In the caatinga biome, on the margins of the São Francisco River in the State of Bahia, the sharp-eyed observer will see burrows approximately 15 centimeters wide covered by spiny leaves. An icon of the backcountry, the macambira bromeliad plants form a thick blanket that characterizes the barren environment of the region, a landscape that protects the creatures that live in this environment. More specifically, this landscape is the home of torch tail spiny rats, the small rodents that are found only in that region. Biologist José Wellington dos Santos had to deal with the spiky leaves and extreme temperatures, which can range from 15 degrees Celsius at night to 43 degrees Celsius during the day, to understand how these small mammals live. He realized that colonies of rodents living in burrows are the key to success.
“A proper burrow is protected by spiny vegetation,” says the researcher, who is currently enrolled in the post-doctoral program at the Federal University of Bahia(UFBA). He started doing research on the rabo-de-facho (Trinomys yonenagae) when he was still an undergraduate student at UFBA, soon after the species had been described in 1995, and classified under the Proechimys genus. He continued doing research on the mammal while he was enrolled in the master’s degree program at the University of São Paulo and in the doctoral program at the University of California in Berkeley. He finished his doctorate in 2010. The first article that resulted from his PhD thesis was published last year in the Journal of Mammalogy. It describes the first case, as witnessed by researchers, of social groups among rodents of the Trinomys genus, commonly found in Brazilian forests.
The Trinomys from the backcountry is quite different from its next-of-kin, which closely resemble ordinary rats. Trinomys has big hind legs, which help the animal leap like a kangaroo when it is in a hurry. The rodent whips its long tail that has tufts of hair on the tip to change directions in the middle of the leap and quickly run away from predators. It is a source of food for many animals: wildcats, owls, snakes, tegus, and wild dogs are some of the predators that visit the sand dunes in search of this “delicacy,” which measures approximately 16 centimeters, minus the tail. The mammal’s tail is longer than its body, the enormous ears seem out of proportion and the front paws are much shorter. In sum, the rodent resembles the typical mammals that live in desert habitats in other parts of the world. The researcher observed that the rabo-de-facho needs to resort to strategies to survive because it lacks the physiological adaptations that allow other rodents to live in semi-arid climates.
The biologist captured, marked and released more than 400 rodents he had found in the vicinity of the village of Ibiraba, next to the municipal region of Barra, which lies on the left bank of the São Francisco River in the State of Bahia. He captured the rodents while he was still doing his PhD. The study revealed that several adult rodents lived in 75% of the burrows observed by the biologist. The biologist equipped some of the animals with radio transmitters attached to collars to allow the mapping of their movements inside the underground burrows. This showed that males and females sharing the same burrow actually use the same space inside, which clearly indicates that they are a couple sharing the same burrow instead of the territory. This life in groups, that sometimes comprise up to ten adults and their offspring, is not a frequent situation among wild rodents, which do not normally adapt well to a typically human “student dorm” situation, where space and food (and sometimes sex partners) are collective assets.
However, the collective digging of burrows in the dunes, where it is impossible for mammals to survive without a humid burrow because of the extreme heat during the day and cold during the night, is a good strategy. “When a rodent starts to dig a burrow, the others go into a digging frenzy,” says Santos. He estimates that some of the tunnel systems can be up to 15 meters long. He also discovered that the rabos-de-facho visit neighboring colonies, sometimes contributing to produce offspring, and that each year some of the rodents move to other groups where they find companions that are not next-of-kin.
These burrows shelter other animals as well, such as small lizards, spiders and grasshoppers. This is why the rabos-de-facho are important for the maintenance of the ecosystem. The rodents also affect the distribution of the araçás-de-boi, a shrub that belongs to the guava tree family. The seeds of this shrub are the rodents’ favorite food. Santos observed that the rats live where there is an abundance of these shrubs, but he does not know to what extent this abundance is due to the animals’ actions. “The rodents bury some seeds next to the opening of the burrow, but I haven´t seen how many of these seeds the rodents retrieve later on,” he says. The fact is that some of the seeds germinate; this, in turn, increases the density of these shrubs, which account for approximately 40% of the vegetation in this area of the dunes. “The action of the rabos-de-facho might help increase the vegetation, protect the sand on the dunes, and create microclimates for other animals,” he suggests, indicating that these are fields of research that still need to be explored. In his opinion, it is also important to take into account the heterogeneous nature of the landscape in order to plan the conservation of this biome: a valley could shelter a genetic asset that differs considerably from another genetic asset, as the tops of the dunes might function as partial barriers.
An interesting aspect of the caatinga is that the climate patterns are unpredictable. The rainy season is supposed to last from November to April, but this does not always happen. From the evolutionary point of view, this unpredictability prevents the organisms that live there from concentrating essential functions, such as reproduction, during more favorable periods. “Like the animals living in the Mata Atlantica rainforest, the rabo-de-facho females actively reproduce all year round,” explains Santos. However, the offspring only survive during the rainy season, when the araçás-de-boi shrubs bear fruit. The biologist concludes that “in a chaotic environment, the strategy is to have no strategy.”
SANTOS, J.W.A. & LACEY, E.A. Burrow sharing in the desert-adapted torch-tail spiny rat, Trinomys yonenagae. Journal of Mammalogy. v. 92, n. 1, p. 3-11. 2011.