Implementing a novel movement-based approach to inferring parturition and neonate caribou calf survival

Abstract

In ungulates, parturition is correlated with a reduction in movement rate. With advances in movement-based technologies comes an opportunity to develop new techniques to assess reproduction in wild ungulates that are less invasive and reduce biases. DeMars et al. (2013, Ecology and Evolution 3:4149–4160) proposed two promising new methods (individual- and population-based; the DeMars model) that use GPS inter-fix step length of adult female caribou (Rangifer tarandus caribou) to infer parturition and neonate survival. Our objective was to apply the DeMars model to caribou populations that may violate model assumptions for retrospective analysis of parturition and calf survival. We extended the use of the DeMars model after assigning parturition and calf mortality status by examining herd-wide distributions of parturition date, calf mortality date, and survival. We used the DeMars model to estimate parturition and calf mortality events and compared them with the known parturition and calf mortality events from collared adult females (n = 19). We also used the DeMars model to estimate parturition and calf mortality events for collared female caribou with unknown parturition and calf mortality events (n = 43) and instead derived herd-wide estimates of calf survival as well as distributions of parturition and calf mortality dates and compared them to herd-wide estimates generated from calves fitted with VHF collars (n = 134). For our data, the individual-based method was effective at predicting calf mortality, but was not effective at predicting parturition. The population-based method was more effective at predicting parturition but was not effective at predicting calf mortality. At the herd-level, the predicted distributions of parturition date from both methods differed from each other and from the distribution derived from the parturition dates of VHF-collared calves (log-ranked test: χ2 = 40.5, df = 2, p < 0.01). The predicted distributions of calf mortality dates from both methods were similar to the observed distribution derived from VHF-collared calves. Both methods underestimated herd-wide calf survival based on VHF-collared calves, however, a combination of the individual- and population-based methods produced herd-wide survival estimates similar to estimates generated from collared calves. The limitations we experienced when applying the DeMars model could result from the shortcomings in our data violating model assumptions. However despite the differences in our caribou systems, with proper validation techniques the framework in the DeMars model is sufficient to make inferences on parturition and calf mortality.

Space use and social association in a gregarious ungulate: Testing the conspecific attraction and resource dispersion hypotheses

“…Variation in home range size may be explained by other factors, including maternal status of females. Females without calves are not restricted by the slower movement speed of their calf (Bonar, Ellington, Lewis, & Vander Wal, ; DeMars, Auger‐Méthé, Schlägel, & Boutin, ), and often have very large home ranges in summer. It is also possible that the distribution of anthropogenic disturbances, such as logging cutovers, could also influence the high degree of variation in home range size we observed in summer (Faille et al, ; MacNearney et al, ; Schaefer & Mahoney, ).…”

Geometry of the ideal free distribution: individual behavioural variation and annual reproductive success in aggregations of a social ungulate

“…We only assessed parturient females, which we determined using a movement‐based approach based on the movement rates of the GPS collared adult females (Bonar et al . 2018). We followed methods outlined by DeMars et al .…”

“…(2013) and validated in Bonar et al . (2018). These capitalised on the fact that mothers are constrained in their movement when they have offspring: (1) a mother stops to give birth; (2) a young calf cannot travel as quickly as its mother, and slows its mother’s movement rate; (3) when a calf dies before 4 weeks of age, its mother returns to a baseline movement rate faster than it would with a calf‐at‐heel.…”

“…The degree of aggregation for most ungulates varies seasonally, with female–offspring aggregations being denser during birthing and post‐parturition, presumably because limited offspring mobility impairs adult movement (Bergerud 1975; Bonar et al . 2018). Birthing aggregations therefore represent an opportunity for selection (Moorad & Wade 2013) on an individual’s propensity to aggregate, as variation in both aggregation behaviour and variation in ARS as a function of aggregation behaviour exist.…”

Analysis of movement recursions to detect reproductive events and estimate their fate in central place foragers

“…Specifically, connecting movement to reproduction, an important component of fitness, is a promising emerging application. For example, recent studies used changes in movement patterns to detect parturition events along ungulate GPS tracks, particularly moose (Alces alces; Severud et al, 2015;Nicholson et al, 2019) and caribou (Rangifer tarandus; DeMars, Auger-Méthé, Schlägel, & Boutin, 2013;Bonar, Ellington, Lewis, & Vander Wal, 2018). However, to our knowledge, no study has attempted to infer avian reproductive fitness from movement data.…”