It’s commonly assumed that animals give birth with the arrival of spring. But a five-year study by researchers at the University of Wyoming has shown multiple factors play a role in the timing of birth in mule deer.
The study, which included animals that migrate short and long distances, revealed that only pregnant does who migrate long distances from low-elevation winter ranges to high-elevation summer ranges could match their births with prime spring green, according to a UW news release.
For instance, mule deer located in western Wyoming typically give birth in early June, just in time for newborns to grow large enough to withstand winter.
But animals that migrated long distances often carried underdeveloped fetuses that coincided with completing the migration process.
Scientists tracked does by attaching GPS collars that monitored their movements; they also monitored body conditions, studied fetus development through ultrasonography and located newborn fawns.
“In contrast to existing theory, which predicts that conditions at the birth site should shape optimal birth timing, our results provide a clear example of birth timing being shaped by trade-offs arising from events occurring away from the birth site and from other parts of the annual cycle,” researchers said in the announcement.
The study also revealed that most deer end their migration on average 23 days before giving birth, and does that completed migration early also gave birth early.
“Thus, movement tactic had profound downstream effects on birth timing,” researchers wrote in the Journal Ecology. “These findings highlight a need to reconsider classical theory on optimal birth timing, which has focused solely on conditions at the birth site.”