Male Australian sea lions will aggressively protect several females simultaneously when breeding. These mate guarding males will vocalise using a series of unique barks to warn off other males.The breeding cycle of Australian sea lions is very unusual – lasting over a 17 month period, rather than breeding annually. The time of breeding is not synchronous between breeding colonies.
For my honours project, I wanted to see whether mate guarding males recognise and respond differently to barks from local males from their own colony compared to foreign males from more distant colonies.
I conducted a playback experiment, where I collected male calls from the same colony and a distant colony 180 km away. These calls were played to males that were actively guarding females and recorded their behavioural response.
What did I find?
Males were able to distinguish between local and foreign males based on the duration of each bark, duration between each bark and the pitch. This was the first study to demonstrate geographic discrimination in vocalisations in sea lion colonies.
I found that males responded more aggressively and quickly to males barking calls from local males. This suggests that males from the same colony pose a greater potential threat – and are more likely to compete for access to females.
An experience never to forget
For this research, I was accompanied by two students to a tiny Island off the coast of South Australia, called Lewis Island. The island was void of any other people. We brought large cartons of drinking water and food to survive our 6 week fieldwork. We were only allowed to bring one camping backpack each to pack our most essential items – clothes, medical supplies, books and laptops.
We scouted the perimeter of the island every morning to record any newly arrived males and count the number of females being guarded in each group or ‘harem’. One section of the island that we had to navigate through was extremely steep, like a gorge. This area was being guarded by a nasty looking male we nicknamed ‘bulls eye’. Unlike most mate guarding males that stay close to their mates, this one had no problem chasing us over long distances.
But with the males being so aggressive, how did we manage to record their calls?
One person would approach the females guarded by the male to get his attention. The male would start to approach the person, barking vigorously. We observed that most males won’t go further than 20 meters away from the females they are guarding. I would sneak forward, holding the microphone out towards the male to record his calls. We had to be really alert and react quickly.