Increasing Intermammary Distance During Estrus in Bottlenose Dolphins ( <i>Tursiops truncatus</i> )

Cetaceans' mammary slits, situated bilaterally to the genital slit, have been identified as an instrumental metric for the quantitative assessment of alterations in genital morphology. Increase in the intermammary distance in bottlenose dolphins (Tursiops truncatus) associate modification of the genitalia, which occurs immediately before parturition. In certain mammalian taxa, estrus phase is characterized by conspicuous swelling of the external genitalia. Consequently, this study aimed to investigate potential fluctuations in the intermammary distance in bottlenose dolphins associated with the estrus with a specific focus on discerning morphological alterations occurring concurrently during estrus. The intermammary distance was meticulously measured across seven ovulation cycles in three female dolphins. The intermammary distance was significantly longer on estrus days than on nonestrus days (p < 0.05). Furthermore, the intermammary distance displayed notable positive correlation with serum estradiol concentrations. These results support the findings that alterations in the intermammary distance are closely associated with the estrus and are indicators of concurrent morphological modifications. Conversely, positive correlations between the intermammary distance and body weight in two subjects underscore the considerable influence of alterations in body shape. Quantification of the intermammary distance represents a promising noninvasive modality for detecting estrus in bottlenose dolphins.