Wednesday, 31 August 2022

Ceratozamia norstogii: Identifying Animals which disperse the seeds of an Endangered Mexican Cycad.

The genus Ceratozamia comprises about 35 species of neotropical Cycads, all but one of which are found only in Mexico. As such they form an important part of Mexico's botanical diversity, and understanding their ecology and evolutionary history is seen as a key to understanding the climatic history of northern Mesoamerica. 

One of the major steps in understanding the biogeography of any Plant is working out how its seeds are dispersed. This is particularly challenging for Cycads, as their seeds tend to be rather large, and often contain methylazoxymethanol glycosides, which are toxic to most Vertebrates. Nevertheless, most Cycad seeds are thought to be dispersed largely by Rodents and other small-to-medium sized Mammals. 

The seeds of members of the genus Ceratozamia are thought to be particularly toxic, and are therefore presumed to be dispersed largely by gravity. However, this theory has never been put to the test, and it is hard to explain the wide distribution of the genus through gravitational seed dispersal alone.

In a paper published in the Biodiversity Data Journal on 24 August 2022, Héctor Gómez-Domínguez of Senda Sustentable, and Jessica Hernández-Tapia and Andrés Ortiz-Rodriguez of the Departamento de Botánica at the Universidad Nacional Autónoma de México, present the results of an experiment in which two specimens of Ceratozamia norstogii in La Sepultura Biosphere Reserve in Chiapas State, Mexico, were observed by camera trap for ten months in order to assess what Animals were feeding on and spreading their seeds.

Ceratozamia norstogii in the study area. On the left, Plant in reproductive phase (prepollination). Top right, habitat (Pine-Oak forest). On the bottom right, a seedling growing amongst the leaf litter. Héctor Gómez Domínguez & Ana Rocha in Gómez-Domínguez et al. (2022).

Ten months is the time taken from the first appearance of the cones of Ceratozamia norstogii until their disintegration. The cones in La Sepultura Biosphere Reserve were observed over this cycle between October 2020 and July 2021. The first seven months of the growth cycle, October-April, were taken up by the pre-pollination growth phase, during which time the cone reaches its maximum size. The second phase lasts around two months, May-June, and is where pollination occurs. During this phase the cones exude a sweet smelling, amber-coloured liquid which attracts Beetles of the Family Erotylidae (Pleasing Fungus Beetles), known to be important pollinators of Cycads. During the final phase, from June to August, the seeds mature to a brown colour, and are either carried away or eaten by frugivorous Animals, or fall to the ground as the cone disintegrates. 

Female cone maturation. Pre-pollination phase. (A) Emergent cone, with a short, straight peduncle and a general reddish-brown colouration. (B) Young cone, a larger brown cone with a straight peduncle. (C) A large, fully developed cone with a greenish colouration, peduncle much longer and bent towards the ground. Pollination phase. (D) A large, pendant cone, with a light brown colouration, and barely separation amongst megasporophylls. Seed maturation phase. (E) A large, pendant cone, with a light brown colouration and with an evident separation amongst megasporophylls. (F) Mature seeds. Ana Rocha in Gómez-Domínguez et al. (2022).

During the whole ten months, seven Animals were recorded visiting the female cones, the majority of them at night. The daytime visitors comprised three Birds of different species who used the cones as perches during the daytime; two during the immature growing phase, and one while a cone was disintegrating. Also during daylight hours, a Badger was seen to approach a cone, pausing to smell it before leaving. 

During the night, several small-to-medium sized Mammals both visited and interacted with the cones. Most notably, a Mouse of the genus Pteromiscus was observed both feeding on the exudate of the cones, and removing seeds and carrying them beyond the range of the camera, on several occasions. A Kinkajou, Potus flavus, was also seed removing seeds from the cones, although it appeared to be doing this in order to access the central axis of the cone, which it spent some time biting. This is a significant observation for the Kinkajou, a small Mammal usually presumed to live and feed almost exclusively within the tree canopy, but which approached the cone from the ground, but does not appear to have aided the distribution of the Cycad's seeds. Finally, a Southern Spotted Skunk, Spilogale angustifrons, was again observed to bite at the seeds and carry several beyond the reach of the camara.

Female cone visitors of Ceratozamia norstogii. (A) Mouse, Pteromiscus sp., collecting seeds; (B) Southern Spotted Skunk, Spilogale angustifrons, biting the cone and collecting seeds; (C) Kinkajou, Potus flavus, removing seeds. Gómez-Domínguez et al. (2022).

Three species of frugivorous Mammal were observed visiting the cones of Ceratozamia norstogii, all of them at night (after 8.00 pm), with most of the activity happening between 12.00 midnight and 3.00 am. This activity peaked in the second week of July, but carried on for 20 nights, during which time the cones were visited on 65% of nights. The Mouse was the most common visitor, visiting the cone on 13 nights, and making a total of 40 visits. The Southern Spotted Skunk made 15 visits on 3 nights, while the Kinkajou made two visits on a single night. Based upon this, Gómez-Domínguez et al. conclude that the Mouse, Pteromiscus sp., is the most effective distributor of the seeds of Ceratozamia norstogii.

Gómez-Domínguez et al.'s results support the hypothesis that the seeds of Ceratozamia norstogii are distributed by small-to-medium sized Mammals, with the Mouse, Pteromiscus sp., being the most effective distributor. The other two species were less frequent visitors, and were probably feeding opportunistically, rather than using the cones as a regular seasonal food resource. Both are primarily insectivorous, but are also secondarily omnivorous, feeding on smaller Animals, carrion, and fruit. The feeding activity of the Kinkajou was particularly surprising, given that the activities of this Animal are usually restricted to the treetops, but probably not significant for the Cycad. The Southern Spotted Skunk, on the other hand, could potentially be an occasional important seed disperser for this Plant, given that it has a much greater home range than the Mouse, making it likely to carry the seeds for greater distances.

The finding that the seeds of Ceratozamia norstogii are distributed by only a small number of species is consistent with other studies of Cycads, whose seeds are typically dispersed by a small number of, or even a single, species. This contributes to the distribution seen in Cycads, which tend to be found in fairly dense colonies, where it is hard to distinguish between gravity dispersed seeds and seeds dispersed by small Mammals who only carry them a few metres.

This short-distance dispersal is likely to have promoted allopatric speciation (the division of colonies into new species after they become geographically isolated) within the genus Ceratozamia. Within this genus, episodes of speciation appear to have been linked to extinction events in which clades of large Mammals disappeared. This could potentially suggest that the seeds have historically been dispersed over long distances by a variety of large Mammals, but that when these species have gone extinct this role has fallen to smaller Mammals with limited ranges, effectively cutting off the widely dispersed colonies from one-another.

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