Marine mammal evolution 2: polar bears, sea otters, seals etc. Flashcards
Polar bears and sea otters
Evolved relatively recently in the pliocine era
Sirenia
Manatees, dugongs and sea cows
Desmostylia extinct ancestor wassimilar to elephant but aquatic
Families grouped by tusks
Trichechidae-the manatees - grazers in rivers and coastal waters, 3 species
Dugongidae -dugong, once diverse but only one surviving species, marine, consume seagrass
Stellars sea cow went extinct recently so now there are only two extant sirenia lineages.
Stellar sea cow (Hydrodamalis gigas) – discovered by Europeans in the Bering Sea in 1741, hunted to extinction in less than 30 years. Much larger than extant species
Unusual forelimb that appeared to have digits and a wrist that bent – hence its distinction in the evolutionary tree
Sirenia Phylogeography: Differentiation among modern sirenian species is defined in part by their geographic distribution, though the west Indian manatee and Amazonian manatee are parapatric. (Different ranges for each species)
Pinnipeds
Otariidae – external ‘pinnae’ ears
Phocidae – no external ears ‘earless seals’
Odobenidae - walruses
Canid – dog-like appearance
Very well adapted to move within water – whales and seals similarly streamlined (fusiform)
Semiaquatic mammals less efficient
Thought to have evolved when the physical distribution of the continents was a bit different. Ancestors evolved into otariids on one side of a continent and phocids on the other side.
Enliarctos mealsi
pinniped ancestor: Similar to modern seal but with some unique characteristics
Early on there was a controversy about the nature of the pinniped phylogeny, based especially on proposed morphological synapomorphies this led to the theory that phocids were less related to bears than other pinnipeds
Diphyly was based on morphological characteristics interpreted as synapomorphies, such as the foramen ovale in the cranium (Koretsky et al. (2016) Vestnik zoologii, 50: 327–354
Pinnipeds are in fact monophyletic (all descendants of bears) However ofcourse there are morphological synapomorphies that distinguish the three pinniped lineages
34 named species of pinniped: 1 extant otobenid(walrus) 19 phocids and 14 otaarids
Walrus (odobenid)
Walrus (Odobenus rosmarus) – A benthic feeder (often using their tusks to dig in muddy substrate feeding on molluscs and other invertebrates) in Arctic waters. Both males and females have tusks, and they are gregarious throughout the year.
Hind limbs can rotate back under their body as for Otariids
Only one extant species, but up to 12 recent diverging species within this group, and ~10 Ma ancestors were without tusks (Tanaka & Kohno 2015, PLoS One 10, e0131856).
Ancestors did not have tusks. Radiation of species resulted from change in sea levels then habitat level increased and 11 out of the 12 became extinct
Comparing otariids and phocids
Comparing otariids and phocids, there are several consistent differences in morphology – the external pinna (ear flap), terminal claws on the flippers in phocids, and reversible hind flippers in the otariids which they can tuck under allowing them to run on land (whilst phocids ‘belly flop’)
Otariids
fur seals and sea lions are very similar genetically but have distinguishing morphological differences
Otariids (fur seals and sea lions) – There are two groups of otariids, though as seen in the phylogeny, their lineages are not clearly differentiated. However, there are phenotypic differences: fur seals have thicker fur, tend to be smaller, have a blunter nose, and the pinna are often larger than for sea lions.
Distribution: Fur seals have some independent distribution but in places over laps with sea lion range
California sea lion, stellar sea lion and northern fur seal are all sympatric along the Californian coast and size is a distinguishing morphological difference
Size is also a distinguishing factor for the sympatric populations of sea lions and fur seals along the coast of South America – the female fur seals are ~50kg, the males ~200kg, while the female sea lion is ~150kg and the male ~350kg.
California sea lion, stellar sea lion and northern fur seal are all sympatric along the Californian coast and size is a distinguishing morphological difference
Its not all about size though
A comparison of sea lions (Zalophus wollebaeki) and fur seals (Arctocephalus galapagoensis) off the Galapagos Islands tested the question of the relationship between size and habitat use directly (Jeglinski et al. (2013) J. Anim. Ecol. 82, 694-706). They compared adult sea lions with adult fur seals, but also juvenile sea lions (which are about the same size as adult fur seals). Take home message – not just about size…
Relationship between size and habitat uses comparing adult sea lions and fur seals as well as juvenile sea lions which are similarly sized to fur seals.
Juvenile sea lions dived similar depths to adult fur seals and Sea lion adults appeared to dive deeper
However clearly fur seals spend more time off shore so size is not the defining factor for behaviour
Reproductive strategies in pinnipeds
bulk and starve whilst feeding young
OR
spend less time tending pup, hunt between feeds
Reproductive energetic strategies consistent among otariids – income breeders (see Champagne et al. (2016) In: M. D. McCue (ed.), Comparative Physiology of Fasting, Starvation,
and Food Limitation, DOI: 10.1007/978-3-642-29056-5_19).
(Otaarids income breeders and phocids capital breeders)
Northern Elephant Seal -Maybe the most successful capital breeders – spend 40 days not eating or drinking, males defend harems of upto 100 females, females spend this time raising their pup and leave when their pup weans
The northern elephant seal (Mirounga angustirostris) is an extreme capital breeder, with all animals on the beach during the breeding season fasting (both food and water) for the duration (~40 days). They conserve water by not breathing when they don’t need to (apnea), and they capture and recycle ~70% of the water in the exhaled air using a bony structure in the nasal passage called the turbinate process (which is cooled when inhaling and, in turn, cools the exhaled air, condensing the water). During fasting females typically loose 40% of their body weight. The males defend harems of females and fight for access, mating just before the female returns to the sea.
How do they go without water during this period?
The turbinate process allows the seals to collect water from cold air condensation collected in their specially adapted nasal cavity.
Phocids around the world
Phocids are one of the most broadly distributed mammalian groups in the world
Phocids (earless seals) – found throughout the world, but separated by species that haul out and give birth on rocks and beaches in temperate zones (e.g. harbour seal, elephant seal) or in tropical zones (monk seals), or haul out and give birth on ice (e.g. harp seal in Artic and Weddell seal in the Antarctic)
For tropical water phocids thermoregulation is a challenge due to blubber and fur - but they have adapted
Adaptive evolution of phocid dentition
and breeding strategies
Tooth type determines groups of species
Crabeater seals have lobed teeth to filter feed on krill
^A similar dentition to that seen in early mysticetes (convergent evolution)
Leopard seals have long canines and lobed teeth are used for tearing rather than filtering
Northern and southern elephant seals use their exaggerated canines for fighting among males in competition for access to females. Callouses from goring during fights are visible as pink discolouration on their chests. These phocid species together with most otariids are highly polygynous.
Ice breeding seals are more typically monogamous, or effectively so. Male weddell seals (Leptonychotes weddellii) produce vocal displays at ice holes, likely to defend territories and gain access to females. Their vocalisations are distinct from other aquatic sounds (sound synthetic!)
Mating behaviour in pinnipeds
See table in notes: Cassini (1999, Behav. Ecol. 10, 612-616) reviewed the characteristics of mating behaviour among pinniped species based on behaviour and environment
(Phocid exceptions shown as the two species on the right)
^Despite the similarities in behaviour the two phocids on the right are not more related to otariids.
breeding on land and feeding offshore often results in polygyny
^ Why are land-breeding seals often polygynous? See Model by Bartholomew in notes (1970, Evolution, 24, 546-559) suggests that the combination of onshore parturition and offshore marine feeding led to polygeny.
Mate availability and distribution defines chances of polygyny.
Emlen & Oring (1977, Science, 197, 215-223) considered the general case and the role of resource in what they termed the ‘evolutionary potential for polygyny’ or ‘EPP’
Mating synchrony in pinnipeds is achieved through delayed implantation of embryos, leading to annual parturition, even though the gestation period is much shorter.
For pinnipeds in particular, the maximum EPP will be related to the number of females available for mating, and the duration of their availability. When the conditions are best, this leads to a selective advantage for male competition (and characteristics that aid competitive abilities).
Some adaptations relevant to both cetaceans and pinnipeds
Deep diving
An adaptation that sets marine mammals apart from terrestrial mammals is their ability for deep diving
Dive response:
One important aspect is about the distribution of oxygen. In deep divers more goes into muscle and proportionally less into the lungs. Air is also evacuated from the lungs to help avoid nitrogen narcosis (the ‘bends’).
^ Nitrogen can leave blood causing narcosis and there is the limitation of stored 02/ CO2 build up.
Seals store more in blood and muscle than in the lungs, collapsing there lungs as they dive reduces chance of nitrogen leakage.
In dolphins a tangle of blood vesicles (called the rete mirabile) can concentrate oxygen where it’s needed, and also be involved in thermal regulations. (rete mirable = miraculous net)
Blood volume also matters, with marine mammals having proportionally higher blood volume (and therefore oxygen binding capacity) per Kg
Another important factor is the concentration of myoglobin in the blood, which binds oxygen more strongly than haemoglobin. Marine mammals have more myoglobin to store more O2
Other adaptations
Thermal regulation:
for pinnipeds, polar bears and sea otter, behavioural thermal regulation is important (as with other mammals), though they have also evolved thick pelage to protect them, including two layers in the sea otter and fur seals. For cetaceans and sirenians (and seals to varying extents) subcutaneous fat provides essential insulation, varying in thickness depending on their habitat.
Blood redirection:
Pinnipeds and especially cetaceans have highly ‘reniculate’ kidneys. Although this feature is found in other mammals, it is especially highly developed in marine mammals. However, it does not appear to be very important for osmoregulation (marine mammals rarely drink sea water, instead getting fresh water metabolically and from within prey). Instead it is likely related to diving behaviour and kidney function when blood is shunted away during a deep dive (see Pfeiffer (1997) Aq. Mamm. 23, 75-84).
^ Reniculate kidneys are basically made up of clusters of tiny kidneys, seals and cetaceans have the most reniculate kidneys, they do not use this to osmoregulate (they don’t drink sea water and get their fluids from prey) this reniculation is related to their diving ability
Vision – the pinniped and cetacean eye is adapted for optimal vision in water by changing the shape of the eye and the lens compared to terrestrial ancestors. Marine mammals also have more rod cells and a well-developed ‘tapetum lucidum’, a layer of reflecting plates behind the retina. These plates act as mirrors to reflect light back through the retina, increasing the light-gathering ability of the rod cells
^pinnipeds are also able to distort their lenses to see better on land
Convergent evolution of dim light vision in owls and deep-diving whales.
See diagram by: Castiglione et al. (2023) Curr. Biol. 33, 4733–40
Dim-light specialists also regularly encounter bright sunlight, creating elevated risk for blindness. Mutation in rhodopsin gene enhances sequestration of toxic bright-light byproducts - prevents blindness
Summary
1)Polar bears and sea otters evolved relatively recently and represent marine versions within their lineages (bears and mustelids respectively)
2)Sirenians evolved earlier and are more completely adapted to aquatic life, and phenotypically quite dissimilar to their extant terrestrial ancestors, the elephants. Differentiation associated with phylogeography.
3)Pinnipeds are a highly evolved group in the carnivore lineage, showing hydrodynamic efficiency comparable to that for cetaceans
4)Pinnipeds can be divided into three lineages, the odobenidae (now only the walrus, but once a diverse lineage), the otariids and the phocids, differentiated according to distribution and life history characteristics
5)Important adaptive specialisations include reproductive strategies (especially monogamy vs polygyny, associated with environment), diving abilities (also seen for cetaceans), vision and thermoregulation (fur or fat)
