Gray-Brown Mouse Lemurs (Microcebus griseorufus) as an Example of Distributional Constraints through Increasing Desertification

Anthropogenic climate change may threaten endemic species commonly found in biodiversity hotspots around the world. One endemic species of particular concern is the Gray-Brown Mouse Lemur (Microcebus griseorufus), a primate species found in Madagascar—a biodiversity hotspot. Currently, climate change is increasing rates of aridity and desertification within the lemurs’ habitats. These climatic changes may affect the species’ end-of-the-wet-season food supply, an important resource that primarily contributes to their survival during the harsh, dry season. Therefore, to assess the impacts of aridity on lemurs and to identify factors that could inhibit the species’ distribution and range expansion under dry conditions, Bohr et al. (2011) compared two populations of lemurs in adjacent habitats that differed in humidity levels. They measured differences in lemur abundance, body mass, body condition, and food type abundance between the two sites, and also determined lemur distributions and feeding patterns. The authors found that the more humid site produced more high-quality food and maintained a higher population density of Microcebus griseorufus, with individuals in better condition compared to the drier site. The results showed that at the end of the wet season, the lemurs adjusted their home range size to local food plant density, indicating that lemurs modify their food intake at the end of the wet season to prepare for the dry season.  A negative, exponential relationship between food plant density and home range size also demonstrated that lemurs had an upper limit for the size of their home ranges. Therefore, primates from the drier habitat were unable to compensate for their reduced food availability by expanding their home range beyond this upper limit. Unfortunately, although lemurs would have the ability to migrate to mesic habitats under drier climate scenarios in search of food, habitat fragmentation in Madagascar could significantly reduce the lemurs’ ability to do so.—Megan Smith
Bohr, Y.E.M.B., Giertz, P., Ratovonamana, Y.R., Ganzhorn, J.U., 2011. Gray-Brown Mouse Lemurs (Microcebus griseorufus) as an Example of Distributional Constraints through Increasing Desertification. International Journal of Primatology 32:4, 901–913.

Microcebus griseorufus occured in southwestern Madagascar and occupied all vegetation formations from spiny bush to evergreen humid forest. They were the only mouse lemurs that inhabited the driest of these habitats (the spiny bush), and therefore represented the arid end of the genus’ ecological niche. In the more mesic parts of its range, it lived with Microcebus murinus, and these two species often hybridized. There were distinct dry and wet seasons within the species’ habitat, and Microcebus griseorufus tended to reduce their day range and activity (and therefore reduce their metabolic rate) during the dry season in response to food shortage. Therefore, lemur energy reserves accumulated during the wet season were crucial for their survival during the dry season.
The study site was located in the Parc National de Tsimanampetsotsa, which experienced highly seasonal rainfall. Recently, this region experienced a shift in maximum rainfall from December and February to March and April. This zone had the highest levels of plant endemism on the island (48% of the genera and 95% of the species were endemic). The majority of the vegetation was xerophytic and was classified among Madagascar’s spiny forest formations. There were two different vegetation formations within the study site that varied according to the underlying soil and the soil’s water holding capacity. The first location was a dry forest on unconsolidated sands (DFS) and the second location was a spiny bush formation on calcareous soil (XBC). The DFS site was more humid than the XBC site. The study period lasted from April until July 2008. April and May were defined as the late wet season and June and July were defined as the early dry season. A map of the study site showing the different vegetation types and the location of both study plots was displayed.
Within each vegetation structure, the authors established one study plot of 6 ha (150 X 400 m) and placed 96 Sherman Livetraps at 25 m intervals in each plot. Traps were placed 0.5–2.0 m high in the vegetation and were baited with bananas. One trapping session was conducted in each habitat in each season and lasted for 4 consecutive nights. This resulted in a total of 384 trap nights per habitat per season. Captured lemurs were anesthetized and marked with a microchip. They were weighed and their tail circumference was measured at the tail base. In addition to body mass, tail circumference represented body condition since gray-brown mouse lemurs store fat in their tail before the dry season.
Twenty-two individuals (DFS: 6 females, 5 males; XBC: 5 females, 6 males) were supplied with radio collars to assess feeding and ranging patterns. The authors studied feeding behavior through focal observations of 2 radio-collared individuals (1 female, 1 male) per habitat per season. The type of food ingested and the lemur’s position each time it moved was recorded. Frequency of feeding on certain food categories was documented rather than time spent feeding on items since the animals time processing and handling food items (fruits, gum, and arthrpods) varied depending on the type of food consumed.  All 22 mouse lemurs were sequentially tracked over a total of 8 half-nights by triangulation to assess their spatial and temporal distribution. Home range sizes were estimated using Animal Movement and the minimum-convex-polygon method. The authors compared the sizes of home ranges to test for seasonal variation in home range size between the wet and dry season. Home range data was also analyzed to test for habitat effects. Two graphs displaying the correlations between the numbers of food plants per ha and home range size (ha) for the wet and dry seasons were constructed.
All known food plants within the study plots were mapped using ArcView 3.2a. The plant data were overlaid over home range polygons to assess food availability within the individual home ranges. Food plants that had a height >1 m or a diameter at breast height >10 cm were included in the study. The researchers checked the plants for flowers and fruits twice a month. A graph displaying the phenology of fruit plants in the studied habitats between March and July 2008 was constructed. All the data were assessed using statistical analyses. Three tables displaying the results of the statistical analyses were constructed.
Bohr et al. found that the population density in the mesic dry forest was 3 times higher than in the drier spiny bush and that at the end of the wet season, mouse lemurs had higher body masses and larger tail circumferences (fat storage) than at the beginning of the dry season. Lemurs from the dry forest were in better condition than those from the spiny bush. Home ranges were also larger at the end of the wet season than during the dry season. Home range sizes did not differ between the two sites, and home range size was positively correlated with tail circumference.
At the end of the rainy season, observed lemurs fed equally on fruits and gum. However, at the beginning of the dry season, the lemurs ingested more gum over fruits. In the dry forest, lemurs consumed gum and fruits equally, whereas lemurs in the spiny bush primarily consumed gum. Arthropods were also eaten more frequently in the spiny bush than in the dry forest. Tables displaying the diet of Microcebus griseorufus in the DFS and the XBC sites, as well as during the wet and dry seasons, were constructed.
Throughout the entire study, the number of fruit-bearing plants was lower and declined faster in the arid spiny bush versus the mesic dry forest. Overall food abundance was high in March, but steadily declined in June and July. A higher total number of fruit-producing versus gum-producing plant individuals were found in the home ranges of the dry forest, and home ranges in the spiny bush had a significantly lower density of fruit-producing plants. The density of gum-producing plants did not differ between sites. Home range size and food plant density correlated negatively at the end of the wet season, indicating that home range size would need to increase exponentially if food abundance was to further decrease. No such relationship was observed during the dry season.
These results demonstrate that there were substantial differences in habitat quality between the two sites, and that the dry forest was the more favorable habitat for Microcebus griseorufus since it contained a larger density of the lemurs’ favored food: fruit. Lemurs’ distribution was therefore linked to food abundance at the end of the wet season, but not during the dry season. The lemurs prepared for the less favorable dry season at the end of the wet season by expanding their home range size and increasing their food intake. They then reduced their metabolic rates and lowered their energy expenses instead of attempting to increase their energy intake. This suggests that the species was limited by bottom-up factors (food resources) rather than top-down factors (predation).
The observed higher population density in the dry forest, with its higher availability of fruit plants, also suggests that the lemur populations were regulated by bottom-up factors (food resources). The lemurs preferred fruit to gum, and arthropods were hunted opportunistically since they were more difficult to locate and defend. Although gum contained concentrations of protein or carbohydrates that exceeded those found in Madagascar fruit, the fruit may have been preferred over gum because the gum contained secondary compounds that inhibit digestion.
The lower density and poorer body condition of the lemurs within the spiny bush indicate that the spiny bush habitat was less favorable than the dry forest habitat. This suggests that the higher proportion of gum-producing plants in the spiny forest could not compensate for the reduced amount of fruit plants at this site. Animals with larger home ranges accumulated more fat in preparation for the dry season. However, if the lemurs were able to extend their home ranges beyond their present sizes, larger home ranges would have been observed in the spiny bush. Clearly, these lemurs were unable to extend their home ranges to include more food resources even when faced with a drier climate and unfavorable food. The animals could have reached a point where home range extension (as compensation for declining food abundance) became unprofitable. Since climate change induced-desiccation will shift food resources toward gum at the expense of fruits, lemurs will need to migrate to more mesic areas to obtain required food resources.
Since Microcebus griseorufus inhabits the dry limit of its ecological niche in the xerophytic spiny bush, it will have to migrate to more mesic areas as climate change-induced desiccation shifts its food resources towards unfavorable gum. However, connectivity between habitats in Madagascar has been extensively disrupted by anthropogenic habitat fragmentation. Therefore, conservation efforts must be made to establish connectivity between lemur habitats.

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