We think of spiders as fearsome hunters, spinners of webs and treacherous mates, but construction workers? Yes, that too. Some groups of spiders – trapdoor and wolf spiders – dig tunnels that they use to ambush passing insects. But these tunnels can also provide shelter and accommodation for other animals, including one of the rarest of Australia’s lizards – the pygmy blue-tongue lizard. It seems that the lizard’s survival depends entirely on the spiders.
The pygmy blue-tongue is a native of South Australia. It’s so rare that zoologists thought it extinct for over 30 years and it re-emerged in the public eye in the most unlikely way. In 1992, a dead specimen of this supposedly extinct animal was found in the stomach of a brown snake, found dead on the side of a road. That unexpected discovery prompted intensive surveys of the surrounding area, which found several lizards living in spider burrows.
Like the spiders, the lizards use the burrows for ambush but they also act as nurseries, cooling stations and defensive forts. To understand the relationship between lizards and spiders, Michael Bull from Adelaide’s Flinders University studied the fates of both species in a single hectare of South Australian land. Over two years, his team spent bursts of two weeks, intensively searching the plot of land for signs of burrows. Each one was probed with a fibre optic camera to see who lay inside.
Earlier studies have found that lizards readily accept artificial burrows and adding these to the local area will halt the decline of the lizard populations. That’s all very good, but as Bull writes, “a sustainable supply of natural burrows would be a better option in the longer term”.
For humans, sex is a simple matter of chromosomes: two Xs and we become female; one X and a Y and we develop into males. But things aren’t so straightforward for many lizards – many studies have found that the temperature of the nest also has a say, even overriding the influence of the chromosomes. But the full story of how the lizard got its sex is even more complicated. For at least one species, the size of its egg also plays a role, with larger eggs producing females, and smaller ones yielding males.
The discovery comes from Richard Shine’s group at the University of Sydney. In earlier work, they showed that if the Eastern three-lined skink (Bassiana duperreyi) incubates its eggs at low nest temperatures, XX carriers develop into males regardless of their chromosomes.
Now, Rajkumar Radder, a former member of Shine’s team, has shown that the amount of yolk also determines the sex of a skink, but only at low temperatures. By deliberately adding and removing yolk from eggs using a syringe, he managed to alter the sex of the hatchlings. This degree of complexity is totally unprecedented – it means that for the skink, sex is a question of its chromosomes, the temperature it was reared under and the amount of yolk it had.