Pity, then, the mole-rat. It is consigned to just such a life. At least it has put its head-thumping to good use. For mole-rats thump their flat heads on the ceilings of their underground tunnels to communicate seismically with one another.
Some new research - studying mole-rats in Israel - shows that the patterns of such seismic signals correspond to differences in territory size and population density between individual species. The patterns employed by the four species appear to have been adapted to suit their individual needs.
The mole-rats were studied by researchers at the University of Haifa, at the Technion-Israel Institute of Technology, and the Jerusalem-based Nature Reserves Authority. The two conducted tests using 98 different individuals in a laboratory mock-up of underground mole-rat tunnels.
Israel's four chromosomal species of mole-rat are found in four climatic regions. From north to south, their locations are: the cool, humid, Upper Galilee Mountains (where they are known as Galilee mole-rats); the cool, semi-dry Golan Heights (Golan); the warm, humid, central part of Israel (Central); and the warm, dry, desert-like southern part of the country (Southern).
Recordings of the seismic patterns of the thumping showed that it was different for each species. The number of pulses of vibrations (or thumps) in a group of such pulses - and the duration - increased in the following species order: Golan, Galilee, Central and Southern. The thumping frequency - the number of pulses per second - decreased in the same order. There was no significant variation between males and females of the same species.
The researchers say the population densities of the four species of mole-rat decrease in precisely the same order. Population density almost certainly depends on how much food is available. Food is more profuse in northern Israel (Golan and Galilee) and less abundant in the south. Individual territory size, on the other hand, shows almost the opposite trend.
It seems likely that the differences in seismic communication between the different mole-rat species reflect each one's adaptability to its ecological - and, hence, demographic - background.
This interpretation was backed up by some of the evidence from measuring the electrical potentials created on the brain surface by vibration signals received by the mole-rats. This gives a measure of the sensitivity of individuals, or species, to vibrations. What the researchers found was that the Galilee mole-rats showed the lowest response and their southern cousins the highest.
So the Southern mole-rats, having the largest territories, produce more pulses - in larger pulse groups - and are the most sensitive to these seismic vibrations. Because they live further apart, they probably need to be.
Underground, auditory communication between mole-rats is limited to perhaps five metres (15ft). So seismic communication is the only effective means of long- distance communication between animals such as these that live in separate territories. As territory size increases, it seems the effort mole-rats have to put into thumping their heads on their tunnel ceilings increases, too.
It works perfectly. Tunnels excavated by different mole-rats never intersect, even though they often come very close to one another. But, one wonders, what would happen if some Southern mole-rats were moved, and set up home in, say, the Golan Heights? That would put a rather large seismic spanner in their subterranean works.
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