Many animals if not all will in some point in be carrying a load which is an addition to normal body weight either through carrying one of their young like most primates. Or needing to carry extra body weight for migration which is seen in many birds which do so to survive the long distances that they must travel without feeding. The energetic costs on the animals is the amount of energy that is required to carry the extra load around with them and how much of a handicap it is on the animal if any. E.g. how it affects the animal's speed, the animal's ability to move (nimbleness) and how it affects the animal's behaviour. This energetic cost may vary with relevance to weight or it may be influenced by the way in which an animal carries a weight e.g. in primates is it more energetically viable to carry the young on the back or front of the parent. Different species of animals have evolved different methods of carrying loads which is influenced by their morphology, the way in which they move and how the animal interacts with its environment. Bumble Bees (Bombus) for example collect pollen on their legs in pollen baskets to bring back to the colony [1] whilst other animals like emperor penguins (Aptenodytes forsteri) will carry food in their stomachs and regurgitate it for the young. Load carrying may give an animal an advantage whether it is more protection for the young e.g. Marsupials have evolved a pouch to carry young, or being able to carry resources back to the colony which is seen in most species of ants in which some species like the Leafcutter ant (Atta cephalotes) can carry loads way over their own body weight. Load carrying can have a big affect on an animal's body, some will increase their body weight dramatically during certain times like the grizzly bear (Ursus arctos horribilis) which must build up its body mass during the summer months in order to survive hibernation during the winter. An animal's body must be capable of tolerating the extra weight without permanently damaging the animal's body. Permanent damage is sometimes caused in humans (Homo sapiens) from excessive load carrying even when previously warned which can lead to implications later in life. It seems that some animals like A cephalotes can carry loads that are much heavier than themselves for long distances of time without any foreseen damage to the animal whilst others like H sapiens may inflict injuries on themselves doing so. It is certain that some animals are much better at carrying loads than others however the sacrifices that they make for this (if any) may detriment the animal in another aspect of its abilities as an animal cannot be good at everything. A cephalotes may have a strong body design in terms of carrying loads however they may be poor swimmers as a result of their design for carrying loads. Another animal that has a strong design in being able to lift and burrow through things is the Rhinoceros Beetle which is part of the family Scarabaeida and are one of the largest species of beetles around reaching 6cm in length. [2] When a male encounters another male the two beetles will fight, it is done by the two beetles interlocking their horns together and using their strength to try and lift their opponent up and throw them aside. For an animal to lift a peer and throw them aside with such ease it must have a lot of strength in proportion to body size. Rodger Kram wanted to see whether the metabolic rate of Rhinoceros Beetles increased as the load that the beetles carried increased [3]. The experiment consisted on the beetle walking in a respirometer chamber whilst on a treadmill carry weights up to 30 times is body mass. Kram found that the beetles could carry the loads cheaply and that the energy consumption had doubled only when the beetle was carrying a load that was equal to 10 times its body mass [3]. R Kram argues that this may have been down to the way in which the beetle moves and its limb posture during walking however Kram finds no evidence for this. This hard to believe as the beetles legs and movement must have undergone some natural selection pressure for load carrying and the life style of the beetle. Kram assumes that the beetles' life style of burrowing through rotting material such as wood [4] and battling other males for mates may have had generated natural selection for the ability to carry heavy loads and exert strong forces. Should this be considered for all arthropods and that all are very good load carriers, capable of carrying loads past their own body weight. For example cockroaches are also capable of carrying loads equal to their own body mass and this only increases their metabolic rate by 50%.[5]
However some orders like Hymenoptera would be unable to carry weights greater than themselves especially in the air as they are so finely balanced in flight. Martin Burd found that leaf-cutting ants do not maximize individual energetic efficiency and that they try to rate-maximize and increase the amount of fragments that enter the colony. [6] The ants think like the colony that they are and look at the overall colony energetic efficiency and try to maximize it which is not through taking the largest leaf fragments.
Hermit crabs (Coenobita compressus) are another arthropod and perhaps have to bear a greater load than most animals as they carry their shells around with them. Hermit crabs will move from shell to shell as they get larger to better suit their size. Herreid II, C. F. and Full, R. J. [8] measure the energy needed for hermit crabs to move with and without shells at different velocities. The experiment showed that when the crabs had no shell and were resting there was no significance difference between the ones with shells. However when the crabs were running crabs with shells required significantly more energy than shell-less ones. It was also found that hermit crabs are especially good at carrying loads that were four times the mass of the crab and that the volume of oxygen used did not increase. No definitive answer could be made for this however it was noticed that the crabs that carried large shells shifted their leg positions to allow them to occasionally drag the shell. This intern allowed the crabs to bear the extra weight without using excessive amounts of energy. Clyde, Herreid and Robert also looked at how the amount of limbs a hermit crab had affected the energy needed to move with and without a shell by amputating a certain number of legs off several crabs. They found that the volume of oxygen used increased when running however they were not satisfied with the results as imbalance may have had affected the crabs which would be corrected over time.
When animals carry a load they may adopt a different posture to compensate for the extra weight to make it more energetically efficient, this could be in the shape of the body or in the leg movement and the way in which they move with the weight. J. R. Grote [7] carried out and experiment looking at the effect of locomotion on crayfish. He classified a loaded crayfish as being out of the water and an unloaded crayfish being in the water and then looked at how the position of the legs changed. When the crayfish were out of the water the legs moved into the body this made the distance of each step shorter allowing the same force to be used however over a shorter distance compensating for the extra weight. The crayfish are more at home in the water where the load of their bodies does not affect them however they are capable of adapting to the situation with changes in its posture to efficiently carry themselves.
The weight of a load is not always as important as the type of load that an animal carries, one type of load may be considerably more difficult to carry than another or a load may have a stimulus on the carrier in some way triggering it to use more energy for example. [9] An experiment looking at the effect of loading on honey bees Apis mellifera found that the when a bee would load itself with either pollen or nectar that the bees flight metabolic rate would increase but only at a small level in comparison to the amount of load that was carried. However when the bees were hovering the bees that were foraging for pollen were on average 10% more metabolically active than bees looking for nectar. The study [9] could find no biomechanical reason behind this and proposed that the foraging bees are more actively stimulated by the pollen foraging than the nectar foraging bees. The study [9] concluded that the bees find the collection of pollen a more rewarding resource and work harder to collect it or that pollen foragers have a higher metabolic rates to collect pollen in the colder mornings when it is most likely done (based on J. Fawells observations). [10]
For animals to be able to migrate they must have a source of food for the journey whether this is pit stops along the way or bringing their food along with them on the journey as extra body weight which can be used as energy. Or an animal may choose to do both and not turn down the opportunity of food along its migratory route The Red Knot (Calidris canutus) is one bird that migrates long distances during different times of the year. The Red Knot will build up energy reserves in body mass before it migrates. How carrying large fuel loads affects sustained flight Red Knots was investigated in a study [11] which found that the metabolic power used increased in proportion to body size. It also found that the flight muscle efficiency increased also to compensate for the weight. The Red Knott adjusts its muscle size to give maximum efficiency in relation to its weight [12] and helps them to maintain manoeuvrability and the speed which they can take off crucial for avoiding predators such as Peregrine Falcons.
Predation is a day to day business for most animals and all have their ways of escaping being eaten for birds ascending as high as possible above the predator and flying fast to gain the advantage or making for cover where it cannot be reached with ease. Many birds have their own individual strategies however this is what the blackcap [13] (Sylvia atricapilla ) uses. A study [13] looking at how body fat affects the blackcap found that as the amount of weight increased the take off velocity decreased being hampered by the extra weight gained. Also the angle of accent decreased with weight both having a negative effect on the birds' performance. However the blackcaps could tolerate the extra weight up to around 30% of the original body mass without having a too greater effect on the performance. Only when load reached around 40% was the affect great on the bird's velocity and angle of accent [13]. The study suggest that migratory birds carrying large fat loads will have a hampered ability to escape predators and with reference to [14] is the reason why passerine birds with 50% or more body weight are only found when they are preparing to make a large journeys in their migration, requiring them to have the extra fat. In order for someone to understand the effects of loading on an animal's performance they need to look at the inner workings of the animal so that a better understanding of what is happening to the animal can be seen at different levels. David J. Ellerby and Richard L. Marsh [15] decided to investigate how blood flow, cardiac output and oxygen consumption change in a guinea fowl (Numida meleagris) leg muscles and how the different types of muscle behave. The experiments results were not surprising and found that blood flow to the muscles increased when the load was trunk loaded or distal limb loaded and the animal was running. Resulting in a 15% increase in metabolic power above the control which was unloaded. Cardiac output and net oxygen consumption also increased, with both loads organ blood flow was also measure and was found to drop significantly when the animal was loaded (trunk & Limb) and running. At the same time there was a reduction blood flow to flight muscles [15]. This supports most modern theories that blood is taken away from areas around the body which are not in the greatest need for it and are given to areas which are. Depending on where the load was on the guinea fowl affected the blood flow, if the load was positioned on their backs (trunk) the muscles that are used for stance had an increased blood flow and more so than the muscles used in the swing phase of leg movement. If the load was then moved to the distal Load (lower leg) the blood flow increased in both stance and swing muscles however less stance muscles had increased blood flow and more swing muscles did. Looking at the finer details of how load carrying effects an animal's allows us to better understand the internal changes that an animal makes when bearing a load rather than just looking at the external effects.
Load bearing itself varies in so many ways from the carrying of young to the extra weight needed to survive migration for some animals it is a matter of survival. The way in which carrying a load affects the energetic cost on an animal and how it affects its body or mind is different for each species. To some animals load carrying can be a big hindrance (e.g. birds) which rely on being light weight for flight. An animal's environment and lifestyle must be the selective pressure behind its ability to bear weight as seen in Rhinoceros Beetles. It is clear that some animals are better at bearing a load than others like the Rhinoceros Beetle capable of carrying a weight up to 30 times its own body mass it is obvious that other species like H sapiens would be unable to bear such a weight. Is this down to what kind animal it is e.g. are arthropods better than chordates at bearing loads? It is hard to say by looking at only a handful of animals which animals are better as there will always be exceptions of the groups. If animals are better than other animals what is it that makes them better is it the design, movement style, carrying method, blood flow, stronger muscles ECT. One thing that is apparent is that there is no single reason or explanation behind bearing a load and the reason for the energetic costs of load carrying are many and complex.
