
I thought about this a bit more and did a bit of digging around on the internet when I got home to see if anyone else had researched the male/female photographer split. I wasn't surprised to find that this topic has not only come up before, but everyone else had reached the same conclusion as me.
So why is this, I wondered? The number of women studying at photography schools and colleges is slightly lower than men and then those that have studied it drop out along the way. Fewer photographers who join the profession later in life are women too. I think the main reason is probably because landscape and travel photography are both very time-consuming and solitary and hence not practical if you have kids. Given that women are usually the primary carers of children, spending hours (at strange times of day) out in the middle of nowhere just isn't possible. Perhaps women are also more nervous about going out on their own too. Perhaps, also, the technical side of things puts women off; men love their gadgets, after all.
I'd love to know how many female travel or landscape photographers (or photo-journalists) have children; I suspect the number who do have children is quite small, as they'd have to leave them for long periods of time in order to be able to do the job. I would expect a large proportion of the women who do end up becoming professional photographers specialise in areas where the hours are more regular - such as portrait and weddings, and where the physical risks and separation/isolation required are lower.

I noticed today that National Geographic has started its own photographic stock agency and that out of their talent pool of 73 photographers and videographers available for commissions only 13 of them are female. No surprise there then.
So in the end, it's not dissimilar from banking - the proportion of women that start out in the industry is under 50%, and an increasing percentage of those women drop out along the way, because of children, higher aversion to risk, etc.
I guess I'm something of an exception to this: I have no kids, just a husband who can look after himself and a dog who can be looked after by my husband or parents, so I can just fly off to take photos for 10 days if I want (and if I had neither the husband nor the dog, I would be flying off a lot more!). Most 42-year-old women would not be in a position to do this, mainly because they do have children. When I spoke to some of the other photographers that I met in Iceland (who were also in their forties) they mentioned holidays with the wife and kids, but the photographic trips were always alone or with other (male) photographers; the wives always looked after the kids back home.
One thing that doesn't really fit in with my theory, however, is money. These days photography is rarely well-paid, so not usually the profession of the breadwinner in the family (who still tends to be male). I wouldn't be surprised that with the traditional stock revenue stream dramatically reduced for pros this might serve as a barrier to entry for both genders and there will be fewer and fewer photographers who are able to make a living out of it in the future, male or female. But that's another story.
I'd be very interested to hear people's thoughts on and experiences about this gender observation...