University Of Wyoming Research Finds Mule Deer Give Birth On A Tight Schedule

Apr 28, 2021

Mule deer commonly give birth to twins, like these fawns near LaBarge Creek.
Credit Tayler LaSharr

Mule deer migrate twice a year, moving between low elevation winter ranges and higher elevation summer ranges. In the spring they "surf the green wave," following the nutritious young spring plants upwards in elevation.

Previously, it was thought that does timed giving birth to peak green up during this wave, giving their fawns easy access to the most nutritious forage.

"In seasonal environments, like in Wyoming, there's periods of great resource abundance and then periods of major resource deficiencies," said Ellen Aikens, a previous PhD candidate at the UniversityUniveristy of Wyoming."And so when animals mismatch important parts of their life cycle, like timing of birth, this could be really bad for the likelihood that the offspring will survive."

Her research has found that mule deer does have to balance a lot of things when it comes to giving birth.

"If you're a deer in Wyoming, you also don't want to give birth too early or too late," said Aikens. "Because if you give birth too late, maybe your offspring doesn't have enough time to grow and develop before you need to move back down to winter range. If you give birth too early, maybe your offspring would be vulnerable to late season storms or things of that nature."

Aikens and her team tracked the movements of GPS collared mule deer does on their annual migrations for five years and compared them to satellite imagery of the spring green up. They also captured the does at the beginning and end of winter to assess their body condition and, during the March capture, to take ultrasounds and assess fetal development.

They found that only does that migrated long distances and surfed the green wave right up until they reached their summer range matched giving birth with peak green up.

"The vast majority of deer actually did not give birth during peak green up, which is when we think that peak resources are available on the landscape. They actually migrated shorter distances, surfed the green wave for a shorter amount of time, let the green wave essentially pass them by, and then they gave birth," said Aikens.

The deer that migrated shorter distances also tended to use summer ranges that were lower elevation and gave birth earlier in the year than the animals that surfed the green wave the whole way up into higher elevations.

"Basically, those shorter distance migrants seem to be giving birth earlier, and basically trying to maximize the amount of time that their offspring had to grow and develop before they're needed to migrate back down to their lower elevation wintering ranges," said Aikens. "Whereas those deer that surf the green wave for a longer period of time were actually maximizing their exposure to important food resources. And so it seems like there's this trade off between time as a limiting resource and forage acquisition."

According to Aikens, fetus size in March was strongly linked to the mother's ability to surf the green wave and to birth timing, with larger fetuses born earlier in the year to does that followed the green up more effectively. But there was no major difference between fawns after they had been born.

"We actually found that there was no relationship between the body mass of fawns and if they were born early or late with respect to their expected birth date, as measured in March," said Aikens. "So that suggests that there's kind of this threshold that fetuses have to reach before they're ready to go, which is really interesting, and a bit different than what's been shown in the past."

The researchers didn't have the data to determine if the does that traveled shorter distances were mating earlier in the year or adjusting their gestation in some way, but to Aikens, it highlights the importance of timing for the deer.

"If the animal needs to switch their migration strategy for some reason, it's not as simple as switching, no problem. There's a complete lifecycle and series of events that allows an animal [and] their reproductive strategy to be matched with their environment," she said. "So if we as humans mess that up, it's not so easy for animals to just adjust that year. There's things that are downstream, happening during mating season and whatnot, that will have negative repercussions. So it's just another important reason why it's really critical that we maintain these open landscapes so that animals can continue to move freely across the landscape and perform these movements that are so finely tuned to the environment that they live in."

Have a question about this story? Contact the reporter, Ivy Engel, at iengel@uwyo.edu.