209
Chapter nine: Thermoregulation
9.6.4 Designer temperature telemetry
The development of ever more sophisticated dive recorders is discussed in the “ToolBox”
sections of many chapters in this book. They are important for the study of thermoregulation, because they can collect skin surface temperatures, stomach, deep muscle, and any
other temperature from wherever the investigator is able to place a temperature probe.
Temperature data from these recorders have been integral in the models of diving blood
flow, identifying tissues that are metabolically depressed during diving, and in understanding the overall thermal balance of animals underwater (Andrews 1998; Ponganis
et al. 2004; Kuhn and Costa 2006; Meir and Ponganis 2010). They can even be used to
measure food ingestion by showing the temperature change in the stomach after a marine
mammals has consumed a cold fish meal. In a very recent development, specialized units
have been designed to be placed permanently in the body cavity of wild Steller sea lions,
transmitting archived data only when the animal dies and the tags are released from the
decomposing or consumed body. These life-history transmitters (LHX) use temperature
as one of a suite of measured parameters being used to provide not only individual known
fate mortality, but data on the diving behavior, birth rate (for females only), and inferences on the cause of death. Upcoming modifications to this technology will also allow
for interim uplinks of core body temperature in live animals to nearby receivers placed at
strategic haul outs (Horning and Mellish 2009, 2012, 2014).
9.7 Unsolved and future questions
All of the marine mammals we have discussed so far have been carnivores that are active
swimmers and strong divers, with a high metabolic rate. The sirenia (dugongs: Dugong
dugon, and manatees: Trichechus spp.) are warm water vegetarians with a low metabolic
rate. They are not phylogenetically related to any of the pinnipeds or cetaceans and are
most closely related to the elephants. They are marine mammals, but in a group by themselves. However, because they live in a marine environment, they must be able to dive,
hold their breath, swim and have many of the same evolutionary adaptation pressures that
face other marine mammals. While sirenia are discussed in several of the other chapters of
this book from those perspectives, we look here at their thermal biology.
Returning to Fick for our discussion, sirenia are relatively large (small SA:V ratio),
tend to inhabit warm water (reduced convective heat loss) and will even move into warm
water springs during the colder winter months (behavioral thermoregulation). They have
a thick blubber layer (insulation), but no fur or hair. Their body temperature is lower and,
therefore, the temperature gradient from body core to water is reduced (Gallivan et al.
1983). However, despite these heat-conserving characteristics, they must still compensate
for a low metabolic rate. Clearly, this balance works well and they are a successful group,
but they have a very low environmental thermal window. That is, they do not have much
ability to move away from warm water. The endangered status of many sirenian populations is a result of human interference and loss of habitat that allows them to stay in warm
waters (Fertl et al. 2005).
9.7.1 Steller’s sea cow
In terms of thermal biology, the extinct Steller’s sea cow (Hydrodamalis gigas) is a
fascinating case study. It was a vegetarian like dugongs and manatees, however, unlike
extant sirenia, the Steller’s sea cow lived in the shallow intertidal areas from southeast
Chapter nine: Thermoregulation
9.6.4 Designer temperature telemetry
The development of ever more sophisticated dive recorders is discussed in the “ToolBox”
sections of many chapters in this book. They are important for the study of thermoregulation, because they can collect skin surface temperatures, stomach, deep muscle, and any
other temperature from wherever the investigator is able to place a temperature probe.
Temperature data from these recorders have been integral in the models of diving blood
flow, identifying tissues that are metabolically depressed during diving, and in understanding the overall thermal balance of animals underwater (Andrews 1998; Ponganis
et al. 2004; Kuhn and Costa 2006; Meir and Ponganis 2010). They can even be used to
measure food ingestion by showing the temperature change in the stomach after a marine
mammals has consumed a cold fish meal. In a very recent development, specialized units
have been designed to be placed permanently in the body cavity of wild Steller sea lions,
transmitting archived data only when the animal dies and the tags are released from the
decomposing or consumed body. These life-history transmitters (LHX) use temperature
as one of a suite of measured parameters being used to provide not only individual known
fate mortality, but data on the diving behavior, birth rate (for females only), and inferences on the cause of death. Upcoming modifications to this technology will also allow
for interim uplinks of core body temperature in live animals to nearby receivers placed at
strategic haul outs (Horning and Mellish 2009, 2012, 2014).
9.7 Unsolved and future questions
All of the marine mammals we have discussed so far have been carnivores that are active
swimmers and strong divers, with a high metabolic rate. The sirenia (dugongs: Dugong
dugon, and manatees: Trichechus spp.) are warm water vegetarians with a low metabolic
rate. They are not phylogenetically related to any of the pinnipeds or cetaceans and are
most closely related to the elephants. They are marine mammals, but in a group by themselves. However, because they live in a marine environment, they must be able to dive,
hold their breath, swim and have many of the same evolutionary adaptation pressures that
face other marine mammals. While sirenia are discussed in several of the other chapters of
this book from those perspectives, we look here at their thermal biology.
Returning to Fick for our discussion, sirenia are relatively large (small SA:V ratio),
tend to inhabit warm water (reduced convective heat loss) and will even move into warm
water springs during the colder winter months (behavioral thermoregulation). They have
a thick blubber layer (insulation), but no fur or hair. Their body temperature is lower and,
therefore, the temperature gradient from body core to water is reduced (Gallivan et al.
1983). However, despite these heat-conserving characteristics, they must still compensate
for a low metabolic rate. Clearly, this balance works well and they are a successful group,
but they have a very low environmental thermal window. That is, they do not have much
ability to move away from warm water. The endangered status of many sirenian populations is a result of human interference and loss of habitat that allows them to stay in warm
waters (Fertl et al. 2005).
9.7.1 Steller’s sea cow
In terms of thermal biology, the extinct Steller’s sea cow (Hydrodamalis gigas) is a
fascinating case study. It was a vegetarian like dugongs and manatees, however, unlike
extant sirenia, the Steller’s sea cow lived in the shallow intertidal areas from southeast