(Presented by Stephanie McPherson and Rachel Carter)

This study was done by Kjartan G. Magnusson and Toshio Kasuya. The two species under observation and study were the Sperm Whale and the Short-Finned Pilot Whale. The Sperm Whale is the largest of the toothed whales and they live in pods. The adult males range in size from 50 to 60 feet, while the female adults are between 33 and 40 feet long. The sperm whale’s skin color is dark gray with a wrinkled texture to it. They are the deepest diving whales and they feed on deep-water squid. The males reach sexual maturity between 7 and 13 years old. The sperm whale’s breeding season is from September to December. The Sperm Whale’s name is derived from spermaceti oil in the melon, which was used as a perfume additive, which lengthened the scent’s duration.

The Short-Finned Pilot Whale, it is also known as the "pot head whale". They are medium sized toothed whales, and they also live in pods. The adults range in size from 12 to 21.5 feet. Their color is jet-black or dark gray. Their dives can last for ten minutes or more, but they are not deep divers. The Short-Finned Pilot Whales feed on squid or octopus, and sometimes fish. The females reach sexual maturity at nine years of age, and they have a calf every 4 to 6 years. An interesting fact is that it’s thought that males and females bang their heads together when they are courting each other.

For Sperm Whale distribution, mating and calving areas include waters off New Guinea and around Hawaii in the North Pacific, and also deep waters off east Australia, and many other tropical areas, but not in polar seas. They are rarely seen near coastlines. For Pilot Whale distribution, they are found mainly among warm temperate waters around the world, although their exact location is unknown. This is because they are often confused with the Long-Finned Pilot Whale. They have no fixed migratory patterns, and some populations are fixed year round (i.e. Hawaii). In 1975-1983, sightings were made during aerial surveys conducted off California. In 1989-1994, they were seen off of Oregon, California, and Washington.

The two strategies observed were the Searching Strategy and the Harem Strategy. In the Searching, each male searches continually for female pods and mates with all the females in oestrus and moves on to other female pods in oestrus. In the Harem, a male finds a pod of females, remains with them, and mates with all the oestrus females until the end of the breeding season. In terms of pregnancy rates, the advantage that one strategy has over the other, depends on four key parameters. The first one is the number of oestrus cycles in a breeding season (m). The second one is the fraction of time in the season when the female is in oestrus (q). The third one is the ratio of the number of males to the number of female pods (s). The fourth parameter is the expected number of pods found by a male in a breeding season (t). The first two parameters are biological and abundance dependent, while the last two parameters are independent of abundance.

For parameter one (m), the length of the breeding season is defined as a hypothetical time period when females repeat oestrus cycles if they fail to conceive. A good indication of the average number of oestrus cycles experienced by a female in a single breeding season is the number of ovulations experienced before conception. This is usually 1-2, on average. If the number of oestrus cycles in a breeding season (m) is low, and the ratio of the number of males to number of female pods (s) is high, it is to the advantage of the female for a male to be available when she goes into oestrus, which is the harem strategy. If (m) is high and (s) is low, then there is still a good chance of pregnancy because cycles are more plentiful, which is the searching strategy.

For parameter two (q), the receptive time, or the period when copulation can end in conception, can be estimated indirectly. This was done by a study of uterine sperm in the Short-Finned Pilot Whales, suggesting that sperm will survive in the uterus for only 3-4 days after copulation. The life of ovulated ova could be short, but is essentially unknown. The length of time when females can conceive in a cycle is approximately 6 days.

The third parameter (s), includes only males joining in the breeding activity and ignores males which might remain outside the breeding ground during the season. There is no proof that males found in the pods actually mated with the females in the same pod. They may be related to the females. The fourth parameter (t), could be measured theoretically by tracking males through a season and finding the number of pods they encounter, or by estimating the average time between encounters.

Male sperm whales are usually found with scars, caused by inter-male fighting. This indicates competition over oestrus females by use of the searching strategy. With Short-Finned Pilot Whales, more than one male is sometimes found in the breeding pods. They show no signs of distinct fighting marks. This could indicate no competition over oestrus females, and also that new males can join the pod at any time. This could mean that males form small groups and keep joint harems, or a "big commune".

Ovulation rate in Short-Finned Pilot Whales declined with age from 0.7 ovulations per year at age 7, to 0.14 over 30 years old. The proportion of ovulations followed by conception is significantly higher below 20 years old. Annual pregnancy can be controlled by a key factor, which is the availability of males, but more importantly, can be the number of females having oestrus cycles and the probability where ovulation is followed by conception. As the searching efficiency of the males increases, or also the number of pods available, then the searching strategy has the advantage. This is then better from the female point of view because as the number of pods increases with a harem strategy, a larger number of males will be captured by other pods and thus unavailable.

Evidence suggests that Sperm Whales employ the searching strategy, while Pilot Whales employ the harem strategy. Searching strategy would work just as well when the length of the oestrus cycle is high, because she has no need for the male to remain to impregnate her. It is to the advantage of the female that the male stays with the pod (Harem), if the length of the oestrus cycle is low, so a male is available when she comes into oestrus. A way to keep a male in the pod is to practice non-reproductive matings, which is called a "trick". The individual employing the trick usually gains with a pregnancy depending on the value of (q), but as the trick spreads throughout the population, one particular female may or may not gain from using it. The gain to an individual female decreases if other females are employing the trick also and keeping males around.

Since it seems that Sperm Whales employ the searching strategy and Short-Finned Pilot Whales employ the harem, it can be concluded that female Sperm Whales have longer oestrus cycles, while Short-Finned Pilot Whales have shorter ones. This study was inconclusive on many substantial results as to whether one strategy was more advantageous than the other. One reason could have been that the parameters were difficult to measure. It was concluded that reproductive mating depends mostly on the oestrus cycle.

1) www.whalewatch.co.nz/sperm.htm

2) www.sci.tamucc.edu/tmmsn/29Species/sfpilot.html

3) Science 15th of July 1997.V 197. Number 4300.

4) Kasuya, T. Marsh. 1984. Life History and Reproductive Biology of Short Finned Pilot Whales, Globicephala machorhynchus, Off the Pacific Coast of Japan. pp 259-310.

5) Kasuya, T. Marsh, H. Amino A. 1993. Non-reproductive Mating in Short Finned Pilot Whales.

6) www.wdcs.org/dan/publishing.nsf/ (al...AA799421F22C9802569F8004F3832? open document)

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