SEASONALITY AND PHOTOPERIODISM
IN VERTEBRATES

LECTURE OUTLINE

OBJECTIVES

1. Demonstrate that seasonal events can regulated by an annual timing mechanism
    or in response to seasonal change in an exogenous cue, such as daylength
2. Establish the diversity of environment cues that can trigger seasonal cycles 
    in animals
3. Explain how daylength (=photoperiod) is measured by animals

I.  INTRODUCTION TO SEASONALITY

    A. Latitude and Seasonality
	1. Timing reproduction to enhance survival of offspring
	    a. Short day and long day breeders
	2. Example of a representative seasonal cycle--Indigo Bunting
    B. Role of endogeneous timers (circannual clocks) or exogenous cues

II.  AN ANNUAL CLOCK (=CIRCANNUAL CLOCK)
   
    A. Characteristics of an circannual clock
	1. Endogenous timer with a period of about a year
	2. Synchronized to an environmental cue
	3. Examples of circannual timing
             a. Annual migration of the Willow Warbler
	     b. Hibernation of ground squirrels

III.  DIVERSITY OF EXOGENOUS CUES

    A. Temperature
	1. Example:  Chameleon reproduction
    B. Food Quantity and/or Quality
        1. Example:  Vole reproduction
    	     a. Importance of fresh grass (6-MBOA) and phenols
	2. Example:  House mouse reproduction
    C. Water Availability
	1. Example:  Peromyscus
    D. Photoperiod (Change in daylength)
    	1. Photoperiod changes with latitude
        2. How is daylength (or nightlength) measured?
             a. Hourglass model measures the absolute length of the night
             b. A daily endogenous clock is involved (=Circadian Clock)
		 

STUDY QUESTIONS

1. List the environmental cues that vertebrates use to control seasonal events. Give an example for each. What factor(s) might determine that one cue is used over another in nature?

2. You have studied the ecology of a desert lizard for many years and have established that this insectivorous species breeds during late April with considerable precision. You decide to investigate the control of seasonal breeding in this species. First, Propose a specific mechanism controlling breeding. Defend your choice. Second, Propose a controlled experiment which tests your hypothesis. Third, Provide the rationale for using this experimental approach. Fourth, What outcome would you predict if your hypothesis is correct?

3. Photoperiodism has independently evolved in many vertebrate taxa which live at temperate latitudes. What is the particular advantage of using photoperiod to cue annual events? But regularly timed events occur in the tropics as well. What factors might regulate tropical events?

4. How are biological rhythms involved in timing of seasonal events? In general, how would you experimentally confirm that a rhythm is involved?

5. Phenolic compounds in the grasses of late summer inhibit reproduction in voles. Set up a couple of experiments to determine the neurophysiological basis for this antigonadal effect. Include appropriate controls in your experimental design. Explain the rationale behind your experiments.