Herbivores Daily Lecture PDF

A day in the life of a herbivore (or, the diversity of herbivore life history strategies) Specific focus: Large mammal herbivores Mainly African savanna, grassland and semi-arid systems The importance of scale – both spatial and temporal TEMPORAL...

A day in the life of a herbivore (or, the diversity of herbivore life history strategies) Specific focus: Large mammal herbivores Mainly African savanna, grassland and semi-arid systems The importance of scale – both spatial and temporal TEMPORAL SPATIAL SCALE SCALE Community Multi-year Biome composition, distributions, abundance, composition etc. distributions etc. Demographic Annual rates, population Demographic rates, trajectory population trajectory Water dependence, Seasonal Plant phenology Key resources migration, reproductive physiology etc. Abiotic Dentition, gut-type, Food (?), feeding rate, Daily constraints, body size, physiology, patch selection etc. defences behaviour etc. PLANTS HERBIVORY HERBIVORES 1. What is the main objective of an individual, from an evolutionary perspective? o Maximise lifetime reproductive success (LRS) o Contribution of longevity (survival) and fertility (fecundity) 1. How can we assess an individual’s (potential) lifetime reproductive success at different scales? o Total number of viable offspring over lifetime (long-term studies) o Body condition or health of an individual is a good indicator of its likelihood of surviving and reproducing A day in the life of a herbivore... Objectives: 1. Adequate nutrition 2. Survive 3. Reproduce All herbivores aren’t equal... 1.How much food is needed each day? 2.What is food? 3.How do you go about obtaining that food? 4.What is a considered a threat? 5.What is your reproductive strategy? 1. How much food is needed each day? Allometric scaling: Kleiber’s law Metabolic rate = B0 * Mass 0.75 Metabolic theory of ecology: Metabolic rate is a function of mass and temperature MR provides the Temperature also has a fundamental constraints +ve relationship with by which ecological metabolic rate processes are governed. Kleiber (1947) – The fire of life An elephant is about 400 times heavier than a steenbok, but only needs about 90 times more kilojoules each day.  2. What is food? Herbivores eat plants – but all plants are not equal Herbivore perspective: Fat reserves – carbohydrates, lipids Protein content Digestibility – cell walls, lignification Which is more NB – fat or protein? Modelling suggests fat maybe more NB, but as environmental conditions become relatively more severe, importance of protein increased (Parker et al. 2009) 2. What is food? continued... General patterns New growth has higher protein content and is more digestible As plants get taller, the need for lignification increases in order to provide structural support Forage quality varies – water, nutrients get withdrawn in the plant dormancy season and in older parts of the plant The quality and quantity of available forage is skewed towards an abundance of low quality forage and a scarcity of high quality forage. Frequency (not scaled) Plants Herbivores Predators Ratio protein to fat and carbohydrate in body Raubenheimer et al. 2009 Functional Ecology 2. What is food? continued... Herbivores can achieve their daily nutritional demands by targeting: an abundant but low quality diet (‘bulk feeders’) or a scarce but high quality diet (‘selective feeders’) Jarman 1974 Behaviour This trade-off closely reflects body size, but there are many adaptations which influence diet selection. Mouth morphology Dentition Gut type 2. What is food? continued... Body size influences diet choice through its effect on: a) Gut volume Scales nearly isometrically with body size The bigger you are, the more food you can process at a time b) Retention time The bigger you are, the longer food takes to pass through you, allowing you to digest it more completely Therefore, larger body size means you can eat a lower quality diet (Jarman-Bell principle) However, just because you can survive on rubbish, doesn’t mean you’ll only choose to eat rubbish! 2. What is food? continued... To graze, browse or be a mixed feeder? Perez-Barberia, F. J. & Gordon 2001 Proc. Roy. Soc. B Phylogeny: diet choice of your ancestors constrains your diet Body mass: browsers tend to be smaller Having controlled for phylogeny & body mass: Dentition o Grazers have more protruding and wider incisors than browsers o Grazers have more bulky and higher-crowned molars than browsers o Browser molars have higher ridges than those of grazers o Grazer muzzles are broader Muzzle width http://etc.usf.edu/clipart/41400/41438/teeth_41438.htm Molar ridge Molar shape surface area Perez-Barberia & Gordon 2001 Proceedings of the Royal Society of London B http://www.thetrigger.net 2. What is food? continued... Having apprehended forage, you then need to digest it: All large mammal herbivores rely on bacteria to breakdown their food More finely chewed = greater surface area for their attentions Two main gut types: 1. Ruminants or foregut fermenters 2. Non-ruminants or hindgut fermenters The main difference is that ruminants have a specialised oesophagus / stomach complex (‘four stomachs’) and hindgut fermenters have a modified cecum.  http://ecodevoevo.blogspot.com http://www.calacademy.org RUMINANTS NON-RUMINANTS http://ayearwithhorses.blogspot.com http://www.calacademy.org  Bininda-Emonds et al. 2007 Nature Non-ruminants: Equidae (zebra), Rhinocerotidae (rhino), Suidae (pigs), Hippopotamidae (hippo), Elephantidae (elephant) Ruminant families: Tragulidae – e.g. kudu Giraffidae – e.g. giraffe Antilocarpidae – e.g. impala Cervidae – e.g. fallow deer Ruminants Bovidae – e.g. buffalo Non-ruminants 2. What is food? continued... Within the ruminants, gut morphology varies with diet Grazers Large fermentation chamber Smaller openings between them Fewer papillae for slower fermenting grasses Browsers Smaller rumen Bigger openings More papillae for more rapid passage and absorption of more rapidly digestible leaves 2. What is food? continued... What are the consequences of being a ruminant versus a non-ruminant? Ruminants Non-ruminants Slower passage rate = eat less Faster passage rate = eat more More complete digestion Less complete digestion Advantage when food availability is Advantage when food availability is not limiting limiting 3. How do you go about obtaining that food? Herbivores are constantly making foraging decisions: Which bite to take? Which patch to feed on? When to move to the next patch?  Seasonal to multi-annual time scale Large-scale distribution patterns Seasonal migrations Habitat choices Daily time scale Bite size within patches Decisions to switch between patches Daily forage intake rates Senft et al. 1987 BioScience ‘The Bite’ As a rough approximation, a herbivore will take: 30 000 bites per day 210 000 bites per week 900 000 bites per month 10 950 000 bites per year Its one hundred millionth bite around its ninth birthday Are you sure you’re doing that right? ‘...by the sheer frequency of their occurrence, any systematic error herbivores make in selecting bites of food will be compounded over days, seasons, and lifetimes with potentially large consequences.’ – Shipley 2007 Oikos 3. How do you go about obtaining that food? continued... Food intake by herbivores is influenced by: Bite size Cropping rate Chewing time Bite size has a major influence on food intake rates: Large bites reduce the amount of cropping time needed per unit of food ingested, so big bites = faster intake rates Large bites are also chewed more efficiently than small bites Cropping and chewing aren’t done very well concurrently, so tend to limit each other.  3. How do you go about obtaining that food? continued... Large bites allow you to take in food quicker while feeding at one spot, but: because you can chew while you’re moving to your next feeding spot; bite size also influences food intake rates at larger scales (i.e. between feeding stations) Thus, when large bites are available, animals can meet energy requirements or fill their gastrointestinal tracts more quickly.  FEEDING MOVING RESTING How much of the day do you need to spend feeding? The net effect of scaling body size, bite sizes, diet qualities etc. translates into smaller individuals needing less feeding Du Toit & Yetman 2005 Oecologia time each day. Du Toit & Yetman 2005 Oecologia Benefits of not needing to forage all day long Lower energy expenditure More time = more selective Thermoregulation Vigilance More time for reproduction related activities... How far have we got? 1.How much food is needed each day? 2.What is food? 3.How do you go about obtaining that food? 4.What is a considered a threat? 5.What is your reproductive strategy? Nutrition = body condition = survival & reproduction = lifetime reproductive success = evolutionary success 4. What is considered a threat? Surviving the day is a critical part of being a successful herbivore! The main day to day risk to survival is predation. Extreme climatic events Disease Poor body condition exacerbates the threat of all of these potential causes of mortality. At the daily timescale, herbivores rely on a few traits to minimise predation risk: Vigilance Herd formation – ‘many eyes hypothesis’ Crypsis – e.g. steenbok, offspring of some species Predator-herbivore dynamics are starting to come under more focus in the literature, but much work remains to be done. Vigilance Most herbivores need to maintain some degree of alertness to the risk of predation whilst foraging Visual scanning of the surrounding regions is one of the main ways of detecting predators. Costs arise where this behaviour interrupts feeding, thus reducing food intake rates. Two types of vigilance are recognised: routine vigilance induced vigilance Routine vigilance Under good / plentiful foraging conditions can be ‘cost free’ Overlaps with chewing time or performed while searching for the next bite Bite size and forage density thus can determine whether it reduces food intake rates Predicted to be relatively more costly for larger animals that need to spend a greater proportion of the day feeding Thomson’s gazelle Trade-off with vulnerability to Illius & Fitzgibbon 1994 Animal Behaviour predation Induced vigilance Response to an actual signal of potential predator presence – smell, sound, alarm call etc. Herding vs. crypsis? Herd formation allows you to benefit from the vigilance efforts of others, thereby reducing the cost to yourself. Compete with your fellow herd members for forage. There appears to be a threshold body size above which herd formation occurs – could this be related to diet quality and abundance? Perhaps small animals with very selective diets can’t afford to form herds due to forage competition? Smaller species tend to rely on crypsis to avoid predators – freeze and hide. 5. What is your reproductive strategy? The last useful thing to think about in a days work is reproduction Longer term processes, but day to day activities will be influenced Timing and magnitude of costs differ for males and females Females Most costly stage is during late pregnancy (± 50 % > than non-pregnant), peaking during lactation (65 – 215 % higher; Parker et al. 2009) Usually timed for onset of plant growth season Protein becomes relatively more important in diet Capital vs. income breeding strategies Males Most costly during rut Also need to maintain territories Usually late in plant growth season Energy i.e. carbohydrate / fat probably more important in diet Yoh yoh, hectic morning... put your feet up for a while. References