The sexiest mammal on earth?
Consider the love life of the meadow vole, the sexiest little mammal on Planet Earth.
Voles are small mouse-like mammals who live all around North America in prairies and meadows.
They are sometimes called field mice.
Like Charles Darwin’s finches on the Galapagos Islands, who show small changes in the shape of their beaks, depending on the food they are specialized to grasp, voles are a wonderful natural experiment in evolution. Like Galapagos finches, they live in strikingly similar places, with similar food, predators, social habits and the like.
But some are voles are monogamous and other are polygamous.
What are the biological pressures that cause some mammals to be faithful to their mates? (pair bonding)
What conditions favor sleeping around?
So you see, meadow voles ARE relevant to human behavior.
This is a fundamental question in evolution, because Darwinian adaptation has only two big drivers — the chances of survival and the chances of reproduction.
To give a human example, humans living in traditional European culture tended to favor monogamy — though not 100% of the time. After the Sexual Revolution — when birth control became practical — the same genetic population tends to favor multiple sexual partners.
(Oddly enough, modern urban cultures do not reproduce faster than their parents did two generations ago. Urban cultures do not replace their members, and having more partners does not necessarily mean more children.)
Voles are a wonderful “model species,” since they live only a few months and reproduce with amazing rapidity. Vole populations can explode very fast under good conditions, and collapse very quickly when predators multiply, or when the weather turns bad. Males compete with each other for access to fertile females, and females may compete with each other for access to males.
The problem with being a mole is reproducing before you die. You average life span is only two months — but if you luck out, dodge all the predators, get lots of food, you could live for 10 months. Still, the AVERAGE mole has to be very busy getting on with business.
In your average two-month lifespan you need to:
1. Grow to sexual maturity: around a week.
2. Mate selection, beating the competition, and insemination: two to three months.
3. Time of pregnancy (gestation): less than one month.
Now, notice that if you add up points 1.-3. above, you get almost twice as long to reproduce as the average life span of the poor meadow vole.
This SEEMS like a contradiction, but it’s not. The average is only the peak of a distribution of life spans. Moles are hunted fiercely by cats, owls, dogs, and other predators. To make up for their short lifespans, moles have multiple litters of about five pups each. Sheer survival pressure selects only some of the moles in each generation to reproduce. The unlucky ones never reproduce.
In other words, if you’re a male vole you need to impregnate a female before some cat or owl eats you at the end of your two-month average life. If you’re a female, you need to get pregnant around one month of age to have at least one litter before the end of your average life.
Everything is a race for time.
Your average vole doesn’t have time to live out their Golden Years in leisurely retirement.
With ultrafast reproduction, local populations can expand very fast, only to collapse when predators catch up with that great new food supply. As Darwin wrote, animal populations expand to use all the available food, space, and other resources, and collapse when they bump into a natural ceiling — they run out of food, or predators multiply — only to repeat the cycle again and again. Each population collapses could mean the end of the species, but somehow mouse-like mammals have been around for about 200 million years! Individual moles have little time, but as a biological system their species is amazingly successful.
Think of it as a high-speed High School dating scene. Ups and downs, every minute of the day.
Voles act can be sociable when they’re not in the mood for love, and very aggressive when they are. Males fight to gain the favors of a female — but only if the female shows readiness by odor signals (from hormones) and by receptive behavior.
Once meadow vole lovers meet, they bond for life. And yes, they do cheat on their partners at times, but once a mate dies, the grieving survivor may never take another mate.
This creates a natural experiment: What is the difference between monogamous and multi-partner voles?
Voles in Love.
The meadow vole is adapted for pair bonding with a single partner. The male vole has a continuous contact with its female, which lasts for all of their lives.
Pair bonding is related to the oxytocin and vasopressin hormones. The oxytocin receptors of the female are located more densely in the brain reward system, and have more receptors than other species, which causes a sort of an ‘addiction’ to the mate.
I’m hooked on you
Oh, oh, oh, oh
I’m hooked on you
What can I do? It’s over you
In the male meadow vole, the gene for the vasopressin receptor has a longer segment, as opposed to the montane vole, which has a smaller segment. This segment is longer in other bonding animals (such as humans) and shorter in other nonbonding animals (such as chimpanzees). (Hat tip to Wikipedia).
So prairie voles are romantics.
… because VOLE spells L-O-V-E backwards!
(So much for science.)
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