark: WHY?? (what’s an alternative to 8L:28D and why is that bad?)

Not dependent on the absolute amount of day light but on whether that day light occurs during the internally set photosensitive phase of the day.

Discussion Question In another experiment with white-crowned sparrows, males that had been on a 8L:16D schedule were held in complete darkness for periods ranging from 2-100 hours before being exposed to a 8L block. Several hours later, researchers measured levels of LH (released from the anterior pituitary and causes testes to grow). How do the following data test the photosensitivity hypothesis further?

Proximate causes of circannual shifts Change in photoperiod does not always reliably predict access to food or mates Dry winter, delayed thaw, etc. Arizona sparrow Testes grow in March, as days lengthen, but following a dry winter, they don’t breed Rainfall during the July monsoons triggers production of LH, which stimulates testes to release testosterone which increases song production and reproductive behaviors Rainfall is a more reliable predictor of resources for offspring than photoperiod changes so this sparrow has adapted to use that proximate cue to regulate behavior

Lunar Cues Foraging only during moonless nights when seed cache is abundant (top, white), some nighttime foraging as seeds start to run out (yellow), more later in season (blue)

Lunar Cues

Effect of food itself Red crossbill finches reproduce whenever food is abundant—take advantage of good conditions regardless of season However, breeding is down in the shortest months December and January even when food is abundant Suggests some internal clock that regulates physiology despite environmental cues In fact, under constant conditions, testes do shrink and hormone levels decline from Oct- Dec, consistent with the photoperiod sensitivity exhibited by other temperate birds

Lack of breeding populations in Dec and Jan.

Social cues Crossbill finches caged with females underwent reproductive growth faster than males kept in solitude One 60 minute exposure to a female was sufficient to raise testosterone levels and initiate singing As distinct from environmental cues like rainfall, photoperiod, food abundance.

Social cues in mice Prior experience determines subsequent interest

Social cues in mice Exposure to a dominant male’s odor triggers the addition of new neurons in the mouse brain, leading to rewiring that allows her to effectively detect a dominant male again should she encounter one. Male mice also respond to social cues: after mating, male mice become infanticidal, killing all pups encountered within 3 weeks of ejaculation. Guess how long the mouse gestational period is?

How does the male control the switch from killer to parental? Internal timing device counting off from time of last mating? If this is true, we should be able to manipulate the infanticidal period by altering the perceived length of a day Fast Day group: Day is 11L:11D Slow Day group: Day is 13.5L:13.5D Predictions??

A timing device in males Detects number of light-dark cycles since last mating Controls male hormone? Testosterone? High Testosterone=high aggression and infanticide Progesterone? High progesterone=suppress parental behavior

Experiment Hypothesis: If progesterone is key to infanticide, then males lacking progesterone receptors should not kill pups Generate a genetic knock-out of the PR, expose males to new pups PR-/- males do NOT kill pups Levels of Progesterone and Testosterone are the same as in wild-type mice Open questions? How does the timing mechanism regulate PR sensitivity? Levels? Regional expression? Female mice are also infanticidal, but measure the pregnancy in absolute time, not dependent on daylight cycles.