Benjamin

[Benjamin]

It seems like a long time since you commented on the WLH post but many thanks both.

Finally back from trips and turning my attention to the things that really matter, starting with my thoughts regarding one of the few species I think I have a pretty good understanding of - the plucky old purple emperor.

‘Plucky’ might seem to undersell such a celebrated butterfly but it works for me. The wonderful hyperbole notwithstanding I think I’ve seen emperors being the victim of other wildlife and weather just as much as I’ve seen them dominate it. They certainly do have a go though and I still readily concede that there is no more beautiful a sight in all of nature than a freshly emerged and grounded purple emperor flashing his scale perfect iridescence around like some nymphet mischievously revelling in her newly acquired powers.

This one in Bulgaria had the whole group on strings - he knew what he was doing…..

The season started with a bang at Knepp, and to some degree took everybody by surprise. After a wet and slow spring emperor larvae were a little behind schedule but the incredible period of warm, settled, dry and sunny weather that commenced around the 22nd of May (presumably triggered by my questionable decision to lay a new lawn so late in the spring) and lasted well into the second half of June had a big impact on larval/pupal development. Under these conditions larval development speeds up significantly and importantly the development range is reduced (or at least doesn’t increase). I think this is because the effects of microclimatic conditions around a site are reduced when the whole site experiences excellent overall conditions for prolonged periods. In the usual mixed spring weather, for example, some particularly well situated caterpillars may still benefit from the small windows of sun and warmth that speed up development whereas others that remain shaded and cool will fall further behind. Under excellent stable conditions everything races along at max speed regardless of location.

At Knepp numbers seemed very high initially and the season has generally been celebrated as being very good. The fine settled weather during the pupal period has been highlighted as the likely cause of the boost in numbers - a reduction in predation due to a shortened pupal period is a major factor being imagined here. There are other benefits that settled weather brings at this delicate time - serious storms for example will certainly cause casualties, but I’ll focus on the idea of reduced predation for now.

I can’t fault the thinking around this, but I tend to think the effect would be smaller than is being imagined.

I’ll quickly lay out my reasoning using the huge population at Knepp as an example.

If we imagine that Knepp produces something like 500 butterflies each year (Neil has suggested this previously - who would argue?) and that pupal predation is around two thirds (the last few years at SW support this) then we give ourselves a starting point of 1500 pupae. This does feel like quite a lot, having spent quite a few hours UV searching there, but the area is vast so ok….

So 1500 are reduced to 500 over the course of the average pupal period, of let’s say, 20 days. If we assume predation to be spread evenly over the 20 days of standard pupation then we can conclude that in a population of this size, 50 pupae are lost to predation each day.

Clearly we now need to look at just how short the pupal period might become under excellent developmental conditions. I think the shortest record in captivity is something like 14 days for a male at the extreme end of the spectrum. In the wild the effect of the conditions will be somewhat reduced compared to captivity (still some mixed weather and cold nights etc) so I think it’s reasonable to suggest a figure somewhere between the average and this extreme record in captivity. If we go down the middle and suggest 17 days as the average under excellent conditions in the wild then we can conclude that three days will have been saved compared to the average pupal period in a standard year.

Saving three days should save 150 butterflies according to the figures we’re working with for Knepp.

That does sound significant, however, my experience (data) and certainly common sense dictates that predation is certainly not spread evenly over the 20 days, and curves away from a high point on day one, to a low point on day 20. Poorly positioned pupae (highly visible) or pupae in areas where birds forage regularly will be picked off early, and those that survive the first few days are much more likely to make it all the way. A pupa that has made it to day 17 is clearly in a very good spot and so predation becomes far less likely over the final three days.

So shaving off these final three days will save far fewer butterflies than the 150 I suggested. How many would still be predated so late on is anyone’s guess, but if we allow 50 (and personally I think it’s likely even fewer than that) then the effect of the shortened pupal period is to produce 550 butterflies instead of 500. A boost for sure, and one that may be noticeable to the most fastidious observers, but not IMO the key factor that I think is being imagined.

Of course at the other end of the spectrum, in a cold year, we can add a few days on to the average 20. This will increase the difference at the extremes but again the extra days will be of increasingly low predation so the effect is again reduced.

As I’ve suggested in previous posts, I believe the impression of a good year has far more to do with weather conditions during the flight period than anything else. This year I think the emergence was condensed following excellent developmental conditions, and this, combined with the excellent weather in the early part of the flight season ensured that everything that emerged was instantly very visible.

Essentially it all happened at once and we got an excellent view of it.

[Pauline]

I'm probably repeating myself Ben but I do so enjoy reading your thought-provoking posts, mainly because your conclusions are based on research you have carried out. That is a lovely image of an immaculate PE too.

[Wurzel]

Really interesting reading as usual Ben I'll consider this before talking about good/bad years in future. So much of does come down to perceptions - people I've talked too suggest that it was a good year for His Nibbs but for me I only saw one this season so, in isolation, I would have surmised (pessimistically) that they hadn't had a good one at all, although I would add the caveat "it seems like" to cover my rear

Have a goodun

Wurzel

[Padfield]

Hi Ben. Which species are the main predators of the pupae? If it's birds, there could be significant behavioural differences in fine weather from in poor weather. In fine weather there are lots of relatively conspicuous flying insects about, providing easy nutrition. In poor weather the birds might spend more time creeping around the sallows looking for caterpillars and pupae. Of the four purple emperor pupae I've seen fail in the wild (I've had just one success), I'm pretty sure one was taken by a squirrel, one by a bird, one by insect parasites and one by disease - it had little dark spots on it which always worried me, but it never erupted with parasites.

Guy

[Benjamin]

Thanks guys - I certainly run the risk of falling foul of ‘a little knowledge is a dangerous thing’ but it does feel like with each passing season I edge closer to something like the true picture.

On the subject of key predators the answer unfortunately is that I don’t know. If I hadn’t signed up to being away at the key time each year then I’m sure I’d have some data by now. Now that pupae are easy to find there’s no excuse really - trail cams can quite easily be used to capture the moment of predation. Naturally I have been able to collect data on predation without knowing who the key players are. This was initially a very small sample of six or so pupae in the pre UV days, that has now grown rapidly to some 40 or so pupae with the aid of UV searches. It still takes a lot of commitment to collect decent data though - finding them is one thing, but monitoring the site closely enough to know when they first appear, and how many days they survive before they disappear is quite another. Intimate site knowledge is at the heart of any success with this approach.

As you suggest Guy, I guess they are predated by a variety of species, and I totally agree that if we knew what the key predator was (if there is one) then we could certainly speculate about how conditions may influence its foraging behaviour. As documented recently I came to the conclusion that the reduced UV signal of the pupae has evolved to combat avian predation and this does seem pretty solid (suggests a diurnal predator) but I would not be at all surprised to find (if I ever get around to it) my trail cams returning images of dormice happily clutching their pupal prizes between their paws as they set about devouring their little pots of nutrition. And if dormice then most likely yellow-necked mice and of course squirrels.

Was he just surprised to see me, or can I detect a touch of guilt there???

I really need to make time for project Trailcam.

[David M]

Fine observations and theories, Ben. All perfectly plausible.

As you say, discovering the identity of the predators would take a huge amount of time and effort. There are so many that could be culpable that it's almost an impossible task without setting up remote cameras.

[Benjamin]

Hi all -

It’s been a while, and although I’ve been increasingly focusing on butterflies a little further afield, I haven’t neglected my investigation into the purple emperor population that this diary was essentially started to document.

This is my fifth year of studying the overwintering population, and will likely be my last. I think five years is plenty. If I still felt that there were more questions than answers, and that further time would edge me ever closer to cracking some enduring puzzle, then I’d have no problem committing more time - but I don’t.

There are still aspects of PE ecology that I’d like to understand more thoroughly, but it now feels a lot more like dotting i’s and crossing t’s, than making any kind of significant jump in understanding. It’s not really a very complicated species, but it is hard to study and its uniquely alluring characteristics mean that the ‘gaps’ in our knowledge are inevitably filled with all manner of fantastical musings, myths and magic. There’s nothing wrong with that, as I’ve said before, but the process of sweeping all the noise aside in order to gain a clear view of things, can leave one feeling like something of a heretic.

I suppose for me the magic and beauty is all there already, and gaining a greater understanding of something only enhances my enjoyment and appreciation of it. I don’t need any input from anywhere else. But anyway…..

I’m now close to completing my autumn/winter search for larvae, and I have something like 40 logged throughout the area. There have been no surprises along the way, with larvae pretty much being found where expected. Numbers are also very similar to previous years. Although I spend 30 or so days completing this search (probably only 20 so far) I could have found 90-95% of the cats in a couple of days. Even after this year I imagine I’ll return to find cats annually (I do love them!) but I’ll only do the couple of days with the high rewards - I’ll not miss the seemingly endless days trudging around with my ladder making sure that the unproductive sallows within my study area have in fact remained just as unproductive for another year.

In keeping with the previous four years I’ll take my winter predation data from 20 caterpillars spread throughout the area. These were all present and correct on December 1st. The theme this year is characters from Nabokov works (seems a little highbrow for me so for balance I should mention that I nearly went with pornstars - maybe next year - oh, I’m not doing it next year - probably for the best).

[Padfield]

So where is Humbert Humbert's protégée ?

As I'm sure you know, Nabokov lived the last 16 years of his life in Montreux, not far from me, and would have hunted butterflies in all the same places I do. In fact, a few years ago, Matt Rowlings and I successfully found Iolas blue at a site near Bex, where the last recorded sighting had been 50 years previously - by Nabokov. I approve the theme for your (possibly) final year's research !

Guy

[Neil Hulme]

Great stuff Ben. No reason there to support a low 2023 egg-lay . I saw plenty of females at work long into July this year, so haven't been convinced by any such concerns.
I really think you owe it to us all to go large just one more time. I need to see an image of a caterpillar named Dirk Diggler.
Catch up soon for some winter pub sessions - much to chew over!
BWs, Neil

[Benjamin]

She’s there Guy ‘She was Dolores on the dotted line’.

Or at least she was there! One of three losses during December - Quilty is also gone which may well be significant. I didn’t tell poor Humbert - he’d never make it through the winter if he knew…..

I didn’t realise you were so close to Nabokov’s hunting grounds - a lovely thing to have in the back of your mind.

Thanks Neil and yes, the site seems to be performing as expected. I’m sure the numbers vary for the various early stages from year to year but it does seem (as we would expect) that the regulating pressures are really very effective at returning things to the expected level by June.

There may be potential for greater annual variation at different sites, depending on the particular combination of limiting factors, but I don’t think I could face starting again somewhere else……(although I agree Dirk Diggler needs to feature at some point!).

So three losses from the Nabokovians during December - a pretty standard level of predation reflected also in the wider sample. We’ve had some lovely chilly winter weather over the last few days but unfortunately no covering of snow to keep them safe - the tits continue to work the sallows and January will inevitably see the distant open door that promises a few dizzying weeks of midsummer hedonism slammed shut for a few more hopefuls….

[Roger Gibbons]

By coincidence, I mentioned Nabokov's obsession with butterflies (in this instance, in America) to someone on Monday, who clearly wondered if I was pulling their leg.

It put me in mind of this article: https://www.newyorker.com/magazine/1948 ... ir-nabokov

Recognise the location?

Roger

[Padfield]

She’s there Guy ‘She was Dolores on the dotted line’.

Of course! I scanned the names too quickly! I'm sorry she was lost.

Roger: that picture puts me in mind of a site in the Val d'Hérens, but the chalet in the background, with the tall, roadside poles, is not right. Maybe this should be my mission for 2024 - to find the place and stand in the great man's footprints ...

I agree with Neil's sentiment, though I had to look up Dirk Diggler !

Guy

[Benjamin]

Quick update from the woods:

The Nabokovians suffered no further losses throughout January, and were all still present when I checked in on them yesterday.

This represents very low predation compared with previous years.

The wider sample also reflects relatively low predation but not as dramatically low as the Nabokovians indicating that the select 20 have, as a group (and up until this point), been fairly lucky.

We’ve had some fairly mild weather recently but nothing that strikes me as too unusual. The sallows are not yet moving and none of the overwintering caterpillars have wandered off or moved position (apart from the usual on-the-spot reorientation).

I always hear a lot about overwintering iris cats moving a lot in captivity around this time of year, and many even desiccating - it’s one of the difficulties of rearing this species. This is just not something I encounter in the wild. As bud burst approaches they move to leaf buds if they’re not already positioned there, but that’s about it. I’m not sure exactly what the crucial factors are here, but it seems females are far better at selecting suitable spots for their young offspring than we are when we force them to make do with the conditions available in our gardens - no surprise there!

[bugboy]

Hmmm, I wonder if the low predation rate has anything to do with all the rain we've had recently? Less hunting opportunities for the Tits to find them and also less practice to 'get their eye's in' in the first place.

[Benjamin]

The LTB data suggesting protogyny is intriguing, especially as Neil is increasingly sure that it’s kosher.

I still want to catch a newly arrived female and run the experiment in tight conditions just to be sure, but perhaps this has been done already? Is this what happened with any of the examples mentioned? If not then I’m doing it this year!

I know I’ve bothered Neil about this previously but I find myself reluctant to accept a switch to protogyny in this case because I struggle to imagine how it would have evolved. I wouldn’t have too much of a problem believing LTB to be a protogynous species (I can’t find any info that says that it is) but it’s the late season switch to protogyny that I’m struggling to come up with an explanation for. As far as I understand it (quite possibly not far enough) males hold all the aces when it comes to developmental speed for simple physiological reasons. They can race ahead of females if this strategy does best for them, which it generally does in butterflies - right? Aiming to be out just ahead of females in order to secure the best mating opportunities where virgin females are the greatest prize. LTB is, however, a good candidate for there to be an absence of protandry, as it is continually brooded and females exhibit polyandry (I think) - both factors that limit the benefits that males would receive from early emergence, but this doesn’t mean that protogyny should automatically follow. Are there any examples of protogynous butterfly species? There must be some interesting ones, but a quick search just reveals some moth families that tend to have unusual and complicated ecologies.

I can start to imagine why it might be beneficial for females to out develop males (Neil has made a few suggestions previously) but I can’t imagine why it would work for males - and I think any explanation we come up with has to work for males, or else they wouldn’t be doing it! Again this is assuming males have the capacity to out develop females if it would be in their interest to do so. And just to clarify, I’m not suggesting that males can choose how they develop but just that if the optimal male strategy is to emerge just ahead of females (or to develop as quickly as possible under marginal conditions) then this is what will occur because females can’t compete in this area. Of course females have advantages in plenty of other areas (decision to initiate mating for example) but just not in this one.

Avoiding inbreeding could be one benefit of altering development speed as Neil suggests, but I can’t see this being too much of a big deal from the male perspective - surely not enough for them to risk slow development in tricky conditions. It is also easy to imagine other methods of achieving this without taking such drastic risks - simply leaving the emergence site prior to mating for example. Getting it on with your sibling is potentially a very big mistake from the female point of view (entire cargo compromised), but probably little more than a relatively minor waste of time and resources from the male’s. Being late out of the blocks, potentially missing all the action, or worse not completing your development before the conditions collapse seems to me to be by far the greater risk for the LTB males that find themselves developing late in the summer at the extreme edge of their migratory range.

To put it another way, potentially shagging your sister is a small price to pay for an early entry ticket to the late summer sex party (Buddhist wisdom I think - we must ask Guy).

I’m certainy not saying it’s definitely not happening - it could quite easily be a switch linked to photoperiod for example, but just that I can’t imagine how it makes sense for male fitness. Any suggestions?

I have previously suggested that delaying development in males in order to emerge at the time of maximum female density could make sense of protogyny from the male perspective (as long as females mate multiple times and are long lived) but again I struggle to make that work in this scenario. The risk of missing out seems to outweigh any potential benefit and females tend to depart pretty soon after emerging anyway. This strategy makes more sense in a scenario where female numbers reliably build in an area.

Because of the huge and obvious peril of developing under marginal conditions at the edge of the migratory range I believe males will develop as quickly as possible. How could anything else be rewarded?

But there are more complex ways that male development could be influenced by factors beyond their control - it seems bacteria can play an intriguing role here. I don’t think there’s much point thinking too much about this until we’ve established beyond doubt that the usual male/female development speeds are reversed in the way that the available data is suggesting. I guess Neil will say that he has already established this, but running a proper experiment still seems the next sensible step.

Anyway - it’s a fascinating topic and one that others might want to chip in on, so I thought I’d put it here rather than clog up Neil’s diary! The downside of this, however, is that nobody will read it, but I figured if I started a new topic then I’d have to clean up my fruity language, and I didn’t want to do that. Hopefully I can revisit it later in the year with some results from captive rearing. Also it’s an excuse to liven up my diary with some of my LTB stick pics that would otherwise never see the light of day…….

[Benjamin]

Hmmm, I wonder if the low predation rate has anything to do with all the rain we've had recently? Less hunting opportunities for the Tits to find them and also less practice to 'get their eye's in' in the first place.

You could well be right Paul - it must have something to do with the tits and there’s certainly no shortage of them in the woods this winter. Conditions for them may well improve, and if they do then no doubt they’ll set about returning predation to the normal levels…..

[Padfield]

To put it another way, potentially shagging your sister is a small price to pay for an early entry ticket to the late summer sex party (Buddhist wisdom I think - we must ask Guy).

I'm quite sure there is an obscure tantric text to this effect, but it doesn't appear in the more orthodox Perfection of Wisdom in 100,000 lines.

You say you struggle to understand a late season switch to protogyny. At the limits of their distribution - for example, southern England - it seems to make good sense to me. While fresh waves of butterflies are streaming north, it might indeed matter little if a male accidentally mated his sister - though in the algebra of evolution, even small differences in survival rate can have a big effect on the gene pool over time. However, at the end of the season, if males emerge first they will surely pick off predominantly their sisters, as there will be no more waves of foreigners coming through. Any butterflies further north will have stopped breeding earlier, and already headed south. A gene that triggered rapid development and early emergence in females would survive to live another day (assuming those females flew south and bred). A gene that triggered rapid development and early emergence in males might sign its own death warrant, if those males then inseminated their sisters.

All very interesting!

Guy

[Benjamin]

A gene that triggered rapid development and early emergence in males might sign its own death warrant, if those males then inseminated their sisters.

I don’t really know whether inbreeding is likely to be a problem for LTB or not. It doesn’t really seem like it would be as one tends to think of isolated populations over several generations when imagining problematic inbreeding. But the migratory nature of LTB does open up the possibility of some somewhat isolated egg laying on patches of food plant at the edge of the migratory wave, even if the outliers will only be isolated for one generation before moving on and most likely mixing back in to the bulk of the population.

But anyway, regardless of what we think about that, all I’m really trying to say on the topic is that:

1. Whether the selective pressure to avoid inbreeding is strong or weak (ie whether this is a problem or not for LTB) I would expect it to act on females to a far greater degree than on the males. If it is strong enough to drive evolution then we should expect female behaviour to be modified in order to avoid this far more than males - as it is females that stand to lose most from a sibling mating.

2. The selective pressure to develop as quickly as possible will be so strong on males and females (certainly at the end of the season but also probably at any time in a continuously brooded migratory species), that if any behaviours evolve to avoid inbreeding then they will not negatively impact development times. There are clearly many possibilities but some kind of sibling recognition system, or dispersal behaviour would IMO be far more likely to evolve than anything that delayed development.

As a result of point 2 i can only make sense of females out developing males if delayed male development has been selected for (i.e., I cant see it being the result of accelerated female development while male development remains constant). And if we are imagining that this selective pressure on males would be the result of fitness limiting encounters with receptive siblings, then this seems incompatible with point 1.

If delayed development ever became the dominant male strategy then I can’t believe that it wouldn’t be invaded by something more successful involving max speed development.

But having said all that there’s no reason why points 1 or 2 should necessarily be true and I’m definitely no evolutionary biologist! I do like to think about these things in an amateurish way though and am happy to have open discussions in the hope that forcing myself to articulate my thoughts will lead to my increased understanding of a topic. I’m definitely not committed to, or even confident in any of my positions on subjects about which I know very little (such as this!) so would be just as happy for someone to explain why I’m completely wrong as I would be for someone to agree with me! I know it shouldn’t really be necessary to write this last paragraph (it’s quite obvious!) but sometimes when I read back what I’ve written I find it sounds a bit too much like I’m trying to convince others, and really I’m just trying to organise and articulate my thoughts.

[Benjamin]

I know I said I wouldn’t speculate further until I’d run my own test to confirm protogyny but I’m expecting Neil to come back with a convincing defence of his data so I might as well….

Trying to come up with plausible theories as to why delayed male development could be selected for is quite a fun game. One idea that I find quite appealing is that point 1 above is completely wrong and this is because virgin females use the initial mating for nutrients only. Polyandrous species seem to have varying degrees of control over sperm allocation so it isn’t completely fanciful that they could use an initial mating primarily to gather nutrients in preparation for a long journey and then mate again upon arrival and favour this subsequent mating for egg fertilisation.

In this way they could be said to be exploiting their poor randy brothers (or any other males at the breeding ground) and the tables would be turned - the pressure would then be on the males to avoid this threat and delayed development could make sense. As unlikely as this is, I do like the logic of it. Until, that is, you try to imagine male strategies that might avoid this threat and do better - like developing as fast a possible and heading off to await receptive females (who will actually use your sperm honestly!) at the next breeding ground on the migratory journey. That seems an awful lot better to me.

Returning to reality for a moment - the fact that LTB males are reported to remain at the breeding ground for a reasonable amount of time before departing suggests to me that inbreeding is not an issue - at least not from their point of view. That is to say, genes for intercepting females at your own emergence site will do better (despite the inbreeding risk) than those that delay mating behaviours until arrival at a new site. Shag everything that moves ASAP and ask questions later might just be as complicated as it gets from the male point of view - who would have thought it?

[Benjamin]

Nothing new from the woods but just to report the data from February. 3 Nabokovians were lost from a group of four close together on three neighbouring sallows.

Overall survival (wider sample) is currently at 57% which compares favourably with previous years, although final survival data won’t be collected until they’re back feeding which may be up to a month away.

Some cats will be feeding again very soon though, which may well set a new record for the earliest wild feeding on record.

2020 was also very early but I didn’t get the exact feeding date. There was a nasty virus going around that had the incredibly dangerous effect of making otherwise perfectly reasonable people comply to bullshit restrictions. It’s all a bit of a blur looking back but I was clearly unwell - I vaguely remember desperately trying to virtue signal by clattering about on my doorstep with pots and pans - a black time. Mercifully I only had a very mild dose and it cleared up very quickly, but it was enough to miss the moment.

When checking on cats recently I found a couple that were active. Several had recently moved to align with leaf buds and a couple were busy silking their new locations. I noticed one laying a lot of silk over a bud a few inches from his resting position. As the silk pathways are all linked together and form one big network it made me realise just how keenly this caterpillar must feel the movements of this expanding bud, or perhaps any bud that falls within the silk network. No doubt the primary function of the copious silking that we see, is to ensure safe passage around the walkways, but this could be an important secondary function.