Australasian Science: Australia's authority on science since 1938

All Creatures Great and Small

Photo: Si-Chong Chen

While it’s true that large animals feed on some large seeds from fleshy fruits, we shouldn’t overlook the fact that large animals, especially ungulates such as rhinoceros, zebras, peccaries, deer and buffalos, also unintentionally vacuum up huge amounts of small, inconspicuous seeds as they browse on short grassy vegetation. These interactions are the primary factor that drives the negative relationship between animal body mass and ingested seed size across all vertebrates. Photo: Si-Chong Chen

By Si-Chong Chen & Angela Moles

Many large animals are rare or under threat, so the discovery that they ingest and disperse both large and small seeds has widespread ecological consequences.

It’s estimated that more than 90% of the plant species in tropical rainforests rely on animal ingestion to disperse their seeds. Some seeds may even need to be processed by the animals’ digestive tracts to be able to germinate.

You might think that larger creatures would gulp down bigger chunks of food. This idea had long been accepted by scientists studying seed dispersal, leading to the assumption that larger fruit-eating animals generally ingest larger seeds than smaller animals. This idea has been supported by studies of some animals, including fruit-eating birds and bats, but do these positive patterns hold true at a broader scale and across diverse taxa?

We have compiled data from thousands of scientific papers and amassed a worldwide database of 13,135 animal–seed interactions from all vertebrate groups, including 224 species of mammal, 313 species of bird, 42 species of reptile, one species of amphibian and seven species of fish.

The smallest seed-ingesting animal in our dataset was the Chatham Islands skink, which only weighs 3.3 grams. The smallest seed in its diet comes from a type of mountain snowberry. In contrast, the largest seed in the African elephant, which weighs nearly 4 tonnes, comes from an African tropical tree and is 9 cm long.

The South American tapir, which weighs more than 200 kg, transports the greatest number of seed species (more than 350 species). And giant Galapagos tortoises can hold seeds in their guts for up to 33 days. Because of the movement of the tortoises during this time, the seeds they consume may be dropped so far away that the seedlings colonise new habitats.

There were also some surprises in our dataset. Species you might not have thought of as seed dispersers, such as aardvarks, armadillos and even some Amazonian fish, actively ingest and disperse seeds. The only amphibian that ingests seeds is a tree frog found in tropical America, and it ingests the seeds of five plant species.

Our most surprising finding was that larger animals generally swallowed smaller seeds (Fig. 1). While this contradicts traditional ecological thinking, we can understand this counter­intuitive result by looking more closely at what different types of animal are doing.

In many groups, including birds, bats, carnivores, marsupials and lizards, larger animals tend to ingest larger seeds. Ungulates such as rhinoceros, zebras, peccaries, deer and buffalos are a clear exception to this trend, with larger animals actually ingesting smaller seeds rather than larger seeds.

One problem with previous thinking about seed ingestion was that scientists tended to focus their attention on fleshy fruits, such as berries, that are actively sought by small fruit-eating animals. However, we found that nearly one-third of the seed species ingested by animals are not fleshy, such as the seeds of grasses and daisies. These dry seeds may be eaten un­intentionally during grazing or browsing. Many large animals, such as ungulates, ratites and giant tortoises, are herbivores that feed on a variety of plant materials. Our data show that small and dry seeds are often accidentally mowed by large animals grazing in grasslands and herb fields.

Further analyses of the relationship between animal body size and the size of seeds ingested emphasised the importance of large animals as seed dispersers. Larger animals fed on a greater diversity of seed species, with larger maximum seed sizes and smaller minimum seed sizes (Fig. 1).

Many large animals are currently rare or at imminent risk of extinction because of poaching, habitat loss and other human activities. If we lose these large animals, both the largest-seeded species and the smallest-seeded species may lose their seed dispersers.

Since larger animals usually have larger home ranges, the opportunities for seeds to be dispersed further may be compromised if they can’t hitch a ride in the digestive tracts of larger animals. This could be particularly important in the current context of climate change, which already has many researchers wondering whether plant species will be able to migrate quickly enough to track their optimal growing conditions.

Our study has corrected our understanding of the interactions between fruit-eaters and the seeds they spread. It turns out that the old theory only holds true for small- to medium-sized animals that actively seek seeds with fleshy fruits.

Science is self-correcting, even for simple and seemingly obvious “common sense” knowledge. From knowledge to wisdom, the avenues of long-held assumptions need to be constantly tested.

Sichong Chen is a PhD candidate supervised by Angela Moles in the Evolution & Ecology Research Centre at The University of NSW.