The mating rhythms described above are called infradian or long period rhythms. Other infradian rhythms include bird migrations, hibernation, the human menstrual cycle, fur color changes, and reproductive cycles. Although some reproductive cycles such as that of the 17-year cicada are very long infradian rhythms, we are all familiar with the yearly springtime population boom of animals. Seasonal rhythms such as this that last about a year are called circannual rhythms and are usually associated with long-lived plants and animals. These rhythms persist for several cycles and under seasonally constant conditions. In a classic study, Davis and Finnie took woodchucks from Pennsylvania to Australia. Under natural photoperiodic conditions, it took about 3 years for their body weight rhythms to resynchronize to the new environment.[See review by 30].
Why do organisms "plan" their reproductive events to occur only at one time of the year? Temperate organisms synchronize reproduction with favorable environments to maximize their fitness. During the time of birth and feeding of the young, good nutrition is quite important. Fortunately, plants are producing nutritious young shoots and flowers and insects are at their highest numbers. Although food is the dominating ultimate factor of reproduction, it is not the only one. Another consideration is weather. Springtime weather conditions are certainly less harsh than those of winter. Availability of appropriate nest sites is yet another factor.
What physiological or proximate factors contribute to seasonal reproduction? The testis size of seasonally breeding birds and mammals increases dramatically from the non-breeding to the breeding season. Female mammals come into estrus such that the birth occurs during peak environmental conditions. Many environmental cues (ex. temperature, food availability, and social cues) function as zeitgebers for these rhythms, but photoperiod is the most important one, at least in males. In fact, "there is no other environmental factor in any climatic region that is of comparable importance for the immediate control of annual cycles".