Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Title Maternal investment strategies of killer whales, Orcinus orca, due to environmental factors. Author Neal Sandy: [email protected] Abstract Next to humans, killer whales (Orcinus orca) are the most wildly distributed mammal on Earth. Orcas have no specific breeding season since data on that subject is inconclusive. However, orca females usually don’t give birth to a large number of offspring over a lifetime due to a 12 to 18 month gestation period and only being sexually active from around 15 to 40 years old (only about 25 years). In almost every case, orca females only give birth to 1 calf at a time. Since these animals are at the top of the food chain, it can be concluded that starvation is the more likely to be the cause of death rather than predation. Introduction The killer whale is found in every ocean and is at the top of the food chain. Contrary to popular belief killer whale are actually the largest species of dolphin, belonging to the family Delphinidae. Female orcas can grow to be up to 7.9 meters long and on average live to be about 50 years old, which by comparison is considerably longer than males (who live only to about 29 years old on average). Females reach sexual maturity around 14 years old and are between 4.9 and 5.5 meters long. However, the youngest ever recorded birth was at only 11 years old. This species doesn’t have any noticeable breeding season and give birth year round however; data from the northeast Pacific Ocean does show a higher number of births between October and March. Usually this species only has 1 offspring per birth with twins being a rare occurrence. "Size estimates of SeaWorld-born killer whales suggest that calves average 2.6 m (8.5 ft.) in length and weigh between 136 to 181 kg (300-400 lbs.)."(SeaWorld, online site) The main goal of any sexually mature female is for the survival of at least two sexually mature offspring to replace her and her mate. Since orcas are a highly social animal, mothers place a great deal of time and energy into their young. Despite this, for reasons unknown orca calves have different mortality rates among different regions. For example, in the Pacific Northwest, 43% percent of newborns die in only the first 6 months. (SeaWorld, online site) This could be due to a variety of factors including starvation, or predation by sharks, pollution, and different diseases and infections. According to the maternal risk management model (Cassill, D. L., 2013), utopian environments favor investments in a few, low quality offspring, predation environments favor investments in offspring quantity, seasonal environments with periods of scarcity favor investments in offspring quality and multi-risk environments with high predation and periods of scarcity favor investments offspring diversity—usually a few high-quality offspring and many-low quality offspring. During this study, we examined offspring quality and quantity for the killer whale and then used measurements to predict the environmental risk factors that shaped the evolution of the orca’s maternal investment strategies. Method Offspring number and relative offspring body-size were plotted on the inner “x” and “y” axes. The outer “x” and “y” axes are qualitative probabilities of predation or starvation. The relative body size of offspring at independence and thus the probability of starvation was estimated as S = m/M where S = expected probability of offspring mortality based on cycles of food scarcity; M = size of mother at the time of offspring independence; mx = size per offspring at the time of its independence. The expected probability of offspring mortality by predation was estimated as P = 1 – (2/N) where P = expected probability of predation; 2 = expected lifetime fitness per mother; N = the number of offspring produced by a mother per clutch or lifetime. The expected probability of offspring mortality in multiple-risk environments is estimated as PS. Results Offspring quality was estimated using 4.9 meters as the average length of offspring at independence (sexual maturity). Since orcas never truly leave their mothers or their pod the size of the mother can be up to an average of 7.9 meters. Relative offspring quality represents the probability of offspring mortality by starvation, which was calculated as 4.9m/7.9m = 0.6203. Offspring quantity was estimated at only 1 per clutch. Since this gives a negative result for the equation for probability of offspring mortality by predation, the life time total of offspring was used. The probability was then calculated to be P=1 – [2/6] = 0.6667. Overall, the percent of the mother’s lifetime offspring that will die of predation or disease (66.7%) exceeds the percent of individual offspring that will die of starvation (62.03%; Fig. 1). Offspring Number (N) R Offspring Size(S) S U P Low Starvation (S) High Low Predation-Disease (P) High Offspring Number (N) Figure 1: The maternal investment strategy of Orcinus orca. The blue circle represents individual clutch number (N=1), while the red circle represents offspring over the mother’s lifetime (N=6). P: Predation environments favor investments in offspring quantity rather than quality. S: Seasonal environments that cycle between abundance and scarcity favor investments in offspring quality rather than quantity. R: Multi-risk environments with high predation and periods of scarcity select for a few high-quality offspring and many-low quality offspring. U: Utopian environments favor few, low quality offspring. Discussion Killer whales are carnivorous and usually eat fish, squid, seals, sea lions, dolphins, and other marine mammals. They have even have been known to hunt large whales (Sperm, Blue, Etc.). This range in diets is commonly attributed to habitat and location, those that live in a primary area (Resident pod) tend to eat mostly fish, while those that travel over a large area (transient pods) tend to eat more marine mammals. The orcas ability to hunt animals larger than itself and animals that can even move onto land show the effectiveness of this organism’s physiology and communication abilities which allow it to coordinate attacks and ambush prey. Just like with humans, the groups success also stems from their ability to teach their young to hunt and problem solve with the group. While fully grown healthy adults have no predators, young orcas can be preyed upon by sharks. According to our calculations, we can conclude that orcas maternal investment strategy tends to be investing more in offspring quality over quantity. This strategy most likely evolved in an environment where the offspring had low risks of predation, but had a limited food supply. References: Anderson, G. (2003). Killer Whales: Reproduction. Retrieved from Santa Barbara City College Marine Science Website: http://www.marinebio.net/marinescience/05nekton/KWreproduction.htm Burnett, E. (2009). Orcinus orca: Killer whale. Retrieved from University of Michigan Museum of Zoology Animal Diversity Website: http://animaldiversity.ummz.umich.edu/accounts/Orcinus_orca/ Cassill, D. L. (2013). A novel risk management model illuminates the evolution of maternal investments in offspring quantity, offspring quality and offspring diversity. Unpublished manuscript, Department of Biological Sciences, University of South Florida St. Petersburg, Florida. Clark, S. T., & Odell, D. K. (1999). Nursing parameters in captive killer whales (Orcinus orca). Zoo Biology, 18(5), 373–384. doi: 10.1002/(SICI)1098-2361(1999)18:5<373::AID-ZOO2>3.0.CO;2-D Good, A. (n. d.). Orca: Orcinus orca. In Northwest Wildlife Preservation Society. Retrieved from http://northwestwildlife.com/wp-content/uploads/2013/12/orca.pdf NOAA (2013). Killer Whale (Orcinus orca). In NOAA Fisheries Office of Protected Resources. Retrieved from http://www.nmfs.noaa.gov/pr/species/mammals/cetaceans/killerwhale.htm