- In Madagascar’s Tsinjoarivo rainforest, adults of the critically endangered diademed sifakas living in the most degraded of forest fragments tend to be skinnier, and young individuals show stunting, compared to individuals living in more intact parts of the forest, according to a new study.
- Skinny bodies in adults could mean that their nutritional intake is compromised in the disturbed areas, researchers say, while young sifakas could be growing more slowly in the most disturbed areas in response to reduced nutrition in the diet.
- Sifakas living in less-disturbed forest fragments, however, don’t appear to be in poorer health than those in continuous, intact forests. This could be because the long-lived sifakas are likely resilient to moderate habitat changes, the researchers say.
- But threats could add up and cause local populations to disappear, the researchers add.
The study’s lead author, Mitchell Irwin, an anthropologist at Northern Illinois University, first started working in Madagascar in 1999. “At that time I was interested in how lemurs were faring outside protected areas,” he told Mongabay.
To do so, Irwin zeroed in on Tsinjoarivo, a region of unprotected rainforest located 80 kilometers (50 miles) south and southeast of Antananarivo, soon expected to become a part of a new protected area. Human pressure was high there, with evidence of hunting, tree-cutting and forest encroachment, Irwin said in an email. But there was a gradient of disturbance: the western parts were more fragmented and disturbed by settlers compared to the more intact, continuous forests of the east.
In Tsinjoarivo, Irwin and his colleagues set up a behavioral study of the critically endangered diademed sifaka (Propithecus diadema), one of the largest living species of lemurs, distinguished by their long, silky coats. Between October 1999 and June 2018, the team, along with a veterinarian, captured 113 individual wild sifakas from the region, some more than once, and measured their body mass, length and body conditions, before releasing them back into the forest.
Diademed sifakas live in small groups that usually include one adult male, one or two adult females and a few immature individuals. The young sifakas tend to disperse at around 5 years of age. The researchers examined 12 such groups: five that resided in continuous forests with limited human disturbance, and seven in forest fragments with varying degrees of human disturbance. Where lemurs live — whether in continuous or fragmented, disturbed forest — can affect what food is available to them, the researchers found in previous studies. This, in turn, can be reflected in their physical condition, they posited.
“The most immediate way is that being fragmented and close to villages means that the lemurs are vulnerable to direct harvesting or hunting,” Irwin said. “The second way is that the extraction of trees has affected the food supply for the lemurs. A lot of their main preferred fruit sources have been largely eliminated in the disturbed fragments, and the fragment groups eat mistletoe (a less-preferred food) almost year-round.”
Irwin’s team found that the physical condition of sifakas in some fragmented forests were quite similar to those of sifakas in continuous forests. But below a certain threshold in the most degraded of the fragments, the researchers found differences: the adults were skinnier and had low body mass, while the immature sifakas showed stunting — lower mass and height for their age — compared to those in the more intact forests.
Skinny bodies in adults could mean that their nutritional intake is compromised in the disturbed areas, Irwin said, while young sifakas could be growing more slowly in the most disturbed areas in response to reduced nutrition in the diet.
In fact, over the years, Irwin’s team saw 11 lemurs from three groups living in the three worst habitats of their study area die or disappear. One group with one adult male, one adult female and one immature individual, for example, disappeared or died by September 2006, within a year of their body conditions being examined.
“The adult male’s remains were recovered in a tree, partially decomposed but intact and articulated; although a post-mortem exam was not possible, the body showed no sign of predation,” the authors write in the study. “This is consistent with a scenario in which poor nutrition contributed to death; this group had the lowest recorded nutritional inputs in the population.”
While it’s hard to directly link the disappearances to poor health, the researchers say the low-quality habitats could have played a role. “It’s sad to actually witness a species’ range shrink — it’s a small step toward extinction,” Irwin said in a statement.
However, the team found that sifakas living in fragments that were less disturbed didn’t seem to be in poorer health than those in continuous, intact forests. This could be because these primates have long life spans exceeding 20 years, possibly making them resilient to moderate habitat changes.
“But over time these threats can add up and cause these populations to be lost,” Irwin said. “This research not only helps identify which groups are at risk, but identifies ways to monitor their health directly through these measurements of size and growth.”
Overall, the sifakas’ health seems to depend on the “quality” of the forest, which in turn depends on how much disturbance it has undergone. “Just to tally up the remaining forest and count how much is left can be misleading, if the quality of that habitat is altered so as to be less suitable for certain species you’re aiming to conserve,” Irwin said.
Since research looking at the link between habitat quality and the health of non-human primates is slim, the researchers say they hope to extend their studies to another lemur species at their study site: the brown lemur (Eulemur fulvus).
“Unlike the sifaka, it’s mostly absent from all disturbed forest and fragments. But we’re not sure if that’s more due to hunting in the past, or if the food supply for them in fragments is simply inadequate,” Irwin said.
Additional reporting by Malavika Vyawahare.
Banner image of a diademed sifaka (Propithecus diadema) by Rhett A. Butler/Mongabay.
Irwin, M. T., Samonds, K. E., Raharison, J. L., Junge, R. E., Mahefarisoa, K. L., Rasambainarivo, F., … Glander, K. E. (2019). Morphometric signals of population decline in diademed sifakas occupying degraded rainforest habitat in Madagascar. Scientific Reports, 9(1). doi:10.1038/s41598-019-45426-2