Time to leave home
The Garden Wolf Spider (Tasmanicosa godeffroyi) is a dedicated mother. Like other members of the family Lycosidae, she carries her newly hatched spiderlings around on her back. And that’s a very large family!
Eventually, her brood will leave their secure mobile home and venture out into the undergrowth to start searching for food. They’ll have large appetites, as up to that time their only nutrient source has been the yolk in the egg from which they developed – another legacy of their mother.
I described this behaviour in our recent Summer Sightings blog. But the discovery in late March of another nesting female immediately in front of our house deck gave me the opportunity to record it in more detail.
I sought answers to questions such as – How long do the spiderlings remain on the mother? Do they all leave home at the same time or in dribs and drabs? What triggers their departure? What advantages accrue to the young (or the mother) from this arrangement?
As Mother’s Day approaches, I dedicate this blog to mothers everywhere ❤️
First sighting of new mother
I was first alerted to the presence of a new female wolf spider on 26th March, when I saw an egg sac in a patch of vegetation. The spider was holding the sac with her spinnerets while facing towards the bottom of her burrow, thereby incubating the embryos in the sac in the sunshine.
It was only after she changed position and advanced towards the entrance that I got a good look at her.
Her nest had an interesting construction. She had made an earthen burrow, 9cm deep, in the standard wolf spider fashion. But unusually, it was sited in an area with a dense growth of grasses and forbs, rather than in the open.
She had woven a long silken extension to position the tunnel entrance near the top of the undergrowth. Quite a work of art!
Spiderlings emerge!
Just 2 days later, on 28th March, the female emerged proudly displaying her new clutch of offspring. The spiderlings were densely packed over her back and sides. They completely covered her abdomen, spilling over onto her cephalothorax. Based on a study of the same species carried out near Canberra, they would have hatched and undergone their first moult inside the egg sac (Humphreys, 1976).
This same author reported that spiderlings appeared on the backs of females about a month after egg sac production. So my female would have been present at that spot long before I first noticed her. Her nest site in the dense undergrowth would have presumably also helped to conceal her from potential predators.
The mother emerged from the burrow periodically to sunbake with her clutch.
I was able to get good photos from a distance of 1.5 to 2 metres with my telephoto lens. But if I came too close or moved suddenly she would dart back down her burrow. Peering out, she would wait until I moved away before emerging again – a very protective parent!
I never saw her venturing far beyond the mouth of the burrow, enabling her to make a quick retreat if threatened.
She was still carrying her egg sac at this stage, but three days later it was no longer present. It’s unlikely that all of her brood emerged from the sac at the same time. There were around 70 spiderlings on her back on my first sighting, while a few days later that number had risen to well over 100.
This raises the question of how the spiderlings get out of the egg sac. Foelix (2011) states that, at least for some wolf spiders, the mother opens the rim of the egg sac with her chelicerae to release the young. It seems unlikely that this happens in the case of Tasmanicosa godeffroyi. I saw no sign of an open seam in our spider’s egg sac and the gradual increase in spiderling numbers over several days argues against them leaving the sac by a single, large opening. How they do get out remains an open question.
Once on the mother’s back, the spiderlings often moved around, as seen in the compilation of videos below, made on 28th March. Some wandered over the surface of the egg sac, but no further afield.
This next video compilation, made on 31st March, shows the spiderlings moving in occasional frantic bursts as the mother lies motionless within the burrow. Note that one little spider ends up at the mouth of the burrow, but does not venture further.
The exodus begins
I checked the mother on a daily basis after my first sighting of the clutch. But it was only on 14th April, 17 days later, that I saw the first clear signs of spiderling departure.
It was a cool 15°C morning at 9:33am and the nest was in the shade. I noticed a few spiderlings clambering over their siblings and moving down the mother’s legs onto the walls of the tunnel.
There were well over 100 spiderlings on her back at that time. It’s difficult to get a precise count as they were at least two deep in places.
Then every minute or so over the next 5 minutes, there was a burst of activity in the clutch as a number of spiderlings started moving at the same time. Some of these left the mother and wandered around inside the tunnel. A few ventured further afield, moving into the surrounding vegetation.
Over the next two hours, there were further periods of increased activity within the clutch, with spiderlings moving around and scurrying up and down the mother’s legs – as seen in the video below. However, few left her body during this time.
Breakouts!
Then at 11:22am there was a sudden change in the behaviour of the clutch, which continued for around 20 minutes.
All at once, a large part of the clutch began moving around rapidly. Many of these spiderlings dashed away from the mother’s back into the surrounding vegetation. They would often follow silk lines, laid down by earlier departing spiderlings.
I’ve called these periods of activity “breakouts”.
Some traffic went in the opposite direction, as spiderlings returned home.
Breakouts occurred in some cases after the mother began moving, but others happened when she was quite still.
On one occasion - seen in this video - a breakout occurred immediately after a couple of spiderlings had returned to the clutch. But was that just a coincidence?
The breakout shown above - at 11:45am - was the last I saw on that day. A spiderling or two would move around occasionally but otherwise all was quiet on the mother’s back, as shown in the video below.
Spiderlings in their new home
While some of the departing spiderlings quickly returned home, many remained in the surrounding undergrowth. This overview video shows spiderlings exploring their new environment.
Clusters of spiderlings were seen climbing to the tops of forbs and grasses - presumably to escape the attention of predators below.
It’s a dangerous world out there! It will be six months before they start burrowing, when only 10% of the spiderlings will have survived (Humphreys, 1976).
Further breakouts reduce the mother’s load
While no video recordings were made between 14th and 17th April, snapshots showed an obvious decrease in the number of young on the mother’s back over that period.
On 18th April, a large number of breakouts took place, beginning at 11:37am. Over a 10 minute period, the number of spiderlings on the mother’s back dropped from around 50 to 30.
When I finished monitoring the clutch on that day, there were only around 20 spiderlings left. That number had dropped to 12 by the afternoon of 19th April.
Some spiderlings are reluctant to leave home
The remaining 12 spiderlings took up to another week to leave their mother.
On the afternoon of 22nd April, there were still 9 present, a number which remained constant until 24th April, when it had dropped to 2.
That final pair hung around until the afternoon of the following day, when the mother was finally free of her load.
It’s likely she died soon thereafter. I saw no sign of her at the mouth of the burrow over following days. A week later, the silk entrance was gone and her burrow was empty.
Humphreys (1976) states that egg sac production has ceased by April and that females probably die after their young have dispersed.
Findings:
a) Leaving home is a protracted affair
The first young left 17 days after they had first taken up positions on their mother’s body. Over 90% of the clutch left within the next 5 days but the rest remained for up to a further week. A few stragglers spent 4 weeks with their mother.
This is much longer than the 8.6+/-3 days reported by Humphreys (1976) for the same species. However, his data was drawn from observations of females housed in the laboratory. He suggests that the spiderlings may remain on the mother for longer in the field. Framenau et al. (2014) state that some wolf spiderlings can spend two weeks or more on the mother.
b) Spiderlings leave in fits and starts
At least on the two days I monitored the mother for an extended period (14th, 18th April), spiderlings departed in short bursts, rather than at a steady rate over the day.
Over a 10-15 second time span, a portion of the clutch started actively moving around and several of these spiderlings left the mother’s body - a breakout.
This was repeated every minute or so over a 10 or 20 minute period. I saw only a couple of such periods of activity on each of the two days in question.
The number of spiderlings left on the mother had dropped significantly at the end of those days.
Unanswered questions:
a) What triggers spiderling departure?
The sudden and erratic pattern of spiderling departure suggests that something is triggering this behaviour. However, the nature of that trigger remains a mystery.
Movements of the mother often lead to spiderlings changing positions on her body, but generally not breakouts. The latter were often observed when the mother was stationary.
The return of spiderlings to the clutch often causes a rearrangement of positions on the mother, but not breakouts.
b) Why does a mother bother?
The mother wolf spider’s task of carting around her mass of youngsters for some weeks appears quite onerous. It is certainly an extreme example of parental care amongst spiders.
So what benefit does she or her offspring actually gain from this association? Foelix (2011) states that “whether the spiderlings gain any particular advantage by being carried around is still unclear” (p.259).
The mother responds instantly to threats by darting back down her burrow, so this would provide protection to her offspring. She rarely ventures beyond the mouth of the burrow while carrying her young.
But after they leave the mother’s body, spiderlings are barely, if any larger than before and would seem to be just as vulnerable to predation.
References:
Foelix, R.F. (2011) Biology of Spiders 3rd ed. Oxford University Press.
Framenau, V.W., B.C. Baehr and P. Zborowski (2014) A Guide to the Spiders of Australia New Holland Publishers.
Humphreys, W.F. (1976) The Population Dynamics of an Australian Wolf Spider, Geolycosa godeffroyi (L. Koch 1865) (Araneae: Lycosidae) J. Animal Ecology 45: 59-80.