Science1 publisher2 min readPublished
Northern wasp spiders mate earlier and their young take harder cold, a European study finds
Researchers raised wasp spiders from opposite ends of Europe under identical lab conditions to ask whether the species' northward spread involves evolution and not only warmer weather, as British sightings arrive further north.
The Scientist · Science desk
What happened
- Around nine or 10 wasp spider sightings have been reported along the River Avon, part of a run of records across Worcestershire and neighbouring counties for a spider more familiar in southern England.
- Farther north, Chester Zoo conservationists found a female wasp spider during a wildlife survey in Cheshire, according to the Guardian.
- In a common-garden experiment, females from the northern end of the species' European range were smaller than southern females but laid similar numbers of eggs, and a similar proportion of those hatched.
- Colder simulated winters killed more spiderlings from both regions with no significant north-south difference, while in separate extreme-cold tests northern spiderlings chilled beforehand survived better.
- An earlier genetic comparison of living spiders against museum specimens found the spreading populations carried ancestry from groups that had once lived apart.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability If warming let long-separated populations meet and interbreed, as Krehenwinkel and Tautz propose, the expanding front can inherit combinations of cold-hardy traits that neither parent population held alone.
- constraint Cold tolerance at the range edge came out as both an inherited difference and an acclimation response, so no single hardiness number can be entered into a range model as a species constant.
- contradiction The experimental evidence for evolutionary change sits in continental Europe while the new sightings sit in Britain, and the British arrivals have not been tested for the same traits.
- decision Worcestershire Wildlife Trust is soliciting photographs from the public, so anyone turning its record counts into a rate of spread first has to separate new spiders from newly recruited observers.
A common-garden design is what separates the two explanations here. The team collected spiders across Europe, from southern France to Estonia [9]. Females from both ends of that range then laid their eggs under the same lab conditions, and their young were put through simulated winters [10]. Identical conditions for the mothers is the point of the exercise: when the offspring still differ, the difference did not come from the weather the mother lived through. The researchers' reading of the northern pattern is a faster life cycle, spiders that grow and mature quickly enough to breed in a short summer without laying fewer eggs [12]. The same study described northern populations mating earlier in the season [8].
Both cold treatments were run indoors. That is how a controlled comparison gets made, and it is also why the survival results are not a forecast for any particular field site.
The earlier work went at the question historically. Henrik Krehenwinkel and Diethard Tautz compared living populations with museum specimens, and tested how spiders from different populations handled temperature change, including whether they could get through a winter away from their home region [17].
For Britain the record is shorter. Argiope bruennichi was first recorded at Rye, in East Sussex, in 1922 [6]. The British Arachnological Society says warmer conditions are helping the spiders spread [5], and that successful breeding depends on a long warm season and a mild winter, conditions becoming more common in Britain [7]. Both of those are climate terms, already inside any temperature envelope a modeller would draw.
The county record counts are small. Wendy Carter, communications lead at Worcestershire Wildlife Trust, had received one wasp spider record for the county the previous year, according to the BBC [2]. Nine or ten reports along one river is nine to ten times that [24], though one count is a county total and the other a single stretch of water.
A female is marked yellow and black, with a body reaching about three-quarters of an inch and a web spanning nearly a foot, a bright band of silk running through the centre [22]. She may eat a male while they mate, and she is unlikely to hurt a person [23]. The females disappear in autumn, leaving brown, flask-shaped egg sacs attached to vegetation, with the young inside until spring [21].
What to watch
- A study that tests British wasp spiders, rather than continental ones, for earlier mating and cold acclimation.
- Whether Worcestershire's record count keeps climbing once the photograph appeal has run a full season.
- Field data on egg sac survival at the northern edge, the step the lab winters only simulated.