Doctor of Philosophy
Department of Biological Sciences
Van Tets, Ian Gerard, Pollen in the diet of Australian mammals, Doctor of Philosophy thesis, Department of Biological Sciences, University of Wollongong, 1996. http://ro.uow.edu.au/theses/1051
Mammals are frequent visitors to flowers in some Australian plant communities. The energy rich nectar is usually considered to be the food sought, because pollen is thought to be of low digestibility. However, earlier studies have reported finding pollen grains in large quantities in the faeces of some mammals and have speculated on the potential value of pollen as a protein source for these mammals. The aim of this study was to investigate the importance of pollen in the diet of the small non-flying mammals that are known to visit flowers in south-eastern Australia.
To achieve this aim, the study was divided into two parts. The first part was a field study, in which the proportion of pollen relative to other food items in the faeces of four non-flying mammal species was determined and their ability to extract the protoplasts from Banksia pollen grains was compared. The species studied were the Eastern Pygmy Possum, Cercartetus nanus, the Sugar Glider, Petaurus breviceps, the Brown Marsupial Mouse, Antechinus stuartii, and the Bush Rat, Rattus fuscipes. The second part involved laboratory feeding trials to determine the ability of one of these species to meet its nitrogen requirements on pollen. The species chosen for this section was C. nanus. The density of Banksia pollen produced in the study sites was also determined. Banksia pollen was produced in the sites from January to October. The highest pollen densities were recorded in July and August, when over 2 kg of pollen was produced per hectare.
In the field study, faeces were taken from trapped individuals from each species over a 16-month period. The faecal samples were examined microscopically to determine the proportion of pollen relative to other dietary items in the faeces and the proportion of pollen grains that had lost their protoplast as they passed through the mammal's digestive tract. The proportion of pollen in the faeces varied between species and between times of the year. Banksia pollen was usually absent from the faeces of all four species between November and February, even though this included the main flowering period for Banksia serrata. It was, however, a major component of the faeces of C. nanus and P. breviceps for the remainder of the year. Banksia pollen occasionally composed a large proportion of A. stuartii faeces during this period, but it always composed less than 1 % of the faeces of R. fuscipes on average. Other major components of the mammals' faeces included invertebrates, plant material other than pollen and fungal spores.
All four species had extracted the protoplast from a large proportion of the Banksia pollen grains. The percentage of empty Banksia pollen grains in the faeces of the four mammals were 66 % for Petaurus breviceps, 65 % for Cercartetus nanus, 55 % for Rattus fuscipes, and 37 % for Antechinus stuartii. The mean percentage of empty grains in the faeces of A. stuartiiwas significantly lower than it was in the other faecal samples (P
In the laboratory feeding trials, C. nanus were fed diets in which nitrogen was provided either by Eucalyptus pollen or by mealworms. The apparent digestibility of the nitrogen (ADN) from both sources was high. The mean ADN was 76 % for the pollen and 73 % for the mealworms. C. nanus was able to maintain nitrogen balance on both food sources and its Maintenance Nitrogen Requirements (MNR) on pollen were very low. The truly digestible MNR of C. nanus on pollen was 43 m g N.kg-0.75.day-1 compared to 127 mg N.kg-0.75.day-1 on mealworms. This difference appears to be related to the composition of the mealworm and pollen protein. The biological value of the pollen nitrogen was exceptionally high for a plant protein at 73 % , whereas the biological value of the mealworm nitrogen was only 43 % , suggesting that the amino acid composition of the pollen corresponded more closely with the requirements of C. nanus than the composition of the mealworms protein did. The truly digestible MNR of P. breviceps fed Eucalyptus pollen was also very low (tdMNR = 73 mg N.kg-0.75.day-1,Smith and Green 1987) and Eucalyptus pollen nitrogen had a high biological value for that species as well (66%, Smith and Green 1987).
Assuming the MNR of these two species on Banksia pollen were similar to the values for Eucalyptus pollen, an individual 25 g C. nanus would require the complete Banksia pollen production of less than 1 ha from May to September to meet its nitrogen requirements on pollen alone. For July and August it required the pollen production of only 0.3 ha. During this period, free-living C. nanus in Barren Grounds Nature Reserve should be able to meet their nitrogen requirements on Banksia pollen alone. Petaurus breviceps has a home range of 3.5 ha (Quinn et al. 1992). Only in July and August, was the area required by a 150 g P. breviceps to meet its maintenance nitrogen requirements on Banksia pollen less than this and, even then, it required the total pollen production of over 2 ha to do so. It is unlikely that P. breviceps could meet its MNR on Banksia pollen alone.
However, it is still capable of meeting a large percentage of its nitrogen needs on Banksia pollen, particularly in winter when the pollen density is high. Based on the M NR values for C. nanus on the mealworm diet, free-living C. nanus should also be to meet their nitrogen needs on insects. When pollen densities are low, insects may be a valuable source of protein for this species.
A wide range of mammals, including non-specialists, are able to extract the protoplast from Banksia pollen. The biological value of Eucalyptus, and presumably Banksia, pollen is exceptionally high for a plant protein source and the maintenance nitrogen requirements of both C. nanus and P. breviceps feeding on pollen are among the lowest recorded for any mammal. When Banksia and Eucalyptus pollen are available in high densities, they are likely to be important sources of protein for the small flower-feeding mammals that are found in Australia.