Download Title Maternal investment strategies of killer whales, Orcinus orca

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
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http://animaldiversity.ummz.umich.edu/accounts/Orcinus_orca/
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maternal investments in offspring quantity, offspring quality and offspring
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