Remember that feeling, when you were allowed to go to the shops without your parents, when you started high school or your first year of college? That deep in the stomach nauseating feeling mixed with intoxicating excitement.
What did you do? Did you spend all your allowance on junk food? How did you make friends? Or find where all your classes would be held? Did you keep yourself out of trouble? Did you learn to cook for yourself?
Food, friends and safety, are some of the decisions we all have to make when we no longer have our parents making them for us. So what do these life choices have to do with dolphins?
Dolphin calves, like most mammals, gradually gain independence from their mothers through a process called weaning. Weaning is a period in which females gradually cease to provide milk for their young. When a calf no longer receives milk from its mother, it is considered fully weaned and now a juvenile. For the dolphins in Shark Bay, Western Australia this occurs around the age of 4 yrs.
Why is this important? Unlike in other mammals, such as primates, where weaned offspring stay with their mother, juvenile dolphins are on their own. This means a newly weaned juvenile bottlenose dolphin must negotiate a complex social and physical environment without maternal care. As a result, they are required to find enough food for growth, avoid sharks, as well as other dolphins that might harm them, and find other dolphins that they can make friends and associate with, all in the name of survival.
This is quite the undertaking for a small, young, newly independent dolphin! No kidding, and no wonder there is high mortality in this age group, especially for males (1). Considering the immediate mortality risk and the importance of acquiring social skills (e.g., development of bonds, fighting abilities, and avoiding aggressive individuals) and ecological knowledge (e.g., foraging preferences, predator avoidance) necessary to be a successful adult, the 'choices' that individuals make to mitigate or meet these challenges during the transition to nutritional independence are vital.
But do dolphin females and males make similar ‘choices’ during the transition from calf to juvenile? We actually don’t know! Juveniles remain the least studied life-history stage.
Few longitudinal data-sets are available in long-lived mammals outside of primates (2–5). This is due in part to juvenile mammals being difficult to observe, especially in natural settings. Juveniles tend to move fast and unpredictably, are small in size, lack distinctive markings, and one sex often emigrates-- making individual identification difficult (5). Moreover, for species who spend many years as a juvenile, it takes many more years of study to collect data.
So what do we know? We know that there are sex differences in foraging, ranging, and sociality in adults in a variety of taxa (6, 7), and that these sex differences are strongly linked to sex-specific reproductive strategies (e.g. socio-ecological theory (8)). For adult females reproduction is usually limited by food and therefore, females tend to focus their efforts on foraging, which includes the use of rare techniques; such as chimpanzees using spear-like sticks to hunt (9), or bottlenose dolphins using sponges to find fish (10, 11). Conversely, male reproduction is often limited by access to mates, and consequently males tend to focus on enhancing their access to mating opportunities. This can be achieved through rank, as seen in macaques (12), or alliance formation, as seen in bottlenose dolphins (13).
Despite a well-established theory predicting sex differences in the behaviour of adult animals, there are few studies that directly examine the development of such behavioural differences in juveniles post-independence from their mother in the wild (but see 5, 14, 15).
So what now? I am super excited to tell you that I, along with my collaborators, have recently published a scientific article in Animal Behaviour on a study that we undertook to bridge this gap in knowledge.
What exactly did I look at?
I wanted to know what ‘choices’ juveniles make when they are no longer under their mother’s supervisory eye and if there is a difference between male and female ‘choices’. I focused on three non-mutually exclusive explanations for differences that could be found between the sexes: a) social skills - the need for friends, b) protection and safety - the need to protect themselves from sharks or other dolphins, and c) energy allocation - the need to perfect skills necessary for growth and health.
Aaaand this means, what? Social skills are important – Watching young children being social we often see lots of physical contact, such as touching, hugging, holding hands, maybe even sticking fingers in each other’s noses! Dolphins are no exception. Juveniles are seen petting, rubbing, and bonding fin to fin (see photo 1). These behaviours may benefit the individual by facilitating the development of important skills; such as motor training for future interactions (16), learning characteristics of the opposite sex and gaining competence in interactions with them (17), training for when members of the same sex compete with each other over access for mates (17), and social bonding (18). It would therefore be in the best interest for juveniles, with their new-found independence, to increase their time socialising and associate with more individuals. Adult males in Shark Bay form alliances of 2 to 3 individuals that compete for access to mate with females. It is likely that juvenile males would socialise and associate more than juvenile females, particularly with other male juveniles to build and cement bonds early on in anticipation of developing future alliances.
So what about protection and safety? “Stranger, Danger” It’s a rhyme we have been taught since we could walk and it’s no different for young, newly independent dolphins. They have to be cautious of other potentially aggressive dolphins, such as adult males, as well as sharks that are abundant in Shark Bay, hence the name! In fact, 74% of Shark Bay dolphins have shark bite scars! Individuals are more vulnerable to predators when alone (19, 20). Therefore, newly independent juveniles need to spend more time with other individuals to gain some protective benefit from predators, and yet still avoid aggressive males, and it can be tricky to navigate both!
What is energy allocation? How did you know what to eat? What was safe to eat raw? What had to be cooked? What went well together? Who taught you how to cook? Maybe no one did and you found out in college! These are just a few skills we are taught when we are young and continue to learn as we age. Finding and procuring enough food is also important for young dolphins; as are the development of foraging skills that are needed for growth and survival. There is no Dominoes delivery for dolphins, so they have to get it right! Because of their size, strength, visual and motor coordination, and experience, juveniles are likely to be less skilled at foraging than adults. It would therefore be important for newly weaned juveniles to increase their time foraging, to not only compensate for the lack of their mother's milk, but to become more efficient at foraging, and possibly specialize in a foraging technique. Given that daughters are more likely to acquire maternal foraging tactics than sons (11, 23), it’s likely that juvenile females would allocate more effort to honing their foraging skills than males (10, 24–26). The specific foraging tactics that males acquire would therefore depend more on who their eventual alliance partners are (13) as opposed to what they learned from their mothers.
How did I do this? How do I know which dolphin is which? How do I know females from males? How do I know how old they are? All great questions!
My collaborators and I at The Shark Bay Dolphin Research Project have collected data as part of a long term study of a resident population of bottlenose dolphins in Shark Bay, Western Australia. Shark Bay is a UNESCO world heritage site with relatively pristine shallow waters and a population consisting of over 1,600 individually recognized bottlenose dolphins since 1984. We identify individuals by the dorsal fin shape and notches (see photo of Eden to the left). To determine if a dolphin is male or female we can use DNA or we have to wait until they come and swim at the front of our boats and roll over onto their back (which they LOVE to do!). This lets us see whether the dolphin has mammary slits or not (see photos below).
And their age? We estimate their birth date from when the mother was last sighted and the first time a new calf was sighted as well as behaviour and body features. Body features we look for are whiskers on their rostrum, which fall out within a few days after birth, and fetal lines (see a photo of Sonic with her mother Surprise below) which are caused by being scrunched up in the uterus for 12 months. Think of them as being similar to the pruney look you have when you have spent too long in the bath!
So what did I find? I found evidence that male and female juvenile behaviour foreshadows adult behavioural differences. Females increased their foraging rates from late infancy to the early juvenile period and even surpass adult female rates. Both male and female juveniles increased same- age and -sex associate preferences from that of the calf period. And each sex avoided aggressive adult males, but neither sex adjusted their time spent in a group to mediate predation risk from sharks.
Hold up, hold up, what does this all mean? Let’s revisit the three explanations given above: a) social skills, b) protection and safety, and c) energy allocation.
Social skills - Juveniles socialised at a similar rate as calves (6-21%) which is much higher than that of adult females in Shark Bay (<2% (27)). Furthermore, calves and juveniles had far fewer associates than has been reported for adults. What this suggests is that development of social skills begins during infancy but continues to develop and be important into the juvenile period, at least for two years post-weaning. The relationships and skills formed when an individual is a juvenile could be critical for current survival, and/or for future social roles such as male alliance membership (29) or the bonds that contribute to female calving success (30).
Protection and safety – It appears that predation pressure does not have an overwhelming influence on juveniles. As juveniles with their new found independence did not spend more time with other dolphins! More plausibly, high rates of time spent alone in juveniles (41.7%) relative to calves (16.1%), could possibly be a strategy to increase foraging efficiency, as dolphins in Shark Bay tend to forage alone, and/or avoid aggression from other dolphins. Interestingly though, juvenile females did not avoid juvenile males, who can also be aggressive (31). It is possible that juvenile males are not particularly threatening to juvenile females, and instead provide juvenile females with opportunities to learn about characteristics of the opposite sex (17).
Energy allocation - Juveniles do increase their time foraging from when they were calves, however, females were the driving reason for this result. The lack of increased foraging rates by juvenile males is intriguing, indicating that foraging rates 2 years pre- and post-weaning (25-30%) may be sufficient for growth for current survival. Whereas the rate that juvenile females foraged (59.6%) was higher than typically found among adult females in Shark Bay (27%-32%,27,32). Highlighting the probable importance for females to learn hunting skills, but also increase fat stores and growth for current survival and future reproduction.
OK, so some cool stuff was found, but why is this important? The differences in activities and associations between male and female juveniles appear to foreshadow their adult activities and social status but still require extra time as a juvenile to become a successful adult. It means that juveniles are challenged in distinctively different ways than adults or calves. This is important baseline for this relatively neglected age group, crucial to protecting this age group differently to that of adults or calves, and essential for future comparisons and on-going conservation management.
The next time you do something that scares you, such as go to a new country where you don’t speak the language and you have to find food and accommodation, hold on to that deep in the stomach nauseating feeling mixed with intoxicating excitement – and remember that newly weaned juvenile dolphins are probably going through something similar themselves, or maybe just playing with some seagrass!