Calf Development

Newborns

A brand new calf is always an exciting event for the dolphins- and the researchers. This is also a difficult time for both mother and calf. The newborn must learn to coordinate his or her movements with the mother, breathe at the surface, dive, nurse, and avoid danger such as tiger sharks. Much of the popular and scientific literature has promoted the view that females “babysit” for each other, referred to as allomaternal care. Births to the provisioned dolphins in the early 1990s allowed us to systematically observe several wild newborns nearly every day from birth. During these and offshore observations, it became clear that during the first week of life, female dolphins that are either inexperienced or who have not successfully calved, attempt to steal the newborn from the mother by swimming rapidly past it. The newborn, with an innate following response, simply swims after the faster moving dolphin. We have called this “natal attraction” since the females seem to be drawn towards the infants. Mothers sometimes attacked females who stole their calves, and they typically had to steal their own calf back using the same method. Adult females are rarely aggressive in any other context. Oddly enough, mothers tolerate those same females swimming away with their calves during the second week of life. We’ve proposed that the first week is an imprinting period and mothers do not tolerate separations from their calves until they’ve learned to recognize the mother (Mann & Smuts 1998).

When calves are born, they will have lumps or folds in their skin, referred to as fetal folds. These folds last for approximately one week. Fetal lines, white lines resulting from the folds, develop and will remain until approximately the third month.

Mothers and calves use distinctive whistles to mediate reunions after separations of up to several hundred meters (Smolker et al 1993). While about two-thirds of calves studied are weaned before their fourth birthday, some calves nurse for up to eight years. Current studies are investigating the reasons behind this high variation in maternal care.

In our population, 44% of calves do not survive to age three, with the highest mortality rates occuring in the first year of life. There are several variables that can contribute to calf mortality as shown in the diagram below (adapted from Mann & Watson-Capps 2005). The majority of calves show signs of poor health prior to their death, and have also been shown to seek out more contact with their mothers when compared to surviving calves.

While calf condition is likely the primary cause of mortality, predation also has significant effects on maternal behavior, with mothers avoiding dangerous deep water habitats during shark season. When it was observed that calves born to provisioned females were experiencing elevated rates of mortality (56% in the first year), our research aided in implementing a management regime that has significantly improved the survival of these calves.

Figure 4: A schematic representing a possible network of factors leading to calf mortality in bottlenose dolphins (in boxes), the direction of their effects (arrowheads), and the strength of their relationships (indicated by the width of the line).

Calves begin catching their own fish at around 3-4 months of age. They usually begin with a technique called “snacking, where they chase small fish belly up, possibly to silhouette their prey against the surface so they can easily snap it up. As calves get older they develop a wide variety of foraging techniques, which they seem to learn from their mothers through observation (Mann et al 2007).