LECTURE 11
SEASONAL REPRODUCTIVE CYCLES--MAMMALS
PLEASE PRINT OUT ALL FIGURES AND BRING THEM TO LECTURE

I. General Introduction

a. Most vertebrates show seasonal variation in their reproduction.

  • This variation is sometimes dramatic, with all mating or birth taking place during a restricted part of the year.



  • Sometimes the seasonality is not prominent. Only a tendency for concentrated activity is observed.





    b. Whether an animal reproduces continuously or seasonally depends on the environment. Seasonality is the most dramatic effect of environment.





    c. Latitude is a good predictor of annual extremes in temperature. There is good correlation among latitude, plant growth, and the tendency to reproduce seasonally.





    d. Patterns of seasonal reproduction

  • Annual only certain time of the year

    -Can be in response to predictable environmental event, the most important ones being photoperiod, climate (e.g., rain fall, temperature) and food source.







    -Found in temperate regions

  • Biennial (every other year)

    -May be every third or fourth year also.




    -More common in temperate regions.




  • Opportunistic

    -Reproduction occurs at a different month from one year to next.




    -Response to environmental event, such as rainfall.




    -Unpredictable.




    -Found in both tropics and deserts, when rainfall varies from year to year.






    -Sometimes the environmental factors optimal for reproduction (e.g., rainfall) is bimodal. Opportunists can also reproduce in bimodal fashion.







    II. Photoperiodic responses of mammals

    a. The pineal gland--the endocrine gland that transduces photoperiodic signal

  • Located in the epithalamus--the dorsal diencephalon.




  • Receives photic signals from the eye via complex neural pathways.






  • Pineal gland synthesizes melatonin






  • Melatonin is secreted in the dark phase of the cycle







  • Melatonin can be used to phase-shift the circadian rhythm and is responsible for the reproductive seasonality of many species of mammals.




  • Pineal gland, in essence, is the intermediary between the external photoperiod and internal environment. It is the site at which light/dark information is translated into a chemical messenger that is responded by the body. Melatonin is the "chemical expression of darkness".





    b. How pineal gland influences the seasonality of reproduction in short-day breeders

  • Occurs in several deers and domestic sheep.




  • Because of the gestational period, fall mating allows the development of young by the following spring.




  • Short-day breeder (sheep) hormone profile






    -When rams were moved abruptly from 4 months of long days (16L:8D) to short days (8D:16L), there was a resurgence of gonadotropins in less than 2 weeks and an increase in testicular size in 2-4 weeks.




    -When rams were returned to the long days, gonadotropins quickly dropped to a minimum, and testicular activity declined slowly.




  • If the animal is given melatonin during summer, they will become reproductive.




  • In short-day breeders, melatonin promotes reproduction.




  • If the animal is pinealectomized (pineal gland removed), it will still respond to seasonality for a short period. However, the seasonality will become less and less synchronized with time, and cycle becomes poorly defined.





    b. How pineal gland influences the seasonality of reproduction in long-day breeders

  • Occurs in hamsters




  • Short gestation period allows them to mate and give birth in the summer.





  • Consequence of pinealectomy in long-day breeders






  • In long-day breeders, melatonin inhibits reproductive activity.




  • Pinealectomy results in continuous reproductive activity.