3 does in wheat field

Summer is in the home stretch. And while we’ve been going to fairs, baseball games, and festivals, bucks and does have been doing what they always do – avoiding one another. 

In the past, we’ve been asked how bucks and does interact in the summer. They answer is they don’t. Sexual segregation is a hallmark of dimorphic ruminants. It is defined as differential use of space or resources by sexes outside of the mating season. It is no more pronounced than in the summer for white-tailed deer. Indeed, it has been proposed that the sexes of dimorphic ruminants behave as if they were separate but coexisting species. 

3 does in wheat field

I know “because that’s just the way it is” isn’t going to cut it as an answer for our readership. So I’m on the case. 

If you want to read about all the competing hypotheses regarding sexual segregation in dimorphic ruminants, chapter 6 (Spatial Use of Landscapes) in Biology and Management of White-tailed Deer takes a deep dive. And when I say deep, I MEAN deep. It slays 6 different hypotheses (reproductive strategy, activity-budget, social-factors, scramble-competition, forage-selection, and Bell-Jarman) in detail. Basically saying those are all trash. I was a bit taken aback by some of the candid and straightforward language used to describe them: “makes its value to either theory or management nil”; “has only peripheral value”; “not a fruitful endeavor”; “Regrettably, this is not an improvement.” Yikes!

You know that saying “go with your gut”? Well, that’s exactly what deer do!

The reason why bucks and does don’t hang out together in the summer or any other time of year (except when they have to) is because their bellies are different sizes. 

Enter the gastrocentric hypothesis. It is based on metabolic requirements, minimal food quality, and digestive retention. Simple anatomy (and associated functions) explains the sexual divide. Honestly, this even explains behaviors in my own household.

The “guts” of this hypothesis are as follows:

Males have a larger ruminal capacity. This allows for prolonged retention and greater use of fiber (plant cell walls) for energy.  Males are predicted to consume abundant forages high in fiber because they can efficiently process it.

Females are better suited for post-ruminal digestion. They don’t have the room for high fiber diets. And during times of high energy demands (lactation!), they focus on moving food through the system quickly extracting nutrients from plant cell contents by increasing in the size of the small intestine and mucosal surface prolonging the passage of forage thereby increasing nutrient extraction. 

Voila! Sexual segregation explained.

Many of you may be looking like Princess Buttercup after Westley explains the story of Dread Pirate Roberts

As a counter to that, I will invoke Inigo Montoya! “Let me explain. No. Let me sum up.”

Males = low quality, high volume

Females = high quality, low volume

This affects the microbiome of the rumen as well which reinforces these differences.

Male and female ruminants are sexually segregated because they don’t have the same needs. 

Because male rumens are acclimated to high fiber, selection of low quality diet by males in late summer is reinforced until quantity of forage is limited. Unlike females who are limited to areas of high-quality forage to meet demands of gestation or lactation.

When high-quality forage is abundant or when reproductive demands of females are low, sexual segregation is not seen. This hypothesis is also consistent with observations of immature males associating with adult females. They have not reached large rumen capacity and use same high quality forage until full grown.

I have to admit when I first started researching this topic this is not the answer I was expecting. However, the gastrocentric hypothesis explains the widespread occurrence of sexual segregation. Which is what a hypothesis should do.

Females aren’t excluding males from high quality habitat. Males aren’t aggregated so they can size each other up over the summer. 

Does and bucks are just following their gut. 

-Jeannine Fleegle
Wildlife Biologist
PA Game Commission

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