Parental care in the three-toed sloth and the greater anis

In the natural world we can observe a wide variation in the type of care and amount of resources that parents invest in the offspring. Parental care is defined as any form of parental behavior that increases the fitness of the offspring. Parental care includes preparation of nests and burrows, production of large eggs (which are generally associated with increase hatchability and the survival of the young), care of eggs or the young inside or outside the parent’s body, provisioning of young before and after birth and care of the offspring after nutritional independence.  Parental investment, however, is generally defined as the action of parents that increases the fitness of their offspring at the cost of the parents’ future reproduction. In many species, investment is limited in producing eggs or live offspring, whereas in other species both parents invest in substantial periods of parental care, sometimes even after nutritional independence, as in several bird species and some mammals including humans (Gonzalez-Voyer, A and N. Kolm, 2010). The energy put into parental care is an investment by the parents to successfully pass their genes from one generation to the next.

Here we briefly describe parental care in the three-toed sloth which we saw in its natural environment and was particularly captivating, and the Greater Ani, which is a fascinating bird which Christie Riehl talked about in her presentation.

In humans, we often see mothers feeding their babies and holding them in their arms providing protection.  In nature, we find mammals that live in tall trees, and also show strong affection and provide protection to their offspring in similar ways to humans. An interesting example is the three-toed sloths which can be found living high up in the forest canopy. Female sloths give birth to a single offspring once a year. After mating, the male leaves and doesn’t provide any parental care to the young. Baby sloths are held ventrally to the mother, which is the way the mother protects the baby from predation and provides protection. At the San Lorenzo Canopy Crane, which is around 52 m high and 54 m long, we saw a three-toed female sloth with her baby sitting at the top of a tree. It was amazing to see a female sloth with her baby and to watching how the mother protected her baby from the intense gusts of winds at the top of the canopy. She even moved from her sitting position as the crane approached the tree she was on.

Three-toed female sloth and her baby. Photo credit: Flor Santiago

 

An interesting example of parental care and investment in birds is the Greater Anis (Crotophaga major). A number of females Anis lay their blue eggs in the nests and then share incubation and feeding. Each breeding female removes any eggs in the communal nest prior to laying her first egg. Each female stops removing eggs after she has laid her first egg, presumably to avoid accidentally ejecting her own egg. Females who lay first, therefore, nearly always lose their first eggs to ejection, while females who lay last typically lose none. The number of eggs that are ejected increases with group size, since multiple females rarely begin laying on the same day.

A communal nest of a breeding group of Greater Anis, containing 10 eggs laid by three unrelated adult females. Photo credit: Christina Riehl

A communal nest of a breeding group of Greater Anis, containing 10 eggs laid by three unrelated adult females. Photo credit: Christina Riehl

 

Interestingly, female anis can also act as nest parasites, “dumping” their eggs in the nests of neighboring groups and providing no subsequent parental care. Host groups do have a defense against being parasitized, however. Ani eggs actually change color over time, allowing adults to distinguish between freshly laid eggs and those that have already been incubated. If a parasitic female manages to sneak into a host nest and lay her eggs during the same time window as the host females, the host group can’t recognize the parasitic egg. But if the parasitic egg is laid asynchronously relative to the host clutch, the hosts can recognize and reject it. Unlike most birds that breed cooperatively, Greater Anis nest in groups with genetically unrelated individuals. Because individuals in breeding groups gain no benefits from kin selection, theory predicts that the direct benefits of cooperation – as well as the incentives to compete and cheat – must be quite high (http://gradworks.umi.com/34/63/3463317.html).

 

 

Thanks for reading,

Anakena and Christina

 

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