Lecture week11 (11-2) - Notes Animal Form and Function

Biol. 114 Fall 2023

Lecture week 11 (10/31)

Animal Form and Function

1. Introduction
   1. Limits and tradeoffs

Hollywood’s giant fire-breathing monsters with extra limbs, bugs the size of skyscrapers, giant squids that can walk around on land – they would probably have a pretty easy time surviving. So why don’t they exist?

1. Designs have plasticity (flexibility) within certain limits. Plasticity is the ability for trait to change based on environmental conditions.
   • • 1. Amphibians like frogs and toads grow throughout their whole lives. The rate at which they grow is dependent upon many factors including resource availability and space. However, even with infinite resources, time, and space, a frog could only ever get to be about the size of a dinner plate. This is because they have no supporting structures on their ventral side. If they were to become too large, they could not support their own internal organs.
     1. There’s never just one selective pressure; as you get better at one thing, you often have to sacrifice something else. Everything is based on compromise.
        1. Example: Giraffes have very long legs and necks which help them reach the tops of trees, avoid predation, and large necks are selected for in males to win fights against other males. However, these long necks require specialized vein systems in order to keep them alive while they drink, and even so they have to put themselves in an extremely vulnerable position to do so. Large males will often get on their knees in order to drink, making it much harder to get away should a predator approach.
        2. Environmental variability: Different environments demand different suites of adaptations, e.g. an animal can’t master terrestrial life without compromising aquatic life. A hippo is a comparatively slow swimmer compared to a killer whale, but much more versatile on land.
        3. Becoming larger to escape predation or to increase one’s competitive edge against other species requires multicellularity, cell specialization, and organ systems for transporting nutrients, water, energy sources and waste products between the outer and inner environment of the organism.
2. Energy
   1. Obtaining energy
3. For animals this means eating, but there are many different things out there to eat as well as ways of getting them and converting them into energy.
   • • 1. Generalists vs. Specialists: A generalist is good in everything, master of nothing. There are advantages to both, but few organisms can shift between being an effective generalist or specialist.
          1. Example: Corvids such as the jungle crow are opportunistic omnivores capable of eating a wide range of foods including fruits, insects, and human garbage. As such, they are widespread and can be found in most habitats across the globe, except for South America. Hummingbirds, like the sparkling violetear, live exclusively in North and South America and almost entirely in the tropics, with many being endemic to only small and very specific habitats. Hummingbirds are known for their unique bills which mirror the flowers from which they get their food. As a result, hummingbirds rarely have to compete for nectar with other animals because they are specifically suited to their local flowers. Even within hummingbirds, some species specialize in specific flowers that only they can get nectar from. Crows are in constant competition with other animals, many of which are better at getting specific food items than they are, but since they have so many options they can always find something to eat.
       2. Herbivores, Omnivores, and Carnivores
          1. Energy source variability: Different energy sources demand different, mostly incompatible adaptations to access them, e.g. evolutionary designs of plants vs. animals: no animal is able to photosynthesize, and no plants eat other organisms. Carnivorous plants kill insects but do so for a nitrogen supplement rather than a calorie source. Plants have systems for accessing the glucose from photosynthesis, but they have no mechanism for eating and internally digesting other organisms to obtain glucose. Another example: a lion can’t digest, much less chew, grasses like a cow (the interlocking canines do not allow lateral grinding movements), nor could a cow capture, kill and rip apart the flesh/meat of another herbivore (to begin with, a cow has no upper incisors or canine teeth).
          2. Digestive System: Breakdown of larger compounds into smaller, useable ones – digestive systems of carnivores are simpler than those of herbivores.
   1. Storing energy
      1. Food isn’t always readily available, and even when it is you can’t spend your all your time eating! (Though some animals devote a good chunk of time to it.) Energy from food needs to be stored for later. How much of it and for what purpose depends on the animal.
         1. Example: Dromedary camels are large mammals that live in harsh environments where energy is hard to come by. As such, they have adaptations which allow them to store as much energy as possible over long periods of time. The hump on a camel is actually filled with fat stores; a well-fed camel will have a large hump which it can use for energy when food is harder to find. They can also drink up to 20 gallons of water at a time, and their urine is extremely thick so as not to waste any more water than necessary.
         2. Endothermy vs. Ectothermy: Even endothermic animals that hibernate and show a temperature near ambient need to periodically awaken to dump metabolic wastes and restock energy supplies. Ectothermic animals (e.g. snake) have a much lower energy lifestyle than endotherms (e.g., bird/mammal), and an ectothermic animal becoming endothermic would be unlikely to obtain sufficient food to sustain it. Most reptiles rarely drink water and find/need prey only once every few weeks.
            1. Respiratory System: Brings in O₂, releases CO₂. Endotherms have more complex lungs for greater oxygen usage.
            2. Cardiovascular System: Transports nutrients, wastes and gases throughout body. Most complex in endotherms.
   2. Using Energy
      1. Once you have the systems in place to get and store energy, you have to decide how, when, and for what purpose you will use it. Some animals travel the circumference of the earth every year while others barely move at all. Some animals put all their resources into getting large or creating ornaments for sexual selection, others remain small and relatively plain.
         1. Example: Octopi generally have a “Live fast die young” life history, the longest-lived octopus is the Giant Pacific Octopus and they have a lifespan of only 3-5 years. In that short period of time they can reach up to 33 lbs. and develop levels of intelligence and problem-solving skills on par with primates. They also use chromatophores in their skin to change the color and texture for camouflage and attracting mates. They only breed once before they die. All of these things take energy to develop and maintain, and they have to do it within a very short time, so they are using energy constantly.
         2. Galapagos giant tortoises, on the other hand, are among the longest lived species on the planet, regularly reaching over 100 years old. While they do become very large, it takes them between 40 and 50 years before they reach their full size. They devote most of their energy to developing those large, thick, shells which keep them safe from predators.
      2. Winter in seasonal environments: How different animals adapt to the same problem
         1. In habitats where winters are very harsh, different animals adapt in different ways. Woodchucks will dig a deep burrow and hibernate for most of the winter, allowing their body temperature to decrease drastically so they do not have to forage for food. Arctic terns spend half the year in Antarctica and half the year in the Arctic. They do this so that they never actually have to experience winter at all - when summer in the northern hemisphere ends, they fly to the southern hemisphere where spring is just beginning. This way there are always plenty of small fish for them to eat, which in turn allows them to make such an extreme migration. Coyotes don’t migrate or hibernate, but instead grow a thick fur coat and hunt for small mammals and birds which are also toughing out the winter.
      3. Surface to volume ratio. Animals are multicellular organisms, and in the process of becoming multicellular, their cells became dependent and specialized for a smaller subset of functions, forming multiples of each specialized cell types (tissues) to provide for a larger organism. Eventually clusters of different tissues formed to perform a more refined subset of functions (organs), and, in turn, the organs became designed to completely take over a subset of life processes (organ systems). Each species of multicellular organism is structurally and functionally different from others, and no one species can morph (adjust/acclimate) into another within a lifetime, so there are limits on the ability of organisms to shift to somewhat different ways of living outside the range of the evolved traits that make each species unique, e.g., elephants can’t fly and birds can’t knock over Acacia trees to access food. In the evolutionary design of each species from one to the next, there were losses of old ways of doing things and gains in other ways of doing things, but there is no case where gains were made without losses. Why is this so? The surface to volume ratio (S:V) explains why.
         1. The volume of a sphere increases with the cube of the diameter, while the surface area increases with the square of the diameter, and so something can’t get larger w/o reducing the S:V ratio. For cells/animals depending on the diffusion of oxygen across their surface to supply their internal cell organelles/tissues, there would be a lot less oxygen available per unit volume of cell/body mass with an increase in size. How can cells/individuals compensate for this loss of surface area with larger size? Animals can evolve to be larger but very flat, as did amoebas and flatworms. However, that adjustment increases the distances between different parts of the body, which, if there is division of labor between these parts, eventually causes resource distribution problems as well. What other things change with increases in size (decreases in S:V ratio)?
            1. Heat loss rate decreases
            2. Dehydration rate decreases
            3. Number of predators decreases
            4. The number of shelter options decrease, but because of 1, 2 & 3, the need for shelter decreases (an elephant doesn’t need much shelter, but a mouse does).
            5. The absolute amount of food and water needed by a larger individual increases, but 200,000 mice with the same combined weight as one elephant need 12x more food and water; why? 200,000 mouse bodies collectively have a much greater S:V ratio than one elephant of the same mass, and therefore the mice lose more body heat and moisture and must consume more calories and water to maintain their collective body temperatures. With ectotherms, the problem is mainly with increased moisture loss at smaller sizes (e.g., young animals are more likely to die of dehydration than larger ones).
4. Survival: We will go into much greater detail about predator avoidance in future lectures. Here, we will only touch on a few of the concepts.
   1. Being unappealing to predators
      1. In addition to lowering your rate of heat loss, being large means that the ultimate number of different predators that can kill/eat you decreases. This may also be accomplished by being armored. Animals such as sea turtles combine both of these: an adult loggerhead sea turtle can get up to 3.5 ft in length and weigh 375 lbs.
      2. Some animals are unpalatable (unpleasant to taste), nausea-inducing or even toxic when eaten. Many of these exhibit aposematic coloration, which acts as a warning to predators. Aposematic coloration is often bright (e.g. poison dart frogs) and aimed at making prey stand out (e.g. reverse countershading in skunks). Distasteful species which may or may not be related often use similar coloration, which is known as Müllerian mimicry.
      3. Species that are not unpalatable can exploit the warning signals of other species to make them appear dangerous in what is known as Batesian mimicry. For example, cockroaches in the genus Prosoplecta mimic ladybugs, which are toxic to many animals. This kind of mimicry is negative frequency dependent and relies on having a small amount of the mimic. If there are too many mimics, predators will learn that most animals with that particular appearance are not toxic, and the coloration will no longer be an effective signal.
   2. Being hard to find
      1. Animals can avoid predators by blending into their environment and becoming cryptic (camouflaged), reducing their chances of detection. Populations of the oldfield mouse (Peromyscus polionotus), for example, have been selected to match the background of their location. Mice that live closer to the beach have lighter coat pigmentation, while mice that live further away and closer inland have darker pigmentation.
      2. Not all predators hunt by sight, so some animals have learned to be quiet. Gulf toadfish are preyed upon by bottlenose dolphins, which orient towards “boat-whistle” sound produced by male toadfish during breeding season. Dolphins produce a variety of sounds when foraging, including low-frequency “pops” which are easy for toadfish to hear. Toadfish exposed to “pop” sounds will reduce their call rate by 50% and maintain their reduced call for a period of around 5 minutes.
   3. Running away
      1. Fleeing may seem to be a relatively simple solution, but animals may have evolved such that it will make it easier or more difficult for them to do so. The flight initiation distance, or how close a predator can approach before prey flee, can be affected by any of the following factors:
         1. Prey condition: body size, reproductive state, sex, age, temperature, group size, crypsis, hunger, morphological defenses
         2. Predator condition: speed, size, directness of attack, predator type, starting distance, number of predators, predator intent
         3. Refuge: distance to refuge, light, time of day, habitat type, patch quality
5. Reproduction
   1. Finding a mate
      1. For an animal to have offspring, it (usually) must first find and secure a mate. This tends to require a form of signalling, which in addition to indicating an animal’s availability may also indicate their quality as a mate. Some examples of signalling in courtship:
         1. Visual: Male bowerbirds build a structure to attract females, decorated with sticks and brightly colored objects. These objects sometimes have a theme such as a single color.
         2. Auditory: Birdsong has many functions including attracting and securing mates. Male songbirds often have not one but many different songs, and the size of their repertoire (how many different songs they can produce) has been correlated with mating success.
         3. Chemical: Pheromones are chemical substances produced and released into the environment by animals and often used for the purpose of finding mates. Male silk moths, for example, can detect them with their antennae and use them to find unmated females.
         4. Tactile: Water striders produce ripples on the surface of the water in different patterns including specific signals for calling mates, courtship, copulation and postcopulation.
      2. The average animal’s ability to find a mate could affect which kind of mating system evolves in the species. If finding a mate is very difficult, it may be more beneficial to be monogamous (mate with only one male/female). If finding a mate is easy, you may be able to produce more offspring by being polygamous (mating with more than one female) or polyandrous (mating with more than one male).
   2. Maximizing reproductive success
      1. Animals are evolved to maximize reproductive success, i.e. the number of reproducing offspring they have within their lifetimes. This means there is often a trade-off of quality vs. quantity in offspring production. Organisms that devote a great deal of energy to producing a large number of offspring will have less energy available to allocate resources to each individual offspring. They also have less energy available to devote to traits that would increase their own survival, e.g. growth, their immune system. In mammals, the greatest litter size is 25 for tenrecs; second is for opossum (17). Many mammals have just one offspring/litter (e.g. zebra).
      2. Trait evolution involves good compromises, not perfection. For example, a red-tailed hawk lays two eggs at different times, resulting in one hatchling beginning development before the other and therefore being larger. When food supplies are low or normal, the first born chick will claim all the food brought back to the nest (insuring its survival), and the smaller chick will get little to eat and die. Why lay two eggs? It seems maladaptive and a waste of maternal resources. Well, when food supplies are plentiful, the older chick eats all it can, but not all of it, leaving enough for the smaller chick to survive. This results in two offspring produced instead of one. So, the trait for laying two eggs is imperfect: there is waste much of the time in order to produce two offspring every now and then. Perfect would be a parental trait that could predict food abundance ahead of time and produce only the number of eggs and young that could survive. So, there is a trade-off: waste energy most of the time to double production some of the time. Laying two eggs all the time is imperfect, but it beats laying one egg all the time.