Large mammals facing climate change

Abstract

The fossil record reveals that it has been the largest mammals that have been the mammals most at risk in previous major global warming events, and they are likely to also be most at risk in the current anthropogenic event, in which global temperature is rising much faster than it has in previous events. The large mammals may succumb to heat disease in what will be increasingly frequent and intense heat waves, or may die of dehydration from the diminishing availability of dietary water (especially in the southern hemisphere), or from the disappearance of their sources of food, or from disease caused by pathogens emerging during global warming. However, what is likely to be more pernicious will be failure of reproduction in the face of warming, drought and food reduction. How cattle are affected by heat stress was a topic of research at Onderstepoort in the days of Sir Arnold Theiler. That ambient warming causes failures of conception, teratogenesis, intra-uterine growth retardation and failure of lactation is now well known to the production animal community, but has yet to make an impact on the wildlife community. If they are to prosper, large mammals faced with global warming in their current environments will have to move, or be moved, to more benign environments, or will have to adjust genetically or phenotypically to their new circumstances. Although some species of large mammal have the capacity to move thousands of kilometres within a year, anthropogenic land fragmentation will prevent migration being the solution to threats of local warming that it has been in the past, when polar bears migrated to the Canadian mainland, for example. Some charismatic large wild mammals with low population numbers, like the rhinoceros, may be able to be rescued by assisted colonisation. Valuable livestock, like racehorses, could be relocated to higher latitudes or altitudes. However, the scale (and therefore cost) required of the operations is unprecedented, and the demand for financial resources will compete with those that will be required to move humans. While it is a viable option for taxa with rapid reproduction like bacteria (although potentially catastrophic for host species), genetic adaptation is an unlikely stay-put solution for large mammals. With large body size comes increased longevity, slower reproduction and reduced offspring numbers. Large mammal species will not be able to go through sufficient generations for genetic adaptation to result in speciation occurring within the 50- or 100-year horizon of current global warming. However, there is some room for optimism arising from the genetic process of micro-evolution, which is much faster than speciation. For more than two decades, my research team has been investigating the feasibility of large mammals employing the other stay-put option, namely phenotypic flexibility. Do large mammals have latent physiological talents, autonomic or behavioural, that they do not necessarily require in their current environments, which could be recruited when their environments become warmer and drier? Exploring that question has required the development of a new experimental approach to conservation physiology. It requires the long-term measurement of physiological variables, including behaviour, in identified individual large mammals that are exposed to natural or induced stress, simulating those that will occur with global warming. In the case of wild mammals, at least, studies need to be conducted in free-living mammals in their natural habitats in the absence of human observers, the presence of whom inevitably distorts the mammals’ autonomic and behavioural functions. Such studies have been made possible by the new technology of biologging, which uses onboard instrumentation to measure variables such as location, orientation, movement and temperature in large mammals living free in their natural habitats. We have used seasonal changes in the environment as a proxy for global warming and drying, but have also explored those physiological variables in large antelope in the current extremes of heat and aridity, the Saudi Arabian desert. We have discovered evidence for latent physiological talents, such as switching foraging from day to night, and implementing processes for reducing evaporative water loss, but these are not distributed uniformly across taxa. Among ungulates, perissodactyls show less flexibility than do artiodactyls. Within the scope of the wild mammalian taxa that we have studied, myrmecophages are the most vulnerable to the consequences of global warming. Biologging has been employed for studying livestock physiology under ambient thermal stress, for example by ourselves with Angora goats, but not yet nearly to the extent that it should be. The future of livestock under global warming is a hot topic, literally and figuratively, with cattle farming being the main focus. Meteorologists point to the surprisingly large contribution of cattle to greenhouse gases via the eructation of methane and generation of nitrous oxide from mismanaged manure, and to the profligate water requirements of beef production. Cows’ milk production is heavily compromised by ambient warming, as is conception. Economists point to the diminishing capacity to grain-feed cattle in the face of increasing human food requirements. Agronomists point to the compounding effect of a massive decline in cereal crop production, which is anticipated under global warming, including in the “maize belt” of South Africa. The attractiveness of cattle farming is waning. Yet, there are huge increases in demand for beef, as well as for cows’ milk, in developing countries. Meteorology, conservation and economics argue for a reduction in red meat consumption, at least in developed countries, and for a switch from ruminants to monogastric mammals for meat production. Climate change biology argues for a switch within ruminants from cattle to goats for milk and meat.