Does enrichment make your rat sexy?

May 16 2012 Published by under Behavioral Neuro

Are you trying to get a date? Have you tried it all, cologne, flowers, playing it casual, nice dinners? And have you considered environmental enrichment?

What is environmental enrichment? Environmental enrichment is the idea that the brain responds to stimulating environments, with things like decreased anxiety-like responses, decreased cognitive decline with aging, increased coping mechanisms, and even increasing cortical thickness and the birth of new neurons in the brain.

And it turns out, it might just also make you dead sexy. At least, if you're a rat.

(This rat wants you to draw him like one of your french girls. Used with permission from MysticGaia, via DeviantArt)

Mitra and Sapolsky. "Short-Term Enrichment Makes Male Rats More Attractive, More Defensive and Alters Hypothalamic Neurons" PLoS ONE, 2012.

The authors of this study wanted to see how environmental enrichment affected some very innate behaviors: sexual selection by a female, and what they call "defensive behaviors" (which I'll get to in a minute. So they set up a group of male rats, one half with environmental enrichment (a larger cage with tubes to crawl in, toys to play with, and fruity chews, because rats are nuts for fruity chews), and one with a normal control cage. After 14 days of this, they looked at how female rats responded to the males, and then looked at defensive behavior and hormone levels.

In order to look at female choice, the authors put a female in estrous in a large arena in what must have looked like a rat version of "The Bachelorette". On each side was a smaller cage containing a rat with his soiled bedding. One was an enriched rat and one was a control. The authors wanted to see how much time the female spent on each side.

(I have to say I really dislike this data display. It's needlessly confusing)

The female rats in general spent more time deal the enriched male rat, preferring him to the control. The authors hypothesize that this was due to higher levels of testosterone (the enriched rats had higher testosterone levels in response to the female), and that the females were smelling this, and finding the male rats more attractive.

I'm not sure I agree with this interpretation of the data. I agree the female rat spent more time over there, but I wonder if having the actual male rat present (as opposed to just his smelly bedding) is the best idea. After all, rats are perfectly capable of making noises to attract females, trying postures, showing activity. The activity of the males during this time was not recorded, did the enriched males perform more "come hither" behaviors? I think the results might be more clearly in favor of the smell if just the bedding were used.

The authors were also interested in what they called "defensive behaviors", behaviors in an open area or in the elevated plus maze. Normally, rats prefer closed, dark environments. How much they approach the center of an open arena, or the open arms of a maze, is a measure of anxiety. These rats, in contrast to other studies on environmental enrichment, showed increased anxiety-like behavior, spending less time in the open areas and more time in the closed.

The authors also looked at responses to a predator odor (bobcat, specifically).

You can see here that the enriched rats spent less time exploring the predator odor directly, but more time in a stretch-attend posture (when a rat stretches himself out, keeping his hind paws in a safe place). The authors interpret this (along with the anxiety-like behavior) as being an increase in defensive activity, and then hypothesize that these enriched rats are more manly (with higher testosterone), and predators can smell the manly, and thus are more likely to go after them, which means they need to be more defensive.

I'm more than a little skeptical about this explanation. I think the data is fine, and I'm interested that they got this increase in anxiety like behaviors, but I don't think it's an increase in "defensive" behavior. And is there any evidence that testosterone increases make male rats more likely to get eaten? They don't refer to any specifically, and I have to wonder if any exists. In fact, the presence of a female (and subsequent increases in testosterone) usually emboldens males, making them less anxious in response to predatory signs.

So what could be going on? I'm very interested by the increase in anxiety like behavior (and corticosterone, a hormone which corresponds to cortisol in humans and results in response to stress), and I wonder why they got these results, especially since previous results have shown the opposite effect. I am also interested in the female response, are the males really sexier? I would be interested in seeing how they behave when mating and how many offspring they produce. I think the data from this study is interesting, but I think the interpretations could use a lot more thought. But in the meantime, consider environmental enrichment. It might makes you sexier than the other guy.

Mitra, R., & Sapolsky, R. (2012). Short-Term Enrichment Makes Male Rats More Attractive, More Defensive and Alters Hypothalamic Neurons PLoS ONE, 7 (5) DOI: 10.1371/journal.pone.0036092

6 responses so far

  • Dr Becca says:

    Great write-up, Sci! And I agree, puzzling results (and maddening figures! Why not just a frickin' bar graph for Fig 1? This is not something that requires a scatterplot.).

    But what happened to the hypothalamic neurons???

    Also, "Attractiveness" and "anxiety behavior" aren't really two behavioral measures that I'd necessarily think of in the same experimental design. My guess is that they enriched these animals, ran a whole battery of tests, and reported the significant findings. If that's the case, then who knows how one could have influenced the other? These male rats were exposed to a female in estrus, and then presumably not actually allowed to copulate. Could that experience influence the way they responded to the predator odor? ZOMG BLUE BALLS LEARNED HELPLESSNESS!!!! *

    *yes I know this isn't actually a learned helplessness study

    • Scicurious says:

      They got increased dentrite growth in the hypothalamus, sorry. I didn't mention it. It's in alignment with previous findings showing increases in dentritic spines and neuronal growth with environmental enrichment.

      And yes, it felt like a montage of a bunch of experiments they did with environmental enrichment, that they tried to put together. The way they tried to relate the sexual attractiveness and "defensive behavior" is particularly odd to me.

  • DNLee says:

    Think I gotta read this paper for myself. However, it brings up some very interesting ideas/hypotheses about what makes a male attractive and how males with different RHP may behave to attract/retain mates. RHP= Resource Holding Potential which is defined as cues to a male's fighting ability or ability to procure/secure resources that females need/want in order to choose a mate.
    Wearing my Hip hop Ethologist hat, RHP is the swagger that males have that come from knowing he has what it takes to attract a female, father awesome babies and keep everyone well-fed and safe.
    I'd say that enriched environments would be good proxies for resources. Let me read the paper and let you know what I think.

    If nothing else, I know I'm over due for a hip Hop Evolution post and RHP would be a great topic to introduce.

  • Tim says:

    " Normally, rats before closed, dark environments." before->prefer?

  • Amanda says:

    Interesting read! I am studying the association between Toxoplasma and mouse/rat behaviour -- some of the papers are by Sapolsky's group so I was particularly interested to read that you had reservations about the authors interpretations of this "sexy rat" paper, as I have reservations about some of the conclusions in the Toxoplasma papers! Anyway, something that bothers me about this study is the sample size - for most of the behaviour tests there were only 6 animals in each group. I know from my own work that lab rats have their own personalities, and there is natural variation in their behaviour. 6 animals is not enough to test these sort of behaviours!!

    Oh and I completely agree with your annoyance at the way the data is presented - another thing this paper has in common with the Toxoplasma papers by the same group. Overly complicated for no reason! Makes me feel like they have something to hide 😛

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