On the evolutionary and ecological value of breaking physical dormancy by endozoochory

• Seeds with impermeable seed coat, i.e. physical dormancy (PY) are capable of surviving the passage of animal gut with little damage to the internal structures.
• The type of herbivores, seed maturation state and amount of seeds available can significantly affect the process of endozoochory.
• Seed movement outside the parent plant community through the process of endozoochory often provides numerous fitness benefits to PY species.
• PY seeds dispersed by endozoochory can establish long-term seed banks or germinate in a novel environment.

Seed dispersal by animal vectors (endozoochory) is ubiquitous, significantly shaping plant communities. Plants that produce seeds with physical dormancy (PY), including many agriculturally and economically important species, constrain the timing of seed germination via impermeable seed coats. PY seeds consumed by animals often receive the dual benefit of dormancy-breaking during passage through the animals’ gut and dispersal outside the parent plant community. In this review we synthesized the current endozoochory literature, paying particular attention to germination ecology and seedling establishment in PY species, before suggesting future research based on critical gaps in current knowledge. To fully synthesize available data, we created a database of 139 PY seed-herbivore pairs from studies focused on dormancy-breaking during gut passage and germination success after defecation. Overall, we draw four main conclusions: (1) the type of herbivores consuming PY species determine the total number of seeds germinating following gut passage; (2) an impermeable seed coat likely protects the internal structures of seeds during chewing and gut passage, however traits previously considered important prerequisites for seed survival during gut passage, such as small seed size (<3 mg) and spherical shape, need to be reassessed; (3) PY does not always break during gut passage and seeds may utilize alternative dormancy-breaking cues in the post-dispersal environment, e.g. seasonal temperatures or secondary dispersal; (4) dormancy-breaking of PY seeds by herbivore ingestion is likely an exaptation, i.e. a trait that performs a function for which it did not originally evolve. However, PY seeds benefit significantly from herbivore dispersal outside the parent community and gut passage may free seeds from risk of predation by insects. We suggest future studies deviate from commonly investigated plant families (e.g. Fabaceae) and incorporate the post-dispersal germination environment to fully elucidate the mechanisms of germination and seedling establishment in PY species.