Avian Malaria Expert
The Toughest of Love in Galápagos Sea Lions and Fur Seals
Anyone with a sibling knows exactly what it’s like to have to constantly fight for the parent’s attention. My younger brother and I are a classic example of sibling rivalry when it comes to who our mother supports and cares for more. Typically, my brother wins as he is younger and less able to take care of himself due to lack of experience and the fact that he is nowhere close to going through puberty yet, but is this the same case for animals?
On my recent trip to the Galapagos Islands, I witnessed a fascinating display of parent-offspring conflict and sibling rivalry within the famous Galapagos fur seals and sea lions. We had just arrived on a small islet surrounded by various shapes of lava rocks littered with beautiful sally light-foot crabs and sandy mini-beaches that piqued our curiosity. The air smelled of saltwater due to the violent waves repeatedly crashing against the rocks and threatening to swallow the outskirts of the island. As we jumped out of the zodiacs and onto the land to follow our guide, we took in the wildlife around us. Magnificent Frigate birds flew over our heads and around the elevated portion of the islet while Galapagos Oystercatchers scurried around the shore in search for their next meal.
To our right was an eye-catching example of lava formations with a huge display of different colors of layers of rocks jutting out of the water. We all gathered as close as possible to listen to our guide explain the scene in front of us, when loud, guttural moans coming from the other side of the beach echoed throughout the salty air. Curious as to what could produce those bizarre noises, we approached the edge of the rocky beach and noticed a mother sea lion and her pup wildly struggling against each other. The mother was simply relaxing on the warm sand while the pup continuously butted its head into her stomach and barked at her (behavior that wouldn’t fly for most humans). Occasionally, the mother would bark back and shoo the pup off, waving her head at it wildly. According to our guide, the pup was attempting to coax the mother into allowing it access to her milk by forcing itself underneath her belly.
The mom, on the other hand, was not having it and repeatedly barked at the pup, pushing it away violently. Most of us were confused as to why a mother sea lion would deny her pup access to nutrients in the form of milk, but when another pup of the same mother struggled up the shore, the story became clearer.
Just as many humans do, a multitude of animals grow up with one or more sibling and have to compete for resources from the parents. Parents in the animal kingdom also deal with an increased need for resources with every child they become responsible for, so sibling rivalry and parent-offspring conflicts generally have an effect on each other. Parent-offspring conflict can be defined as differences between the amount of parental care the parent wants to invest into the offspring and the amount of parental care the offspring wants from the parent. For example, weaning a child off of breast milk is defined as parent-offspring conflict because the offspring wants to benefit off of the mother’s milk for as long as it can, but the mother doesn’t want to expend extra energy.
This can interfere with sibling relations as more siblings produced leads to less resources allocated for each sibling, causing the siblings to compete with each other. Being the older sibling has its benefits, like getting a car first or being able to boss your younger brother or sister around, but do older animal offspring get the same treatment as humans do? Almost every household experiences fights between the children over the last cookie or last turn on a video game where the parents simply let the two fight it out, but studies show that 75% of fur seal and sea lion mothers tended to defend the younger sibling against the aggression of the older one. The mother will do this by using “threats, but also escalated to biting, lifting the older offspring by its skin and sometimes causing open wounds .”
As intense as this sounds to us, it is necessary behavior to keep the younger offspring alive because typically, the younger, less developed sibling is at a disadvantage because it is not as aggressive as its older sibling. Just as in humans, the older sibling has the power to boss the younger one around and chase it away from getting milk from the mother, leaving it to waste away and possibly die. Sibling conflict can be extremely dangerous for the younger offspring with some cases even leading to siblicide – the death of an offspring caused by another. This is almost always the case with Nazca boobies as the stronger sibling forces the weaker one out of the nest, leaving it to slowly waste away and perish. As a whole, “sibling relations can be expected to play a significant role in shaping individual phenotypes, whether morphological, physiological, or behavioral.”
In the case of the Galapagos fur seals and the Galapagos sea lions, parent-offspring conflict and sibling rivalry occur in a unique fashion. These species are especially important in studying the particular evolutionary concepts mentioned above because of their late weaning, overlap of successive young, and resource uncertainty . Galapagos fur seals and sea lion mothers tend to breastfeed their young until they reach around two years old, around the same time as humans. Due to this late age of feeding, 23% of pups are born while another is still feeding . Any mother with twins knows it’s hard to feed two children at once and most resort to baby formula, but this is not an option for the Galapagos pinnipeds. Shared breastfeeding causes the younger pup to grow less in the case of the fur seals and for both species to have higher mortality rates at a young age . Referring to resource uncertainty, the Galapagos Islands experience extreme changes in weather patterns, El Niño and La Niña, which alters resource availability frequently.
In most cases of parent-offspring conflict and sibling rivalry, limited resources leads to the mother throwing all of her support into the older, healthier sibling. In the Galapagos pinnipeds, if the older sibling is male, then the effects of these conflicts are stronger due to sexual dimorphism in these species. Sexual dimorphism refers to the fact that male fur seals and sea lions are born larger and require more nutrition than their female counterparts. This is often the case in humans as well, as males are often much larger than females and eat much more. Funny enough, “he’s a growing boy” applies to the pinnipeds of the Galapagos Islands as well. Due to this difference in the demands of each sex from their mother, the mother is most likely to invest more in the son if anything goes wrong, and in turn, causes the males to become lazy while demanding more. When I say they become lazy, I don’t mean they just sit around and watch TV like many brothers do. I mean they don’t go out and find food on their own like their sisters do. In a study by Piedrahita, more than two-thirds of female pups between the ages of one and two engaged in “independent foraging,” the act of independently searching for food. Sound familiar?
In an evolutionary sense, you may be asking why these species don’t just reproduce once every two years to avoid sibling overlap and conflict. The answer is simple. In the Galapagos where resource availability is constantly fluctuating, it is evolutionarily beneficial to produce more offspring as a sort of insurance. If the stronger offspring dies, which has been proven to happen to males when food is scarce, then the mother can switch her investment to another offspring and assure her reproductive value. Think of it this way: many of us update our pictures to the infamous Cloud as back up in case our phone spontaneously blows up or we accidentally crack the screen. It’s always better to be safe than sorry, and the fur seals and sea lions act in this way as well.
Reanalyzing the noisy scene from my trip to the Galapagos, it became apparent that the mother was responsible for two pups, thwarting off the older pup in an attempt to save some milk for the younger, weaker one. All at once, the fur seals on that hot, rocky beach were demonstrating a unique case of parent-offspring conflict, sibling rivalry, and parental interference in the latter. The mother could have been avoiding the older sibling with interest of the younger sibling because resources were plentiful and she could afford to invest in both equally. The demanding sibling could have been a female, and the mother could have already decided to solely invest in the male because resources were limited. There are many possible reasons why this scene of parent-offspring conflict could have occurred, but these two evolutionary concepts are very evident and unique in the Galapagos fur seals and the Galapagos sea lions. What do you think was the case?
 Trillmich, F. & Wolf, J.B.W. Behav Ecol Sociobiol (2008) 62: 363. doi:10.1007/s00265-007-0423-1
 Godfray, H. C. J. (1995). Evolutionary theory of parent-offspring conflict. Nature, 376(6536), 133-8.
 Hudson, R. & Trillmich, F. Behav Ecol Sociobiol (2008) 62: 299
 Trillmich, Fritz. "Field Metabolic Rate of Lactating Female Galápagos Fur Seals (Arctocephalusgalapagoensis): The Influence of Offspring Age and Environment."
 Piedrahita, Paolo. "Lazy Sons, Self-sufficient Daughters: Are Sons More Demanding?"ScienceDirect.