When the thrill of love begins in WINTER
At the end of winter, while the cold still lingers and the landscape seems frozen in time, many animals in the Northern Hemisphere are already beginning their reproductive behaviours: courtship displays, songs, territorial marking, fights between males, or the first copulations. At the Zoo, red pandas grow closer, tigers become more vocal, and dholes start seeing one another in a different light.
At first glance, this timing can seem surprising. Yet it is a remarkably precise biological strategy, shaped by evolution to maximize the survival chances of the young.
Day length: the master signal
The primary trigger of seasonal reproduction in animals is neither temperature nor the immediate availability of food, but rather day length, a mechanism known as photoperiodism. Animals detect the lengthening or shortening of days through a finely tuned hormonal system centered on the pineal gland (in the brain) and the hormone melatonin.
As days begin to lengthen toward the end of winter, nighttime melatonin production decreases. This hormonal shift acts as an internal signal that favorable conditions are approaching. It leads to the release of hormones that make animals fertile and trigger courtship behaviours. This mechanism is especially reliable because day length changes predictably from year to year unlike the weather, which can be unpredictable!
Strategies aligned with the seasonal calendar
In the Northern Hemisphere, most species are known as long-day breeders. In these animals, the gradual increase in daylight starting in late winter initiates reproduction. This is the case for many birds, several rodents, and large mammals such as horses.
Why reproduce so early? Because gestation (and the subsequent development of the young) takes time. By mating at the end of winter or very early in spring, animals ensure that births will occur in spring or early summer, a period characterized by:
- Abundant food,
- Milder temperatures,
- Greater availability of nesting sites or shelter.
In birds, for example, territorial songs and courtship displays often begin as early as February or March, while snow may still be on the ground. Eggs, however, are laid later, when insects and other resources needed to feed the chicks are plentiful.
Conversely, some species are short-day breeders, such as deer, sheep, and goats. In these animals, decreasing day length in autumn triggers reproduction. Gestation occurs over the winter, and births take place in spring, once again at the most favorable time for offspring survival.
The role of secondary cues
While photoperiod is the primary signal, it does not act alone. Other factors can modulate or fine-tune the reproductive response, including:
- Actual food availability,
- Temperature,
- Precipitation,
- Individual health.
In certain extreme or unpredictable environments, these factors can delay, accelerate, or even suppress reproduction. However, they rarely replace the light signal entirely, which remains the fundamental temporal reference.
For example, the red panda uses delayed implantation, a reproductive strategy in which embryo development is paused at the blastocyst stage after fertilization. This “pause,” which can last from a few days to several months, allows births to be synchronized with favorable environmental conditions.
The special case of hibernators
In hibernating animals, such as chipmunks, the situation is even more fascinating. Their emergence from hibernation and the onset of reproduction are governed by an internal annual clock known as a circannual rhythm. This clock is calibrated by the day length experienced the previous year and allows the animal to anticipate the coming season, even while isolated from external conditions during hibernation.
As a result, these animals can begin reproductive activity almost immediately upon waking, without waiting for full spring conditions to set in.
In short, when reproductive behaviours begin before winter is over, it is not a timing error but a strategic anticipation. Animals do not reproduce for the conditions of the moment, but for those that will prevail at the time of birth and early growth of their young.
Reproducing at the end of winter is an investment in the coming spring, so that offspring can take advantage of summer’s abundance to grow, gain strength, and prepare for their first autumn.
