Abstract
Many plants and invertebrates rely on internal transport by animals for long‐distance dispersal. Their dispersal capacity is greatly influenced by interactions with the animal's digestive tract. Omnivorous birds adjust their digestive tract morphology to seasonally variable diets. We performed feeding trials in waterfowl to unravel how changing organ size, in combination with seed size, affects dispersal potential. We subjected captive mallards to mimics of summer (animal‐based), winter (plant‐based), and intermediate diets, and analyzed gut passage of seeds before and after the treatment (trials 1 and 2). To test the effect of gut morphology on seed digestion, we measured digestive organ size after euthanasia. Three hours before euthanasia, differently sized seeds were fed to test how seed size affects gut passage by determining their relative position in the digestive tract (trial 3). Trials 1 and 2 showed that intact seed passage was lower in the plant‐based than in the animal‐based diet group. Retention time changed only within groups, decreasing in the animal‐based, and increasing in the plant‐based diet group. No post‐diet differences in organ size were detected, probably due to large between‐individual variation within groups. Digestive tract measures did not explain variation in seed survival or retention time. Trial 3 revealed that small seeds pass the digestive tract rapidly, while large seeds are retained longer, particularly in the gizzard. Differential retention in the gizzard, the section where seeds can be destroyed, is likely why larger seeds have a lower probability to pass the digestive tract intact. Our results confirm that rapid, flexible adaptation to diet shifts affects seed digestion in waterfowl, although we could not conclusively relate this to organ size. Large interindividual variation in digestive efficiency between mallards feeding on the same diet may provide opportunities for seed dispersal in the field throughout the annual cycle.
Highlights
Animals like the mainly omnivorous dabbling ducks play an import‐ ant role in the long‐distance dispersal of many plant and invertebrate species, in particular through internal transport following ingestion (Brochet, Guillemain, Fritz, Gauthier‐Clerc, & Green, 2009; Figuerola & Green, 2002; Viana, Santamaría, Michot, & Figuerola, 2013)
Endozoochorous dispersal is crucial to the population dynamics of numerous plant species in a wide range of ecosystems (e.g., Pakeman, 2001; Jordano, García, Godoy, & García‐ Castaño, 2007; Sasal & Morales, 2013; Lovas‐Kiss, Vizi, Vincze, Molnár, & Green, 2018) and has the advantage over other disper‐ sal mechanisms that it is often directed toward habitat patches that are suitable for establishment and not necessarily physically con‐ nected (Howe & Murray, 1986; Kleyheeg et al, 2017; Wenny, 2001)
By performing postmortem digestive tract analysis, we expected to find that this difference in digestive parameters is directly medi‐ ated by different digestive organ sizes between diet groups (Kehoe et al, 1988; Miller, 1975)
Summary
Animals like the mainly omnivorous dabbling ducks play an import‐ ant role in the long‐distance dispersal of many plant and invertebrate species, in particular through internal transport following ingestion (Brochet, Guillemain, Fritz, Gauthier‐Clerc, & Green, 2009; Figuerola & Green, 2002; Viana, Santamaría, Michot, & Figuerola, 2013). Experiments with captive mallards demonstrated that gizzard size and small intestine length increased within 10 days when switching from an animal‐based (low‐fiber) to a plant‐based (high‐fiber) diet (Kehoe et al, 1988; Miller, 1975), with potential ef‐ fects on the efficiency of digestion of ingested plant seeds and other small organisms (Charalambidou et al, 2005; Figuerola et al, 2004) Theory predicts that this mechanism enables birds to switch sea‐ sonally between different food types without sacrificing assimilation efficiency per unit time (Leeuwen, Tollenaar, et al, 2012), while main‐ taining a constant retention time (Gils et al, 2008). We expected that seed size‐dependent variation in retention time is mostly determined by differential retention in the gizzard, which could explain why large seeds generally have a lower gut passage survival
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