Vipera berus movement distances during breeding season

France, British isles

Vipera berus movement distances during breeding season

Postby Daniel Kane » Fri Apr 12, 2013 8:55 pm

From what I gather, adults hibernate in the same areas each year and young snakes apparently select hibernation dens at random, perhaps by following scent cues from adult snakes. During the breeding season adders are reported to remain in the vicinity of hibernation areas, where breeding females will be courted by amorous males. The gravid females will then stay close to hibernaculas for the rest of the active season, whereas all other adders will move off into the surrounding habitat to feed throughout the summer.

So does anyone have an explanation as to why today I found one of the adders from my local site, individual M9, so far from where he was 2 years ago? I came across him on a wall around 450m as the crow flies from where I last saw him, mating, in April 2011. If he avoided going through the conifer forest and took the more 'adder-like' habitat of brambles and grass the the distance is more like 700m. My understanding of this species was that the mating and associated behaviour takes place close to hibernaculas, not half a kilometre away.

The site were these snakes live seems to be divided into 2 sub-populations, separated by around 700m of seemingly suitable habitat. Could it be that this year, if there are no breeding females in one sub-population, male snakes may move from one area to the other in search of a receptive female? Andren et al. (2007) stated that berus males were able to detect female pheremones at a distance of 500m, so as the crow flies these two sub-populations are within this limit. However, the Phelps paper does note that "over the years ... the adults, both male and female, were found to exhibit total site fidelity". I'm not so sure what to think, and would like to hear your opinions and thoughts on this.
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Re: Vipera berus movement distances during breeding season

Postby Matthijs Hollanders » Fri Apr 12, 2013 10:15 pm

I would try to look at things with an open mind and try to refrain trying to match the behavior to certain other study populations. Herps don't read field guides. ;)

It might just be something as simple as dispersal. If no snakes ever moved away from breeding sites, eventually you'd have one huge Mt. Everest of snakes piled up. Competition might have led your male to venture to new spots.

By the way, why do you think the population is made up of two sub populations (i.e., why would the "seemingly suitable" habitat be unoccupied)?
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Re: Vipera berus movement distances during breeding season

Postby Daniel Kane » Fri Apr 12, 2013 11:12 pm

The 'seemingly suitable' habitat may well be populated by adders at certain times of the year. What I meant when I said the snakes exist in two sub-populations was that the adults appear to congregate in two main areas for hibernation, breeding, and summer activity. Until today I had never found any individual snake initially identified from one area in the other area, which is why I used the term 'seemingly suitable' habitat (because I did not know if snakes ever used it to migrate between areas).

I see what you mean about dispersal, and obviously most snakes must move away from hibernation areas at some point in the year if they are to be able to hunt effectively. To me it was interesting to observe this male so far from his previous known 'mating area', having never seen or read about this, and I wondered why he might move from one area containing females to another given the time of year (assuming he is looking for the girls). But, as you say, nature is by no means rigid.
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Re: Vipera berus movement distances during breeding season

Postby Matthijs Hollanders » Sun Apr 14, 2013 12:47 am

When you monitor this place, do you always check on the places you know or do you survey every bit of the place with equal effort each visit? If the former, I'd say its observer bias.
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Re: Vipera berus movement distances during breeding season

Postby Daniel Kane » Sun Apr 14, 2013 11:43 am

The former. Undoubtedly I am biased in where I look for the snakes, as I'd much prefer a trip where I spend maximum amount of time observing snakes as opposed to seeing nothing.

My estimated area of adder-occupied habitat at this site (i.e. not in the conifer forest where no light hits the floor) is around 3.64 ha. The areas I regularly visit (in the last 4 years) are roughly 0.67 ha and 1.92 ha in area, so potentially I am missing out on a little over 1 ha of potentially occupied habitat. This habitat is made up of about 80% dense deciduous trees and the rest grass, brambles, etc and, at least to me, seems to be unsuitable for hibernation dens as it is low-lying and shaded for most of the day. Therefore, at least in my head, I am not missing out on finding individual adders, as 'all' individuals in this population seem to spend the first and last portions of the year in one of these two areas.
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Re: Vipera berus movement distances during breeding season

Postby Magnus Karlsson » Mon Apr 15, 2013 8:07 am

In the swedish archipelago adders migrate between islands during spring and summer. It's no problem for a snake to cross patches of unsuitable habitat between patches of suitable habitat.

That males may disperse during the breeding season doesn't surprise me. Several factors can be hypothesized to trigger the behavior. For instance according to the ideal distribution theory males should distribute so that the level of competition is equal between neighboring hibernacula. Thus, If the competition for females is high at one site, some males may migrate to close by areas where competition is smaller. Similarly, if a male has already successfully mated with a high number of females at one site he may migrate to new areas to mate with additional females. And yes, if there are no reproductive females at one site I would expect males to disperse in search for females.
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Re: Vipera berus movement distances during breeding season

Postby Jeroen Speybroeck » Mon Apr 15, 2013 11:38 am

Nice contribution by Magnus.

While these matters are not within my professional domain, I tend to disagree that your sampling effort has to be biased. If not, research would only deal with small, overseeable populations. You just have to deal with the recapture rate and detectability of the animals. Imho, the key is to standardize your effort. It is more important to be consistent than to do the hard-to-repeat effort of sampling every single crumb of habitat. That way, you won't find all animals, but you will re-encounter the same ones more readily. However, I struggle with this myself - by walking the same salamander transect each time, I am likely to have little chance of discovering large distances traveled by individual animals. If I would randomise my route, I might increase that chance, but the chance of finding a specific animal again would drop greatly, thus enhance the uncertainty on a lot of other things I am interested in. If you want to follow individuals through their life cycle, it seems wise to sample their home range. Depending on your sample effort size, the area, ... , random sampling would land you clearly less often in that animals home range. With animals exhibiting migration and different hides during different seasons, I'd sample those to get data, rather than allocating a wealth of time to potential places where you never find a thing. Note that this is my pragmatism speaking - if Matthijs wants to spend a lifetime in empty potential habitats, he can. Surely, this will lead to cool discoveries, but it won't form the basis of research on population dynamics, individual growth etc.

A colleague of mine monitors a (very) big population of adders and has a very good scientific view on these matters. Feel free to contact him, Daniel - dirk dot bauwens at inbo dot be.

On the topic at hand, I here from that same colleague that most(!) animals are indeed of limited adventurous spirit. Very recently, I believe some other colleagues of mine showed that this is even corroborated by genetic differentiation of two subpopulations (site of 2 km by 8 km).
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