Discussion of Adaptation to High Latitudes
When most people think of the modern high latitudes they think of the extreme temperatures. Most of those people however do not live in the Arctic. Those people that do live there tend to think of the annual cycle in terms of extreme changes in the light regime (D'Oro 2006). Therefore, despite a generally milder Cretaceous polar environment compared to that of today's high latitude (Otto-Bliesner and Upchurch 1997;
Parrish and Spicer 1988;
DeConto et al. 1999;
Takashima et al. 2006), the annual variance in light regime should also be considered in discussing the Cretaceous Arctic.
Modern arctic organisms are remarkably complex and demonstrate high levels of adaptation (Irving 1960;
Pielou 1994). With vertebrates those adaptations are either structural such as issues related to insulation, behavioral such as use of shelter, body orientation or posture, or metabolic.
Irving (1960 p. 345) noted, for example, in arctic birds that contour feathers differed from those birds farther south, with the net result being that arctic feathers retained more air than feathers on birds from warmer areas. Arctic hares (Lepus arcticus) use posture and body orientation to adapt to extreme changes in Arctic climate (Gray 1993). And arctic foxes (Alopex lagopus) adjust their metabolic rates to survive periods of low temperature and food scarcity (Prestud 1991), a strategy also employed by some arctic birds (Pielou 1994) and caribou (Rangifer tarandus,
Jeffries et al. 1992). In Rangifer, it is now recognized that, due to the lighting conditions of the northern high latitudes, commonly but mistakenly referred to as six months of darkness and six months of sunlight (e.g.,
Bell and Snively 2008), that circadian activity is suppressed within this group (van Oort et al. 2005). Further,
van Oort et al. (2005) speculate that a reduction in circadian organization may be an adaptative response for all resident polar vertebrates. This adaptation may also be advantageous in assisting animals with adjusting to the variation in the light/dark cycle. The breakdown in dependence on circadian rhythms may contribute to an animal's ability to reduce its metabolic rates during the winter.
I speculate further regarding the significance of the similarity in microwear patterns across all assemblages, independent of latitude, and the implication of similar food use. To accommodate similar food use in the dark part of the year in the northern latitudes hadrosaurs may have dropped their metabolic rates much as has been observed in modern vertebrates during times of food scarcity. Given that most of the conifers of Cretaceous Alaska are thought to be deciduous (Spicer 1987,
2003), this reduction in metabolic rates in northern hadrosaurs would allow them to survive periods of food scarcity. This reduction in metabolic rate may not have ultimately led to hibernation as it is difficult to imagine these animals living in such large numbers burrowing. Rather, this reduction may have been more similar to torpidity often seen in birds.
If the conifers of Cretaceous Alaska were deciduous (sensu
2003), thereby making food extremely scarce, the pattern of food use may provide insight into the metabolism of hadrosaurs in Alaska. In reviewing reptile life history strategies,
Shine (2005) explored the question of how ectothermy may have shaped life history evolution in reptiles. A key point in reptilian ectothermy is the ability to decouple the time of energy acquisition (feeding) from energy expenditure (reproduction), allowing reptiles to withstand months or years of starvation. In contrast, within endotherms these parameters are intimately linked, providing the need for example for most migrating birds to refuel to complete the journey for breeding. If hadrosaurs ate the same type of foods, independent of latitude, then the source of calories was the same even if the quantity fluctuated seasonally for arctic hadrosaurs. Alaskan hadrosaurs may have essentially starved during the winter and reproduced during the short summer. In contrast to the trend in thinking that dinosaurs were endotherms, perhaps dinosaurs as reptiles represent the large-bodied end members of this ectothermic life history phenomenon.
Van Valkenburgh and Molnar (2003) put forth a supportive discussion for high-latitude dinosaurs' physiology. They recognized the compaction of niche space in small theropod guilds throughout western North America during the Cretaceous in comparison to modern ecosystems. They argued that this niche compression in high-latitude sympatric theropods implied reduced metabolic rates for these taxa, or in other words that high-latitude small theropods may have been ectothermic.
It should be clear from this discussion that high-latitude dinosaurs present intriguing challenges to what we think we know about dinosaur physiology and behavior. It is so often stated in paleontological studies that more work and more specimens are needed to resolve the specific topics of discussion in those studies. It is repetitive to suggest that more work and more specimens will add further insight into the discussion of the paleobiology of high-latitude dinosaurs, but with respect to this paleontological problem, it is appropriate to repeat what has been so commonly been stated before. That said however, given that the most important question regarding high-latitude dinosaur faunas remains, how they survived the long periods of darkness during the winter, potentially the most important aspect of further work would be to determine if there are patterns of seasonal variation in food use.