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Chapter fifteen: Conclusions and questions
forage. But, how do you know if they are foraging while diving in a certain area, or just
passing through? While stomach temperature sensors are a start, they only can tell when
the first fish enters the stomach, not an actual measure of how much is consumed, or what
species is ingested. Sea lions also tend to regurgitate, or eliminate, these sensors in a matter of mere hours. Other clever approaches have used sensors to measure the jaw angle,
allowing you to know when an animal opens its mouth to catch prey. But then, more information is needed to confirm that the animal was successful. Having read this textbook,
you can likely now better understand the numerous pieces of information, all that require
different measurement techniques, that have to come together to answer the seemingly
simplest of questions.
15.4 Other physiological questions
There are multiple further physiological and biochemical studies of marine mammals that
occur at the next deeper levels of study and form many research theses and postdoctoral
studies. Many of the authors in this textbook were chosen specifically for their expertise
relating to their specific chapters, but also because they are on the forefront of research and
teaching in diverse areas of marine mammal physiology, including the particularly challenging topics we will just briefly mention.
15.4.1 Pre-partum physiology
Pinnipeds and otters exhibit a phenomenon known as embryonic diapause. Within a very
short time period after giving birth (roughly 4–10 days), the female mates again. The egg is
fertilized immediately and divides to about the 16-cell stage. At this point, it enters embryonic diapause, and sits dormant for about 3 months. The embryo is reactivated to result in
a pregnancy period of 9 months, which results in a predictable, annual cycle that can take
advantage of prime periods of prey abundance, pupping substrate (e.g., pagophilic seals),
or breeding opportunities. For a physiologist studying these species, it leads to challenges
in the standard approach of using blood hormones to detect early pregnancy. The female
may very well be carrying a fertilized egg, but the effects of the diapause period will prevent the appearance of the fairly standard hormone profile found in other mammals.
The period of embryonic diapause also comes into play as trial period for the female.
There is evidence that if the female does not accumulate sufficient energy reserves to support the pregnancy for whatever reason, the pregnancy may terminate and she re-absorbs
the embryo. This results in a divergence in adult females during the breeding season.
Those females who have had a successful year will be present at the rookeries to give birth
to their pups, and to breed. Those females who did not maintain their pregnancy may also
come to the rookery solely to breed, or they may not be present at all, opting to wait a year
to improve their body condition before they breed again. For a review of marine mammal
reproductive biology (see Schroeder 1990; Stewart and Stewart 2003).
15.4.2 Digestive physiology
It is very easy to measure how many fish, squid, or other food items a marine mammal
consumes per day in a controlled setting. The difficulty is in measuring how efficiently
those food items are digested and converted into energy, muscle mass, etc. If a small
dolphin consumes 10,000 cal/day of raw fish, what is the digestive and assimilation
efficiency of those 10,000 cal? It turns out that digestive efficiency is roughly 90%, but
Chapter fifteen: Conclusions and questions
forage. But, how do you know if they are foraging while diving in a certain area, or just
passing through? While stomach temperature sensors are a start, they only can tell when
the first fish enters the stomach, not an actual measure of how much is consumed, or what
species is ingested. Sea lions also tend to regurgitate, or eliminate, these sensors in a matter of mere hours. Other clever approaches have used sensors to measure the jaw angle,
allowing you to know when an animal opens its mouth to catch prey. But then, more information is needed to confirm that the animal was successful. Having read this textbook,
you can likely now better understand the numerous pieces of information, all that require
different measurement techniques, that have to come together to answer the seemingly
simplest of questions.
15.4 Other physiological questions
There are multiple further physiological and biochemical studies of marine mammals that
occur at the next deeper levels of study and form many research theses and postdoctoral
studies. Many of the authors in this textbook were chosen specifically for their expertise
relating to their specific chapters, but also because they are on the forefront of research and
teaching in diverse areas of marine mammal physiology, including the particularly challenging topics we will just briefly mention.
15.4.1 Pre-partum physiology
Pinnipeds and otters exhibit a phenomenon known as embryonic diapause. Within a very
short time period after giving birth (roughly 4–10 days), the female mates again. The egg is
fertilized immediately and divides to about the 16-cell stage. At this point, it enters embryonic diapause, and sits dormant for about 3 months. The embryo is reactivated to result in
a pregnancy period of 9 months, which results in a predictable, annual cycle that can take
advantage of prime periods of prey abundance, pupping substrate (e.g., pagophilic seals),
or breeding opportunities. For a physiologist studying these species, it leads to challenges
in the standard approach of using blood hormones to detect early pregnancy. The female
may very well be carrying a fertilized egg, but the effects of the diapause period will prevent the appearance of the fairly standard hormone profile found in other mammals.
The period of embryonic diapause also comes into play as trial period for the female.
There is evidence that if the female does not accumulate sufficient energy reserves to support the pregnancy for whatever reason, the pregnancy may terminate and she re-absorbs
the embryo. This results in a divergence in adult females during the breeding season.
Those females who have had a successful year will be present at the rookeries to give birth
to their pups, and to breed. Those females who did not maintain their pregnancy may also
come to the rookery solely to breed, or they may not be present at all, opting to wait a year
to improve their body condition before they breed again. For a review of marine mammal
reproductive biology (see Schroeder 1990; Stewart and Stewart 2003).
15.4.2 Digestive physiology
It is very easy to measure how many fish, squid, or other food items a marine mammal
consumes per day in a controlled setting. The difficulty is in measuring how efficiently
those food items are digested and converted into energy, muscle mass, etc. If a small
dolphin consumes 10,000 cal/day of raw fish, what is the digestive and assimilation
efficiency of those 10,000 cal? It turns out that digestive efficiency is roughly 90%, but