Killer whale mothers demonstrate an extraordinary level of parental commitment; a recent study published in Current Biology revealed that they are willing to forgo their reproductive success to continue caring for their adult sons. This level of sacrifice is far beyond what is typically seen in other species and highlights the unique bond between mothers and their offspring.
Michael N. Weiss of the University of Exeter in the UK and the Center for Whale Research in the US recently discovered that adult male killer whales rely on their mothers to survive. The southern resident population of killer whales in the coastal waters of Washington state and British Columbia have been monitored since 1976, so Weiss and his team were able to examine the effects of motherly care on female whales' chances of further reproduction.
Motherly Sacrifice
Through their research, they found that there is a measurable cost for mothers caring for adult sons and daughters. Weiss noted that the southern resident killer whale community provides a unique chance to explore these types of questions. Not only do males and females of this species remain with their mother for life, but they are also among the most comprehensively studied wild mammals on Earth.
The researchers discovered a strong negative correlation between the number of surviving weaned sons a female had and her yearly chance of bringing forth a viable calf.
The costs of this were not reduced as their sons grew older. The scientists found that the cost associated with taking care of sons into adulthood was not due to lactation or group composition, which provides evidence for the costly reproductive strategy that mothers take on to care for their weaned sons.
According to the lead researcher, Weiss, the results showed that for each additional surviving son, a female's chance of having a new calf in a given year can be reduced by more than 50 percent, making it one of the most significant costs of caring for adult sons. The research analysis suggests that there are clear evolutionary advantages to females when their sons can survive and reproduce. This could have major implications for conservation efforts for the critically endangered southern resident killer whales, as the social system of the whales appears to be a major factor in determining population demographics. Further research may help to explain the cost to mother whales of providing food to their adult sons, especially in a food-stressed environment.
Graphical Abstract of Costly lifetime maternal investment in killer whales
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Strong, Unique Mother-Offspring Bond
Weiss pointed out that the bond between a mother and son in killer whales is especially strong and likely unique. He added that research on how social systems affect animals' population numbers is growing, and this study is another significant contribution. In the future, more research is planned to investigate the costs associated with mothers sharing food with their mature sons, especially for the food-stressed southern resident killer whales. To help protect these whales, conservation efforts must focus on recovering Chinook salmon for them to feed on, as reported by Science Daily.
At zoological parks, the gestation period for killer whales ranges from 15.7 to 18 months, with an average length of 17 months. There is no statistical evidence for birth seasons because calves are born throughout the year. There is only one calf born at a time. Water is used to birth calves. However, in North American zoological facilities, peak testosterone and sperm production occurred from March to June, even though males produced sperm throughout the year. There may be peak birth months in particular areas. For instance, numerous calves are born in the northeast Pacific Ocean between October and March.
A female may conceive every 3 to 5 years, according to limited data from populations at SeaWorld and zoological facilities. A female may not have another calf for ten years in some instances. Some juvenile killer whales' light areas may be tan or pale yellow, rather than white. Most of the time, they become white by the end of their first year, but some killer whales stay yellowish until they are adults.
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