Sunday 22 August 2021

Life Lessons from Monogamous Mice

As a general rule, male mammals don't have much to do with raising their young. In many cases, that's because the species in question prefers the solitary life, and the two sexes only meet up to mate before parting ways again. In more social species, males and females may still live in different herds for most of the year, often because they have different feeding requirements and want to avoid competition. Even where are there mixed-sex groups, the males often spend more of their time fighting off rivals than they do actually looking after the young they sire - although, in this instance, they may be helping indirectly by protecting the herd from predators or the like.

But there are plenty of exceptions. 

This is most likely to occur where raising young is a lengthy or otherwise arduous process, where it really helps to have two parents who'll stay the distance. This leads to the development of monogamy, where long-lasting bonds are formed between two individuals. This, of course, by no means unique to mammals. It's considerably more common among birds, for example, due to the demands of incubation and the need to protect and feed fledglings, and a surprising number of fish species are also monogamous.

While the primary purpose of this is to allow the parents to share duties, it isn't necessarily the case that they do so equally. A common pattern is for the mother to stay close to the young (she needs to suckle them, if nothing else), while the male patrols further afield, foraging for food and staying on the lookout for predators. It's not very enlightened from a human perspective, but these are animals... and, in any case, there are several exceptions to this rule, too.

The poster child for sexually egalitarian monogamous mammal species is surely the California deer mouse (Peromyscus californicus). Deer mice are a group of over fifty species of mouse-like animal native to North and Central America, including the widespread and common North American deermouse (P. maniculatus) that's found across most of Canada, the contiguous US, and Mexico. I say "mouse-like" because whilst they look extremely similar to house mice (they tend to have more distinctly white underparts, but that's about it) they are not members of the true mouse family, but are instead cricetids, placing them closer to hamsters and packrats than to house mice and sewer rats. Which is true of rather a lot of American "mouse" and "rat" species.

The California deer mouse is far less widespread than its better-known kin. In the US, it's found only in California, from about San Francisco southwards, and it reaches its southern extent about a quarter of the way down the Baja California peninsula in Mexico. It lives in both forest and chaparral scrubland, avoiding the deserts and mountain ranges of eastern California and, for that matter, the irrigated lowlands of the San Joaquin Valley region. 

Unlike most other deer mouse species, the California kind is highly territorial. In an unusual twist on the usual mammalian pattern, when the young are old enough to leave home, males tend to stay close to where they were born, and quickly establish their own territory to defend. Females, in contrast, travel much further, in search of an unrelated male to settle down with. Once they find a life partner, they remain faithful to them; DNA tests of parentage in 28 families failed to find a single instance in which one parent had cheated on the other. Only if one partner dies will the survivor try to find a new one.

Moreover, once they do establish a territory and start to produce litters of children, both parents share in defending the territory from threats. Exactly how this is done seems to vary between pairs. In some cases, if a possible threat is spotted, both parents will head out to investigate it, but in others, only one will venture forth, while the other stays behind with the pups... but it's as likely to be the mother going out to do the manly adventuring while the father stays at home as the other way around. But, if it isn't decided by pre-defined gender roles, how do the mice decide who does which? Is it just the case, perhaps, that some mice are braver than others, or does something change between them when they pair up?

The first thing we need to know if we're going to try and answer this question is whether some California deer mice are, in fact, naturally brave while other's aren't. In fact, we already know that this is true in some closely related species, where it seems to be an inherent part of their individual personalities, not something foisted upon them by their current circumstances. But, even if this is the case, circumstances can, of course, change, and it may be that paired California deer mice change their behaviour to accommodate that of their partner and provide the best division of labour and potential risk.

To tease that apart, we need to see how the mice behave before and after they have found a partner. Researchers recently did just that, taking lone deer mice in captivity and seeing how they responded to playbacks of other mice making aggressive barks. If they were given the choice, when hearing what seemed to be an aggressive individual approaching, would they head towards the sound to see what has happening, and drive off the supposed intruder, or would they go and hide in a prepared "safe" place?

There was, as expected, considerable variation between individuals. It wasn't a simple split between brave and timid, of course, but a sliding scale of behaviour. Nonetheless, it seems that California deer mice do have varying personalities, with some more risk-averse than others... and no real distinction as to which sex is typically the braver one. And this means that it also possible to decide whether a particular mouse is braver than average or not.

So, in the second part of the experiment, the researchers paired up males and females, giving them time to grow attached to one another and form the monogamous pair-bonds for which the species is so well-known (at least among behavioural scientists). And then they performed the original test again, to see whether or not the mice behaved differently now that they had a partner.

Where both members of a pair had previously been scored as having above-average bravery, then nothing much changed. When they were both timid, they tended to become slightly braver - perhaps feeling that there was safety in numbers - but only to a limited extent. But, where the pair was mismatched, regardless of whether it was the male or the female that had previously been the brave one, both members of the pair altered their behaviour to become more like their partner. 

That is, an adventurous mouse paired with a meek one would become less willing to put himself (or herself) at possible risk, while the timid partner would do the opposite. Now, these pairs did not have pups to defend, so both of them retreating to safety might be a better option for them than for a pair who really do need to protect a litter. But it does show that a brave individual can change their behaviour to stay safe with their mate when their natural inclination would be to see what's going on, while a meek one can be coaxed into heading out so long as their partner is with them.

And 'coax' may well be the right word here, since they don't seem to be just deciding spontaneously to do something different now they have a partner. We can tell this because the researchers also recorded the ultrasonic calls made between the partners, and they were making more of the long, sustained, calls that they use to signal affection than they do normally. While we can't know exactly how the mice are interpreting these calls, it seems as if these may be attempts at persuading their partner to stay with them - regardless of whether they're retreating or advancing. 

And, regardless of which one is the male and which one the female, they're actually arriving at a happy medium, with both partners adjusting their behaviour to be more like that of their mate. Long-term pair-bonding, as many humans will know, has a lot to do with making compromises for the sake of the relationship.

[Photo by "Whatiguana", from Wikimedia Commons.]

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