The Benefits of Commitment in Black-headed Gulls

By: Karina Rodrigues, Joanna Soliman, Yenifer Oseguera, and Giana Youssef (Stonehill College, BIO323: Evolution, Spring 2023)


Loyalty, a quality some lack and some do not. Have you ever thought about the positive effects loyalty has on people, better yet species? In terms of evolution, loyalty translates to how committed a species is to its breeding partner. Originating from the Netherlands, black-headed gulls (Chroicocephalus ridibundus) were used to study the benefits of long-term relationships and if it plays a role in how parents invest more with their partners as well as their pair bond with one another. These seabirds were studied in a model environment that fits the birds natural needs in order to exhibit their natural behaviors. After testing different mechanisms, selection for mate retention reduces parental care conflicts between a pair and also sheds light to selecting traits that increase individuals fitness throughout evolution.

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In the animal kingdom, biparental care, where both breeding partners work together in raising their offspring’s, is rare compared to other reproductive systems. Its rarity is due to the cost of energy and resources the parents need to invest into their children, which in result could negatively affect their health, survival rate, and even their ability to reproduce again. Selection is expected to affect male and female parents by restricting their investments towards their offspring’s to which will force their breeding partner to take part in providing the care their offspring’s need in order to become independent.

Evolutionary Biologist Kat Bebbington and her team studied black-headed gulls Chroicocephalus ridibundus and tested if selection would favor individuals that lower the burden on long-term partners which in return would lower evolutionary conflicts inflicting on parental care. Biparental care theory assumes that the parents fitness increase when the partner invests in parental care. However, when partners only stay together for a short amount of time after breeding, the main parent’s care will solely be given to the offspring, which reduces the conflicts of parental care and instead forces the parent to give its best needs and care to its offspring rather than depending on a breeding partner. This made Bebbington and her team test if individuals only invest in compatible new-partners or in partners they find a long-term bond with.

They tested 42 breeding pairs from 115 black-headed gulls that were captured and monitored at the Groningen Institute for Evolution Life Science since 2010. Historical data was used to find the duration of the relationship of the partners, as well as their age. The courtship data was taken around mid- April by observing the birds every 3-4 days and eventually recording 110 courtship displays. Nesting data was recorded late April by checking for newly built nests every 3 minutes, as well as the clutch sizes for every nest. Incubation period ended around the beginning of May and data was collected by taking pictures of nesting areas every 2 minutes for 14 hours a day which helped them discover the sexual dimorphism between both sex.

Some of the hypothesis established during this experiment was the behavioral coordination, partner commitment, and partner compatibility. Behavioral coordination suggested that pair bond worked better in younger birds because they are able to coordinate responsibilities better, while older birds displayed a lower behavior coordination in their pair bond. Partner commitment showed that males were more committed to nesting-related behaviors and displayed a high level of pair display participation with its partner. Extra-pair displays, which are the other males females mate with outside their relationship, did worse in nesting-duties, making them not good breeding partners. Partner compatibility concluded that new partners of high compatibility do not invest in their partners so they are not reliable breeding partners. However, long-term pairs had high commitment to their relationship. 

In conclusion, pairs with long-term relationships were found to have less intense courtships and were more coordinated with their behaviors in comparison to pairs that displayed short-term relationships. The pairs that were in long-term relationships invested more in their partners which led to females to lay larger eggs and coordination in incubation between the pair to be equally split. Synchronization between the long-term pairs indicated their commitment to their relationship which ties into their investment in parenthood. Bebbington and her team’s results proposed that long-term breeding partnerships gathers mutual benefits which increases their investment in breeding partners which decreases the amount of parental conflicts breeding partners face while taking care of their offspring’s.

Article: Bebbington, Kat, and Ton GG Groothuis. “Partner retention as a mechanism to reduce sexual conflict over care in a seabird.” Animal Behaviour 197 (2023): 15-26.

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