the online meeting place for all who love our amphibians and reptiles
Home Page Live Forums Archived Forums Site Search Identify Record Donate Projects Links
Forum Home Forum Home > General > UK Reptiles and Amphibians
  New Posts New Posts RSS Feed - Adder sexual dimorphism
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Adder sexual dimorphism

 Post Reply Post Reply
Author
Message
Dean G View Drop Down
New Member
New Member


Joined: 14 Nov 2013
Location: United Kingdom
Status: Offline
Points: 5
Post Options Post Options   Thanks (0) Thanks(0)   Quote Dean G Quote  Post ReplyReply Direct Link To This Post Topic: Adder sexual dimorphism
    Posted: 14 Nov 2013 at 4:03pm
Hi,

I'd like to pose a question. Adders have pronounced sexual dimorphism, with the female being larger.
Male adders compete with each other in a wrestling match known as a 'dance'. So, wouldn't it be adaptive for males to grow to larger size and win more fights? It can't be physiologically impossible, because females are already growing to a larger size.
What underlies the observed dimorphism?
Back to Top
will View Drop Down
Senior Member
Senior Member


Joined: 27 Feb 2007
Location: United Kingdom
Status: Offline
Points: 1830
Post Options Post Options   Thanks (0) Thanks(0)   Quote will Quote  Post ReplyReply Direct Link To This Post Posted: 15 Nov 2013 at 7:15am
Hi Dean

nice question. Some thoughts, not definitive by any means:

1 - not sure males are that much smaller than females in terms of overall length, though girth certainly. The girth difference is due to gestating young or presence of developing eggs in the female or general female 'bits' - ovaries etc. My experience of really large female adders is in populations where mating opportunities are few and far between (low numbers of males, population heading for extinction. Females put all their energies into growth rather than reproduction. eg one site in London in the 1990's with no remaining males had 3 venerable old ladies each around 75 - 80cm long.)

2 - 'winning' males are more frequently those already 'in possession' of a female and presumably stoked with hormones, lust etc rather than always the biggest males (unlike in most mammal male-male combats)

3 - possibility of increased mortality of larger males perhaps due to reduced agility, less conspicuous than smaller males making them liable to more risk from predators especially birds of prey (though personally I would expect smaller males to be equally at risk, eg from ground predators like pheasants)

4 - resource partitioning - do males gain reduced competition for food if smaller than females ? (females could feed on mammals and males take more in the way of lizards, perhaps?)

5 - the biggest reptiles in a population are usually the oldest, so smaller males may offset size advantage with increased vitality, fitness (not in evolutionary sense but in terms of 'health') and vigour. Having said that, this wouldn't mean you couldn't have a population where the mean length was larger than in a normal length population, with 'smaller' males still being larger than typical male adders, I suppose.

6 - possibility that there is greater mortality for males generally when compared with females, so smaller males reflects differences in age structure rather than anything else. ie there are simply fewer older, hence larger, males than older females in the population.

Not saying I am persuaded by any of these, but just some idle thoughts. Nice to have a topic to cogitate during the wintertime...

Cheers

Will
Back to Top
Dean G View Drop Down
New Member
New Member


Joined: 14 Nov 2013
Location: United Kingdom
Status: Offline
Points: 5
Post Options Post Options   Thanks (0) Thanks(0)   Quote Dean G Quote  Post ReplyReply Direct Link To This Post Posted: 15 Nov 2013 at 12:04pm
Hi Will

Thanks for your speedy and thoughtful reply. I accept most of what you say, except perhaps point 4.

I'm familiar with resource partitioning as a route to speciation; Galapagos finches and so on. I can't quite see how it would work between sexes within a species.

 Males aren't in competition with females for mating opportunities, they're directly competing with rival males. A gene for 'bigness' would make a male win fights and  be able to take larger prey. They would then compete with females for large prey items, sure.

But you've got to win the race to breed first. 

(I realise, of course, that there is no 'bigness' gene. It's polygenic and environmental.)

Thanks for the discussion.

Dean
Back to Top
GemmaJF View Drop Down
Admin Group
Admin Group
Avatar

Joined: 25 Jan 2003
Location: Essex
Status: Offline
Points: 4349
Post Options Post Options   Thanks (0) Thanks(0)   Quote GemmaJF Quote  Post ReplyReply Direct Link To This Post Posted: 15 Nov 2013 at 7:13pm
I've often observed that the larger males tend to be to me at least more noticeable at many sites. (i.e 'the old friends' one usually finds in the same old places etc) They bask a lot, are spotted more easily when on the move etc. I wonder if there is a factor that being a large male also leads to a greater risk of predation? Notably the females have a totally different tactic early season with hidden mosaic basking being the normal.

So just perhaps the gene for bigness is being selected out of some populations depending on many factors such as available cover/basking sites etc?

I would though think in all this is all very population specific, I've seen sites where males are generally smaller than females, others where the bigger males easily compete in size with the females of the same population.

I think also fairly immature males tend to be seen often (trying to win mates etc), where females of this size are rarely spotted at all. This could give a bias to observations in a given population towards the males being smaller on the whole than the females.
Back to Top
Dean G View Drop Down
New Member
New Member


Joined: 14 Nov 2013
Location: United Kingdom
Status: Offline
Points: 5
Post Options Post Options   Thanks (0) Thanks(0)   Quote Dean G Quote  Post ReplyReply Direct Link To This Post Posted: 16 Nov 2013 at 1:56pm
Hi again,
I've been thinking about Will's point 6. Now I'd like to rephrase my dimorphism question; why do males sport more visible patterns?
If they were Banded Kraits I wouldn't need to ask; they're advertising their extreme lethality. But a snake that can be predated by a pheasant? They'd be better off in a more cryptic brown/ginger like their sisters, surely.
I realise that reptiles grow throughout life, so if they lived longer that would redress the size disparity. One question answered at the expense of another:)
I wondered if the bright black/cream was a sexual display, but I don't think females look much at potential suitors, I think it's more to do with nudging and tongue-flicking.
Any advice welcomed


Edited by Dean G - 16 Nov 2013 at 2:00pm
Back to Top
GemmaJF View Drop Down
Admin Group
Admin Group
Avatar

Joined: 25 Jan 2003
Location: Essex
Status: Offline
Points: 4349
Post Options Post Options   Thanks (0) Thanks(0)   Quote GemmaJF Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 2013 at 2:43pm
Hi again Dean,

I would offer that it very much is to advertise lethality. 

There is a paper showing that the stripe results in less bird attacks on Plasticine snakes than those with no stripe.

The males are highly visible and mobile whilst mate searching, whilst the females are usually hidden and mosaic basking during that time. The males will be seen by predators for sure as they move about quite openly at this time. At many sites they are very easy to spot on the right days by humans for example.

So as that is the case, bright colours and very distinctive stripes can only be saying in my mind, you can look at me but don't bother me! It though would be a percentage thing I guess. I seem to remember that the Plasticine snakes with stripes were not entirely left alone, just attacked less often.

I think most of the points raised would make good investigations for a student, but giving definitive answers from casual observations can only really be opinion.

There is a clear link though that the males are brightest during mate searching. One could think it was a sexual display, but then again one cannot factor out that it is also the very time that the males are most clearly visible to predators.


Edited by GemmaJF - 17 Nov 2013 at 2:49pm
Back to Top
will View Drop Down
Senior Member
Senior Member


Joined: 27 Feb 2007
Location: United Kingdom
Status: Offline
Points: 1830
Post Options Post Options   Thanks (0) Thanks(0)   Quote will Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov 2013 at 6:34pm
I'd agree with Gemma, Dean.  More mobility exposes males to higher chance of being seen, so best to be more visibly harmful at this time of year.  There have been several plasticine-type studies which seem to show fewer pecks to viper-style fake snakes.  I also think that the zig zag is cryptic on bracken and other backgrounds, but once spotted the pattern could deter a predator. 

Interesting how the brighter male adder in spring is there to communicate information to predators, whereas the brighter green in male sand lizards is to communicate fitness to other sand lizards (ie for sexual selection purposes) and by making them more visible to predators, is raising the chance they will be eliminated by natural selection...
Back to Top
GemmaJF View Drop Down
Admin Group
Admin Group
Avatar

Joined: 25 Jan 2003
Location: Essex
Status: Offline
Points: 4349
Post Options Post Options   Thanks (0) Thanks(0)   Quote GemmaJF Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 2013 at 2:10pm
Just to add another reason for the stripe, it can confuse a predator as to which direction the snake is moving in. It is like the effect one sometimes observes when driving past railings when the animal is moving quickly through undergrowth. The arrangement of the stripes and side dots can give the illusion the snake is moving in reverse.

I agree also the stripe is cryptic in bracken and it may well be why it developed. 

There is though an advantage when living on a chalk grassland to having a marking which might confuse a predator regarding which end your head is attached whilst on the move. It would be interesting to set-up some sort of mock up of the effect and establish if higher contrasts give a better illusion of reversed motion. I think from my own observations of black railings and light backgrounds whilst driving it might! 
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 11.06
Copyright ©2001-2016 Web Wiz Ltd.

This page was generated in 0.094 seconds.