Amber Snail Puzzle

While removing an old pot containing a lot of grass and a dead Agapanthus, next to a rainwater filled pot, I stumbled upon this little snail. I was quite surprised as initially, I thought it was a pond snail, but closer inspection revealed the tell-tale eyes-on-top-of-tentacles characteristic of land snails and slugs, while aquatic snails have their eyes at the base of their tentacles. After sifting through a Molluscs guide I found out it was a Common Amber Snail, Succinea putris. Although not aquatic, it usually lives near water or in waterlogged habitats, and it is often found on the stems of aquatic plants. It cannot retract its body completely inside the shell, and the lower pair of tentacles is vestigial. I have no idea how this snail got into our garden, but snails, despite being slow and strongly dependent on humidity, are known to disperse widely. In the Origin of the Species, Charles Darwin believed birds were the most common long range dispersal agents of snails and other aquatic animals and plants. In 1893, Harry Wallis Kew reviewed the dispersal of land and water molluscs, and discusses the evidence for external transport on the feathers of birds:

Sir C. Lyell, remarking on the wide range of Succinea putris, a land-shell which inhabits moist places on the borders of pools and streams suggested that water-fowl might have distributed its ova entangled among their feathers and it seems quite likely that ova of certain terrestrial kinds may be occasionally thus carried, either in the feathers or on the feet of birds; indeed, we have a near approach to proof of such transportal, the Rev. Canon Tristram, as we have seen, having once found ova, believed to be those of a Succinea, upon one of the feet of a mallard shot by him, on the wing, in the desert of Sahara. It is doubtful, however, whether Succinea, from the nature of the localities they often or usually inhabit, ought not, for the present purpose, to be classed with fresh-water, rather than with land-shells. Mr. Darwin suggested that the just-hatched young, possibly, might sometimes crawl upon the feet of ground-roosting birds, and thus get transported; and it certainly seems in the highest degree probable that such is the case, but, as far as I know, no observations in support of such a supposition have yet been made.

A tantalizing possibility, also first put by Darwin, is that of internal transport of organisms in the digestive tract of birds. Many bird species feed on snails, and given that no gastric juices occur in bird's crop, they can potentially survive for a while and maybe be discharged later elsewhere by the bird, or regurgitated by raptor if the bird falls prey to it. Kew stated:

 Twenty specimens of a Succinea, peculiarly packed together, and four of Pupa viuscornvi were once found by Mr. W. H. Dikes in the crop of a bearded titmouse (Parus biarmicus); all the shells, it is said, were uninjured, but it is not stated that any were observed lo be alive.

In 1968, Biggs reported on the recovery of a living Succinea putris from a pigeon's crop at least 8 h after the bird had died. Indeed, although it seems even more unlikely, some snails can survive passage through the whole digestive tract of birds provided the shell is more or less unbroken. This has recently been shown to happen to a small estuarine snail, Hydrobia ulvae, which can survive passage through the digestive tract of Shelducks, and also in some small Japanese terrestrial snails, Tornatellides boeningi a proportion of which were recovered alive in the feces of two species of terrestrial bird they had been fed to.
I don't think any of these forms of dispersal apply to the particular little snail in the above photo. Maybe it travelled on a pot plant we bought some time back in a garden centre, or, stuck to our clothes or shoes during an outing into some wetlands. Maybe, but just that any of the forms of transport Darwin and Kew discussed actually happen to snails shows you don't need wings to fly high.

References
Biggs, H. E. J. (1968). Succinea putris (L.) in a pigeon's crop Conchologist Newsletter, 24: 36.

Gerhard Cadée (2011). Hydrobia as "Jonah in the whale": shell repair after passing through the digestive tract of shelducks alive. Palaios, 26 (4), 245-249 DOI: 10.2110/palo.2010.p10-095r

Darwin, C.R. (1959) On the Origin of Species. Read the book here.
Kew, H.W. (1893) The dispersal of shells: an inquiry into the means of dispersal possessed by fresh-water and land Mollusca. K. Paul, Trench, Trübner & Co., Ltd. Read the book here.
Shinichiro Wada, Kazuto Kawakami and Satoshi Chiba (2011). Snails can survive passage through a bird's digestive system. Journal of Biogeography : doi:10.1111/j.1365-2699.2011.02559.x

Racing male tree bumblebees

In the last two weeks, the garden has been overtaken by frenzied male bumblebees. They follow a set circuit, round and round, racing from bush to bush and then having a little bumble in each. If you wait for a bit on a particular spot on the route, you are likely to see another bumblebee a few minutes later passing by in the same direction, doing exactly the same. Most of the males I have been able to identify doing this are Bombus hypnorum. Today, a male B. hypnorum - which can be distinguished from the female by his white moustache - got trapped inside the conservatory and I had his portrait taken (above). Many male bumblebees have recently emerged from their nests, never to return, and their mission is to find queens and mate with as many as possible. In many bumblebee species, males' strategy consists on tracing a route, sometimes hundreds of meters long, often circular, depending on the species, and marking certain places along the route with pheromones produced by scent glands in their jaws. Males join already set routes and therefore many males, some of them probably their siblings, go round the same routes every day, stopping to feed occasionally. Queens encountering a route are attracted by the pheromone and are then intercepted by males. The discovery and first description of these male bumblebee flight paths - from Bombus hortorum males - dates back to Charles Darwin, from observations he carried out at Down House. Although he didn't realise pheromones were involved, he noticed bumblebee routes and them stopping and bumbling at places he called "buzzing places", and marveled at the fact that the same or very similar routes were used year after year:

I then followed their route for about a hundred and fifty yards until they came to a tall ash, and all along this line they buzzed at various fixed spots. At the far end, near a pollard oak, the track divided into two as shown in the plan. On some days all the bees flew in the direction I have described, but on others some arrived from the opposite direction. From observations made on favourable days, I think that the majority of individuals must fly in a wide circle. They stop every now and then to suck at flowers. I confirmed that whilst in flight they move at about ten miles an hour, but they lose a considerable amount of time at the buzzing places. The routes remain the same for a considerable time, and the buzzing places are fixed within an inch. I was able to prove this by stationing five or six of my children each close to a buzzing place, and telling the one farthest away to shout out " here is a bee " as soon as one was buzzing around. The others followed this up, so that the same cry of " here is a bee " was passed on from child to child without interruption until the bees reached the buzzing place where I myself was standing.

  This sketch of the grounds of Down House shows the part of the male bumblebee flight route studied by Darwin with the help of his children. What fun must have been to have him as a dad!

References

Freeman, R.B. (1968). Charles Darwin on the routes of male bumblebees. BULLETIN OF THE BRITISH MUSEUM (NATURAL HISTORY) HISTORICAL SERIES , 3 (6), 177-189.


Stiles, Edmund W. (1976). Comparison of Male Bumblebee Flight Paths: temperate and tropical. Journal of the Kansas Entomological Society., 49 (2), 266-274.

Darwin on bumblebees: the quick and clever workmen

We could be tempted to think about insects as little 'robots', behaving blindly and rigidly responding to external cues following an internal instinctive program. However, in the last few decades, research on insect learning has taken off providing an astonishing amount of data on the behavioural plasticity and learning abilities of these animals. Much of the research has taken place using bumblebees as models as they can be easily kept in the laboratory and many individuals can be produced and used in experiments. Bumblebees, as social bees, have a rich behavioural repertoire. After emerging from their natal nests, queens explore their environment to find a suitable nesting site, feed themselves with nectar and pollen, build their nests, make cells, lay eggs and provision their offspring with more nectar and pollen. Foraging involves taking decisions as to which species of plant and which individual flowers to visit, the available flowers changing continuously as the season progresses or from site to site. In addition, not all nectar and pollen is easy to collect: in many nectar-rich flowers, nectar is located deep inside the flower (e.g. Honeysuckle, Nasturtiums) and pollen from different flowers has to be gathered in various ways. Recent studies illustrate the flexibility of individual bee learning, but also the less known aspect of social learning: bees not only learn on their own (from trial and error) but also from each other - and even from other species. Despite the flurry of recent research, many of the facts had been observed and hypothesized by Charles Darwin over a century earlier. Darwin never ceases to amaze me: he was a meticulous, keen observer, paying attention and being enthused by the littlest things; never ceasing to test hypothesis, clearly illuminated by his encyclopedic knowledge in all things related to plants and animals. He gathered observational and experimental data painstakingly, often in his own garden and sometimes even enlisting his own children.
He wrote on social learning:
"One day I saw for the first time several large humble-bees visiting my rows of the tall scarlet Kidney Bean; they were not sucking at the mouth of the flower, but cutting holes through the calyx, and thus extracting the nectar. I watched this with some attention, for though it is a common thing in many kinds of flowers to see humble-bees sucking through a hole already made, I have not very often seen them in the act of cutting. As these humble-bees had to cut a hole in almost every flower, it was clear that this was the first day on which they had visited my Kidney Beans. I had previously watched every day for some weeks, and often several times daily, the hive-bees, and had seen them always sucking at the mouth of the flower. And here comes the curious point: the very next day after the humble-bees had cut the holes, every single hive bee, without exception, [...] sucked through the cut hole; and so they continued to do for many following days. Now how did the hive-bees find out that the holes had been made? [...]  I am strongly inclined to believe that the hive-bees saw the humble-bees at work, and well understanding what they were at, rationally took immediate advantage of the shorter path thus made to the nectar." (Darwin, 1857)
And on the use of experience and memory:
"I observed also bees flying in a straight line from one clump of a yellow-flowered Oenothera to every other clump of the same plant in the garden, without turning an inch from their course to plants of Eschscholtzia and others with yellow flowers which lay only a foot or two on either side. In these cases the bees knew the position of each plant in the garden perfectly well, as we may infer by the directness of their flight; so that they were guided by experience and memory." (Darwin 1876)

A fragment of a description of social learning in bees by Darwin in his work "The effects of cross and self fertilisation in the vegetable kingdom" (1876)

A recently emerged Bombus lapidarius queen feeding on Agapanthus

A Bumblebee Bombus terrestris, worker gathering pollen from a poppy

Links
All of Darwin's work, books, articles and letters are now freely available and fully searchable online in The Complete Works of Darwin Online.
Social learning in insects, a recent summary