Allometrie der Kotpartikelgröße von pflanzenfressenden Säugern, Reptilien und Vögeln

Among different factors – such as food characteristics and ingesta
retention time -, ingesta particle size (measurable in the faeces)
is an important parameter that influences digestion in herbivores.
In the literature, there are only four comparative studies on the
faecal particle size in herbivores. These studies either
concentrate on only a few different species, or are restricted to
one taxonomic order. Comparative data on reptiles and birds are
totally lacking. Thus the present study represents the first
attempt to quantify the faecal particle size across broad range of
herbivore species within all three taxonomic classes, and to
investigate whether there is an allometric relationship to body
mass. Relevant factors influencing ingesta particle size, such as
body mass, type of dentition, phylogenetic affiliation and
digestive strategy, are discussed in context. In total, more than
1100 faecal samples from herbivorous mammals (205 species),
reptiles (12 species) and birds (14 species) are analysed with a
standardized wet-sieving procedure using nine sieves ranging from
0,063 mm to 16 mm mesh size. For the description of faecal particle
size two different particle dimensions (in mm particle length) are
used, the mean particle size (“Mittelwert”: MW) obtained by curve
fitting, and the weighted average (WA) obtained by calculation. The
following results are obtained: It is confirmed that faecal
particle size is a reliable measure for the extent to which food is
processed by mastication. However, this only applies to terrestrial
forage plants. Faecal particle size increases with increasing body
mass in all three classes. For all mammal groups faecal particle
size scales positively to body mass to the power of 0.34 (MW) or
0.31 (WA), respectively. Given the literature assumption that tooth
size scales isometrically to body mass, these results underline the
fact that there is a functional relationship between tooth size and
the achieved ingesta particle length. Considering their body mass
range, ruminants - and also equids - achieve particularly small
faecal particles and therefore are peculiar among the mammals.
Species adapted to grazing (Przewalski horse, auerochs) showed
comparable faecal particle sizes in the wild and in captivity,
whereas species adapted to browsing (tapir, giraffe) had smaller
faecal particles in the wild compared to captivity. Reptiles have
distinctively higher mean faecal particle sizes than mammals and
birds. Ingesta comminution in the gizzard of herbivorous birds is
comparable to that achieved by chewing in non-ruminant mammals, as
faecal particle size does not differ between these two groups.
Before it enters the gizzard, however, bird ingesta have a similar
particle size as reptile faeces. With respect to the ongoing
paleobiological discussion on the competition of two dinosaur
groups – the sauropods and ornithopods – the present study supports
the theory that comminution by gizzard is not less effective than
comminution by mastication. The differences in the comminution
efficiency between and within mammal groups can partly be related
to the evolutionary success of certain species.