El carácter procesual de la mediación: fases, modos y tipos

Some early researchers questioned the ability of the platypus to forage in water for very long time. Burrell (1927) for example reported that platypuses venture out in the early morning and evening only, spending not more than two hours in water in any one day. Also, Smyth (1973) postulated that in the field platypuses are poor thermoregulators and probably spend only brief periods in the water in winter, returning frequently to the bank or burrow to warm up. However, it is now widely accepted that platypuses can spend extended periods in the water and that the thermoregulatory mechanisms of the animal are excel- lent (Grant and Dawson 1978a, Grant and Dawson 1978b).

Reports on the duration of active periods in platypuses differ and range from 7 to 19 hours (Tab. 6-9). Grant et al. (1992) investigated platypuses in the lower Thredbo River, NSW, from several days up to five months and found that individuals rested in burrows for between 5.25 and 15.1 (mean 10.7) hours at a time. Gust and Handasyde (1995) recorded mean active periods of about 10 hours per day. Serena (1994) reported an individual mean duration of burrow occupancy between 11.6 and 16.7 hours per 24-hour period. Also, Grant (1983b) found that platypuses spent 40 to 51 % of their time in water. Otley et al. (2000) reported that platypuses at Lake Lea foraged for between 8.5 and 16 hours per 24-hour period (Tab. 6-9).

Mean individual active periods (10.0 to 15.75 h) found in this study are in line with findings of Grant et al. (1992) (Thredbo River, NSW) and Otley et al. (2000) (Lake Lea) but are slightly higher than records from Badger Creek (Serena 1994) and Goulbourn River (Gust & Handasyde 1995) in Victoria as well as Shoalhaven River, NSW (Grant 1983). This observation is consistent with Serena (1994), who stated that the mean time spent in dens by Thredbo

platypuses (Grant et al. 1992) was less than for any of the platypuses monitored along Badger Creek.

Mean active period [h] Habitat Source

9.6 - 12.2 Shoalhaven River, NSW Grant (1983)

8.9 - 18.8 * Thredbo River, NSW Grant et al. (1992)

7.3 - 12.4 * Badger Creek, VIC Serena (1994)

9.8 - 10.5 Goulbourn River, VIC Gust & Handasyde (1995)

8.5 - 16 Lake Lea, TAS Otley et al. (2000)

10.0 - 15.8 Lake Lea, TAS this study

Tab. 6-9: Comparison of active periods recorded for platypuses (*derived from burrow times)

The differences in foraging duration between habitats could well be re- lated to different climatic conditions. Both Lake Lea and the Thredbo River in Australia’s Southern Alps are very cold habitats where water temperatures fre- quently approach 0ºC in the winter months. Platypuses at Lake Lea foraged on average one hour longer in winter than in summer. Activity levels were signifi- cantly higher during the winter months than during summer (Fig. 6-8). Consis- tent with this, Lamm (1993) found that water temperature affected the active time of a female platypus on her study site with activity decreasing with higher water temperatures. Also, Otley et al. (2000) observed longer foraging bouts during winter than during summer.

Longer foraging durations in winter are plausible not only because of in- creased energetic demands but also because item size and diversity of benthic organisms over the winter months are most likely reduced (Faragher et al. 1979, Lamm 1993). Other factors which may contribute to seasonal variation in the length of time that platypuses forage include the degree of food competi- tion with other platypuses or other species including fish or water rat, or for example incidence of flooding and high water levels (Temple-Smith 1973, Grant and Carrick 1978, Faragher et al. 1979, Grant and Temple-Smith 1983, Benson 1997, Otley et al. 2000).

Methods used in this study allowed the constant observation of platypus behaviour in detail over a number of consecutive days. Data-logger recordings show a surprisingly high variability in the duration of the active period (3.40 to 30.83 hours) with continuous foraging (i.e. time with diving activity as mea- sured by time-depth recorders) of up to 29.83 hours. Consistent with earlier reports (Grant 1995, Otley et al. 2000), animals foraged continuously with only short breaks (most of them less than three minutes duration) between feeding bouts.

The use of activity loggers also allowed elucidation of the temporal or- ganisation of the active period. Consistent with results for a number of other species (Refinetti 2000), platypuses at Lake Lea showed the highest activity levels at the beginning and towards the end of their foraging trip while the level of activity did not change significantly during the rest of the active period (Fig. 6-10). Consequently, in nocturnal and diurnal animals (65 % of all observed animals), highest activity levels occurred during twilight, a time where activity levels of many animals are found to be increased (Cloudsley-Thompson 1980). For small carnivores, twilight is generally regarded as the best time to hunt. The relatively dim light provides partial protective cover from predators. At the same time, visibility is still better than at night (Cloudsley-Thompson 1980). In platypuses, increased activity during twilight (crepuscular activity) is well established (e.g. Grant 1983a, Grigg et al. 1992) and might relate to increased prey availability. Many macro-invertebrate insect families show active periods just after sunset and before dawn in river systems (Williams and Feltmate 1992) and this might also be true for lake systems.

Light levels also had an influence on dive cycles of platypuses at Lake Lea. Dive profiles of some animals revealed that they clearly explored greater depths during daylight hours than during the night (12-98-1, 12-99-2 and 01- 00-1, see Appendix A), a behaviour that most probably relates to a change in foraging location between day and night time. In general however, light condi- tions resulted in shorter dive cycles with both dive duration as well as surface duration being reduced. Also, the number of foraging breaks, i.e. resting

periods of more than one minute duration, was less during the day compared to nighttime (Fig. 6-7). This behaviour is likely to be a strategy to avoid predators more effectively during light conditions as time spent at any one place on the lake surface is reduced.

Finally, it is interesting to note that platypuses seem to start and end their day with some minor activity in their burrows. Activity loggers showed that the beginning of each active period was characterised by low activity levels lasting for an average of 50 minutes. Correspondingly, at the end of each active pe- riod, low activity levels for an average of 21 minutes were observed. Activity levels appear too low for burrowing or some other energetic behaviour, which suggests behaviours like grooming or waking up during these periods.