Why is there a difference among female home range sizes?

The difference in home range size between adult female dolphins is a particularly interesting result and the most plausible explanation for this is prey availability and prey type. While prey data is not available for this area, I hypothesise that prey may be more

concentrated in the inner waters, particularly the estuary, resulting in the sub-set of females exclusively using this area and not needing to range as far as the other females. Elsewhere, dolphins inhabiting open coastal areashave correspondingly larger home ranges to encompass the ephemeral and patchy nature of schooling fish (Acevedo- Gutierrez & Parker 2000) and that long distance movements are likely foraging trips exploring for patchily-distributed prey (Silva et al. 2008). This would be applicable to those adult females with large home range sizes using the coastal zone in Bunbury. Elsewhere, differences in prey selection have been reported in bottlenose dolphin populations (Berens McCabe et al. 2010) and prey, that is the quantity and type, best explains the factor driving the difference in female home range sizes. The affect of prey availability on individual dolphin home range size should be tested when further data are available.

As demonstrated in the previous chapter (Chapter 3), adult female dolphins form looser bonds compared to adult male dolphins. This may result in them being solitary feeders and lead to them inhabiting smaller areas where prey is more abundant, easier to catch and does not require cooperative effort with other dolphins. Solitary behaviour has been shown to increase rather than decrease predation risk in other animals, and that to reduce this risk, animals forage closer to refuge or form larger groups (Banks 2001) but in this case ecosystem type and prey availability may be the limiting factors. Prey availability has been shown to differ seasonally between estuarine and coastal areas for bottlenose dolphins (T. truncatus) in Charleston, South Carolina (Olin et al. 2011). Here, dolphins in the estuarine ecosystem feed on estuarine prey in the cooler months then shifted to a more coastal marine forage base in the warmer months. This separation of an estuarine versus coastal prey diet has been confirmed through dolphin stomach contents analyses in the Charleston, South Carolina population (Olin et al. 2011). Resource partitioning based on habitat and prey has been reported in other dolphin species inhabiting similar inshore habitats elsewhere (Parra 2006, Fernandez et al. 2011).

Bottlenose dolphins have a wide distribution, inhabit a variety of habitats and show a high degree of plasticity and specialisation in their foraging behaviour (Lewis & Schroeder 2003, Torres & Read 2009). Dolphins use different foraging strategies for different prey types and habitats (i.e. schooling versus solitary and deep, open water versus shallow, complex habitat (Barros & Wells 1998, Connor et al. 2000b, Mann &

Sargeant 2003). In Bunbury, foraging strategies in the inner waters, particularly the estuary, are likely different from those in the coastal zone. These differences may also explain the differences between adult female dolphins and the ecosystems they utilise and thus the difference between their home range sizes.

Two other possible explanations for the difference between female home range sizes are predation risk and calving. Density and distribution of predators varies across habitat. For example, shallow sub-tropical waters have been shown to have a high density of sharks in summer and hence a higher predation risk to dolphins at this time. Dolphins avoid these areas to reduce predation risk (Heithaus & Dill 2002). In contrast, dusky dolphins (Lagenorhynchus obscurus) seek refuge from predators in shallow water (Weir et al. 2008) and mothers and calves, that spend more time in very shallow water may be able to detect and avoid sharks easier than those in deeper water (Mann et al. 2000). Preference for nearshore, shallow water has also been documented for bottlenose Tursiops sp. and humpback dolphins Sousa sp. in South Africa, that avoid killer whales (Orcinus orca) and white sharks (Carcharodon carcharias) inhabiting deeper water (Saayman et al. 1972, Saayman & Tayler 1979). Bunbury waters are temperate and therefore predation risk would be expected to be higher in deeper waters, similar to the latter examples. This could partly explain why some female dolphins have smaller home ranges restricted to the estuary and avoid the open coastal waters. It may be expected that the presence of a calf would further explain this restriction in home range in adult females but those that had large home ranges in the coastal waters also had calves. Therefore, it seems that another factor is driving the home range of adult females. Lastly, geographical restrictions such as internal lagoons and narrow passages between the inner and coastal waters may prohibit larger ranges.

Competition has been investigated between Shark Bay bottlenose dolphins but there was no evidence of aggression between adult females (Scott et al. 2005). Nonetheless, it is theorized that competition may lead to differences in foraging specialization and home ranges between adult females in Shark Bay (Mann et al. 2000). Competition between individuals and social groups is apparent in bottlenose dolphins in Scotland which has resulted in a stratified spatial distribution (Wilson et al. 1997, Wilson et al. 2004). Despite this, home range overlap was evident between individual dolphins in Scotland. It is thought the presence of abundant prey items in this location results in

lack of competition and therefore coinhabitance of the same area seasonally (Lusseau et al. 2005), which could be the case for female dolphins in Bunbury.

4.5 CONCLUSION

These findings illustrate a sex difference in home range size between adult male and female dolphins, with males having larger home ranges than females. I hypothesis that this difference is due to the polygynous mating system of this species, but this remains to be tested. Adult females using the coastal waters need a larger area than those in the inner waters. Similarly, males using the coastal area have this same requirement for a larger area where their food is sparsely distributed. Management of the dolphins using the coastal zone needs to take into account the need to protect a larger area in order for the dolphins using this area to survive. The female dolphins whose home ranges were exclusively within the inner waters may be at greater risk of coastal threats such as development, vessel strikes, entanglement in fishing gear and compromised water quality. Given these findings, both ecosystems are important and management strategies should incorporate both areas to effectively conserve the dolphin population.