The negative impacts of parasites on wildlife population dynamics, and the cascading effects of these parasites on ecosystem function, are increasingly recognized across a broad range of host taxa (Koltz et al., 2022, Shanebeck et al., 2022). In wild ungulates, gastrointestinal trichostrongyline nematodes (Nematoda: Trichostrongylina), particularly species found in the abomasum, impact host body condition, and host fitness through effects on pregnancy rates, and offspring survival (Albon et al., 2002, Stien et al., 2002, Craig et al., 2008, Debeffe et al., 2016, Aleuy et al., 2018). Although the measurable effects of nematode infection on host fitness are often small (e.g. Stien et al., 2002, Aleuy et al., 2018), they can have consequences for host survival and population dynamics (Koltz et al., 2022, Shanebeck et al., 2022). The diversity of gastrointestinal nematodes present in wild ungulate populations means that it is challenging to quantify the effects of individual parasite species (Aleuy et al., 2018). Longitudinal studies are rare (see Albon et al., 2002, Stien et al., 2002, Carlsson et al., 2017), and most studies on free-ranging wildlife are limited due to being opportunistic cross-sectional sampling of small numbers of animals (e.g., Hughes et al., 2008; Turgeon et al., 2018). Few wildlife studies have been successful in directly addressing differential impacts of closely-related parasite species on host health. However, parasite species that are closely related may affect their hosts in different ways and time frames due to parasite species-specific life history strategies which can influence the timing of transmission and effects (Barger, 1993; Irvine et al., 2000).

Rangifer sspp., a genus comprised of wild and domestic caribou and reindeer, are ecological keystone species and play an important role in the lives of northern Peoples (Hummel and Ray, 2008, Festa-Bianchet et al., 2011). Across the Arctic, populations of Rangifer sspp. have experienced steep declines and the specific drivers for these declines remain poorly understood (Vors and Boyce, 2009, Russell et al., 2018). Recent modelling studies suggest that gastrointestinal parasites in Rangifer sspp. can have important population level impacts (Albon et al., 2002, Koltz et al., 2022, Peacock et al., 2022), and therefore may contribute to detrimental cumulative effects (White et al., 2014). Studies on Svalbard reindeer (Rangifer tarandus platyrynchus) and caribou from the Dolphin-Union herd in Canada (Rangifer tarandus groenlandicus x pearyi) demonstrated that the intensity of abomasal nematode infection had negative effects on both body condition and fecundity (Albon et al., 2002, Stien et al., 2002; Hughes et al. 2008). These effects were mainly attributed to Ostertagia gruehneri, the most common nematode in Rangifer sspp. (Irvine et al., 2000). Infection intensity for O. gruehneri peaks during the summer when caribou are maximising food intake to gain body condition, which is critical for survival during winter (Irvine, 2000; Richert et al., 2021; Fig. 1). However, the effects of other abomasal nematode species that have different life history strategies, such as Marshallagia marshalli, which is transmitted during the winter, have been difficult to distinguish (Carlsson et al., 2012, 2017). Peacock et al. (2022) suggest that life histories of caribou and their parasites can be a stronger driver for population dynamics than climate change, highlighting the importance of investigating species-specific impacts of parasites in a system where co-infections are common.

The two largest populations of caribou (R. t. groenlandicus) in West Greenland, the Kangerlussuaq-Sisimiut (KS) and Akia-Maniitsoq (AM) herds, are functionally isolated from each other by the Sukkertoppen ice cap (Cuyler et al., 2011). A history of sequential colonization and partial parasite loss as these populations established in West Greenland, followed by subsequent spill-over from different introduced sympatric species, has resulted in each herd hosting different abomasal nematode species (Steele et al., 2013). This provided a unique opportunity to investigate potential species-specific effects on host body condition and fitness. The only abomasal nematode of caribou in the AM herd is O. gruehneri, present at 100% prevalence. In contrast, M. marshalli and Teladorsagia boreoarcticus were present in caribou in the KS herd at 100% and 94.1% prevalence, respectively, with M. marshalli dominating the abomasal fauna (Steele et al., 2013). Using data on the abomasal nematode diversity presented in Steele et al. (2013), the objective of this study was to determine the relationship between infection intensity, and the presence of a calf at heel, body condition and reproduction status (i.e. the presence of a fetus) in these two West Greenland herds infected with different abomasal nematodes, and interpret these results in the context of the life histories of the different nematode species.