VIGNETTE: GHOSTS IN THE DARKNESS: STUDYING THE ECOLOGY OF PALLID FAT-TAILED OPOSSUMS

ALBANESE, MARIA SOLEDAD

Secret lives of carnivorous marsupials

CSIRO Publishing

Año: 2018; p. 186 - 188

When I moved to Mendoza Province (Argentina), more than 10 years ago, I never thought I would be fascinated by such a little creature. Actually, I didn´t even know this mouse-sized marsupial inhabited the desert landscapes of Argentina. But with their large ears and big and expressive eyes, once I was introduced to the species, I was quickly hooked. I started my PhD with the main objective of studying several aspects of Thylamys pallidior ecology: where they live (habitat use), what they eat (foraging strategies), how their population is structured throughout the year. But first I had to understand why they had been so challenging to study for other scientists. I soon learned it was not because of lack of attention from researchers, but rather because they were quite difficult to catch. This marsupial coexists with many species of rodents, and while rodents were easily caught during trapping efforts, T. pallidior was so elusive to be almost mythical! I accepted the challenge and got on board for a sometimes demanding, occasionally overwhelming, but always exciting project. Ten years later I can say that it was totally worth it! The more I learned, the more I became fascinated with these intriguing little animals. This mouse opossum, commonly called the White-bellied or Pallid Fat-tailed Opossum, or as I like to call it in Spanish ?Marmosa pálida? meaning pallid mouse opossum, has the largest distribution among all Thylamys species. They inhabit mostly arid or semiarid regions and are found in a great variety of landscapes, from low and flat Patagonian steppe to high elevations in the Andes of South America. Within these landscapes, the species inhabits diverse habitats such as Mesquite woodlands, shrublands, sand dunes, grasslands, and also can be found near rocky habitats. They have amazing adaptations which enable them to exist in harsh desert environments. The Monte desert in Argentina, where this little marsupial occurs, is no exception. The climate is strongly seasonal with freezing cold and dry winters where temperatures readily drop below 0°C. This contrasts the extremely hot and wet summers where temperatures commonly reach and sometimes exceed 40°C. I remember in summer afternoons during my PhD fieldwork in Ñacuñan Reserve (Mendoza), the feeling of opening the door of a giant oven every time we left the Field Station! Unlike us, to cope with these temperature extremes, mouse opossums are active only at night, when temperatures often drop 20 degrees or more. This is the time when many of their preferred prey are also active. About 70% of the diet of Pallid Fat-tailed Opossums is comprised of insects and spiders. The agility of these little carnivorous marsupials enables them to easily capture moths, crickets, grasshoppers, beetles and spiders. But perhaps their most impressive feat is the extraordinary accuracy and speed in catching and handling threatening opponents such as scorpions! During foraging trials performed in Ñacuñan Reserve, we wanted to evaluate which prey Pallid Fat-tailed Opossums consume and which they don´t. We offered the animals different kinds of insects and spiders, and I recall being a little bit concerned when it was time to test them with scorpions. But that feeling only lasted a couple of seconds! In the blink of an eye this Thylamys immobilized its prey by grabbing the scorpion´s tail with one paw and the rest of its body with the other. It was all over in just a few frantic moments ? dismembered scorpion! I was impressed to watch how this, in appearance shy and inoffensive animal, turned into a lethal and efficient predator at the instant a potential prey ítem appeared. During the day, when Pallid Fat-tailed Opossums are not hunting for food, they remain in refuges. They use holes in trees, under the ground, or sometimes even occupy bird nests. In some cases the nests are abandoned, but in others the marsupials will prey on eggs they find inside. Even though this desert mouse opossum is mostly solitary, two or more individuals sometimes share the same nest. The composition of these groups is variable regarding sex and age. They can be composed of only males, only females or both sexes. On one memorable occasion during nest monitoring, we were stunned to find nineteen animals of different sexes and ages in the same nest! It was a big, living ball of fur and tails! We don´t yet know if these groups are formed by relatives sharing the same territory or if unrelated individuals just get together to take advantage of the benefits of group thermoregulation (keeping warm). In fact, this is one aspect of Thylamys pallidior biology we are investigating at the moment. In deserts, winters are definitely an energetically demanding time of the year when, not only temperatures are low, but also resources are scarce. In this sense, Pallid Fat-tailed Opossums show interesting adaptations to deal with this challenging season. For example, they use daily torpor (i.e., decreased physiological activity) as a way to reduce energy expenditure by decreasing body temperature and metabolism. I will never forget my first experience with a torpid animal ? the poor beast looked like it was frozen! Not a movement, not a sound and its little furry body was extremely cold to the touch. You would not even know it was breathing! I carefully wrapped it with my glove and put it in my pocket. Less than an hour later, the little opossum was completely awake and restless, ready to go back where I had found it. Another interesting feature of this species is their tail. As their common name indicates, fat-tailed opossums are able to store fat in their tails to reabsorb as an energy source in lean times. The tail reaches a maximum thickness by the end of Autumn. By this time, tail width may have changed from 3-4 mm to 11 mm in only a couple of months. By the end of the winter, all animals show thin and wrinkled tails as consequence of their fat reserve´s consumption. At this time of the year, individuals that survive winter are ready to reproduce. In Ñacuñan´s population, reproduction occurs only once in a life time. Females are pregnant in spring and give birth in the most favorable time of the year, when resources are abundant. Most males disappear from the population by the beginning of Summer and females just a couple of months later, after weaning their young. At this time, the population is totally composed of young animals from the same generation, born in the same season. This reproductive pattern has been found in other small opossums from South America, and is defined as a semelparous strategy. Even though this is well studied for Australian marsupials, more research is needed to understand how the process compares in their Neotropical counterparts. Working with the pallid mouse opossum in the wild was certainly a great challenge for me. However, it was a very gratifying and enriching experience in many ways. When so little is known there is much to learn, and on this journey I have certainly learned a great deal about this secretive carnivorous marsupial.