Parenting is never easy, and fur seal mothers don't have the luxury of constantly remaining by their pups providing protection. They must abandon their offspring and head to sea in search of food, leaving them initially within the first week of birth. And when Cape fur seal (Arctocephalus pusillus pusillus) mums pull themselves back onto the beach at Pelican Point, Namibia, they are faced with the unenviable task of relocating their pups amongst the colossal colony of up to 90,000 neighbours. Fortunately, Cape fur seal mothers can recognise their youngster's voice against the cacophony within the first 4 h of birth, and pups can distinguish their mother's voice soon after. But which aspects of the moan-like calls do the seals cue in on when reuniting? Mathilde Martin, from Université Paris-Saclay, France, Isabelle Charrier (Université Paris-Saclay) and Antonia Immerz (currently at University of Pretoria, South Africa) travelled to Namibia, where they were supported by Tess Grindley and Simon Elwen (Sea Search Research and Conservation, South Africa), to find out how well Cape fur seal mothers and pups recognized each other's voices after the scientists electronically altered them.
To identify which pups and mothers belonged together, Martin and her colleagues hid in the sand, cautiously recording the seals as they called with a directionally sensitive microphone. ‘Once we had been close to them for a while, the seals did not really notice our presence’, recalls Martin. Then the team electronically synthesized artificial mother and pup calls based on the seals’ husky braying cries: making them slightly higher pitched (increasing the frequency by 10 and 50 Hz); removing the natural rise and fall of the moan's volume, setting it steady from start to finish; retuning the call to one note so it lost its sing-song quality; and finally randomly switching some of the tones, making the calls sound completely artificial. The team then played the altered pup calls back to the mothers and the synthesized mother calls to the youngsters, filming the seals’ reactions to find out whether they still picked out their relative, despite losing some of the distinguishing acoustic features. ‘The most challenging aspect of this study was to test each individual with several playback series’, says Martin, recalling how difficult this was in the noisy densely packed colony.
Impressively, the mothers could still pick out their pups’ calls when they were slightly higher pitched and the volume did not vary, but as soon as the call became monotone or highly pitched, the mum could no longer recognise her offspring's cry; the mums were tuned into the sing-song quality of their pups’ calls. And when the scientists altered the mothers’ calls, the pups struggled to recognise them, with the high-pitched calls and randomised tone calls causing the most confusion. It seems that the Cape fur seal mothers and their pups depend on the unique blend of pitch, tone and volume variation in their cries to identify each other amongst the tens of thousands of frantically calling mums and pups around them. And each pup's ability to distinguish its mother's calls from those of other females is extraordinarily sophisticated, given how young they are.
Additionally, the team estimated the distance over which the seals’ calls carry, revealing that the calls begin to lose some of their essential features beyond 8 m, making them difficult to distinguish from the cries of other mothers. However, when the team monitored the distance over which the youngsters roamed while their mothers were at sea, it was clear that they could cover a 60 m range, making it even more impressive that mums and pups can reunite by ear amongst the din of 90,000 neighbours.
