Seal voices

For the first four months of its life, an Antarctic fur seal pup depends on its lactating mother for sustenance. The mother seal spends the majority of her time in the ocean on foraging trips, returning to the land every four to seven days to feed her pup for a few days or less.

During the breeding season, Antarctic fur seals can congregate in dense colonies of over a thousand individuals – and, because mother seals will only nurse their own pups and can be aggressive toward pups that are not their own, a pup’s ability to recognize its mother is crucial.

Scientists know that seals in the family Otariidae, or eared seals (so named because they have external ears), including the Antarctic fur seal, use vocal cues to communicate and recognize one another. Mothers and pups may also use sight and smell to find each other, but auditory clues appear to be their most effective means of reunion. A new paper recently published in the journal PLoS ONE elucidates new details of how an Antarctic fur seal pup recognizes the voice of its mother, and some limits to that auditory recognition.

Researchers working on Courbet Peninsula in the Kerguelen Islands in the southern Indian Ocean studied a colony of 750 pairs of mothers and pups. They recorded the calls of mother seals and played them back to the pups, sometimes with modifications to amplitude and frequency, to determine which acoustic aspects of the calls the pups were using to recognize their mothers. The scientists recorded the number of calls the pups made in response to the recordings, as well as how long it took the pups to respond, and how long it took the pups to look at the loudspeaker standing in for the mother seal.

The researchers also noted that the pups often gathered in groups of about 10 individuals while waiting for their mothers to return to land; they took advantage of these gatherings, and played mother seal calls to them from about 25, 100, and 200 feet away to see how well the pups could discriminate mother seal calls at progressively longer distances.

The pups responded best to mother seal calls that were not modified in amplitude or frequency, suggesting that they use both of those signals in recognizing their mothers’ voice. They also appeared to be better at discriminating mother seal calls at shorter distances – when the scientists played the vocalizations of one of their mothers to the groups of about 10 seal pups, about four pups typically responded from 200 feet away, three from 100 feet away, and just one from 25 feet away. The researchers also note that “[f]or all tested distances, the filial pup of the female chosen for the playback always responded.” In other words, while some pups got it wrong, the pup whose mother they were actually listening to always got it right. 

Antarctic fur seal pups at Salisbury Plain, on South Georgia Island, in the southern Atlantic Ocean.

(Image by Liam Quinn via Flickr/Creative Commons license)

Seal meals

Ecology is the study of how living organisms interact with their environment, and with each other. One of the most commonly studied interactions among different creatures is the flow of ingested energy – in other words, mealtimes.

An investigation into an animal’s eating habits might involve many questions – not just, ‘what does it eat?’ But also, ‘how often?’ And ‘where?’ And ‘at what time of day (or night)?’ And ‘under what conditions might it not eat at all?’

Direct observations can answer some of these questions, but the problem becomes more complicated if all the action occurs underwater – for instance, if you’re talking about harbor seals (or other marine mammals). Harbor seals can dive hundreds of meters underwater to catch the fish and other seafood that make up their diet.

Underwater video footage is one way to spy on harbor seals as they hunt for their meals, but, as a team of researchers who recently reported an alternate method in The Journal of Experimental Biology points out, the presence of the required light source may influence the very behaviors videographers attempt to witness and record during deep dives.

By strapping an accelerometer – a device that measures changes in speed – to the head of a harbor seal using a small neoprene headband, the scientists were able to record a characteristic jerk of the seal’s head each time it captured a fish. The researchers were working with a single harbor seal in a controlled environment for the purpose of testing the accelerometer; however, they say the method has the potential for use in months-long studies in the wild, partly because the battery demands of the accelerometer are so low.

“Such long records of foraging behavior will help us to understand how free-ranging aquatic predators search for and acquire energy from their dynamic environment in time and space,” the scientists write. By answering questions like, ‘when and where do seals find their meals?’ the researchers will be able to investigate the rest of the food web, too.

Harbor seal swimming in shallow water on the California coast.

(Original image by Tewy via Wikimedia Commons)