Plants and animals can also live in marriage

When we think about the life of plants it is difficult to imagine without interaction with the animals, as they establish different symbiotic relationships day after day. These symbiotic relationships include all the herbivores, or in the contradictory way, all the carnivorous plants. But there are many other super important interactions between plants and animals, such as the relationships that allow them to help each other and to live together. So, this time I want to present mutualism between plants and animals.

And, what is mutualism? it is the relationship established between two organisms in which both benefit from living together, i.e., the two get a reward when they live with the other. This relationship increase their biological effectiveness (fitness), so there is a tendency to live always together.

According to this definition, both pollination and seed dispersal by animals are cases of mutualism. Let’s see.

POLLINATION BY ANIMALS

Many plants are visited by animals seeking to feed on nectar, pollen or other sugars they produce in their flowers and, during this process, the animals carry pollen from one flower to others, allowing it reaches the stigma in a very effective way. Thus, the plant gets the benefit of fertilization with a lower cost of pollen production, which would be higher if it was dispersed through the air. And the animals, in exchange, obtain food. Therefore, a true relationship of mutualism is stablished between the two organisms.

 “Video:The Beauty of Pollination” – Super Soul Sunday – Oprah Winfrey Network (www.youtube.com)

The extreme mutualism occurs when the species evolve depending on the other organism, i.e., when there is coevolution. We define the coevolution such as these evolutionary adaptations that allow two or more organisms to establish a deep relationship of symbiosis, due that the evolutionary adaptations of one specie influence the evolutionary adaptations of another organism. For example, this occurs between various orchids and their pollinators, as is the well- known case of Darwin’s orchid. But there are many other plants that also have co-evolved with their pollinators, as a fig tree or cassava.

In no way, this should be confused with the trickery produced by some plants to their pollinators, that is, when they do not obtain any direct benefit. For example, some orchids can attract their pollinators through odours (pheromones) and their curious forms that resemble female pollinator, stimulating them to visit their flowers. The pollinators will be impregnated with pollen, which will be transported to other flowers due to the same trickery.

Bee orchid (Ophrys apifera) (Autnor: Bernard DUPONT, flickr).

SEED DISPERSAL BY ANIMALS

The origin of seed dispersal by animals probably had occurred thanks to a co-evolutionary process between animals and mechanisms of seed dispersal in which both plants and animals obtain a profit. The most probably is that this process began in the Carboniferous (~ 300MA), as it is believed that some plants like cycads developed a false fleshy fruits that could be consumed by primitive reptiles that would act as seed dispersers. This process could have intensified the diversification of flowering plants (angiosperms), small mammals and birds during the Cretaceous (65-12MA).

The mutualism can occur in two ways within the seed dispersal by animals.

The first case is carried out by animals that eat seeds or fruits. These seeds or some parts of the fruits (diaspores) are expelled without being damaged, by defecation or regurgitation, allowing the seed germination. In this case, diaspores are carriers of rewards or lures that result very attractive to animals. That is the reason why fruits are usually fleshy, sweet and often have bright colours or emit scents to attract them.

For example, the red-eyed wattle (Acacia cyclops) produces seeds with elaiosomes (a very nutritive substance usually made of lipids) that are bigger than the own seed. This suppose an elevated energy cost to the plant, because it doesn’t only have to produce seeds, as it has to generate the award too. But in return, the rose-breasted or galah cockatoo (Eolophus roseicapillus) transports their seeds in long distances. Because when the galah cockatoo eats elaiosomes, it also ingest seeds which will be transported by its flight until they are expelled elsewhere.

On the left,  Galah  cockatoo (Eolophus roseicapillus) (Autnor: Richard Fisher, flickr) ; On the right, red-eyed wattle’s seeds (black) with the elaiosome (pink) ( Acacia cyclops) (Autnor: Sydney Oats, flickr).

And the other type of seed dispersal by animals that establishes a mutualistic relationship occurs when the seeds or fruits are collected by the animal in times of abundance and then are buried as a food storage to be used when needed. As long as not all seed will be eaten, some will be able to germinate.

A squirrel that is recollecting som nuts (Author: William Murphy, flickr)

But this has not finished yet, since there are other curious and less well-known examples that have somehow made that both animals and plants can live together in a perfect “marriage.” Let’s see examples:

Azteca and Cecropia

Plants of the genus Cecropia live in tropical rain forests of Central and South America and they are very big fighters. The strategy that allow them to grow quickly and capture sunlight, avoiding competition with other plants, resides in the strong relationship they have with Azteca ants. Plants provide nests to the ants, since their stems are normally hollow and with separations, allowing ants to inhabit inside. Furthermore, these plants also produce Müllerian bodies, which are small but very nutritive substances rich in glycogen that ants can eat. In return, the ants protect Cecropia from vines and lianas, allowing them to success as a pioneer plants.

Ant Plants: Cecropia – Azteca Symbiosis (www.youtube.com)

Marcgravia and Bats

Few years ago, an interesting plant has been discovered in Cuba. This plant is pollinated by bats, and it has evolved giving rise to modified leaves that act as satellite dish for echolocation performed by these animals. That is, their shape allow bats to locate them quickly, so they can collect nectar more efficiently. And at the same time, bats also pollinate plants more efficiently, as these animals move very quickly each night to visit hundreds of flowers to feed.

Marcgravia (Author: Alex Popovkin, Bahia, Brazil, Flickr)

In general, we see that the life of plants depends largely on the life of animals, since they are connected in one way or another. All the interactions we have presented are part of an even larger set that make life a more complex and peculiar one, in which one’s life cannot be explained without the other’s life. For this reason, we can say that life of some animals and some plants resembles a marriage.

REFERENCES

• Notes from the Environmental Biology degree (Universitat Autònoma de Barcelona) and the Master’s degree in Biodiversity (Universitat de Barcelona).
• Bascompte, J. & Jordano, P. (2013) Mutualistic Networks (Chapter 1. Biodiversity and Plant-Animal Coevolution). Princeton University Press, pp 224.
• Dansereau, P. (1957): Biogeography: an Ecological Perspective. The Ronald Press, New York., pp. 394.
• Fenner M. & Thompson K. (2005). The Ecology of seeds. Cambridge: Cambridge University Press, 2005. pp. 250.
• Font Quer, P. (1953): Diccionario de Botánica. Editorial Labor, Barcelona.
• Izco, J., Barreno, E., Brugués, M., Costa, M., Devesa, J. A., Fernández, F., Gallardo, T., Llimona, X., Parada, C., Talavera, S. & Valdés, B. (2004) Botánica ªEdición. McGraw-Hill, pp. 906.
• Murray D. R. (2012). Seed dispersal. Academy Press. 322 pp.
• Tiffney B. (2004). Vertebrate dispersal of seed plants through time. Annual Review of Ecology, Evolution and Systematics. 35:1-29.
• Willis, K.J. & McElwain, J.C. (2014) The Evolution of Plants (second edition). Oxford University Press, pp. 424.
• National Geographic (2011). Bats Drawn to Plant via “Echo Beacon”. http://news.nationalgeographic.com/news/2011/07/110728-plants-bats-sonar-pollination-animals-environment/

Cold blood vs warm blood? Neither one nor the other

When we are at school and at science class we are taught about the different groups of animals, we are taught that animals can be divided into “warm blooded” and “cold blooded”. Even though this refers to the different thermoregulation mechanisms found in the different animals, this differentiation between cold and warm blood is not completely right. In this entry we’ll explain, in a more scientific way, the different temperature-controlling mechanisms present in the animal kingdom and we’ll give you examples of different species that cross the line between cold blood and warm blood.

WHERE DOES IT GET HEAT FROM?

The first thing we must ask ourselves is where body heat comes from. This may come from two different sources:

Endothermy: Endothermy (“endo”, inside) is the mechanism of obtaining body heat by intern production. Endotherm animals have metabolic mechanisms that generate heat (thermogenesis). To generate heat is energetically costly, which means that these animals have high energetic and nutritional requirements.

The blue-footed booby (Sula nebouxii) and the Galápagos sea lion (Zalophus wollebaeki) are two good examples of endothermic animals. Photo by Peter Stuart Mill.

Ectothermy: Ectothermy (“ecto”, outside) happens in animals that get their body heat from the environment, basking in the sun (heliothermy) or staying in contact with heat sources like sun warmed rocks (tigmothermy), etc… These animals do not present any metabolic mechanisms to generate inner heat, but ectotherms have many behavioural adaptations to obtain or release heat. Therefore, as they do not spend any energy to generate heat, their energetic requirements are usually lower and they do not need to eat as often as endotherms.

A Catalonian wall lizard (Podarcis liolepis) basking in the sun, is a good example of an ectothermic animal. Photo by David López Bosch.

These two concepts only refer to the source of heat, independently of the animal’s ability to regulate or not its body temperature.

IS ITS BODY TEMPERATURE CONSTANT?

The next pair of concepts refers to whether the body temperature of an animal is constant or if it varies over time:

Homeothermy: Homeotherms (“homeo”, same) control their body temperature, making it relatively constant while the environmental temperature varies. As they have constant body temperature, their activity is not conditioned by environmental conditions.

Range of temperatures at which an homeotherm and a poikilotherm can be. while the temperature of the rat is usually around 37oC, the temperature of the lizard is much more variable. Image by Petter Bockman.

Poikilothermy: Poikilotherms (“poikilo”, varied) present a body temperature similar to the environment. Their inner temperature varies as the environment temperature does, and so their activity is pretty much conditioned by environmental conditions. For example, on the Iberian Peninsula, lizards have their optimum temperature at 33-38^oC and snakes at 28-34^oC. Still, as they are used to sudden changes in body temperature, they have an advantage over homeotherms in some habitats, as the latter can die if their body temperature increases or decreases a few degrees.

Thermogram of a snake aroung a human arm, showing how the snake's temperature is similar to that of the environment. Photo by Arno.

Yet these are relative concepts. Homeotherms, while having pretty constant average body temperature, do not have exactly the same temperature in all their body parts (temperature in the trunk and internal organs is usually higher than in the extremities). Similarly, poikilotherms do not always have exactly the same temperature as the environment, because, as most of them are ectotherms, they can increase their body temperature using external heat sources.

Thermograms of a cat, showing that homeothermic animals do not have exactly the same temperature on different body parts. Photos by Yellowcloud.

Using these four definitions, birds and mammals are endotherms (generate metabolic heat) and homeotherms (constant body temperature) while reptiles, amphibians and the rest of animals are ectotherms (obtain heat externally) and poikilotherms (body temperature varies with the environment). But in practice, this isn’t an impassable line. Hereafter we’ll show you various examples of animals that make the line dividing these four concepts even more blurred.

HETEROTHERMY

Heterothermy (“hetero”, different) occurs in animals which are able to switch from endothermy to ectothermy. This usually happens in small birds and mammals with high metabolic rates (quite active and with very high energetic requirements), which decrease their body temperature during inactivity periods. These inactivity periods usually are yearly or daily.

American black bear (Ursus americanus) and cubs hibernating. Photo by National Park Service.

Yearly inactivity periods are usually known as hibernation (or estivation if it happens in summer). Usually hibernators are endothermic mammals such as squirrels, small primates, hedgehogs and many marsupials, and even though bears also hibernate and decrease their metabolism, their body temperature is not so much lowered (only one or two degrees).

The fat-tailed dwarf lemur (Cheirogaleus medius) usually estivates during the malgasy dry season. Photo by Frank Vassen.

During hibernation, the animal’s metabolic rate is drastically lowered and so, they stop regulating their body temperature, making it similar to that of the environment (that’s why hibernator mammals seek refuge in places where the temperature is not as cold as the exterior). When typically homeotherm and endotherm animals aren’t able to find enough food, they pass to a poikilotherm and ectotherm state to save energy.

Graphic of body temperature variation during daily torpor on the Syrian hamster (Mesocricetus auratus). Source Fatemeh Talaei et al.

Other homeotherm animals go through daily poikilotherm periods called torpor, during which their metabolism is also greatly reduced. This typically happens in many species of bats, which have a high body temperature during the night (when they are awake) but this decreases during the day (when they are sleeping). Yet as bats sleep during daytime when temperatures are higher, their temperature does not decrease as much as it would if they slept at night.

Cluster of hibernating Virgina big-eared bats (Corynorhinus townsendii virginianus). Bats usually congregate during hibernation, helping them to stay warm enough during cold weather. Photo by Stihler Craig, U.S. Fish and Wildlife Service.

Hummingbirds can also lower their temperature during night time, but in their case it depends on the quantity of nectar they have consumed during the day. Hummingbirds feed exclusively on nectar, which contains mainly sucrose, which cannot be stored in the organism and passes directly to blood and tissues. This means hummingbirds have an extremely high metabolism and that they need to feed very often to be able to keep up their activity every day.

Purple-throated Carib hummingbird (Eulampis jugularis) feeding on nectar. Photo by Charles J. Sharp.

If during the day they have been able to consume a lot of nectar, at night they are able to maintain their typical body temperature of 39^oC. However, if during the day they haven’t consumed enough nectar, at night they enter a state of torpor, during which their metabolism is greatly reduced and body temperature drops to 12-17^oC. This allows them to save energy to be able to look for more nectar the following day.

Video of a hummingbird that fell into torpor during the night in an artificial feeder in Tennessee. Video by Chip Curley.

Next, we have the case of the naked mole rat (Heterocephalus lager), a rodent from east Africa which is the only known poikilotherm mammal. The naked mole rat’s body temperature is the same as the environmental temperature. Yet some scientists have studied that while its body temperature is exactly the same as the environmental temperature from 12 to 36^oC, because it is not able to thermoregulate, at 28^oC and over its metabolism becomes homeothermic, even though it is not known how it generates or regulates its body heat.

Female naked mole rat (Heterocephalus lager). Photo by Jedimentat44.

REGIONAL AND FACULTATIVE ENDOTHERMY

Finally there are typically ectotherm animals which are able to generate heat and increase their body temperature using various adaptive strategies. Most of these animals use muscle activity to generate body heat.

Many oceanic fish present a complex of veins and arteries called rete mirabile. This complex is found typically in mammals and birds, and is a countercurrent exchange system (artery-vein) which is used to level different parametres (temperature, pH or gas concentrations) in different body parts.

Drawing about the functioning of the rete mirabile. The thick orange arrows indicate heat exchange that returns to the body. Image by Arne Hendriks.

In many oceanic fish like sharks, tunas and marlins, this system allows them to raise their body temperature because their inner muscles are very strong and their temperature is very high. The rete mirabile allows them to distribute through all the body and to keep the heat generated while swimming, making them practically endotherms.

Drawing where we can see the high temperature on the inner muscles of a shark. Drawing by Vittorio Gabriotti.

In fact, the fish called opah (genus Lampris) are the first fish known to be completely endothermic, because thanks to their rete mirabile located in their gills and to a layer of fat covering their bodies and insulating them from the exterior, they can keep their temperature 5^oC above water temperature. Yet they are not homeothermic, as their body temperature can still vary depending on the environment.

Photo of an opah fish (Lampris guttatus). Photo by USA NOAA Fisheries Southwest Fisheries Science Center.

Muscle-driven heating is not found only in fish. Mammals also shiver when we are at risk of hypothermia, because muscular contraction generates heat. Even if it cannot be said that they shiver, many insects and some reptiles also use muscle activity to raise their temperature. When insects need to activate their metabolism, they flutter violently to increase their temperature. This, together with a counter current system (similar to the rete mirabile), makes the insect’s body temperature raise considerably compared to that of the environment.

Thermograms of an insect increasing its body temperature by fluttering. Photos by Crespo J.

Similarly, some reptiles can generate heat. Many pythons incubate their eggs after laying them. To do so, they roll around their eggs and start contracting their body muscles voluntarily to raise the temperature of their clutch.

African rock python (Python sebae) brooding eggs. Photo by Tropicario.

We’ve seen mammals and birds reducing their body temperature, fish generating heat, non-thermoregulating rodents, and reptiles and insects that get warmer moving. As you can see, each species is a unique example of adaptation to its habitat and, even if for an elementary school class it may be useful, dividing animals into warm-blooded and cold-blooded not always the most suitable.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Cover photo: Anudeepanudterrell.
• https://www.ncbi.nlm.nih.gov/pubmed/11824403
• http://www.nature.com/nature/journal/v429/n6994/full/429825a.html
• http://www.nature.com/news/dinosaurs-neither-warm-blooded-nor-cold-blooded-1.15399
• http://jeb.biologists.org/content/214/8/1276.full
• http://www.datasci.com/solutions/thermoregulation/uncovering-the-secrets-of-the-naked-mole-rat
• http://onlinelibrary.wiley.com/doi/10.1046/j.1469-7580.1997.19030321.x/abstract;jsessionid=D9047BAFF5C5A7E048926A2CA53EC554.f01t03