What if it gets crowded? Density‐dependent tortuosity in individual movements of a Neotropical mammal

Abstract

Effects of density dependence on animal movements have received much attention in ecology, but it is still debated to what extent dispersal and movements in general are density dependent, and their potential contribution to population regulation processes. Here, we determine the occurrence and nature of density dependence in the movements of a Neotropical marsupial, the black‐eared opossum Didelphis aurita Wied‐Neuwied 1826. Using spool‐and‐line tracking devices, we estimated the tortuosity of fine‐scale movements of 149 individuals by their fractal dimension D. We evaluated the relative importance of population size, reproductive or climatic seasons and reproductive maturity of individuals as determinants of movement tortuosity, using a model selection approach. Population size was the most important determinant of movement tortuosity, with season (climatic seasons for females, reproductive seasons for males) and reproductive maturity as secondary but also important variables. We detected a positive density‐dependent effect on movement tortuosity, resulting in more intensive use of areas by individuals during periods of high population size. This positive association between movement tortuosity and population size is more likely to result from intraspecific competition, which forces individuals to explore their environment more intensively during high‐density periods. Therefore, despite being density dependent, movements in D. aurita apparently do not contribute to population regulation mechanisms.

Negative or positive density‐dependence in movements depends on climatic seasons: The case of a Neotropical marsupial

“…In Neotropical marsupials, females and males seem to have different behavioural patterns of movements, driven mostly by (i) food acquisition to deal with the high costs of lactation, and (ii) finding mates for reproduction respectively (Loretto & Vieira ; Almeida et al . ). Previous studies with D. aurita found higher movement tortuosity in the reproductive compared to the nonreproductive season for males, and higher tortuosity in the humid season compared to the super‐humid season for females (Loretto & Vieira ; Almeida et al .…”

“…The same positive association between movement tortuosity and population density was detected in a previous study with D. aurita (Almeida et al . ), in the same sites of continuous forest that we studied. Interestingly, however, this previous study detected consistently positive density‐dependence irrespective of the climatic season considered.…”

“…Movements may also be directly affected by density, either if individuals avoid or are attracted by co‐specifics, which determine, for example, the tortuosity of movement paths: more spatially restricted movements imply higher tortuosity of movement paths, whereas more linear and longer movements imply low path tortuosity (Almeida et al . ). If movement paths become less tortuous with increasing density (negative density‐dependence in tortuosity), movements may ultimately contribute to population regulation, by spreading individuals over a larger area, thus reducing density when it is high.…”

“…with higher tortuosity) during high‐density periods (Almeida et al . ). However, no density‐dependence effects were found on daily home range sizes for another didelphid in the same study area, the Brown four‐eyed opossum, Metachirus nudicaudatus (Ferreira et al .…”

“…), thus probably affecting both population sizes and movement behaviour of individuals (Almeida et al . ; Ferreira et al . ).…”