Pollen digestion by new world bats: effects of processing time and feeding habits

Ecology, Dec, 1998 by L. Gerardo Herrera, Carlos Martinez del Rio

DISCUSSION

Excretion patterns and chemical reactor models

In general, the grastrointestinal tracts of the bats we studied seemed to function as CSTR-PFRs with some longitudinal mixing in the PFR. In an ideal CSTR-PFR, MRT equals stomach mean retention time (1/k) plus MGT. This condition is almost satisfied in the digestive processing of pollen of Hylocereus undatus by Anoura geoffroyi and of columnar cactus pollen by all of the bats that we studied: MGT and 1/k together account for 85-99% of the MRT. There were small to nearly large amounts of longitudinal mixing in the digestive processing of these types of pollen. In the case of Pseudobombax ellipticum pollen processing, MGT and 1/k represented only 68-74% of the MRT. The amount of longitudinal mixing in the processing of this type of pollen was intermediate. Pollen grains were apparently retained in the stomach for slightly more than one-third of the MRT and only one-fifth, in the case of A. geoffroyi. Rainbow Lorikeets process nectar in a similar fashion: the food remains in the stomach for about one-fourth of the MRT and then it flows along the intestine, probably with some longitudinal mixing (Karasov and Cork 1996). Longitudinal mixing also occurs in the intestine of frugivorous birds (Levey and Duke 1992).

The only exception to this pattern of a gastrointestinal tract that functions as a CSTR-PFR, with short mean retention times in the stomach and longitudinal mixing in the intestine, was Artibeus jamaicensis when given H. undatus pollen. According to the PFR-CSTR model, this species had a very large mean retention time in the stomach (60 min), which accounted for three-quarters of the MRT; we consider this rather unlikely, given that most digestion of pollen occurs in the intestine (Law 1992a). We suggest thai the gastrointestinal tract, in this case, functioned as a single PFR with a large amount of longitudinal mixing. PFRs with large amounts of longitudinal mixing produce output distribution curves similar to the curves of CSTRs (Martinez del Rio et al. 1994). In this case, the vessel dispersion number was slightly lower than the value characteristic of gastrointestinal tracts with high amounts of longitudinal mixing.

It is important to mention that there were a few cases in which the first excretion contained nearly 100% of the total number of grains excreted, and the bat gastrointestinal tract seemed to function as a simple PFR. In Queensland blossom bats (Pteropodidae: Syconycteris australis), two different kinds of pollen were excreted in different fashions: Callistemon grains were excreted almost completely in the first excretion, whereas the first excretion of Banksia grains contained only 60-70% of the total number of grains ingested (Law 1992a).

Pollen extraction and processing time

Extraction of pollen contents was higher in bats that are specialized flower visitors (L. curasoae and A. geoffroyi) than in seasonal flower visitors (A. jamaicensis and Sturnira lilium), even though they processed pollen grains at approximately the same rates. L. curasoae and A. geoffroyi emptied 46-90% of the pollen grains with a MRT that ranged from 105 to 127 min and a MGT ranging from 50 to 88 min, whereas A. jamaicensis and S. lilium extracted the contents of 32-68% of the pollen grains with a MRT of 101-166 min and a MGT of 32-82 min. These results suggest that phyllostomid nectarivores have a digestive system that allows them to feed more efficiently on pollen grains than do their frugivorous relatives. In contrast, Wooller et al. (1988) found no differences in pollen extraction when they compared species of birds that regularly ingest pollen with species that do not. They concluded that pollen eating did not require a specialized digestive system, but their study failed to use a standard methodology for the comparison.