Age and interpulse interval relation from newborn to adult sperm whale (Physeter macrocephalus) off Mauritius

Sperm whales (Physeter macrocephalus) have been studied for decades, but the development of their clicks during the animal growth is not yet well known. The click they emit during socialization and echolocation contains information about the length of their acoustic organs and, therefore the length of the body through the interpulse interval (IPI). This paper provides the first IPI/age relationship for juvenile male and female sperm whales (Physeter macrocephalus) based on field recordings of individuals whose age is largely known. Across 9 years, audiovisual recordings of a Mauritian sperm whale social unit were carried out. Adult female and juvenile sperm whales were identified and aged. The dataset made from those recordings is publicly available. The interpulse interval was measured for individuals whose ages ranged from 7 days to around 38 years. The growth of the acoustic organ of juveniles showed an early inter-individual variability as well as sexual dimorphism. Usual growth models were also fitted, predicting a mean <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>I</mml:mi> <mml:mi>P</mml:mi> <mml:msub><mml:mi>I</mml:mi> <mml:mi>∞</mml:mi></mml:msub> </mml:mrow> </mml:math> of 3.5 ms for adults and a physical maturity reached at around 30 years old. The use of passive acoustic monitoring (PAM) is one of the main tools used to study sperm whales. This IPI-age relationship may aid demographic studies on sperm whales by enabling PAM to assess the ages of recorded sperm whales.