Animals are among the most important vectors for long distance dispersal of plant seeds (e.g. Howe & Smallwood, 1982; Nathan et al., 2008, Fricke et al., 2022). Besides contributing to seed dispersal via endozoochory (i.e. passage of seeds through their digestive system), animals (notably mammals) also transport a large number of seeds of many plant species via attachment to the exterior of the body. This mechanism is called epizoochorous seed dispersal (Fischer et al., 1996; Benthien et al., 2016). While our mechanistic understanding of endozoochorous dispersal has been increased thanks to studies of animal feeding behaviour, animal movement and gut retention times (e.g. Picard et al., 2016, González-Varo et al., 2017, Lepková et al., 2018, Fricke et al., 2022), we still have comparatively little knowledge about the process of epizoochory. The major challenge in this respect is the description and quantification of the process of seed detachment. As a consequence, we lack information on the retention time of seeds in animal fur, a crucial parameter for quantifying seed dispersal and dispersal distances. This study aims at quantifying and explaining shaking movements on different parts of mammal bodies, relevant for seed detachment, and thus, identifies the link between the process of seed detachment to available collar data, frequently recorded on wild animals.
The three phases of seed dispersal are initiation (for epizoochory seed attachment to an animal body), transport (movement of the animal at a certain velocity) and termination (seed detachment) (Nathan et al., 2008). Seed attachment can be studied by moving animal furs through/along vegetation (Fischer et al., 1996) and by combing furs of wild animals (e.g. Heinken et al., 2002). Thanks to recent advances in tracking technologies and modelling, our understanding of animal movement, and hence, potential seed transport, has increased (e.g. Jonson et al., 2008, Gurarie et al., 2017, MacCurdy et al., 2009, Wright et al., 2020) and plenty of movement data are available, e.g. on Movebank (movebank.org). The process of seed detachment poses, however, a greater challenge since it cannot be easily observed and studied in the wild.
Due to the difficulty to investigate seed detachment in the wild, previous studies have chosen an experimental approach. They found that for this process the interplay of the animal surface (e.g. fur properties) and seed morphology (e.g. appendages like hooks) are crucial factors, determining the so-called contact separation force that is needed to detach the seed from the fur (Gorb & Gorb, 2002, Couvreur et al., 2004; Tackenberg et al., 2006; de Pablos & Peco, 2007). On an animal body forces to release seeds can be created via shaking of the body, causing acceleration of the fur (F = m x a), for example during animal movement. It has been shown that seed release can be induced by fur acceleration (Römermann et al., 2005) and that the strength of fur shaking determines the proportion of seeds released (see Figure S1).
Accelerometers, that measure acceleration at high frequency can quantify the strength of acceleration which directly translates into the force that seeds experience in animal furs. To transfer the above-mentioned insights on seed detachment in dependence of force/acceleration into natural systems it is therefore essential to measure acceleration over time on wild animals. Thanks to rapid advances in GPS telemetry in the last decade (e.g. Cagnacci et al., 2010, Hallworth & Marra, 2015) more and more studies are conducted on animal movement, often including measures of acceleration. These data can be used to infer animal activity, energy budgets or even specific behavioural patterns or syndromes (e.g. Brown et al., 2013, Gleiss et al., 2011, Kröschel et al., 2017, Rast et al., 2020). Nowadays many commercially available GPS tracking collars (e.g. Eobs, Vectronic aerospace, Biotrack) are equipped with accelerometers and the recording of acceleration data does not cause much additional effort and costs (apart from battery life and storage space). Still, analysing acceleration data is challenging (e.g. training algorithms to detect behaviour patterns e.g. Giovanetti et al., 2017, Valletta et al., 2017, Chakravarty et al., 2019), which might be one reason for large differences in the number of publications using only GPS data and those using acceleration data in movement ecology. These reasons, together with own experiences in previous projects leads us to believe that considerable amounts of acceleration data are recorded and therefore theoretically available without actually being used and published (yet).
In principle, these acceleration data bear the potential to inform epizoochorous seed dispersal. However, these data are, without exception, recorded at the neck of animals, since the 3D-accelerometers are included in the GPS neck collars used in these studies (e.g. Weegman et al., 2017, Chakravarty et al., 2019). Seeds are to some extent attached at the head and neck but especially at lower parts of the animal torso and the legs (compare Albert et al., 2015, Liehrmann et al., 2018, Petersen & Bruun, 2019). To be able to use available acceleration data from animal necks to infer acceleration at the torso and leg, there is a clear need to identify relationships between acceleration between these different body parts. Besides acceleration at the neck body size can be expected to be an important determinant for acceleration at lower parts of the body, and hence, relevant for seed detachment. Larger animals have a higher muscle mass with larger cross-sectional area, thus more strength, and faster (limb) movements which should result in higher acceleration at their body (compare Calder, 1984, Kilbourne & Hoffman, 2013, Cloyed et al., 2021).
In this study we measured acceleration simultaneously at different body parts of mammals of various body size in order to quantify i) how acceleration at the breast/torso and the leg of mammals is related to acceleration at the neck of animals, and ii) how this relationship depends on animal body mass.