Saturday, 14 August 2010

Rodents of Bristol 9:The weirdest of them all

To draw this series on the rodents on show at Bristol to a close, I will finish with what is arguably the weirdest mammal (certainly the weirdest rodent) in the world, the Naked Mole Rat Heterocephalus glaber. Belonging to a sub-Saharan family of burrowing rodents, the Bathyergidae, Naked Mole Rats are confined to the horn of Africa from Kenya to the Sudan, where they can be locally common depending on the environment.

As you might guess from their range, they are essentially aridland animals, living almost entirely on the starchy tubers of desert plants, which provide all the water they need. Some of these tubers can be 60cm across, and the colony will feed on them for months or years. Quite often, when they finish they will use the hollow in the still living plant as a latrine site, thereby providing the plant with a fertilizer boost so they can return in a year or two’s time when it has regenerated.

They have so many unique features that it is very hard to know where to start, but what they are most famous for is their social structure, which is that of a eusocial insect like bees or ants. A single breeding queen, plus up to three breeding males, is the mother of all the other animals in the colony, usually around 80 strong, which have two castes – workers and soldiers.

Worker rats do most of the digging, at which they are extremely effective (tunnels systems can be several kilometres long), while soldiers protect against predators (mainly snakes). When the queen dies, potential successors fight it out (often to the death) until a new queen is triumphant.

While Naked Mole Rats were the first mammals known to be eusocial (by which is meant that they have distinct castes) the Damaraland Mole Rat (another Bathyergid) is now known to be eusocial as well. Other species have not been studied sufficiently to be sure, but it may be that several other species in the family also have this pattern.

The metabolism of Naked Mole Rats is as strange as their society. Living in burrows in cement-hard soil as they do, oxygen levels are low and CO2 levels are high. Possibly as a response to the high CO2, they have lost their ability to produce substance P, which is used in the transmission of pain sensations and is triggered by acid. Their lungs are reduced, but their blood has a high affinity for oxygen.

The main adaptation however, is that they have essentially abandoned the mammalian homoeothermic metabolism. Like reptiles, their body temperature varies with their surroundings, and although they still generate internal body heat, this is insufficient for an isolated individual to avoid fatal chilling. Consequently, they use behavioural means to regulate their body temperature, either piling together in a single nest chamber, or basking just under the surface of ground in the heat of the day. It has even been speculated that these warmed up individuals then go back down to the nest to warm the others like living hot water bottles.

Perhaps because of this reptile-like metabolism, they also have a reptilian life span. Queens have been recorded as living to 26 years in captivity, and it is quite possible that a similar age is reached in the wild.

Naked Mole Rats can be seen in many collections these days, but unsurprisingly require care different from the average small rodent. Feeding is straightforward – a staple diet of sweet potato (yam) plus similar root vegetables and some apple or pear is all that is required, but accommodation and environment requires careful planning. The usual display environment is a series of clear glass or plastic tubes, interspersed with nest chamber big enough to hold all the animals in the colony, but it is vital that they have ample opportunity to dig. At Bristol we have a devise off show which enables them to dig through a tank of sand each day for exercise. They tend to use a single latrine chamber, so cleaning out is straightforward, but this must not be done too often as the colony scent is important in maintaining a group identity. Additional water is not required.

Mole Rats are nearly, if not completely, blind, but low light levels do not disturb them and allow viewing. What is important is the auditory environment, as they are sensitive to unexpected sounds. Playing a radio (we have a recording of a burrow) 24 hours per day desensitizes them to the public and means they behave normally. The temperature must be strictly controlled by a thermostat to between 26C and 31C –temperatures outside this range cannot be tolerated for long. Humidity needs to be between 30-50% to avoid skin problems.

The queen rat of a new colony will produce litters until there are enough surviving young to maintain and defend the burrow system, then stop. The only way to restart breeding is to remove individuals or expand the burrow system space. New colonies start in the wild when an old colony divides, and all mole rats are highly similar to each other as a result of queens usually mating with close relatives in the colony.

next time: parrots at Bristol part 1


Images: Wikipedia

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